Enrichment mechanism and exploration and development technologies of high coal rank coalbed methane in south Qinshui Basin, Shanxi Province

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 43, Issue 2, April 2016 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2016, 43(2): 332-–339.

RESEARCH PAPER

Enrichment mechanism and exploration and development technologies of high coal rank coalbed methane in south Qinshui Basin, Shanxi Province ZHAO Xianzheng1, 2,*, YANG Yanhui1, SUN Fenjin3, WANG Bo3, ZUO Yinqing1, LI Mengxi1, SHEN Jian1, 4, MU Fuyuan3 1. PetroChina Huabei Oilfield Company, Renqiu 062552, China; 2. PetroChina Dagang Oilfield Company, Tianjin 300280, China; 3. PetroChina Research Institute of Petroleum Exploration & Development-Langfang 065007, China; 4. School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China

Abstract: Based on analysis of the basic geologic characteristics and enrichment controlling factors of the high coal rank coalbed methane (CBM) in south Qinshui Basin in China, its enrichment mode, and exploration and development technologies are studied. Practices on the CBM exploration and development proved that the CBM reservoirs in this study area have the following three major properties: (1) High coal rank, strong adsorption ability, and good resources condition; (2) Low porosity, bimodal porosity structure, and obvious “bottleneck” of flow condition; (3) low reservoir pressure gradient that can constrain production. Based on deep analysis of high coal rank coal properties, this study proposes a coexistence and complementarity concept of structure, sedimentary, thermal power and hydro-geological conditions, and establishes a CBM dissipation model, which can simplify CBM enrichment problem and directly guide the region selection of CBM development. Five major critical technical systems have been formed for the CBM exploration and development in south Qinshui Basin: (1) Comprehensive geophyisical exploration and evaluation technologies; (2) Well drilling and completion technologies for high coal rank coal reservoirs; (3) Major reservoir treatment technologies; (4) Intelligent drainage and production control technologies; (5) Digital technology of coalbed methane field. These have effectively provided technical support for an orderly productivity construction of new CBM blocks. Key words: Qinshui Basin; high coal rank coal; coalbed methane; enrichment characteristics; enrichment condition; exploration and development technologies

Introduction Scale development of coalbed methane (CBM) in China has made breakthrough firstly in the Qinshui Basin. During 2003-2013, the CBM industrial base has been built; the CBM exploration and development has changed from “low return, low efficiency” to “higher benefit and efficiency” since 2014. By the end of 2014, there were 10500 CBM wells in the basin, accounting for 71% of the total number in China; the proved reserves were 4 350×108 m3, accounting for about 65% of proven reserves of CBM. The gas production in 2014 was 30×108 m3, accounting for about 81% of CBM production in China. Summing up the CBM accumulation theory and exploitation technologies of Taiyuan Formation and Shanxi Formation of Carboniferous Permian in Qinshui Basin, and analyzing

major issues encountered will be helpful for the development and improvement of China CBM geological theory of high coal rank coal and providing reference for the exploration and development of CBM blocks in this basin and other regions.

1. Overview of the study area and basic features of high coal rank CBM reservoirs 1.1.

Geological background

The study area is located in the southern Qinshui Basin, adjacent to the Taihang uplift and bordered with Jin-huo major fault to the east, adjacent to Huoshan uplift to the west, and Zhongtiao uplift to the south (Fig. 1). The main structure in the basin is a NNE large-scale synclinorium, with axis generally located in Qinxian-Qinshui, and two basically symmetric wings. The west wing is relatively steep in dip angle, while

Received date: 28 Apr. 2015; Revised date: 16 Feb. 2016. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the China National Science and Technology Major Project (2011ZX05061; 2011ZX05043-006; 2011ZX05028-002); PetroChina Science and Technology Major Project (2013E-2205). Copyright © 2016, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

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1— Xiadian block; 2—Qinnan block; 3—North Shizhuang block; 4—South Shizhuang block; 5—Mabi block; 6—Zhengzhuang block; 7—Daning block; 8—Panzhuang block; 9—Fanzhuang block; 10—Zaozhuang block; 11—Zhangzi block

Fig. 1.

