Scholarly article on topic 'Potential implementation of underbalanced drilling technique in Egyptian oil fields'

Potential implementation of underbalanced drilling technique in Egyptian oil fields Academic research paper on "Earth and related environmental sciences"

CC BY-NC-ND
0
0
Share paper
Keywords
{"Underbalance drilling" / "Drilling fluids" / "Drilling cost" / "Hole problem"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — K.A. Fattah, S.M. El-Katatney, A.A. Dahab

Abstract The need to increase productivity and to reduce drilling damage favors the use of underbalanced drilling (UBD) technology. In highly depleted reservoirs, extremely low-density fluids, such as foams or aerated mud, are used to achieve circulating densities lower than the pore pressure. In such cases, the induced modification of the in situ stresses has to be supported mainly by the rock, with little contribution from the drilling fluid pressure. The application of underbalanced drilling depends on the mechanical stability of the drilled formation, among other factors. In general, poorly consolidated, depleted formations are not suited for that technology. In this paper, 23 UBD worldwide cases have been analyzed; two of which are from Egyptian fields and the others are from Iran, Algeria, Kuwait, Oman, Texas, Mexico, Indonesia, Canada, Libya, Middle East, Qatar, Saudi Arabia and Lithuania. From these analyses, the reasons of failure or success have been stated. The reasons of success included depleted reservoirs and highly fractured carbonates formation while, the reasons of failure include over pressurized shale, highly tectonic stress areas, and downhole failures. The main attractive application of this technology was proposed to be only in the reservoir section, and the target was to prevent the reservoir damage and hence increase the productivity and recovery factor. A proposed underbalanced drilling program is developed based on these analyses to be used in the three main regions in oil and gas producing Egyptian fields. The aerated mud was selected as a drilling fluid to drill the reservoir section in Western Desert and Gulf of Suez region whereas the single phase fluid was selected as a drilling fluid in the Nile Delta region.

Academic research paper on topic "Potential implementation of underbalanced drilling technique in Egyptian oil fields"

Journal of King Saud University - Engineering Sciences (2011) 23, 49-66

King Saud University Journal of King Saud University - Engineering Sciences

www.ksu.edu.sa www.sciencedirect.com

REVIEW

Potential implementation of underbalanced drilling technique in Egyptian oil fields

K.A. Fattah a *, S.M. El-Katatney b, A.A. Dahab b

a Petroleum and Natural Gas Engineering Department, College of Engineering, King Saud University, P.O. Box 800,

Riyadh 11421, Saudi Arabia b Petroleum Engineering Department, Faculty of Engineering, Cairo University, Egypt

Received 20 October 2009; accepted 10 February 2010 Available online 7 December 2010

KEYWORDS

Underbalance drilling; Drilling fluids; Drilling cost; Hole problem

Abstract The need to increase productivity and to reduce drilling damage favors the use of under-balanced drilling (UBD) technology. In highly depleted reservoirs, extremely low-density fluids, such as foams or aerated mud, are used to achieve circulating densities lower than the pore pressure. In such cases, the induced modification of the in situ stresses has to be supported mainly by the rock, with little contribution from the drilling fluid pressure. The application of underbalanced drilling depends on the mechanical stability of the drilled formation, among other factors. In general, poorly consolidated, depleted formations are not suited for that technology.

In this paper, 23 UBD worldwide cases have been analyzed; two of which are from Egyptian fields and the others are from Iran, Algeria, Kuwait, Oman, Texas, Mexico, Indonesia, Canada, Libya, Middle East, Qatar, Saudi Arabia and Lithuania. From these analyses, the reasons of failure or success have been stated. The reasons of success included depleted reservoirs and highly fractured carbonates formation while, the reasons of failure include over pressurized shale, highly tectonic stress areas, and downhole failures. The main attractive application of this technology was proposed to be only in the reservoir section, and the target was to prevent the reservoir damage and hence increase the productivity and recovery factor.

Corresponding author. E-mail address: kelshreef@ksu.edu.sa (K.A. Fattah).

1018-3639 © 2010 King Saud University. Production and hosting by Elsevier B.V. All rights reserved.

Peer review under responsibility of King Saud University. doi:10.1016/j.jksues.2010.02.001

A proposed underbalanced drilling program is developed based on these analyses to be used in the three main regions in oil and gas producing Egyptian fields. The aerated mud was selected as a drilling fluid to drill the reservoir section in Western Desert and Gulf of Suez region whereas the single phase fluid was selected as a drilling fluid in the Nile Delta region.

© 2010 King Saud University. Production and hosting by Elsevier B.V. All rights reserved.

Contents

1. Introduction........................................................................................................................................................50

2. Studied cases........................................................................................................................................................54

2.1. Case 1: gulf of Suez area............................................................... 54

2.2. Case 2: western desert gas field area....................................................... 54

2.3. Case 3: Iranian oil field................................................................ 55

2.4. Data analysis....................................................................... 56

3. Proposed UBD program to be implemented in Egyptian fields................................................................................59

3.1. Gulf of Suez oil field area.............................................................. 59

3.2. Western desert oil field area............................................................. 60

3.3. Nile delta oil field area................................................................ 65

4. Conclusions ........................................................................................................................................................65

Appendix A........................................................................................................................................................65

References..........................................................................................................................................................65

1. Introduction

Drilling cost is considered one of the major components of operating cost in the petroleum industry. Improving the penetration rate of drilling operation and reducing drilling problems, such as pressure differential pipe sticking and lost circulation, have long been considered an effective way of decreasing drilling costs. The overbalance pressure, generally recognized as the most important among the many factors affecting penetration rate, is often defined as the pressure differential between the borehole pressure and formation fluid pressure (Murray and Cunningham, 1955; Eckel, 1957; Cunningham and Eenink, 1959; Gamier and van Lingen, 1959; Vidrine and Benit, 1968; Bourgoyne and Young, 1974; Black and Green, 1978). Formation pressures lower than the static pressure of a column of fresh water require the use of a lighter fluid, such as air, injected with liquid to obtain lower overbalance pressure to enhance penetration rate and to minimize lost circulation and pipe sticking as well as formation damage. Therefore, aerated mud drilling "implies the use of air or natural gas as the circulating medium instead of the regular mud'' is becoming an attractive practice in some areas. The commercial use of aerated mud drilling began only in recent years (Rankin et al., 1989; Claytor et al., 1991). Low-density drilling fluids used in underbalanced drilling consist of air, mist, stable foam, and aerated mud foam with back pressure. Whereas the term ''aerated mud'' implies the simultaneous introduction of air and mud together into the standpipe in order to drill special types of formations (Godwin et al., 1986; Boyun and Rajtar, 1995; Salah El-Din and El-Katatney, 2009).

