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ScienceDirect
Natural Gas Industry B 2 (2015) 270-276
www.elsevier.com/locate/ngib
Research article
Pilot tests of microbe-soil combined treatment of waste drilling sludge
Chen Lironga*, Huang Mina, Jiang Xuebina, Li Huia, Chen Qiangb, Zhang Mina, Li Shenglina
a Safety, Environment and Quality Surveillance & Inspection Research Institute, Chuanqing Drilling Engineering Co., Ltd., CNPC, Guanghan, Sichuan 618300,
b College of Resources and Environment, Sichuan Agricultural University, Ya'an, Sichuan 610051, China
Received 3 November 2014; accepted 8 April 2015 Available online 3 September 2015
Abstract
Microbe-soil combined treatment is a newly developed technology in view of the defects of the curing process and waste drilling mud slag properties. In particular, 0.3%—0.5% bioremediation reagents were fully mixed with the waste drilling sludge according to its wet and dry degree, and 1.5 folds to twice weight of more finely ground soil was added in the mix, which was covered by soil of 5—15 cm thick and thereby grasses or greeneries were planted on the soil. The process was successfully applied to some fields of Well Danqian 001-8, Well Lianhua 000-X8, etc. After three months of such treatment, the main indexes of the drilling solid waste such as the degradation of COD and the oil-degrading ratio reached more than 90%, the index of leaching solution met the requirement of the first grade in the national "Integrated Wastewater Discharge Standard'; heavy metal ion concentration in soil did not change significantly with the indicators meeting the requirement of the third grade in the national "Soil Environmental Quality Standard" (Dry Land); and no harmful effects of heavy metals have ever been found on the planted grasses and greeneries. In conclusion, with this microbe-soil technology, the soil property will recover its background values without any other chemical additives, realizing the ecological restoration and reuse of land covered by wellsite wastes, so it is in line with the energy-saving and environmentally-friendly treatment way.
© 2015 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Drilling; Waste mud; Waste sludge; Microbe-soil; Combined treatment; Treatment process; Chemical oxygen demand (COD); Oil; Heavy metals; Standard
ELSEVIER
1. Introduction
At present, direct landfill or curing is mainly adopted to dispose of the solid waste generated in oil and gas drilling in China. Since waste drilling sludge contains a great amount of organic and inorganic pollutants, direct landfill would result in severe contamination and damage to surface water, underground water and soil. Curing process is a method widely used in onshore oil/gas fields, especially in the Sichuan Basin, which involves adding hardening agent such as cement in the waste drilling sludge to convert it into soil or solids with high cementing strength, which are buried on the spot or used as constructional
* Corresponding author.
E-mail address: clycly555@163.com (Chen LR).
Peer review under responsibility of Sichuan Petroleum Administration.
material. In this way, the contamination damage of waste drilling sludge can basically be solved for a short period. Technically, through solidification, the absolute majority of contaminants in the drilling fluid sludge are fixed in the solidified blocks, so effective treatment can be achieved on the waste drilling sludge. However, in the long run, this curing process only fixes the pollutants in the solidified blocks, rather than completely counteracts them. When buried underground for a long time, the solidified blocks under physical, chemical and biological actions, will undergo a series of variations, and thus result in formation pollution. From the perspective of resources utilization and sustainable development, curing process consuming substantial resources and raw materials, is not an energy-saving disposal mode. Furthermore, the solidified land will lose cultivation value due to the change in soil structure. Therefore, curing process has intrinsic defects such as not really saving and recycling resources,
http://dx.doi.org/10.1016/j.ngib.2015.07.021
2352-8540/© 2015 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).
and not conforming to the requirements of the national energy-saving and sustainable development policy.
Harmless treatment of waste drilling sludge has been paid attention to both at home and abroad. Early in the 1970s, in order to solve the pollution of soil by oil leaked or spilled due to the failure of oil pipelines and storage tanks, American Esso Research and Engineering Company started to hunt for clean biological solutions, and found an effective "bacteria-seeding method" in laboratory research, which set a precedent of biological remediation of soil polluted by oil [1]. The biological remediation technology of contaminated soil has drawn increasingly attention since the 1980s; it has also made great progress and become gradually mature. Some American drilling companies built biological treatment pits near the wellsites before drilling, where the wood cuttings and sawdust were used as biological bacteria culture carrier to cultivate bacteria ahead of drilling; in the course of drilling, waste drilling sludge, when produced, was sent into the biological treatment pit of wood cuttings and sawdust for remediation treatment [2-6].