Location of the study area.

the east wing is relatively flat. Divided by Shitou fault, the Qinshui-Yicheng area west of the fault has a series of EW high angle normal faults and NE and NNE secondary folds, while the section east of the fault has superimposed NNE folds and EW folds[1]. 1.2.

Resource of high coal rank CBM

High in metamorphic degree, the coal in south Qinshui Basin has a maximum reflectance of vitrines (Ro,max) of 1.95% to 3.49%[2]. The coal has very high adsorption capacity for methane, with Langmuir volume on air dried basis ranging from 26.58 m3/t to 44.90 m3/t, mostly between 30 m3/t and 40 m3/t, and on average 37.02 m3/t. The coal has a CBM content from 6.16 to 30.43 m3/t in general and about 20.02 m3/t on average (measured by air dried basis, the same below), which is much higher than the average value of China. The coal has a CBM content of more than 15 m3/t in general, 72.5% of wells have a CBM content of more than 20 m3/t, and 20% of wells have a CBM content between 10 to 20 m3/t. High gas content, thick coal seam and large coal-bearing area make CBM resource abundant, laying a good resource base for CBM development. 1.3. Porosity and permeability of high coal rank CBM reservoir The exogenous fractures can be divided into NE and NW two groups, which cut the coal into mesh. Endogenetic frac-

tures developing in bright coal and semi-bright coal, can be divided into two groups, one group, face cleats striking NE between 33° and 66°, at linear density from 27 to 120 per meter, and the other group striking NW between 42° and 54°, at linear density from 24 to 60/m[3]. The porosity test results of 82 samples in south Qinshui Basin show the coal has a porosity from 2.91% to 10.74%, 5.43% on average. Meanwhile, a large number of test results show that the pore structure of coal in the study area has a bimodal distribution, with micropore in absolute dominance, accounting for approximately 77% of the total pore volume, macropore accounting for about 16.42%, and mesopore volume the least, accounting for approximately 6.29%. The specific surface area of micropores is about 99% of the total on average, and that of mesopores is about 0.5%, and that of macropore is negligible. The pore structure distribution pattern leads to double effects, firstly, high proportion of micropores taking up almost all specific surface area of the coal results in strong methane adsorption capacity and thus high gas content; secondly, the bipolar distribution pore structure and less developed mesopore cause seepage bottleneck in mesopore segment, decreasing the coal reservoir permeability. Permeability test of 39 coal layers in the study area show the coal has a permeability from 0.01×103 to 0.91×103 μm2, on average 0.19×103 μm2. 64% of wells have a permeability of less than 0.1×103 μm2, and 36% of wells have a permeability of (0.11.0)×103 μm2. Obviously, the permeability of thecoal reservoirs, low, is mainly controlled by pore structure model. 1.4.

Stratum energy of high coal rank CBM reservoir

Data of 46 wells from well testing shows that the coal reservoirs have a pressure gradient from 0.153 to 1.080 MPa/100 m, on average 0.62 MPa/100 m, indicating most part of the coal reservoirs is severely under-pressure and under-pressure, and part slightly underpressure coal reservoir, and only a small part is normal in pressure. Among the tested wells, five have serious underpressure coal reservoir (pressure gradient is less than 0.50 MPa/100 m), accounted for 11%; 23 wells have underpressure coal reservoir (0.500.75 MPa/100 m) accounting for 50%; 10 have slightly underpressure coal reservoir (0.750.90 MPa/100 m), accounting for 22%; 8 wells have normal pressure (0.901.10 MPa/100 m), accounting for 17%. The critical CBM desorption pressure in the study area is from 0.1 to 6.7 MPa, on average 2.1 MPa. The ratio of critical desorption pressure to reservoir pressure varies from 0.3 to 1.0, with an average of 0.5. The adsorption time of coal varies from 2.07 to 45.09 days, with an average of 10.76 days. The longer the adsorption time, the longer the time needed to reach gas production peak will be.