The main advantage of air as a circulating fluid is that being the lowest density fluid. It imposes minimum pressure on the formation to be drilled. High penetration rates have been achieved in hard and dry formations with the use of air as a circulating fluid. In addition to high penetration rate, longer

bit life results through the use of this medium as compared to mud. Drilling rates as high as 90 ft/h have been attained in shales. Air drilling, however, is restricted to areas where high volume water sands are not present ahead of the producing zone. The rate of water influx that can be handled in the case of air drilling is also not well known. Other inherent disadvantages of using air or natural gas as drilling fluids include possibility of downhole fires and explosions, and sloughing of formations due to underbalance of stresses around the wellbore. Possibility of downhole explosions are of particular concern in air drilling operations. Small dust-like particles are generated as a result of rock cuttings (chips) being ground and pulverized by the drill string in the annulus, and collision of cuttings with each other, the tool joints, and the wall of the borehole due to the high velocity forces. In the presence of moisture, seal rings may form at tight places in the annulus, which create pressure chambers. With additional influx of natural gas from gas-bearing zones being penetrated by the bit, an explosion may easily occur.

Besides having formations suitable for air drilling, the most important consideration in drilling with air is the volume of air required. Air drilling often fails because of insufficient volume of air necessary to clean the hole efficiently under certain conditions, e.g., wet hole, sloughing shales, and influx of formation water. A practical rule of thumb for determining adequate air volume is that the volume required achieving 1000 ft per minute annular velocity to clean the hole properly (Godwin et al., 1986; Boyun and Rajtar, 1995).

Drilling with foam has some appeal due to the fact that foam has some attractive qualities and properties with respect to the very low hydrostatic densities, which can be generated with foam systems (Hooshmandkoochi et al., 2007; Moore and Lafave, 1956; Maurer, 1998; Bentsen and Veny, 1976). Foam has good rheology and excellent cutting transport properties. The fact that foam has some natural inherent viscosity

Table 1 Change in BHCP versus mud rate and N2 rate.

Duration (h) Mud rate (GPM) N2 rate (SCF/m) N2 (%) SPP (psi) BHCP (psi) ECD (kg/lit) Gain (bbls) MWD Signal

1.5 250 250 4.8 2250 2887 0.86 31 mud Ok

3.0 250 500 9.1 2250 2660 0.79 34 mud Ok

2.0 240 500 11.4 1800 2652 0.79 0 Ok

1.5 230 500 13.2 1600 2620 0.78 0 Ok

1.5 210 500 15.5 1450 2590 0.77 0 Ok

1.5 180 500 21.6 1120 2549 0.76 0 Ok

Figure 1 Well profile diagram for case 2: western desert gas field area.

Choke pressure = 39D psi Hole Size ]№ Inch

s/Kj...........

\ : > ................ !

\ xs. Working wi ndow^

250 500 750 1000 1250 1500 1710 2010 2259 2509 2750 39D0 Gas rale (scft/mmj

-flowiata =80 gpm -flcwraie = 100 gpm--■ilowiale = 12Q gpm -RessrmirPressura - -Target Pressure

—Mai Motor ELY IVin Motor ELV -Min Horizontal Velocity -Cultings Transport Ralio

Figure 2 Working window for case 2: western desert gas field area.

Figure 3 Well profile diagram for case 3: Iranian oil field area.

6HCP vs Oil and Nitrogen Injection Rates

NIOC. GS 333 Gachsaran Field. Iran

Z 2400 o

Sialic Reservw 5ressure 2,622 psi

Win. Up x$ Vebcty \ 32Cgm a* Motor FToiv Rate LJ

IfltlmK 1-'

Un Motor Ft jw Rale

1000 1200 N2 Injection rate (>cfm)

Figure 4 Operational envelope - native crude for case 3: Iranian oil field area.

Table 2 Recorded ROP in Algeria.

Algeria sandstone reservoir

Well number ROP overbalanced ROP underbalanced

(ft/h) (ft/h)

1 10.4 19.5

2 10.4 17.6

3 19.3 22.5

4 19.5 22.3

5 13.5 45

6 17 26.6

□ Overbalanced

□ Underbalanced

10 --I

1 2 3 4 5 6 Well number

Figure 5 Comparison between ROP in OB and UB cases.

Figure 6 Relationship between ROP and pressure drop.

as well as fluid loss control properties, which may inhibit fluid losses, makes foam a very attractive drilling medium. During connections and trips, the foam remains stable and provides a more stable bottom hole pressure. It is a particularly good drilling fluid with a high carrying capacity and a low density. The foam normally remains stable, even when it returns to the surface, and this can cause problems on a rig if the foam cannot be broken down fast enough. In earlier foam systems, the amount of defoamer had to be tested carefully so that the foam was broken down before any fluid entered the separators. In closed circulation drilling systems, stable foam could cause particular problems with carry over. The recently

Table 3 ROP versus pressure drop for UBD wells.

Reservoir Pressure Rate of Lithology

pressure (psi) drop (AP) (psi) penetration (ft/h)

2900 290 45 Sandstone

3000 360 38 Sandstone

1350 540 16 Sandstone

3200 640 27 Sandstone

5500 990 30 Sandstone

Table 4 Recorded data for UBD wells.

Pressure ROP Production while Production after Lithology

drop (psi) (ft/h) drilling (%) test (%)

290 26.6 0 1.2 Sandstone

320 44.7 0.8 3.9 Sandstone

350 19.45 1 2 Sandstone

406 22.5 1.5 1.8 Sandstone

435 17.6 2.7 3.4 Sandstone

o c o a

MO a) re CC

50 45 40 35 30 25 20 15 10 5 0

280 380 480 580 680 780 Pressure drop (psi)

880 980 1080

Figure 7 ROP versus pressure drop for UBD wells in different

reservoirs.

ÏS 30

£ 10 o

3 0 a.

330 380

Pressure drop (psi)

Figure 8 ROP versus pressure drop for UBD wells in one reservoir.

developed stable foam systems are simpler to break, and the liquid can also be refoamed so that less foaming agent is required and a closed circulation system can be used. These systems, in general, rely on either a chemical method of breaking and making the foam, or the utilization of an increase and decrease of pH to make and break the foam. The foam quality at surface used for drilling is normally between 80% and 95%.

The quality of foam means that the system is 80-95% gas, with the remaining 5-20% being liquid. Downhole, due to the hydrostatic pressure of the annular column, this ratio changes as the volume of gas is reduced. An average acceptable bottom-hole foam quality (FQ) is in the region of 50-60%. Fluid densities for foam range from 1.6 ppg to 6.95 ppg (0.20.8 S.G.) (Godwin et al., 1986; Boyun and Rajtar, 1995). The density ranges are adjusted with the make up of the foam by adjusting the Liquid Volume Fraction (LVF) through the injection of liquid and gas by adjusting the backpressure on the well. The backpressure adjusts the downhole pressure and slows down the velocities in the annulus. Experience has proven that foam is able to handle over 100 bbl/h of water influx (Godwin et al., 1986; George and Waston, 1956; Boyun and Rajtar, 1995).

So, the objective of this research work is to investigate and analyze many worldwide applications of underbalanced drilling and state the reasons of success or failure of this application. Based on these analyses, a proposed underbalanced drilling program is developed. In this proposed program, the method of selecting the appropriate technique to be applied for these candidate areas are selected according to the geology of the area and the bottom hole conditions inside the wells.

