Methane Wells Research Articles

Geochemical Characteristics of Water Produced from Coalbed Methane Wells in the Southern Qinshui Basin and Construction of an Associated Model: Implications for Coalbed Methane Co-Production

The geochemical characteristics of water produced from coalbed methane (CBM) wells contain rich information about the associated geology, environment, and production. This study was conducted in the Southern Qinshui Basin, where produced water samples were collected from 10 typical CBM wells and their ionic compositions and water quality parameters were tested. The differences in the chemical characteristics of the produced water between different producing coal seams and between single-seam production wells (SPWs) and multi-seam co-production wells (MCWs) were compared, and the geochemical formation process of the produced water was revealed. The following conclusions were obtained: (1) the water produced samples that were mainly Na-HCO3-type and were generally weakly alkaline and moderately mineralized. The water produced from No. 15 coal seam was more enriched in SO4, Ca, and Mg compared to that of No. 3 coal seam, and the variations were more intense, reflecting a more complex water chemistry formation. (2) The ionic data of the water produced from MCWs do not lie between the coal seams of SPWs, nor do they satisfy the linear relationship between the ionic compositions of SPWs, reflecting the differences in the water sources between MCWs and SPWs. Water from MCWs tends to communicate with active water sources outside the coal seams, and the produced water contains small amounts of Cl and total dissolved solids, thus inhibiting the pressure reduction efficiency and limiting the effect of CBM co-production. (3) Based on a principal component analysis of the ionic compositions, two characteristic components were extracted, and these represented two types of hydrochemical formation processes. The first type is pyrite oxidation and carbonate dissolution, and its opposite represents sulfate reduction. The second type reflects the groundwater retention and confinement characteristics, and its opposite represents active groundwater or stronger recharge conditions. (4) A geochemical formation model of the water produced from CBM wells in the study area was constructed. Cation exchange adsorption and sulfate reduction were found to be the main water–rock interactions in the coal measure, and they determine the overall water quality of the produced water. Recharge has a relatively significant influence on water produced from MCWs. Pyrite oxidation exists in the water produced from No. 15 coal seam of the Taiyuan Formation, and the higher sulfur content in the coal contributes to this reaction. The results of the study will assist in deepening our understanding of the geochemical formation mechanisms of water produced from CBM wells, and they provide the main reasons for the poor CBM co-production effect from the Shanxi and Taiyuan Formations.

The Influence of Fracturing Fluid Volume on the Productivity of Coalbed Methane Wells in the Southern Qinshui Basin

Hydraulic fracturing is the main technical means for the reservoir stimulation of coalbed methane (CBM) vertical wells. The design of fracturing fluid volume (FFV) is mainly through numerical simulation, and the numerical simulation method does not fully consider the water block damage caused by the leakage of fracturing fluid into the reservoir. In this work, the variance analysis method was used to analyze the production data of 1238 CBM vertical wells in the Fanzhuang block and Zhengzhuang block of the Qinshui Basin, to clarify the relationship between the FFV and the peak gas production (PGP) under the different ratios of critical desorption pressure to reservoir pressure (Rc/r), and to reveal the controlling mechanism of fracturing fluid on CBM migration. The results show that both the FFV and Rc/r have a significant impact on gas production. When Rc/r < 0.5, the PGP decreases with the increase of the FFV, and the FFV that is beneficial to gas production is 200–500 m3. When Rc/r > 0.5, the PGP increases first and then decreases with the increase of FFV. Specifically, the FFV that is favorable for gas production is 500–700 m3. Excessive FFV does not significantly increase the length of fractures due to leaks in the coal reservoir. Instead, it is more likely to invade and stay in smaller pores, causing water block damage and reducing gas production. Reservoirs with high Rc/r have larger displacement pressure, which can effectively overcome the resistance of liquid migration in pores, thereby reducing the damage of the water block. Therefore, different reservoir conditions need to match the appropriate fracturing scale. This study can provide guidance for the optimal design of hydraulic fracturing parameters for CBM wells.