2. Controlling factors on high coal rank CBM accumulation

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ment degree in southern Qinshui Basin, and the deepening and improving of understanding on high coal rank CBM accumulation geologic theory, we have reached the findings that sedimentation controls coal distribution, combination of plutonic metamorphism and magmatic thermal metamorphism control hydrocarbon generation, tectonism controls accumulation condition configuration, hydrogeological process controls continuous accumulation adjustment, and multiple actions worked together to determine the accumulation of CBM. 2.1. Control of sedimentation on coal reservoir distribution Qinshui Basin was located in the tropical and subtropical regions[4] near 13.9 degrees north latitude in late Paleozoic, with humid and rainy climate conducive to the occurrence of coal accumulation. Before the formation of the No.15 coal seam in the early depositional stage of Taiyuan Formation, the north China plate shifted from uplifting in south and subsiding in north to uplifting in north and subsiding in south, making the overall terrain of the area higher in the north and lower in the south, and resulting in the feature of overall slow fluctuating subsidence of the area[5], which provided a stable structure background for coal accumulation, so multi-layer of recoverable coal seams deposited in this area; among them, the Taiyuan Fm. No.15 and Shanxi Fm. No.3 coal seams are thicker and more stable than the others. Coal seams of Taiyuan Formation were mainly formed in sand barriers-lagoon and shallow water delta front, the Taiyuan Fm. No.15 coal seam is thin in north and south and thick in middle part, and the zone thicker than 3 m of it is located in the southeast Zhangzi-Zhengzhuang lagoon- tidal-flat area and local barrier sand bar phase region. Coal seams of Shanxi Formation were mainly formed in interdistributary on delta plain, such as the coal seams in Yushe, Anze, Qinshui, Yangcheng, Jincheng; the Shanxi Fm. No.3 coal seam is 4 to 8 m thick, and simple in structure (Fig. 2). Meanwhile, the sedimentary system affected the seal of CBM by controlling the lithology combination. The coal bearing formations have a ratio of mudstone and siltstone of over 50%, and the sandstone is tight with certain integral strength, providing strong sealing. The direct roof and floor of the Shanxi Group coal seams have a high shale content, thick cap rock, high breakthrough pressure, favorable for CBM storage. For example, the direct roof in Fanzhuang-Panzhuang, Mabibei-Zhengzhuang, Qinnan-Xiadian and other blocks of southern Qinshui Basin is rick in mudstone, where CBM content of coal seams is higher than 15 m3/t in general[6]. 2.2. Plutonic and regional metamorphism jointly control hydrocarbon generation Reconstruction of burial history of the southern Qinshui Basin coal seams shows that hydrocarbon generation evolution of organic matter in coal has experienced two critical stages[78].

Fig. 2. Shanxi Formation sedimentary facies and thickness of No. 3 coal seam in Qinshui Basin and neighboring area.

Phase 1, Hercynian-Indosinian, maximum buried depth of the late Paleozoic coal was shallower than 4 330 m, and the geothermal gradient, about 2.8 C/100 m was normal, and the coalification obeyed plutonic metamorphism and reached the coal rank of gas coal, with a cumulative hydrocarbon generation volume of 46.4781.45 m3/t. Phase 2, Yanshan period, the geothermal gradient was about 69 C/100 m, indicating abnormal ground temperature, and coalification was subordinate to the regional magmatic thermal metamorphism, and reached super anthracite at most, and the total hydrocarbon amount reached 97.86359.10 m3 /t when the coalification stopped. The two key thermal evolutions, on the one hand, made the coal rank increase, effectively improving gas productivity of the organic matter in coal and rapidly increasing the quantity of hydrocarbon generation; on the other hand, resulted in well-developed micropores and large specific surface area due to fast thermal metamorphism, so the coal seams have strong methane adsorption capacity, and large storage space for CBM. The increase in hydrocarbon generation volume and reservoir storage capacity at the same time laid a solid foundation for CBM enrichment in this basin. 2.3. Tectonics control on configuration of CBM accumulation conditions Tectonism is an important factor affecting CBM accumulation in Qinshui Basin. (1) Qinshui Basin is a fault basin in craton with relatively weak tectonic movement[9]. It is different from the Ordos Basin (long-term and stable subsiding after Permo Carboniferous coal measure deposition with thick