2. Studied cases

In this section, three case studies from Egyptian fields and other places are analyzed in detail and a summary of 20 cases from other worldwide fields are given with a brief discussion about their objectives, problems and results (Salah El-Din and El-Katatney, 2009).

2.1. Case 1: gulf of Suez area

The well is located at onshore Belayim oil field. The well target was sandstone of zone III (Belayim formation, Feiran member) at a total depth of 2335 m TVD, 2854 m MD. The pressure in Zone III (sandstone) was estimated to be 30003500 psi (0.3917-0.4569 psi/ft). The objectives of UBD were to increase rate of penetration, enhance Well control, reduce occurrence of lost time incidents, and increase well productivity. The 20 m of the new hole at 7 in. liner shoe at 2659 m MD was drilled with only mud, then the MWD signal test was performed (inflow test and also to test the optimum rate combination for better MWD signal) as shown in Table 1. Based on this test, the formation pressure was estimated to be less than 2500 psi that was confirmed at 2400 psi from vacuum test and

300 <u

0 12 3

Production while drilling

Figure 9 Production while drilling versus pressure drop for UBD wells.

the MWD can work up to 21% nitrogen. Nitrified mud (500 SCFM + 230 gpm diesel) was applied while close balance drilling the six in original and side-track lateral section. The six in hole was drilled to depth 2830 m utilizing UBDS and powerpack motor of 1.15° BH c/w MWD Impulse, VPWD, ADN tools (inclination at bit, annulus and string pressure, GR resistivity, density-neutron) with 2 x 3-1/2 in. W.FORD float valve + motor restriction sub (nozzle 14/32 in.) for improving MWD signal. The analysis of this well results showed that, The ROP was enhanced drastically in sand from 4 m/h while sliding to 50 m/h, and in anhydrite was 8-10 m/h (experienced 2-4 m/h in normal overbalance drilling), the use of rotating head helped to control well while tripping and also in case of separator carry over problems, and the Crew acquired UBD work experience.

2.2. Case 2: western desert gas field area

The well is located at the central part of the western desert block. The well target was to drill 3-7/8 in. x 500 m horizontal

■ Production while drilling □ production after test

Well Number

Figure 10 Comparison of production while and after UBD drilling.

Table 5 Drilling time and cost savings for 8 5-1/2" hole section

drilled underbalanced conditions.

Well Real cost Clean cost (just drilling)

Days K$ Days K$

8-1/2" hole - conventional

1 27 1171 27 1171

2 25.7 1146.3 24.4 1114

3 30.4 2125.3 21.6 1771.9

4 19.3 1360.1 17.6 1230.8

5 31.9 2215.7 16.7 1629.3

6 23.3 1058.5 22.4 1035

7 31.4 1385.1 23 1005.6

8 21.6 1241.5 17.8 989.9

9 20.7 899.1 17.2 667.4

10 34.1 1551.6 30.3 1300.1

Average 26.5 1415.4 21.8 1191.5

8-1/2" hole - underbalanced

1 20.5 1652 14.8 1395.6

2 19 1458 13.7 1243.5

3 21.2 1998.6 16.5 1541.5

4 17.8 1193.6 15.7 728

5 12.9 597 12.2 553.9

Average 18.3 1379.8 14.6 1092.5

section in unit 3 of the Mesozoic Lower Safa reservoir. They are composed of low to medium permeable (1-500 md) micaceous sandstones deposited in a strong tidally influenced estuary, Fig. 1. Lower Safa formation comprises a high-energy sequence of Estuarine deposits with a total average thickness of 110 m in the area where is planned, although only 29 m of these thickness are considered productive. The objective of UBD was to prevent reservoir damage. Gasification was through drill pipe injection technique.

The well was completed as open hole. Average ROP during overbalanced drilling operations on offset wells has been historically 2-3 m/h in the horizontal section. Historical data for UBD wells suggested that there will be an improvement in ROP due to the elimination of the chip hold-down effect.

It was estimated that the ROP will be between 5 and 10 m/h. The drilling fluid of choice was produced water. The drilling fluid could be separated from the produced hydrocarbons and re-used. Due to the CO2 content of the reservoir (up to 9%) and the use of nitrogen (up to 5% O2), corrosion mitigation was required. Once the well started to produce during the drilling phase, the N2 was stopped, which in turns eliminated excessive use of corrosion inhibitors. Water and nitrogen gave the desired underbalanced margin when kicking off the well, and water was treated with suitable chemicals for corrosion mitigation. It became apparent that the Lower Safa formation was normally pressured. Hence by using just water, the BHP will be 260 psi underbalanced. Nitrogen was required to create a greater draw down than the 260 psi as it is unknown at what draw down the matrix starts to contribute to the inflow.

As soon as the well produced, nitrogen was cut down to zero rates. Nitrogen injection was required again every time the drill string tripped through the Down-hole Deployment Valve (DDV) to remove the water from the reservoir section.

Fig. 2 shows the working window (operating envelope) for the well (case 2) with no reservoir inflow for, 3-7/8 in hole, 3-1/ 2 in. x 2-7/8 in. drill pipe design, 2 x 500 m legs, and bit at TD. Also plotted on the operating envelope, are the various constraints that must be fulfilled during underbalanced drilling operations. After drilling 200 m, the drilling had been stopped due to failure of downhole equipment due to high temperature.

2.3. Case 3: Iranian oilfield

The target reservoir for this well was Asmari formation, the formation was fractured carbonated formation. The reservoir drive mechanism was gas cap. Shale strings were not expected in this formation. Expected reservoir pressure and temperature were 2622 psi and 141 0F, respectively. Reservoir fluid was oil with API gravity of 250, GOR 564 SCF/STB, and H2S concentration of 240 ppm. The permeability of the reservoir was 0.1-1000 md with a porosity of 9% (Hooshmandko-ochi et al., 2007). The well was drilled from m (9-5/8 in shoe depth) to a total depth of 2938 m MD (2567 m TVD), Fig. 3. The primary objectives of this underbalanced drilling project were to: minimize drilling induced formation damage, eliminate drilling fluid losses, and improve drilling performance. The drilling fluid selection was one of the most critical decisions in planning an underbalanced well. The right fluid(s) selection will not only lead to suitable BHCP but will also minimize pressure transients and thus eliminating/minimizing formation impairment. The deviated underbalanced section of this well was to be drilled with a Gachsaran field native crude oil and a membrane nitrogen generation circulating system. Liquid Phase, the native crude oil, was chosen over Diesel and other drilling fluids because it is the natural reservoir fluid for this well. This minimized chances of formation

Table 6 Drilling time and cost savings for 6-1/2" hole section drilled underbalanced conditions.

Well Total cost Drilling cost

Days K$ Days K$

6-112" hole - conventional

1 9 886.6 9 886.6

2 11.8 591.8 11.8 591.8

3 20.7 1186.4 18.1 1082

4 29.6 1596.7 17.8 644.7

5 33.5 2074.1 20 1531.9

6 21.9 928.1 19.7 779.9

7 19.1 995.5 17.8 938.3

8 14.1 778.5 11.8 650.6

9 16.4 800.8 16.4 800.8

Average 19.6 1093.2 15.8 878.5

6-1/200 hole - underbalanced

1 7.4 507.8 6.6 471.9

2 24 1664.6 11.9 998.9

3 22.4 1804 17.2 1057.7

4 14.8 545.1 10.8 387.57

5 9.5 580.6 9 560.6

Average 15.6 920.4 11.1 695.3

Table 7 Gulf of Suez reservoir characteristics.