2. Major property of waste drilling sludge
With the rapid development of China's economy, the energy demand is increasing constantly, and oil and gas, as the major energy resources, have become one of the major motivations in the economic development of China. However, in the course of oil and gas drilling, about 200-4000 m3 waste water in a well would be generated depending on the drilling depth from 1000 m to 7000 m, and about 100-1000 m3 waste sludge in a well would be generated during waste water treatment, together with the waste drill cuttings and waste drilling sludge, would end with the drilling solid waste (waste drilling fluid, sludge, drill cuttings, etc.) of 500-2500 m3 a well depending on well depth (about 0.30 m3 per meter of drilling). The waste sludge with all the pollutants in the drilling waste water concentrated in, is very high in contaminant concentration, e.g. COD can reach as high as 10000-50000 mg/L, content of heavy metals like Cd, Pb, Cu, As, Hg and Cr6+ is relatively low; SS can reach as high as 20000 mg/L. Besides, the sludge contains a certain amount of oil and different components and is usually dark brown.
1) Waste drilling fluid: mainly comes from the discarded contaminated drilling fluid and drilling fluid which is not transferred and recycled at completion. Its volume differs in different wells, and about 50-200 m3 a well may generally be generated by a drilling crew. Because drilling fluid is usually prepared with 20-40 kinds of different inorganic and organic drilling fluid additives, including clay, drill cuttings, weighting materials, various chemical additives, inorganic salt and oil, so it is a multiphase stable suspension mixture very complicated in composition, with generally pH value as high as 8.0-10.0, higher content of harmful organic and inorganic pollutants, and COD as high as 20000-60000 mg/L.
2) Waste sludge: mainly originates from drilling fluid tank clearing and waste water treatment. Its volume, closely
related to waste water treatment volume and well depth,
and generally stands at 200-1500 m3 a well by a drilling crew. The sludge produced from drilling fluid tank clearing has similar properties with the waste drilling fluid, but the sludge resulted from waste water treatment concentrates all the contaminant components in the waste water. Therefore, the waste sludge is also very complicated in composition, with a pH value usually of 8.0-9.0, a certain amount of harmful organic and inorganic pollutants, and a COD of 5000-20000 mg/L.
3) Waste drill cuttings: mainly originate from formation cuttings. Its volume, differing with various well depths, is about 200-800 m3 a well generally in a drilling wellsite. Waste drill cuttings, usually carrying drilling fluid, have the same polluting property of drilling fluid, only with lower contaminant contents.
All these three kinds of drilling solid wastes are very harmful to the environment.
3. Principle of microbe-soil combined treatment of waste drilling sludge
It is well-known that microbes possess very strong metabolic diversity, participating in the material cycle and energy metabolism in nature. So, they have a great potential in degrading wastes, possessing such advantages as rapid decomposition, low cost, thorough degradation, enabling the reutilization of wasted resources; by converting some complicated organic matters in the waste drilling sludge into humus component, degrading some others into simple inorganic matters or even CO2 and H2O, they can remove the contaminants in the waste drilling sludge and make the waste drilling sludge harmless.
Soil is basically composed of soil grain, water, air and other minute associations of plants and animals, in other words, soil is composed of three types of matters: solid, liquid and gas [7]. Solid matter consists of minerals, organic matters and microbes, etc. The humus in soil generally accounting for 85%-90% of the total volume of soil organic matter, can stimulate the activity of soil microbes, and thus is favorable for the microbial metabolic activity. There are a great many sorts of microbes in soil, including bacteria, fungi, actinomycete, alga and protozoa, etc., and they are in huge numbers too, about hundreds of millions to tens of billions of microbes in 1 g soil. Microbes can decompose organic matters and minerals as well as fix nitrogen in soil. Soil air can improve the soil aeration status, which is favorable for the action of aerobic bacteria, and can stimulate the growth of plants.
Based on the soil composition property and performance, it can stimulate the action of microbes, therefore, microbes and soil have a synergistic effect, and the combination of them is can improve their capacity of contaminant degradation [8].
4. Domestication and cultivation of microbial degradation bacteria
The domestication and cultivation process of microbial degradation bacteria used for waste drilling sludge disposal is
as follows: waste drilling sludge — bacterination — domestication — isolation — purification — screening — obtaining dominant bacteria — making solid bacteria — field application. Domestication conditions: normal temperature bacteria, 28 °C, 150 r/min, shaking culturing for 10 d in rocker; isolation and purification conditions: culturing until the occurrence of single colony at 28 °C, choosing single colony to conduct streak purification in beef extract peptone culture medium, confirming the absence of infectious microbes under microscope, transferring to beef extract peptone slant culture medium to culture for 24—48 h.