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cap strata and relatively simple structure) to the west, and also from North China fault block coal bearing area east of Taihang Mountains which was strongly reformed by tectonic movements after Carboniferous – Permian deposition, with structural transformation degree in between those two area. This tectonic environment caused appropriate tectonic deformation and moderate development of fractures in the coal bearing strata, which allows the CBM preservation and gives comparatively high coal permeability to the coal. (2) In Yanshan – middle Himalaya, the coal seam was uplifted to escape zone for a time span of about 027 Ma[9], and the late and short uplifting and return time caused short gas escape time, which is conducive to CBM preservation[10]. (3) Himalaya period, Qinshui Basin was uplifted and returned, with the regional tectonic stress environment changing from compression to tension[11], which was conducive to opening of coal fractures and in turn improvement of permeability. 2.4. Continuous adjustment of gas content by hydrological geology Researchers in China generally believe that the flushing, dissolution and migration of groundwater are not conducive to the CBM enrichment, and hydraulic sealing and blocking effect are important conditions for CBM preservation[1218]. The CBM accumulation in south Qinshui Basin is controlled by stagnant water in depression[19]. Groundwater is mainly recharged from the northwestern region of the study area, and groundwater condition changes from active to stagnant from the edge to the shaft portion of the basin. The basin margin with strong runoff condition is often low in gas content. For example, the coal seams with burial depth of less than 1 000 m in the mid-western Qinyuan area are generally less than 10 m3/t in gas content; And in Xiangyuan – Gaoping in the eastern margin of the basin where underground water discharges, the CBM content is generally low. The weak runoff areas are located in the middle and southeast of the study area, where groundwater is stagnant, the CBM preservation condition is good, and the gas content is generally high. For example, Daning, Panzhuang, Zhengzhuang, Fanzhuang blocks etc shows typical stagnant groundwater conditions due to the high groundwater resistance of Sitou fault and south of Jinhuo fault and depression geomorphology of the coal seam, where mineralization degree is higher than 1 000 mg/L and runoff condition is very weak, very favorable for CBM preservation, and thus gas content is generally above 15 m3/t. 2.5. Combined control of multiple factors on CBM accumulation During the actual CBM accumulation, multiple factors instead of a single factor, worked jointly in a complementary and synergetic manner. “Synergy” means that four kinetic geologic processes including tectonism, deposition, geotherm and underground water worked jointly, and in fact multiple geology factors

worked in any accumulation period. “Complementation” means different influence degrees on CBM accumulation at different geology ages due to different evolutions of geological factors. For example, CBM pool can be formed under condition of poor parent organic material for hydrocarbon generation but strong thermal effect , or formed under conditions of favorable sedimentation and tectonism but poor thermal effect, or formed under poor preservation condition due to the well-developed fractures induced by tectonics but favorable blocking environment due to hydrogeological condition. Besides, constructive and destructive factors would often occur and work simultaneously. Due to the above three characteristics, it’s difficult to quantify the contribution of a geologic process to the CBM accumulation, and accumulation process of a basin or even a block can’t be characterized by one or multiple accumulation patterns, and that is, the concept of “CBM pooling” is hard to define. Therefore, if “CBM pooling” is defined in a way of conventional oil and gas trap, it is difficult to construct CBM patterns and instruct CBM development due to complex processes of multiple geologic factors. Conversely, the geologic factors cause the escape of CBM are few, mainly including CBM zone of weathering, extension faults and its surrounding area, collapse column and strong hydrodynamics. Therefore, it is easy to find CBM enrichment area by firstly defining none enrichment areas of CBM and then ticking them out, and the method is very simple and can be used to pick favorable CBM areas directly. CBM in weathered zones has no commercial value, extension faults intersecting earth surface or aquifer can cause massive gas escape, and the locations where the coal seam directly contacts with aquifer are usually low in gas content due to its strong hydraulic dissipation. Accordingly, none enrichment patterns in this area have been established, including gas dissipation zones around large extensional faults (I), gas dissipation zones where faults connected with overlying (underlying) aquifer (II), CBM weathering belts near outcrops (III), zones with high permeability coal roof (floor) and strong hydrodynamics, for example, in Fig. 3, in IV1, the coal seam and aquifer is connected by high permeability argillaceous sandstone, in IV2, the coal and aquifer is connected by a fracture zone; and V is a karst collapse pillar (Fig. 3). Excluding these non-CBM enrichment areas, the rest regions are CBM enriched.