Parameter Belayim Kareem

Pressure 1500 psi 1700 psi

Temperature 180 °F 190 °F

Gas-oil ratio (GOR) 15-17 SCF/STB 20 SCF/STB

Porosity (md) 18-20% 20- 22%

Permeability 200 md 500 md

API0 gravity of oil 20-23 20- 30

H2S concentration No No

Table 8 Gulf of Suez formation characteristics.

Formation Lithology Top (m) Thickness (m) Pore pressure (psi)

Belayim

Hammam Faraun Shale-sand 2160 35

Ferran Shale-sand 2195 140

Sidri Mainly sand 2335 65 1500

Babaa Anhydrite 2400 15

Kareem Limestone 2415 195 1700

Table 9 Underbalanced drilling design parameters for Gulf of Suez area.

Rig modification

Well plan

Drill string design

Drilling fluid selection A-liquid phase B-gas phase Operating envelope

Hole cleaning Motor performance

Production sensitivity

Data acquisition Completion

> No essential modifications to be made on the rig to suite UBD operations

The substructure has to be high enough to allow Rotating Control Head (RCH) to be installed on top of the Hydril As shown in Fig. 11

■ Use a 5" DP and 5" HWDP on 6-3/4" DC

■ The BHA consists of 6-1/2" mud motor and MWD to drill 8-1/2" hole

■ An 8-1/2" bit size of 3 x 13/32" nozzles

The deviated section will be drilled using an oil bas mud and a membrane nitrogen generation circulating system

> Drilling fluid is native crude oil with density 7.6 ppg (0.91 S.G. or 20° API)

> Liquid flow rates were selected to achieve a drawdown from the reservoir pressure Nitrogen was selected as the injection gas

Nitrogen will be obtained from the surrounding air and generated onsite

> A minimum drawdown at the bit of 100 psi is required to ensure adequate underbalanced conditions in the well

> Using 300 gpm and more than 2400 scfm of Nitrogen will provide maximum 100 psi drawdown from the expected reservoir pressure, as shown in Fig. 12

In case the real reservoir pressure will result below the expected value, then the liquid injection rate should be reduced increasing the risk for a hole cleaning issue

Minimum annular liquid velocities in deviated holes of 210 ft/min when crude oil is used as the drilling fluid to ensure that the drilled cuttings are effectively removed from the wellbore A wiper drilling trip will help clear the problem of hole cleaning The motor should be suitable for oil/nitrogen two-phase application

> A maximum Equivalent Liquid Volume through the motor of 600 gpm was used as reference

> A pressure loss of 800 psi between downhole motor and MWD was considered The motor should not have a bypass valve on top of it

As more reservoir fluids (oil and gas) introduced into the wellbore, the bottomhole circulating pressures (BHCP) will decrease

BHCP will therefore be controlled by increasing liquid injection and/or decreasing nitrogen injection, based on real-time BHCP data from the MWD tool BHCP could also be controlled with surface backpressure

Choking will be necessary in stabilizing the circulating system during and after drill string connections The software for the rig data acquisition has to be able to interface with the UBD equipment software The well can be completed with barefoot completion technique, or installing a slotted liner completions

damage in event of pressure transients and/or from fluid imbibitions. The well was displaced with the produced fluid after getting enough oil production. Gas Phase, nitrogen, was selected as the injection gas because of its inert nature, economic availability and suitability for this specific underbalanced drilling project. Nitrogen was obtained from the surrounding air and generated onsite, by nitrogen production unit (NIOC's). The multiphase flow behavior in the wellbore during underbalanced drilling was very complex. The response of the downhole conditions to changes in various flow parameters must be characterized prior to the commencement of underbalanced drilling operations in order to maximize chances of success. Fig. 4 contains a plot of the bottom hole circulating pressures induced by a variety of nitrogen rates and the Gachsaran native crude oil injection rates. This plot was referred to as the operating envelope. Also plotted on the operating envelope, are the various constraints that must be fulfilled during underbalanced drilling operations. The range of flow rates that satisfy all of the constraints, defined the acceptable operating region. A minimum drawdown at the bit of 200 psi was required to ensure adequate under-balanced conditions in the well, with a maximum drawdown of 300 psi to minimize any near wellbore depletion effects. The target bottom hole circulating pressure at the bit for this well was 2300-2400 psi.

UBD on this well experienced some typical logistical and start up problems associated with a steep learning curve, this being the first such operation in Iran. Despite all the problems encountered in this well, the following performance had been achieved: drilled to 308 m of total open hole depth, no loss circulation was encountered while drilling, successfully implemented UBD technology, and no Quality, health, safety and environment (QHSE) incidents were recorded. Data for case 4 to case 23 are given in the Appendix A (Azeemddin, 2006; Bates, 1965; Bennion et al., 1998; Dorenbos and Ranalho, 2002; Gordon, 2005; Gray, 1957; Hongren et al., 1999; International Association of Drilling Contractor, 2005; Kuru, 1999; Louison et al., 1984; Maclovio, 1996; Meng, 2005; Moore et al., 2004; Nas, 2004; Negra et al., 1999; Parra et al., 2003; Qutob, 2007; Qutob and Ferreira, 2005; Sunthan-kar, 2001, Weatherford Company, 2006; Westermark, 1986; Whiteley and England, 1986; Zhou, 2005).

2.4. Data analysis

The following analysis is carried out based on some actual wells drilled underbalanced worldwide. As mentioned before, the main advantage of underbalanced drilling techniques is to increase the rate of penetration as compared with overbalanced drilling techniques.

Table 2 gives the recorded data that were collected from successful underbalanced drilling cases in which the aerated mud was used to drill sandstone reservoir sections (Moore and Lafave, 1956).

From Fig. 5, there is an observed increase in ROP in all cases that were drilled by underbalanced techniques. In under-balanced drilling, ROP was increased due to the disappearance of chip hold-down effect. So the normal trend includes that an increase of the ROP resulted from a decrease in the hydrostatic pressure of drilling fluid as compared with the pressure of the formation that drilled by UB, as shown in Fig. 6.

Table 3 gives the recorded data of ROP (ft/h) and pressure drop (psi) for different reservoirs that were drilled by aerated fluid as an UBD drilling fluid. These reservoirs have the same lithology but having different reservoir pressure.

Table 4 gives a recorded data for different wells drilled by aerated fluid in a reservoir that has a constant pressure and same lithology compared to those wells drilled in overbalanced environment (Moore and Lafave, 1956).

Fig. 7 illustrates that ROP initially decreases with an increase in pressure drop and increases with further increase in pressure drop. Whereas, Fig. 8 shows that ROP has no

Figure 11 Well schematic of Gulf of Suez oil field area.