5. Field application
Soil-microbe combined treatment of waste drilling sludge was successively tested in Wells Danqian 001-8, Lianhua 000-X8, Yue 101-72-X1X2 and Pingluo 006-U3 in the Sichuan oil/ gas fields from August 25, 2011 to April 26, 2013. After treatment, the indicators of pollutants in the mixture were periodically (2—3 months) sampled and analyzed to investigate the degradation effect.
5.1. On-site treatment status
In this paper, Wells Danqian 001-8 and Lianhua 000-X8 are taken as examples to demonstrate the treatment effect. The former is a 3-well cluster, which produced a total of 1200 m3 waste sludge after completion (Fig. 1). With a total depth of 4444 m, the latter produced a total of 1100 m3 waste drilling sludge after completion.
5.2. On-site treatment process
Based on the wetness of the waste, 0.3%—0.5% degradation bacteria inoculum was added to and mixed fully with the waste drilling sludge (as bacterial inoculum was added, an excavator was used to agitate and blend them repeatedly for 10 times, until they were completely mixed). After the bacterial inoculum and the waste drilling sludge were completely mixed, 1.5—2.0 times (adjusting as per the wet and dry degree
Fig. 1. Contaminant status after completion of Well Danqian 001-8.
of soil and waste) weight of finely-grounded soil was added and mixed in the same way as the mixing of bacterial inoculums. After mixing, the mixture was covered with 5—15 cm thick soil on top. Finally, grass seeds were sowed and vegetation was planted [9]. Treatment status and post-backfill status are shown in Fig. 2.
5.3. Sampling method of treated mixtures and plants
5.3.1. Sampling method of treated mixtures
Soil samples were taken with boring machine or shovel for test at several points (points along diagonals). After being mixed evenly, the soil sample was divided into 4 portions, separately packed in sterile plastic bags to be brought back or sent to a qualified laboratory for analysis. "Technical Specification for Soil Environmental Monitoring" (HJ/T166-2004) was referred to when sampling.
5.3.2. Sampling method of plants grown after treatment Plant samples were mainly taken from the treating pits of
Wells Danqian 001-8 and Lianhua 000-X8, and plant samples were also taken from the site of Well Lianhua 000-X5 where medium-scale treating test was conducted in 2009 for analysis.
5.4. Testing items, methods and evaluation criteria
5.4.1. Content of principal heavy metals
The indicators of principal heavy metals like Pb, Cd, Cu, As, Hg, Zn and Cr6+ of the treated mixtures and the plants grown on the treated mixtures were mainly analyzed by the local Environmental and Farm Produce Quality Monitoring Institution on a commission basis.
5.4.2. Content of principal nonmetal contaminants
For nonmetal contaminants, the indexes including pH value, content of COD, sulfide, oil and Cl_ of leach solution from the treated matter were measured. The measuring method and standard adopted for each index are as follows:
pH value: glass electrode method (GB 6920-1986). COD: dichromate process (GB/T 11914-1989). Oil: infrared spectrophotometry (HJ 637-2012). Sulfide: gas phase molecular absorption spectrometry (HJ/T 200-2005).
Chloride: chromatography of ions (HJ/T 84-2001).
5.4.3. Primary standards of evaluation
1) The national "Environmental quality standard for soil" (GB 15618-1995) was referred to for the indexes of principal heavy metal testing items, in which, Dry land (third grade) (unit: mg/kg): Pb < 400; Cd < 1.0; Cu < 400; Zn < 500; Cr6+ < 300; Hg < 1.5; As < 40. Natural background (first grade) (unit: mg/kg): Pb < 35; Cd < 0.2; Cu < 35; Zn < 100; Cr6+ < 90; Hg < 0.15; As < 15 [10].
2) The first grade (unit: mg/L) of the national "Integrated Wastewater Discharge Standard" (GB 8978-1996) was referred to for the indexes of principal nonmetal testing items, in which, the water should reach a pH value of 6—9, COD of 100, oil of 5.0, and sulfide of 1.0 [11].
3) The standard limitation of vegetables and related products stipulated in the "Food Contaminants of National Food Safety Standard" (GB 2762-2012) was referred to for indexes of harmful heavy metals in the plants grown on the treated mixture [12]. Although Pb, Cd, Cu, Zn, As and Hg were analyzed, only Pb, Cd, As and Hg had standard values.
5.5. Results and analysis of field tests
Fig. 3 shows the onsite sampling photos of Well Danqian 001-8 after biological treatment four months later (on February 3, 2012). The monitoring results of physicochemical property indexes are listed in Table 1.