3. Five key technologies for high coal rank CBM exploration and development After nearly 10 years of CBM exploration in southern Qinshui Basin, a series of CBM exploration and development achievements have been made, strongly supporting the construction of CBM production base there. 3.1. Geophysical exploration and evaluation technologies for high coal rank coals

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The 3-D seismic exploration arranged in the CBM devel-

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Fig. 3.

The patterns of high coal rank coalbed methane non-accumulation (The profile position is shown in Fig. 1).

opment stage is mainly used for selecting CBM production construction areas, which is intended to describe the minor structures, predict distribution and physical properties of coal seam, select “sweet” spots, and guide CBM production well deployment. Based on the above demand and in line with the complex surface conditions featuring well-developed gully/valley and big changes in elevation and lithology in southern Qinshui Basin, the seismic data acquisition, processing and accurate interpretation technologies suitable for the shallow and mountain area have been selected, forming a synthetic seismic exploration and evaluation technology system based on the workflow of “locating the position, finding fractures and predicting gas content”. The technology system includes evaluating structure and coal seam spatial distribution based on seismic frequency extending, attribute analysis, spectrum decomposition and wave impedance inversion, finding fracture zones with seismic coherence attribute, curvature analysis and inversion of resistivity difference, and predicting gas content prediction by AVO inversion, adsorption and attenuation attributes, multiple attributes clustering analysis, wavelet decomposition and reconstruction. Using this technology series, 210 faults with over 3m throw have been find out, and the nature, type, occurrence and extension length of the faults have been described in detail; distribution of more than 30 possible collapse columns with a diameter of over 20 m have been analyzed; and development degree of fractures and CBM content have been evaluated, which have not only effectively guided deployment of CBM development wells, but also provided geological guarantee for horizontal well drilling. Taking Zhengzhuang Dongda block as an example, 9 horizontal well locations were designed before, and the well location and track of 7 wells out of the 9 have been adjusted according to the new fine seismic data, and the newly drilled wells have much better drilling effect (well type, effective drilling ratio of coal seam).

3.2. Well drilling and completion technologies for high coal rank coal reservoir 3.2.1.

Drilling technology for vertical (cluster) wells

A two-section well structure has been adopted in the vertical CBM well drilling[20]: the 1st section is drilled into bedrock from earth surface with 311.2 mm bit , and 244.5 mm casing is run into 1020 m below bedrock top face and cemented; the 2nd section is drilled into 5060 m below the bottom of the coal seam with 215.9 mm bit, and then 139.7 mm casing is run into and and cemented. This well structure has simplified vertical (cluster) well drilling process and reduced the cost of drilling significantly. Based on the characteristics of CBM drainage and well pattern arrangement, the cluster wells should be less than 30º in deviation angle and between 250 m and 300 m in bottom hole displacement. Cluster vertical wells can not only drill multiple target layers to satisfy commingling development from multi-coal seams, but also decrease well quantity, land occupation, cost of surface construction and operating, realizing low-cost CBM development. For example, 948 cluster wells in south Qinshui Basin reduced 601 well sites and land occupation area of approximately 600 000 m2. 3.2.2. wells 3.2.2.1.

Drilling technology for multiple types of horizontal Engineering design optimization technology

Optimization of updip trajectory design of horizontal wells: based on production characteristics and drainage rules, water and coal fines should be expelled as much as possible in the beginning of CBM well drainage to expand the depressurization area and improve CBM desorption efficiency. Based on this concept, potential theory has been applied to design the updip trajectory of horizontal wells. Optimization of profile design of horizontal wells: the main horizontal wellbores take middle curvature radius, profile of continuous ascent combination of “vertical–buildup–buildup–