600 -400 ■ 200 -

0 -i-1-i-i-1-1-

0 200 400 600 «00 1000 I200 1400 1600 1800 2000 2200 2400 2600 Nitrogen Injection Rate (scfm)

Figure 12 Operating window, multiphase fluid injection of Gulf of Suez oil field area.

Table 10 Underbalanced drilling design criteria for western desert area.

Rig modification

Well plan

Drill string design

Drilling fluid selection

A-liquid phase

B-gas phase Operating envelope

Hole cleaning

Hydraulic modeling

Pressure while drilling

Data acquisition Completion

> No essential modifications to be made on the rig to suite UBD operations

> The substructure has to be high enough to allow Rotating Control Head (RCH) to be installed on top of the Hydril

> As shown in Fig. 13

■ Use 5" DP, 5" HWDP and 6.5" DC No downhole motor used

■ An 8-1/2" bit size of 3 x 13/32" nozzles size

> Based on the pore pressure and formation depth, the reservoir formation is below the normal pressure regime

> The subnormal pressure requires the use of a multiphase (liquid + gas) drilling fluid system in order to obtain on Underbalanced drilling condition

■ Drilling fluid is native crude oil with density 6.84 ppg (0.82 S.G. or 41.7° API)

> Liquid flow rates were selected to achieve a drawdown from the reservoir pressure

> Nitrogen was selected as the injection gas

> It is displayed as the area of the graph between the targets BHCP's, bound by the maximum motor throughput, the minimum annular liquid velocity, Fig. 11

Using 300 gpm and more than 2200 scfm of Nitrogen will provide maximum 200 psi drawdown from the expected reservoir pressure

> Depends on several variables such as cutting size and shape; liquid properties; drill string rotation; liquid velocities; flow regime, etc.

> Minimum vertical annular liquid velocities of 180 ft/min when crude oil is used as the drilling fluid to ensure that the drilled cuttings are effectively removed from the wellbore

Using a multiphase hydraulic simulator, the required underbalanced drilling parameters could be evaluated in detail

Graphs can be created to incorporate the limiting factors of minimum annular liquid velocity required for hole cleaning and the desired BHCP range

> When the maximum gas volume fraction (GVF) inside the drill pipe is bellow, 20% conventional mud pulse tools (MWD/LWD/PWD) can be used

> Otherwise, electromagnetic transition tools have to be used in order to obtain downhole data real time The software for the rig data acquisition has to be able to interface with the UBD equipment software The well can be completed with barefoot completion technique, or installing a slotted lined

Figure 13 Well schematic of western desert oil field area.

definite relation with pressure drop if other drilling parameters are ignored. However a continuous increase in formation fluid production while drilling was observed with the continuous increase in pressure drop as shown in Fig. 9.

Fig. 10 illustrated that all wells drilled by UBD have an increased in fluid production rate compared to those wells drilled in overbalanced environment. In addition, there is no clear relation between the amount of fluid production while drilling

and the amount of fluid production after the well is put on production as shown in Fig. 10.

Table 5 highlights the savings in total rig days and cost for conventional versus underbalanced drilling wells in Iran (Roving and Reynolds, 1994). It is clear that big savings in drilling cost was realized.

The cost savings ranged between $90,000 and $110,000 for 8-1/2 in. hole section and between $170,000 and $190,000 for the 6-1/2 in. hole size (Table 6). A total of approximately

Nitrogen Injection Rate (scfm)

Figure 14 Operating window, multiphase fluid injection of western desert oil field area.

Table 11 Proposed UBD program in Nile Delta area.

Rig modification • No essential modifications to be made on the rig to suite UBD operations

• The substructure has to be high enough to allow Rotating Control Head (RCH) to be installed on top of the

Hydril

Well plan • As shown in Fig. 15

Drill string design • Use a 5" DP, 5" HWDP and 6.5" DC

• An 8-1/2" bit size of 3x13/32'' nozzles

BHA • The BHA consists of 6-1/2" PDM mud motor and MWD to drill 6" hole

• If MWD signal doesn't observed, use electromagnetic MWD tools

Drilling fluid selection • Water based fluid (flow-drilling operation)

• Drilling fluid is water with density 8.75 ppg (1.05 S.G.)

• Liquid flow rates and surface choke backpressure were selected to achieve a drawdown from the reservoir

pressure

Operating envelope • It is recommended to pump at least 400 gpm of liquid phase to avoid any operational problem related with hole

cleaning

• The drawdown is 200 psi to prevent wellbore collapse

Motor performance • A maximum equivalent liquid volume through the motor of 600 gpm was used as reference

• A pressure loss of 800 psi between downhole motor and MWD was considered

Hole cleaning • Minimum annular liquid velocities in deviated holes of 180 ft/min to ensure that the drilled cuttings are effec-

tively removed from the wellbore

• A wiper trip will help clear the hole cleaning problem

Tripping • Some type of snubbing device can be used, or a downhole isolation valve can be installed

• Balancing the well for trips seemed the simplest and least expensive method

Data acquisition • The software for the rig data acquisition has to be able to interface with the UBD equipment software

Completion • The well can be completed with barefoot completion technique, or installing a slotted lined

$1.4MM has been saved (drilling only) and about $1MM (overall), for the five wells drilled.

3. Proposed UBD program to be implemented in Egyptian fields

Based on the experience and the problem faced discussed in the previous discussions, a proposed UBD program is given here-below.

3.1. Gulf of Suez oil field area

The selected example includes drilling through the reservoir section, which consists of two production formations (Belayim and kareem formation from Miocene age). The reservoir and formation characteristics are given in Tables 7 and 8.

The selected reservoir can be drilled by underbalanced drilling technique and the proposed UBD program is given in

Figure 15 Well schematic of Nile delta oil field area.

4000 3900 3800 3700 3600 3500 3400 3300 3200 3100

Reservoir Pressure

-100 psi BP 200 psi BP 300 psi BP 400 psi BP 500 psi BP 100 psi prod 200 psi prod 300 psi prod 400 psi prod 500 psi prod

300 350 400 450 Liquid flow rate (gpm)

Figure 16 Operating window, flow-drilling operation for Nile delta oil field area.

Table 9. Fig. 12 shows the operating window, multiphase fluid injection of Gulf of Suez oil field area.

3.2. Western desert oil field area

The selected example includes drilling through the reservoir section, which consists of Alam El Buieb formation of Cretaceous

age. The lithology of this formation is sandstone with depleted reservoir pressure 1600 psi, reservoir temperature 219 of, porosity 19%, permeability 200 md, GOR 95 SCF/STB, 41.70 API gravity of oil, and there is no H2S concentration. The selected reservoir can be drilled by underbalanced drilling technique as given in Table 10. Fig. 14 shows the operating window, multiphase fluid injection of western desert oil field area.

Table A.1 Summary data of case 4 to case 23.