Study on Key Parameters for Jet Impacting Pulverized Coal Deposited in Coal-Bed Methane Wells

Cleaning out the pulverized coal deposited at the bottom of a coalbed methane (CBM) well is key to achieving continuous CBM drainage and prolonging the workover period. In this study, Fluent is used in conjunction with the standard k-ε model and the Eulerian-Eulerian model to simulate and analyse jet erosion of deposited pulverized coal particles. The depth and width of the stable erosion pit that is formed by jet-impacting deposited pulverized coal under different conditions are determined and provide a theoretical basis for the cleanout of pulverized coal in the bottom of a CBM well. In this paper, the three parameters of the jet target distance, nozzle diameter and nozzle outlet flow velocity are selected to perform an orthogonal simulation. The change trends in the depth and width of the scouring pit with time are determined. The results show that jet impacting of deposited pulverized coal can be categorised into four stages, periods of rapid growth, stability, jet swing and dynamic stability. A sensitivity analysis shows that the nozzle outlet flow velocity has the strongest influence on the depth of the scouring pit among the selected parameters. The depth of the jet impact pit can reach the maximum depth at t = 3 s, while the width of the impact pit can reach the maximum after t = 7 s. This can provide key design parameters for CBM well pulverized coal impacting operation. It is of great significance for capacity damage control during CBM well workover operation.

The inflow performance relationship in fractured, vertical, and dewatered coalbed-methane wells

Although not greatly studied, the inflow performance relationship (IPR) in dewatered and vertical coalbed methane (CBM) wells is essential in the development of a CBM reservoir. The dynamics of the stress sensitivity effect (SSE) and the matrix shrinkage effect (MSE), as well as the hydraulic fracture propagation, have all been neglected, especially for the exterior region, which is larger than the drainage radius. A novel IPR model has now been built that integrates dynamic SSE–MSE and hydraulic fracture propagation into the skin factor, and is validated with real production data from the Qinshui Basin, China. The absolute open flow rate given the SSE–MSE is 60.5% larger than without SSE–MSE. If the positive effect of the SSE–MSE on the permeability in the exterior region is neglected, the absolute open flow rate is reduced by 21%. The MSE has a greater effect on the fracturing skin factor than the SSE and tends to lower the fracturing skin factor, benefitting the production of CBM. Moreover, in light of the SSE or MSE, the non-Darcy flow effect is weaker due to restraints on the velocity of flow or the permeability. Useful and essential theoretical guidance for real CBM production can thus be gleaned from this novel IPR model.

Revealing the Effects of Water Imbibition on Gas Production in a Coalbed Matrix Using Affected Pore Pressure and Permeability

The effect of water imbibition on characteristics of coalbed methane reservoirs, such as permeability, gas occurrence state, and gas production, is controversial. According to the mechanism of imbibition, gas and water distribution in blind pores is reconfigured during the fracturing process. Therefore, a new comprehensive model of pore pressure and permeability, based on the perfect gas equation and the weighted superposition of viscous flow and Knudsen diffusion, was established for micro- and nanoscale blind pores during water drainage. Using the numerical simulation module in the Harmony software, the effects of imbibition on coal pore pressure, permeability, and gas production were analyzed. The results showed that (1) water imbibition can increase pore pressure and reduce permeability, and (2) water imbibition is not always deleterious to gas production and estimated ultimate reserve (EUR), when the imbibition is constant, the thicker water film is deleterious to coalbed methane wells; when the thickness of water film is constant, more imbibition is beneficial to gas production and EUR. This research is beneficial to optimize the operation of well shut-ins after fracturing and provides methods for optimizing key parameters of gas reservoirs and insights into understanding the production mechanism of coalbed methane wells.

Experimental evaluation of the dynamic water-blocking effect in coalbed methane reservoir

Field observations indicate that almost 59% of coalbed methane (CBM) wells in the southern Qinshui Basin showed insufficient daily gas production (∼600 m3/t); this is caused by the water-blocking effect (WBE) attributed to the invasion of fracturing fluids or the accumulation of formation water. WBE can cause serious damages to the low-permeability reservoir but has not been paid sufficient attention to in CBM studies. To enhance understanding of the dynamic WBE in the multiphase flow process, we conducted core flooding experiments on two typical Chinese bituminous coal and anthracite coal samples. During the flooding experiments, real-time nuclear magnetic resonance (NMR) measurements of the change of water signal in the core were used to calculate the gas-water relative permeability and to evaluate the dynamic WBE. The results highlight that a more pronounced WBE occurred in anthracite coal compared with bituminous coal. The gas breakthrough model can calculate the gas breakthrough time, which is a useful parameter for evaluating water blocking. The water-blocking threshold pressure gradient (TPG) was determined based on experimental data. The results reveal that the water-blocking TPGs for both samples follow a positive power-law relationship with the normalized water saturation. The water-blocking TPG is approximately an order of magnitude higher than that of the conventional gas TPG. A high water-blocking TPG value reflects stronger gas/water interference during multiphase flow, exerting a negligible impact on the gas migration through the coalbed, which should be prioritized in CBM production. The implications of this study are important for a better understanding of the interaction mechanism of gas and water transport in coal reservoirs during the gas production process.