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holding (horizontal section)”, with kickoff point on top of coal seam and the curvature radius of 50100 m in buildup section, resulting in smooth well trajectory, small friction and torque. Optimization of bottom hole structure design of combined “V” multi-lateral horizontal wells: based on the CBM geologic characteristics in south Qinshui Basin, coal adsorption-desorption model and seepage theory, bottom hole structure of combined “V” type (geometrical morphology like multiple “V” combination) have been designed. Key parameters for the bottom hole structure have been optimized to improve the CBM production of combined “V” multi-lateral horizontal wells through numerical simulation and comparison analysis (Fig. 4). The detail parameters include: each horizontal well has two main branches with intersection angle of 20º30º and 5001 000 m length each, and the direction of main branch should be perpendicular to or intersect at large-angle at least with the main strike of fracture in coal; each main branch include 3 or 4 branches of about 200 m long at an intersection angle of approximately 30 with the main branch; each well with a total footage in coal seam of more than 3 000 m, and control area of more than 0.32 km2. Vertical wells are deployed in the blank area or down-dip location of bottom hole of horizontal wells to increase the recovery of CBM resources and assist discharge of water or coal fines. 3.2.2.2.

Process optimization

In view of low permeability, low pressure and strong heterogeneity of high coal rank CBM reservoirs, comprehensive mud logging real-time acquisition software, measurement while drilling (MWD) software, real-time analysis of logging & mud logging while drilling software, real-time trajectory control and track system have been developed. Low cost horizontal well technologies for CBM development, including real-time acquisition of drilling and logging and mud logging, real-time analysis, control and optimization of borehole trajectory, prevention and treatment of accidents of drilling and logging have been formed.

With the improvement of engineering design and process technology, CBM development with combined “V” multi-lateral horizontal wells in south Qinshui Basin has made some accomplishment. 102 wells have been completed, among which 78 are producing gas with an average daily production of 4 498 m3 per well, about 4.6 times of the average daily production of the vertical wells. Furthermore, fracturing stimulation in horizontal wells and assisting discharge of vertical wells in structural low have been tested and made some achievements. Meanwhile, the “U” and “L” type wells have been optimized in design and drilled, and at initial water pumping stage. 3.3.

Currently, the technology of “variable displacement, large liquid volume, active water and sand added fracturing” has been established to solve the challenges of easy reservoir damage, high filtration of fracturing fluid and high treatment pressure in coal seam fracturing. The keys of the fracturing technology include adopting active water and sand-added fracturing fluid with low damage to coal, enlarging the fracturing fluid displacement to decrease filtration and improve the effect of fracturing, and increasing hierarchically the liquid volume to control fracturing pressure, which has helped the realization of high-efficiency CBM development, making the average daily production increase from 1 000 m3/d per well to 1 300 m3/d. Particularly since the year of 2009, in view of the different coal structure and reservoir damage types of low gas production wells, different scales of re-fracturing have been tested in the area, and re-fracturing process for plugging removal of different types of low gas production wells have been established. The technique can effectively remove coal powder near wellbore, open the micro-fractures closed by stress change and remove gas lock effect. It has been widely applied and achieved good reservoir treatment effect and economic benefits. For example, 80 low gas production wells in the Fanzhuang block have adopted the re-fracturing treatment to remove plugging, resulting in the increment of daily gas production of 400 to 1000 m3/d per well and 44 032 m3 of average daily gas production of the block in total, with a treatment efficiency of 78%. 3.4.

Fig. 4. Sketch map of well bottom structure of combined “V” multi-lateral horizontal wells.

Main technologies of reservoir simulation

Intelligent drainage control technology

Theories and technologies including double hump production curve, single well development curve, and “five stages three pressures” had been widely used in CBM well drainage[21]. Based on drainage experiments and simulation analysis, the understanding on CBM production has been deepening, and four key control points affecting CBM drainage have been further recognized, and “five stages, three pressures and four times” drainage technology widely used now has come about. Its technical contents include insisting on the drainage criterion of “continuous, gradual change, stable and long-term”, rigorously monitoring four key times “initial water produce

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time, initial gas desorption time, initial deflated gas time, and stable production time”, and properly controlling key indices of bottom hole pressure, desorption pressure, abandon pressure and gas production rise during five stages of production, water pumping stage, casing pressure building stage, bottom hole pressure control stage, high and stable productions stage and exhaustion stage divided by four key times. Intelligent CBM drainage control technology and equipment based on “double circle and three control” method have been developed, which aims to adjust pumping parameters intelligently to realize pressure drop control, stable bottom hole pressure and casing pressure control through automatically monitoring casing pressure and bottom hole pressure. Through field tests, the refinement and intelligent CBM drainage technology can fulfil the demands of “continuous, gradual change, stable and long-term” CBM well drainage through bottom hole pressure control, and thus realizing stable and high CBM production. At present, the CBM drainage technology has been widely used in new CBM development blocks. 3.5.