Information Objective(s) Results

Case 4 - SE (U.S.) area

Location SE (U.S.) area Improve production rate by eliminating Production rate increased from 6 MMcfd to

formation damage 24 MMcfd

Formation Smackover & norphlet carbonates

Depth 18,300 ft TVD Reduce/eliminate fluid losses to expedite well Hostile operating environment (H2S and

clean-up 350 °F BHT) safely drilled using UBS

techniques (no QHSE incidences)

Pore press 2700 psi

Well type Vertical New Drill

Hole size 6-1/2

Case 5 - Texas Panhandle area

Location Texas Panhandle Remove barium sulfate scale from liner/ Increased gas production rates from 800 Mcfd

perforations to restore production to over 5000 Mcfd

Formation Hunton limestone

Depth ±22,000 ft TVD Avoid fluid losses to formation Minimized tubular corrosion in the presence

of CO2 and H2S

Pore press 1100 psi, + 380o F

Well type Vertical cleanouts Carry metal cuttings back to surface

Hole size 3-1/16 in.

Case 6 - Texas Panhandle area

Location Texas Panhandle Mist drill 400 in. new hole to eliminate Successfully drilled target interval in fewer

formation damage days than planned

Formation Hunton limestone

Depth 19,322 ft TVD/19,700 ft MD Minimize corrosion by effective implementation Project cost $1,000,000 less than budgeted

of corrosion program

Pore press 800-1000 psi Gas production rates substantially higher

than previously drilled wells in the same field

Well type Horizontal re-entry Capture real-time surface flow and pressure

Hole size 4-3/4 in.

Case 7 - West Texas (Pecos County) area

Location West Texas (Pecos County) Sidetrack and drill lateral section in a severely Maintained underbalanced environment in a

depleted gas reservoir deep, 550 psi reservoir using UBS techniques

Formation Ellenburger

Depth 13,100 ft to 14,100 ft TVD Minimize fluid losses and differential sticking Encountered no lost circulation or stuck pipe,

using nitrogen mist systems

Pore press 550 psi

Well type Horizontal, ee-entries Increase rate of penetration over conventional

methods

Hole size 6-1/4 in. and 4-1/2 in.

Case 8 - NE (U.S.) area

Location NE (U.S.) Increase ROP relative to conventional Vertical deviation controlled

techniques

Formation Hard rock (surface hole)

Depth 3900 in. Minimize footprint of surface equipment to Hammer drilling increased rates of

reduce location size in an environmentally penetration from 10 ft/h to more than 50 fph

sensitive area in 28-1/2 in. hole. Realized up to 75 ft/h ROP

in 24 in. interval

Well type Vertical new drill, gas storage

Hole size 28-1/2 in. and 24 in. Minimize vertical deviation

Case 9 - Permian Basin, Texas area

Location Permian Basin, Texas Maintain an underbalanced condition in a No fluid losses recorded during

depleted sandstone reservoir while drilling 1000 underbalanced horizontal drilling operation

foot lateral

Formation Keystone field (Holt)

Depth 5634 ft TVD Reduce/eliminate formation damage due to Realized a 66% increase in rate of penetration

fluid loss compared to previous well drilled

conventionally

Well type Horizontal re-entry

Hole size 6-1/8 in.

(continued on next page)

Table A.1 (continued)

Information Objective(s) Results

Case 10 - Lea County, New Mexico area

Location Lea County, New Mexico Minimize formation damage due to Fluid losses reduced by 50%

fluid losses compared to wells drilled

conventionally

Formation Greyburg sandstone

Depth 4100 ft TVD Maintain underbalanced conditions Realized up to 97% increase in rate

in depleted sandstone reservoir with of penetration, with average rig time

pore pressure of 200 psi per well reduced by 22%

Well type Multiple vertical new drills & re-

entries

Hole size 4-3/4 in. re-entry deepenings; 7-7/

8 in. New Drills

Case 11- Java, Indonesia area

Location Java, Indonesia Drill 500 m lateral out of 7 in. liner Significant decrease in formation

while maintaining underbalanced damage due to maintaining BHCP

conditions using nitrified water less than pore pressure in the lateral

section

Formation Jatibarang (Volcanic)

Depth 2287 m MD Minimize formation damage due to

lost fluids and solids invasion

Well type Horizontal (New Drill) Lateral section terminated at 175 m

displacement due to limitations of

customer production facilities to

handle production during drilling

Hole size 6 in.

Case 12 - Gargzdai Field, Lithuania area

Location Gargzdai field, Lithuania Increase reservoir productivity by The IP estimated to be 3250 BOPD.

minimizing formation damage Stable production after 3 months

exceeded 2700 BOPD

Formation Cambrian sandstone + siltstone

Depth 1976 m TVD, 2426 m MD Complete well while flowing To eliminate need of snubbing unit

during completion, reservoir pressure

was balanced with 134 bbl of

formation fluid. Well started flowing

after running 21 joints of 2.875 in

tubing. Finished running tubing with

well flowing

Well type Horizontal - Type 1 New Drill

Hole size 6 in.

Case 13 - Central Alberta, Canada area

Location Central Alberta, Canada Underbalance drill the lateral section Gas rates as high as 22 MMcfd

in a severely depleted gas reservoir productions while drilling

Formation Elkton

Depth 9700 ft MD (8400 ft TVD) Increase well productivity compared Nitrified diesel drilling fluid was very

to conventional methods compatible with the formation

Well type Horizontal, coil tubing Significant production increases over

offsetting vertical and horizontal

wells drilled overbalanced

Hole size 4-3/4 in. Minimize fluid losses and differential

sticking

Case 14 - Indonesia area

Location Indonesia Underbalance drill the lateral section Oil rates as high as 400 BOPD

an under-pressured oil reservoir production while drilling

Formation Upper bata

Depth 6249 ft MD Minimize fluid losses and differential Significant (±10-fold) production

sticking increases over offsetting vertical and

horizontal wells drilled overbalanced

Pore press <650 psi Formation evaluation and real-time

fracture identification

Table A.1 (continued)

Information

Objective(s)

Results

Well type

Hole size

Case 15 - OME area Location

Formation Depth

Pore press Well type

Hole size

Case 16 - Libya area Location

Formation Depth

Pore press Well type

Hole size No. of wells

Casse 17 - Eastern middle east area Location

Formation Depth

Well type Hole size

No. of wells

Directional, oil

OME Asmari

2241 m MD (2212 m TVD)

2240 psi Deviated

8-1/2 in Libya

Beda C, Facha C 7000-8900 ft

1050-3000 psi Oil wells

Eastern middle east

Thebes, Risha and Dubeidib 3300 m TVD

Deviated 5-7/8 in.