Experimental study on coal blockage removal based on pulverized coal blockage

The generation and transport of coal powder has caused great harm to coal reservoir channeling conductivity, affecting the capacity of coalbed methane wells, causing the failure of engineering equipment and great distress to production, while current unblocking methods have many shortcomings. In this study, a series of laboratory experiments on coal samples' plugging and reverse-water/gas injection replacement of pulverized coal to unblock coal samples under three-dimensional stress conditions were carried out using a gas–liquid–solid three-phasedisplacement simulation system. In addition, the characteristics of the change of coal permeability and coal powder output under different plugging pressures and unplugging media and pressures were explored. Furthermore, the influence of medium difference on coal powder migration and its mechanism were analyzed. The results were as follows: (1) under the same pressure, the gas carrying capacity for pulverized coal in the pores and fissures of raw coal is much greater than that of water; (2) only when the reverse-gas injection pressure is greater than or equal to the plugging pressure can the coal be unblocked. The reverse-gas injection unblocking the coal body has three stages such as gas-drive-water, gas-drive-water and pulverized coal, and gas production stages (complete unblocking stage of the coal body). The migration difficulty determines the output order of products; (3) during reverse injection hydrolysis of coal plugging, the liquid-phase permeability (LPP) of coal showed “fluctuation–stability” characteristics. The final liquid-phase permeability is lower than the initial value, which is related to the competition between the secondary bridging blockage of pulverized coal particles (PCP) at the pore shrinkage of coal caused by the change in water flow seepage velocity and backwashing, and the pulverized coal's unblocking, redistribution, and capture caused by the local pressure build-up.

Interference mechanism in coalbed methane wells and impacts on infill adjustment for existing well patterns

Inter-well interference, a superimposed effect of pressure propagation from adjacent wells, can enhance the dynamics of methane desorption in pores and gas transport in fractures through energy migration. To better understand the variation pattern of seepage efficiency, the inter-well interference mechanism of different types of reservoirs and productivity changes after infilling coalbed methane wells were investigated. The results showed that the average productivity of old wells increased by 627 m3/d after infilling a horizontal well in the high-permeability reservoirs without surrounding rock water supply (HP-WW); however, the newly infilled horizontal well produced gas at 4996 m3/d. The infill well in HP-WW reservoir resulted in 11％-39％permeability increment compared with initial permeability (the average permeability changes from 1.58 to 2.18 mD). While, much higher permeability increment was observed (varies from 29％-50％) in relative low permeability coalbed formation due to infill well. After the formation of interference, increased confining pressure increases effective stress, which is smaller than the positive effect formed by the fracture expansion and the increase in the effective diffusion area. Additionally, well interference promotes coupled superposition of the pressure drop funnel, which effectively suppresses the influence induced by velocity sensitivity, water sensitivity, and water lock, resulting in an increased pressure difference between the reservoir and the wellbore. This enhances the dynamics of methane desorption from the pore surface and gas migration from the fracture. For gas diffusion, the mean free path of gas molecules decreases with well interference, implying that the main diffusion resistance changes from collisions between gas molecules and the pore wall to collisions among gas molecules. Therefore, the findings of this study can help for better understanding of the response mechanism of inter-well interference to efficient CBM production.

Geomechanics impact due to in-situ and induced stresses during drilling of horizontal and highly deviated coal bed methane wells