It can be seen from Table 1 that, before treatment, the major pollution parameters such as COD and oil of the waste drilling sludge in the waste water pit are all higher. Two months after treatment, these two indexes dropped significantly below the first grade standard values in the national "Integrated Waste-water Discharge Standard". Four months after the treatment (on February 3, 2012), the degradation rate of COD and oil reached 95%. Over the time after treatment, the contaminant content variation becomes smaller and basically keeps stable, and the sulfide content and pH value also fell within the
standard range. On August 9, 2013, samples were taken from the two biological treatment pits at different depths; the monitoring results showed that COD, oil, sulfide and pH value were all within the standard ranges, and these indexes showed no significant differences at different depths. Compared with the samples taken on February 3, 2012, some indexes increased to some extent, which possibly resulted from the different sampling points in the pits, but all of them were lower than the values stipulated in the national "Integrated Wastewater Discharge Standard". Obviously, the microbe-soil combined treatment can effectively degrade and treat the drilling solid waste.
Table 1 shows that after treatment, the heavy metal content of the treated composite samples did not change significantly at different stages, but were all lower than the limitation requirements of third grade standard (dry land) of the national standard "Environmental Quality Standard for Soil" (GB 15618-1995). Except for As, the indexes of other heavy metals tested all approach the standards of natural background values stipulated in the "Environmental Quality Standard for Soil", which possibly attributes to the lower harmful heavy metal content in the waste drilling sludge. On August 9, 2013, samples were taken from the two biological treatment pits at different depths and were tested. The monitoring results showed that the harmful heavy metal indexes were all within the standard ranges, and these indexes showed no significant differences at different depths. Individual anomalous values occurred in the test, i.e., Cu value exceeded the original base value of sludge, and the causes of which are still not clear, but
Fig. 3. Surface effect of biological treatment at Well Danqian 001-8.
Table 1
Physicochemical properties of treated soil samples of Well Danqian 001-8.
Sampling Sampling Principal non-heavy pH Principal heavy metal indexes/(mg $ kg Remarks
location time metal indexesAmg-L"1)
COD Oil Sulfide Pb Cd Cu As Hg Zn Cr6+ Cl"
Soil background 2011-09-16 18.1 0.85 0.116 7.2 0.02 1.73 3.46 83 2660 Soil used in treatment
Waste sludge 2011-09-16 3620 58.3 0.159 8.8 390 58.7 21.7 206 9530
Pit 1 2011-11-13 43.2 5.43 0.058 8.34
2012-02-03 46.6 1.51 0.106 8.59 91 0.46 25 4 0.12 107 1193
2012-03-28 100 0.27 28 3 0.1 114 1063
2012-04-12 49.2 0.63 0.119 8.07
2012-07-27 31.9 0.25 0.128 7.35
2012-11-10 87.0 1.64 — 8.67
2013-06-16 64.0 3.26 0.098 8.39
Pit 3 2011-11-13 74.0 2.28 0.005 8.68
2012-02-03 32.0 1.59 0.101 8.02 76 0.3 25 4 0.1 92 1810
2012-03-28 90 0.24 29 2 0.08 110 1400
2012-04-12 50.0 0.7 0.112 7.98
2012-07-27 48.0 0.45 0.128 8.15
2012-11-10 79.0 2.35 — 8.71
2013-06-16 27.8 1.38 0.097 8.35
41.5 0.33 0.023 7.73 73.8 0.18 47.8 7.6 0.06 108.4 Soil 50 cm underground
2013-08-09 36.5 0.44 0.023 7.74 69.2 0.16 32.6 10.6 0.24 71.0 Soil 90 cm underground
49.8 0.67 0.017 7.77 80.8 0.46 51.3 2.5 0.08 134.2 Soil 140 cm underground
2013-11-14 23.9 0.77 0.026 8.23 77.2 0.13 25.2 4.7 0.1 100.5
Pit 4 2011-11-13 74.3 3.75 0.066 8.24
2012-02-03 39.3 3.31 0.111 7.98 95.0 0.38 26.0 4.0 0.11
2012-03-28 113.0 0.3 30.0 3.0 0.11
2012-04-12 48.9 0.64 0.111 8.07 103 1816
2012-07-27 46.0 0.36 0.134 8.92 113 1916
2012-11-10 72.3 2.35 — 8.50
2013-06-16 49.6 0.21 0.098 8.12
52.4 0.24 0.015 8.17 14.2 0.36 41.7 10.4 0.13 24.0 Soil 50 cm underground
2013-08-09 35.8 0.40 0.021 8.90 30.4 0.30 41.5 12.5 0.1 124.4 Soil 90 cm underground
59.0 0.61 0.028 8.22 54.0 0.36 41.0 14.0 0.13 133.6 Soil 140 cm underground
2013-11-14 27.9 0.64 0.024 8.14 61.3 0.14 22.8 3.2 0.08 92.3
Note: "—" denotes lower than the limit value detected by this method, listed values are those detected by this method.
possibly due to the different analysis pretreating process adopted by different analysis units.