Increase well productivity compared to conventional methods

Minimize fluid loss and NPT while drilling

Nitrified diesel-mist fluid was very compatible with the formation

Total production of 12,757 bbls oil while drilling

Eliminate the use of drilling fluid No additives or LCM added to additives the drilling fluid (formation oil)

while drilling

Minimize formation damage

Saved approximately 10 days of drilling time

To eliminate/minimize possible First ever dual lateral to be lost circulation drilled UB in Libya

To access the required reservoir Wells drilled with zero LTI's To eliminate any impairment of Successfully drilled the wells to the reservoir formation by any- TD non native fluid or material

To increase PI compared to other Positive results helped in conventionally drilled wells promoting UBD technology in

To evaluate and characterise the reservoir production and to increase ROP

Increase production rate by reducing formation damage

To increase ROP relative to conventional drilling Capture real-time surface flow and pressure data

Case 18 - Hassi Massoud oil field - Algeria area

Location

Formation Depth

Well type

Hole size

Hassi Massoud oil field - Algeria To increase oil production by

minimizing formation damage

Re Cambrian/Cretaceous

Up to 4581 m MD Increase ROP compared to

conventional overbalanced drilling

Deviated

Eliminate NPT associated with conventional drilling problems

1st UBD campaign consisting of 3 wells was successfully completed in February 2003

Increased production rates & Reduced formation damage Excellent safety and operational performance led the operator to plan for a 2nd UBD campaign All the wells delivered safely with zero LTI's

ROP reached a maximum of 9 m/ h as compared to an average of 2 m/h for conventional drilling

To date 18 wells have been drilled using UBD technique

Significant increase in ROP compared to offset conventional wells

Successfully spread the UBD technology in the North Africa region

Encouraging production rates were observed while drilling and conducting production flow tests. The best of 27.5 m3/h has been observed so far while drilling the well MDZ 550

(continued on next page)

Table A.1 (continued)

Information Objective(s) Results

Case 19 - Offshore - Qatar area

Location Offshore - Qatar To create moderate under An air injection rate of 750-850 scf/m

balanced conditions necessary to via the parasite string created an

achieve returns to surface while appropriate level of UB conditions to

drilling the 24 in surface hole eliminate losses in the UER

through massive loss zones formation and other zones

Formation UER, Simsima,

Fiqa, Halul, Laffan

Depth 1000-3000 ft To achieve UB conditions by To date 9 wells have been drilled

utilizing air injection via parasite using the air drilling technique

string on the 30 in conductor pipe

set at 500 ft

Pore press 600-1200 psi

Well type Gas well The low degree of UB conditions

successfully avoided massive sour

water flows from flow zones and

limited bore hole instability problems

Hole size 24 in.

No. of wells 9

Case 20 - Ghawar field - Saudi Arabia area

Location Ghawar field - To eliminate formation damage Increased Injectivity rates by more

Saudi Arabia caused by the loss of than 2 to 3-fold

conventional drilling fluid to the

formation and therefore avoids

differential sticking

Formation Arab D

Depth 11,400-11,850 ft To date 4 power water injection wells

(MD) have been drilled Underbalanced

Pore press 3520-3735 psi

Well type Horizontal Maximizing water injectivity

Hole size 6-1/8 in. To increase "on bottom'' rate of All wells were delivered safely

penetration without LTI's

No. of wells 3 To increase bit life

Case 21 - Greater Oman area - North East Syria

Location Greater Oman area Reduce typical drilling non- 13 wells have been drilled using the

- North East Syria productive time (NPT) by Flow Drilling technique

depleting the Shiranish Gas zone

while drilling

Formation Shiranish/Mulusa

Well type Straight & deviated Eliminate an intermediate casing Average drilling rig time of 45 days,

string from the drilling program has been reduced to an average of

21 days

Hole size 6 in. & 8.5 in.

Develop UBD technology, Intermediate casing string has been

practices and procedures for eliminated

future Syrian activity

ROP improvements and excellent bit

performance were experienced

All wells were delivered safely with

zero LTI's

Case 22 - North East British Columbia, Canada area

Location North East British Increase well productivity PIWD as high as 4 MMscf/d/1000 psi

Columbia, Canada through

Formation Jean Marie Technical management of

bottom hole pressure

Depth 2,047 m MD Minimize fluid losses and Gas rate up to 1.5 MMscfd

differential sticking

Table A.1 (continued)

Information Objective(s) Results

Pore press 4560 kPa Monitor reservoir through PIWD

Well type Horizontal Evaluation while drilling

Hole size 156 mm

Case 23 - Lithuania area

Location Lithuania Increase reservoir productivity by IP estimated to be 3250 BOPD.

minimizing formation damage Stable production after 3 months

exceeded 2700 BOPD

Formation Sandstone

Depth 6480 ft. TVD; Complete well while flowing To eliminate need of snubbing unit

7960 ft. MD during completion, reservoir

pressure was balanced with 134 bbl

of formation fluid. Well started

flowing after running 21 joints of

2.875 in tubing; finished running

tubing with well flowing

Well type Horizontal new drill in-fill

Hole size 6 in.

No. of wells 3

3.3. Nile delta oil field area

The selected example includes the reservoir section, which consists of one production formation (Qawasim from Miocene age). It has a sandstone lithology with reservoir pressure 3800 psi, reservoir temperature 185 0F, GOR 1100 SCF/STB, average porosity 25%, average permeability 400 md, gravity of oil 500 API, and there is no H2S concentration.

The selected reservoir can be drilled by underbalanced drilling technique as given in Table 11. Fig. 16 shows the operating window, multiphase fluid injection of nile delta oil field area.

4. Conclusions

Planned and applied correctly, underbalanced drilling technology can address problems of formation damage, lost circulation and poor penetration rates. The ability to investigate and characterize the reservoir while drilling is another important benefit of under balanced drilling. Based on the analysis of the real cases studied during the research, the following conclusions could be cited:

1. Underbalanced drilling technique is a very useful technique especially when applied in reservoir section. It prevents formation damage, increases ROP, increases reservoir productivity and reduces the total cost of the well.

2. Candidate screening is a rigorous and is a critical first step in the design of a successful underbalanced drilling operation. Although UBD has many advantages, it is not a magic solution for all fields or drilling problems. Poor screening and planning would result in an over-enthusiastic misapplication of the technology, and possibly failure.

3. Many issues must be considered when designing an under-balanced drilling project including but certainly not limited to rock properties, reservoir pressure, borehole stability, drilling fluid type, injection method for gas assist, effect of compressible fluid on MWD, downhole motor require-

ments, bit type, corrosion, equipments availability, separation and fluid handling requirements especially when dealing with hydrocarbon drilling fluid, tripping procedures, data acquisition and completion procedures. Proper planning and design work, addressing these parameters, is essential to successfully conduct an underbalanced drilling project.

4. UBD with stable foam through depleted reservoirs can be conducted safely and successfully in both vertical and horizontal wells. Drilling with foam has some appeal because foam has some attractive qualities and properties with respect to the very low hydrostatic densities, which can be generated with foam systems. Foam has good rheology and excellent cutting transport properties.

5. Real time capture of production data while drilling should provide information about the reservoir not otherwise available.

6. A proposed UBD program to be implemented in Egyptian fields is developed.

Appendix A

See Table A.1.

References

Azeemddin, M. et al., 2006. Underbalanced Drilling Borehole Stability Evaluation and Implementation in Depleted Reservoirs, a Joaquin Field, Eastern Venezuela. IADC/SPE99165, February, 2006.

Bates, R.E., 1965. Field Results of Percussion Air Drilling. SPE 886, March, 1965.

Bennion, D.B., Thomasand, F.B., Bietz, R.F., 1998. Underbalanced Drilling: Praises and Perils. SPE Drilling and Completion, December, 1998.