Hydrocarbon exploration is the arduous task. Urgency of hydrocarbon and avengement of technology push operators to recover the unexplored unconventional hydrocarbon like coal bed methane (CBM) which has been discontinued. Limitations such as misunderstanding of geomechanical stress, formation behaviours- abrasiveness, mud properties etc; impaired the drilling performance and leads to well failure. Therefore, wellbore stability due to geomechanical stresses is considered as one of the major stages in well planning and required extensive study. The dominant elements of fascinating the CBM extraction are cost and the currently available technology and poor understanding of reservoir as compare to conventional reservoir. CBM wells fails due to exceeding the limit of tensile and shear strength which includes wellbore collapse, pipe sticking, caving, loss circulation and leads to ruin the operator’s money and time. The stress regimes in induced, in-situ stresses are natural and cannot be change; however study of these stresses and implementation of findings are essential to implement while planning the well and monitoring stresses behaviour during drilling of highly deviate or horizontal wells are key element to successfully develop the coal bed methane reservoir. Effect of geomechanical stresses are experienced while drilling horizontal and highly deviated wells. By considering the effects of in-situ and induced stresses; suitable operational window can be design to reduce the CBM wellbore failure. The present work analyse the geomechanical in-situ and induced stresses which are contributing towards instability of wellbore along with matrix shrinkage effect, outcome of this study can be utilized for the efficient planning of failure-free wellbore operating envelopes for CBM wells.

Pore Structure Characteristics and Adsorption and Desorption Capacity of Coal Rock after Exposure to Clean Fracturing Fluid.

In the hydraulic fracturing process, fracturing fluid contacts coal rock and physical and chemical reactions occur, which inevitably damage the pore structure of the coal rock and affect the adsorption and desorption capacity of the coal rock. In this paper, a low-temperature N2 adsorption method and scanning electron microscopy (SEM) were used to characterize coal samples. Using gas adsorption/desorption tests, high-, medium-, and low-rank coal samples before and after the clean fracturing fluid treatment were systematically studied. According to the relationship between coal pore structure parameters and gas adsorption/desorption characteristics, a correlation between the microscopic pore structure and the macroscopic gas adsorption/desorption characteristics of coal was obtained. The results show that the number of closed pores in high-, medium-, and low-rank coal samples increased after the clean fracturing fluid treatment. The micropore volume increased by 0.0009, 0.00143, and 0.0035 mL/g, respectively, and the specific surface area increased by 4.87, 9.06, and 57.60%. The fractal dimension also increased compared with that of raw coal. SEM analysis indicated that the influence degree of clean fracturing fluid treatment on the pore structure of different-rank coal samples was Gengcun low-rank coal > Pingba middle-rank coal > Jiulishan high-rank coal. The experimental results of methane adsorption and desorption showed that the adsorption capacity of the coal samples after clean fracturing fluid treatment was enhanced, which is related to increases in the micropore proportion, micropore volume, and specific surface area of the coal. The desorption capacity of the coal samples was also enhanced. The desorption rate of medium- and high-rank coal samples increased after the clean fracturing fluid treatment but that of low-rank coal samples decreased. The main reason is the increase in the number of micropores in low-rank coal, which enhances the gas adsorption ability and makes gas desorption difficult. Therefore, clean fracturing fluid is suitable for medium- and high-grade metamorphic coalbed methane mines. These research results provide a theoretical basis for the application of clean fracturing fluid in different coalbed methane wells.

Coalbed Methane Industry Development Framework and Its Limiting Factors in China

The current average daily gas production of single coalbed methane (CBM) wells in China is less than 1,300 m3. The low production rate of CBM wells dramatically limits the sustainable development of the CBM industry in China. To promote CBM industry development, three universal geological factors, i.e., gas sources, channels, and driving force of the low production wells are analyzed, considering the various and complicated geological conditions of CBM reservoirs in China. Additionally, we find that the growth in proven CBM reserves should be greatly promoted as the CBM proven rate is only less than 3% by the end of 2020. The widespread CBM extraction mode based on depressurization via dewatering generates very limited effects in coal seams with notable deformation, high in situ stress, or little water. Therefore, a specific design of CBM development modes is proposed, according to the applicable geological conditions of different CBM development modes. It covers depressurization via dewatering, depressurization via stress release, and coal and CBM comining. Moreover, the regional governments play a key role in the development of the regional CBM industry, and favorable policies should be formulated to promote the multipoint development conditions across China. This study is concluded to provide a recommended development framework involving sustainable CBM production in China and its development processes.