Digital technology for CBM fields

3.5.1. Simplification and optimization of the ground gathering and transportation engineering construction In view of low pressure, low yield and low cost of CBM development, the CBM gathering process model of “wellhead metering, concatenation of valve group, separate gas and water transportation, pressure boost as required, centralized treatment” has been established. On the basis of this model, the optimization and simplification of CBM gathering and transportation system have been realized with the goal of reducing cost: (1) Promoting the systematic and standardized construction: modular design is used for CBM recovery well, gathering station, treatment plant, etc, so these facilities all have common and standard technological processes and layout; typical standardization design maps have been compiled and widely applied to shorten design period and improve construction quality. (2) Exploring skid-mounted construction mode: in line with the rolling development of CBM and construction reality in mountainous area, on the basis of standardized design, skid-mounted construction has been attempted on field rodless CBM drainage device, temporary CBM compressed station, and CBM gathering station, which has shortened the construction period and reduced the construction cost further. (3) Selecting pipes for gas production system: according to the analysis of gas properties, pipeline network, gas pressure level, gathering and transferring process, based on the widely use of PE100 pipe of SDR11 series, pipes used have been further selected by considering economic application conditions: PE100 SDR11 series pipes (thick) have been chosen for pipelines less than 110 mm in diameter, and PE100 - SDR17.6 series pipes (thin) have been chosen for pipelines 125400 mm in diameter, which has greatly reduced the investment on

CBM pipe network construction. (4) Optimizing ground layout of CBM gas field: a low cost construction mode of “one station with multiple wells, well concatenation, gas gathering at low pressure” has been established, and the pipe network construction thought of “uniform CBM gathering pipeline network, CBM producing wells providing fuel supply for temporally none CBM production wells” has been put forward. CBM production practice shows with the implementation of the above optimization technologies, the gathering and transportation process mode has been greatly improved, making it more suitable for CBM “low pressure, rolling development” and construction and production construction reality, with the CBM development process simplified, and the safe and smooth running of the gas field ensured. 3.5.2.

Intelligent CBM field

Remote automatic monitoring and management have been realized using intelligent control and internet of things in digital gas fields, including automation production monitoring and command, and data accessible communication between single wells, CBM gathering station and treatment center, and remote fine control and full life-circle management of instruments and equipment. This has greatly reduced labor and improved utilization ratio of instruments and equipment. Further, the demonstration area of internet of things for whole CBM development process has been constructed to support overall coordination of CBM drainage, intelligent decision and full life-cycle management of production system based on control and application of an intelligent platform. The promotion of intelligent CBM field has greatly reduced the labor intensity of workers, saved cost, improved the timeliness of accident pre-warning and failure diagnosis, strengthened life-circle management of goods and materials, ensured the safety, reliability and continuity of the systems and greatly improved the management level and benefit.

4.

Conclusions

The CBM reservoirs in south Qinshui Basin are characterized by high coal rank, strong methane adsorption capacity, high gas content, low porosity and bimodal pore distribution, low permeability and low reservoir pressure gradient. CBM accumulation there is controlled comprehensively by the relatively stable tectonic environment, abnormal thermal events in Yanshanian, favorable coal accumulating environment, stagnant or weak runoff underground water. Non-CBM enrichment models have been established to simplify the identification of CBM enrichment area and pick CBM favorable areas directly, which has guided CBM exploration and development in south Qinshui Basin. Focusing on the “bottleneck” issue of improving individual well production, geophysical exploration and evaluation, well drilling and completion, reservoir stimulation, drainage process and digital gas field construction technologies have been

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developed and improved to provide strong support for CBM production base construction in southern Qinshui Basin.

CBM. Journal of China Coal Society, 2011, 36(7): 1129–1134. [11] WU Caifang, QIN Yong, FU Xuehai, et al. Macroscopic dynamic energies for the formation of coalbed gas reservoirs and

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