Bentsen, N.W., Veny, J.N., 1976. Preformed Stable Foam Performance in Drilling and Evaluating Shallow Gas Wells in Alberta. SPE 5712-PA, Formation Damage Conference held in Houston, October, 1976.

Black, A.D., Green, S.J., 1978. Laboratory simulation of deep well drilling. Pet. Eng., 40.

Bourgoyne, A.T., Young Jr., F.S., 1974a. A multiple regression approach to optimal drilling and abnormal pressure detection. SPEJ, 371.

Bourgoyne, A.T., Young Jr., F.S., 1974b. A multiple regression approach to optimal drilling and abnormal pressure detection. Trans. AIME, 257.

Boyun, Guo, Rajtar, J.M. 1995. Volume Requirements for Aerated Mud Drilling. SPE 26956-PA, Drilling and Completion, California Regional Meeting held in Ventura, September, 1995.

Claytor, S.B., Manning, K.J., Schmalzried, D.L., 1991. Drilling a Medium-radius Horizontal Well with Aerated Drilling Fluid: A Case Study. Paper SPE 21988 presented at the 1991 SPE/IADC Drilling Conference, Amsterdam, March 11-14.

Cunningham, R.A., Eenink, J.G., 1959. Laboratory study of effect of overburden, formation, and mud column pressures on drilling rate of permeable formations. Trans. AIME 216, 9.

Dorenbos, Roelien, Ranalho, Jone, 2002. Underbalanced Drilling Primer. Shell International Exploration and Production B.V., June, 2002.

Eckel, J.R., 1957. Effect of pressure on rock drillability. Trans. AIME 213, 1.

Gamier, A.J., van Lingen, N.H., 1959. Phenomena affecting drilling rates at depth. Trans. AIME 216, 232.

George, E., Waston, Ralpha, A., 1956. Review of Air and Gas Drilling. SPE 703-G, Petroleum Branch Fall Meeting in Los Angeles, October, 1956.

Godwin, A., Lokpobiri, Ikoku, Chi U., 1986. Volumetric Requirements for Foam and Mist Drilling Operations. SPE 11723-PA, Petroleum Branch Office, California Regional Meeting held in Ventura, February, 1986.

Gordon, D. et al., 2005. Underbalanced Drilling with Casing Evolution in the south Texas Vicksburg. SPE Drilling and Completion, June, 2005.

Gray, Kenneth E., 1957. The Cutting Carrying Capacity of Air at Pressure above Atmospheric. SPE 874-G, October, 1957.

Hongren, G.U., Walton, J.C., Stein, D.A., 1999. Designing under- and near-balanced coiled-tubing drilling by use of computer simulations. SPE Dril. Comp. 14 (2).

Hooshmandkoochi, A., Zaferanich, M., Malekzadeh, A., 2007. First Application of Underbalanced Drilling in Fractured Carbonate Formations of Iranian Oil Fields Leads to Operational Success and Cost Saving. SPE 105536-MS, Middle East Oil and Gas Conference held in Bahrain International Exhibition Center, Kingdom of Bahrailn, March, 2007.

International Association of Drilling Contractor, 2005. IADC Well Classification System for Underbalanced Operations and Managed Pressure Drilling <http://www.iadc.org/committees/underbal-anced/>, March, 2005.

Kuru, E. et al., 1999. New Directions in Foam and Aerated Mud Research and Development. SPE 53963-MS, Latin American Caribbean Petroleum Engineering Conference held in Caracas, Venezuela, April, 1999.

Louison, R.F., Reese, R.T., Andrews, J.P., 1984. Case History: Underbalance Drilling the Midway and Navarro Formations Successfully in Hallettsville, TX. SPE13112, September, 1984.

Maclovio, Yanez M., 1996. PEP Region Norte and Valenzuela J. Marten, Tecominoacan 408: First Underbalance application in MEXECO. SPE 35320, March, 1996.

Maurer Engineering Manual, 1998. Underbalanced Drilling and Completion Manual, November, 1998.

Meng, Y. et al., 2005. Discussion of Foam Corrosion Inhibition in Air Foam Drilling. SPE 94469-MS, International Symbosium on Oil Field Corrosion held in Aberdeen, United Kingdom, May, 2005.

Moore, C.L., Lafave, V.A., 1956. Air and Gas Drilling. SPE 494-G, February 1956.

Moore, D.D., Bencheikh, A., Chopty, J.R., 2004. Drilling Underbalanced in Hassi Messaud. SPE/IADC 91519, October,

Murray, A.S., Cunningham, R.A., 1955. Effect of mud column pressure on drilling rates. Trans. AIME 204, 196.

Nas, S., 2004. Leading Edge Advantage Ltd - Introduction to Underbalanced Drilling Manual, February, 2004.

Negra, A.F., Lage, A.C.V.M., Cunha, J.C., 1999. An Overview of Air/ Gas/Foam Drilling in Brazil. SPE 56865-PA, Drilling and Completion 14 (2), Drilling Conference held in Amsterdam, June, 1999.

Parra, J.G., Cells, E., Gennare, S., 2003. Wellbore Stability Simulations or Underbalanced Drilling Operations in Highly Depleted Reservoirs. SPE Drilling and Completion, June, 2003.

Qutob, H.H. et al., 2007. The Successful Application of Underbalanced Drilling Technology for Reservoir Evaluation and Drilling Performance Improvement in Kuwait. SPE 106680, June, 2007.

Qutob, Hani, Ferreira, Horacio, 2005. The SURE way to Underbal-anced Drilling. SPE 93346, March, 2005.

Rankin, M.D., Friesenhahn, T.J., Price, W.R., 1989. Lightened Fluids Hydraulics and Inclined Bore Holes. Paper SPE 18670 presented at the 1989 SPE/IADC Drilling Conference, New Orleans, Feb. 28-March 3.

Roving, J.W., Reynolds, E., 1994. Underbalanced Drilling Through Oil Production Zones With Stable Foam in Oman. IADC/SPE 27525, February, 1994.

Salah El-Din, M.A., El-Katatney, S.M. (2009). Implementation of Underbalanced Drilling Technique in Egyptian Fields. M.Sc. Thesis, Cairo University, Egypt, 2009.

Sunthankar, A.A. et al., 2001. New Developments in Aerated Mud Hydraulics for Drilling in Inclined Wells. SPE67189, March, 2001.

Vidrine, D.J., Benit, E.J., 1968. Field verification of the effect of differential pressure on drilling rate. JPT, 676.

Weatherford Company, 2006. Operational Sequence in UBD (ROAD MAP). Weatherford Controlled Pressure Drilling and Testing Services.

Westermark, R.V., 1986. Drilling with a Parasite Aerating String in the Disturbed Belt, Gallatin County, Montana. lADC/SPE 14734, February, 1986.

Whiteley, Maxwel C., England, William P., 1986. Air Drilling Operation Improved by Percussion-Bit/Hammer-Tool Tandem. SPE Drilling Engineering, October, 1986.

Zhou, L. et al., 2005. Hydraulics of Drilling with Aerated Mud under Simulated Borehole Conditions. SPE/IADC 92484, February,