Fig. 4 shows the photos of Well Lianhua 000-X8 site after biological treatment for six months (on May 6, 2012). The monitoring results of physicochemical property indexes of treated soil showed that the treated soil have basically restored to the level of soil background value, fulfilling ecological remediation and reutilization of land occupied by drilling waste.
To find out the transferring status of harmful heavy metals in the waste drilling sludge, samples were taken from the plants grown on the treated mixture on August 12, 2013 and were entrusted to the local Agricultural Products Quality Monitoring and Inspection Center for monitoring analysis, and the tested results are listed in Table 2. It can be seen from Table 2 that, Hg content exceeded slightly the standard only in locust tree leaf taken from pit 4 of Well Danqian 001-8 and exceeded 2 times of the standard in trefoil taken from pit 3 of
Table 2
Content of principal heavy metals in plants grown on the treated mixture.
Sampling point Sample name Pb/(mgkg ) Cd/(mg$kg ) Cu/(mg$kg ) Zn/(mg$kg ) As/(mg$kg ) Hg/(mg$
Pit 4 of Well Danqian 001-8 Weed 0.047 0.083 19.752 36.842 0.054 7.750
Locust tree leaf 0.068 0.044 5.696 32.752 0.381 11.210
Pit 4 of Well Lianhua 000-X8 Maize leaf 0.075 0.075 6.939 15.852 0.326 9.436
Maize grain 0.035 0.016 0.899 14.341 0.082 0.250
Maize cob 0.054 0.025 2.758 8.643 0.106 1.800
Pit 3 of Well Lianhua 000-X8 Wormwood 0.100 0.135 13.676 33.815 0.203 2.300
Trefoil 0.030 0.059 6.058 39.963 0.271 29.720
Well Lianhua 000-X5 Bamboo leaf 0.094 0.067 6.447 21.092 0.147 4.790
Alder tree leaf 0.047 0.049 4.167 19.310 0.168 3.990
GB 2762-2012 "Food Contaminants of National 0.2 1.0 0.5 10.0
Food Safety Standard"
Well Lianhua 000-X8, and the Hg content of rest samples of the other two kinds of plants grown on the site of Well Lianhua 000-X8 did not exceed the standard (sampling at 4 points).
6. Technical and economic analysis
1) The integrated treatment unit price of the test wells of this technology is around RMB ¥ 250/m3, which is acceptable. More importantly, no other treating chemicals or curing agents are needed, so substantial curing materials are saved, mainly cement used in the original curing process, which is highly consistent with the national energy saving policy.
2) After being treated by the soil-microbe combined treatment system, the waste drilling sludge generated in drilling activity can be turned from solid waste to "treasure", i.e., the organic matter in the drilling solid waste can be degraded and converted into humus, increasing the soil fertility (observation of the soils treated by Chinese style test and field test shows that the effect is obvious), making the soil suitable for growing common plants, and gradually integrating into the local ecological system, which conforms to the environmentally-friendly treating tenet.
3) The biological bacteria screened out can be used to treat water-base drilling fluid and sludge in other blocks, showing wide applicability.
7. Conclusions
1) Field test application results show that after three months of soil-microbe combined treatment of drilling solid waste, the primary indicators like COD, oil (degradation rate exceeds 90%) and leach solution of the treated waste all met the first grade index requirements of the national "Integrated wastewater discharge standard".
2) The heavy metal ion concentration in the treated mixture has not changed significantly, but all the indexes have
reached the third grade standard (dry land) stipulated in the national "Environmental Quality Standard for Soil" (GB 15618-1995).
3) The test results of plants grown on the treated mixture show that no harmful heavy metal transferring phenomenon occurs.
4) This technology can make the drilling solid waste harmless, and turn the complicated organic matter into earth humus component, and thus "changing waste into treasure".
5) No other chemical treatment additives are needed in the treatment. Three months after the treatment, the property of combined treatment system basically restored to the level of soil background value, fulfilling ecological remediation and reutilization of land occupied by drilling wastes, and cyclic utilization of land resources; clearly, this technology conforms to the national energy-saving and environmental protection policy. It is also good for sustainable development of land, and has good economic, social and environmental benefit, so it is worth promoting.
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