An optimal selection method of wells for secondary fracturing in a single coal seam and its application

At present, methods including mathematical modeling, physical or numerical simulation, and in-situ monitoring have been generally adopted to determine evaluation parameters for coalbed methane (CBM) wells for secondary fracturing. These conventional methods either entail many assumptions, or some parameters are difficult to obtain, resulting in a certain deviation between the evaluation results and reality, or the application cost is high, preventing the monitoring of each CBM well. In view of this, an evaluation index system for the gas production potential, effective length of cracks formed by fracturing, and supporting length of proppant in cracks was established based on the system theory. The evaluation indices were characterized through production data, such as logging, fracturing and drainage, which could avoid potential bias in evaluation when only considering a certain parameter and ensured accuracy and practicability of the evaluation parameters for each well. Principal component analysis (PCA) and the entropy weight method (EWM) were used to obtain weights of evaluation parameters, which avoided the contradiction of contributions of various parameters to optimal selection and the rationalized results. In this way, a method for step-wise optimal selection of wells for secondary fracturing integrating construction of evaluation parameters, determination of critical values, and entropy evaluation was proposed. The results of an evaluation of the Shizhuang South Block of Qinshui Basin (Shanxi Province, China) indicate that wells whose three evaluation indices are satisfied are most preferable; wells that only meet the effective length of cracks formed by fracturing or effective supporting length of proppant in cracks can be selected; wells which do not meet the gas production potential or all of the three parameters cannot be selected.

Iterative Ensemble Kalman Smoother Applied to History-Matching Coalbed Methane Wells

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper URTEC 208291, “Introducing Iterative Ensemble Kalman Smoother to History-Matching Horizontal Coalbed Methane Wells,” by Rubakumar Sankararaj, SPE, and Sylvain Ducroux, SPE, Resoptima, and Dan Kuznetsov, SPE, Arrow Energy, et al. The paper has not been peer reviewed. This study presents the iterative ensemble Kalman smoother applied to a low-permeability coalbed methane (CBM) field in Australia. All wells in the studied reservoir are completed with an artificial lift system and permanent downhole gauges. A forecast study was conducted to validate the history-matched ensemble. The results showed a good match of 12 months of the new production data not used in history matching, which highlights the robust prediction capabilities of the approach. Geology of the Study Area Because of its strategic geologic and geographic situation in Queensland State, the Bowen-Surat area constitutes one of the more economically significant sedimentary provinces of the Australian mainland and one of the world’s major coal and coal-seam-gas-producing regions. This study considers the Q seam from the Late Permian. Although its thickness varies in the basin because of merging and splitting, it is continuous locally, with an approximate thickness of 5–6 m.

Gas Distribution and Its Geological Factors in the No.5 Coal Seam of the Weibei Field, Southeastern Ordos Basin, North China

The distribution of gas contents in the No.5 coal seams of the Weibei Field of southeastern Ordos Basin, North China, is highly variable and its mechanism remains unclear. In this study, systematic evaluation of the gas content and its geological control factors are conducted based on the field investigation together with 16 coalbed methane (CBM) wells in the Weibei Field, southeastern Ordos Basin, North China. The gas content variability is determined from the perspectives of gas generation, migration, and preservation. The results indicate that the gas generation is largely relevant with the subsidence and fluctuation of the coal seam during the middle-late Yanshanian orogeny, which controls the gas content variability of the Weibei Field. Besides, gas migration and preservation determine the gas content on a regional scale, significantly related to fault types, roof lithology, burial depth, and hydrodynamic conditions, but scarcely affected by the roof thickness. In the Weibei Field, the geological models controlling gas content are identified as (1) hydrodynamic trapping of gas in the deep burial depth and thrust faults, and (2) gas loss by groundwater flushing and normal faults. Basically, the first mechanism causes the high gas content of the east zone, whereas the other one is responsible for the low gas content in the west zone of the study area.

Numerical Simulation of Fracture Propagation Characteristics of Hydraulic Fracturing in Multiple Coal Seams, Eastern Yunnan, China

As a reservoir reconstruction technology, hydraulic fracturing is a key method to improve the production of coalbed methane (CBM) wells. The CBM reservoir in eastern Yunnan, an important CBM exploration and development zone in China, is characterized by multiple thin coal seams. Compared to the fracturing of the single-layer coal seam, the combined seam fracturing technology is more difficult and complex. To study the fracture propagation characteristics and influencing factors of hydraulic fracturing in multiple coal seams, taking No. 9 and No. 13 coal seams as the research objects, the fracturing process was numerically simulated by using the finite element method and ANSYS software in this work. Based on the mathematical model of low permeable coal-rock mass, a two-dimensional hydraulic fracture model was established. In addition, the fracture geometries of combined seam fracturing were studied quantitatively. The results indicate that although No. 9 coal and No. 13 coal seams have similar rock mechanical properties, the propagation process and final geometry of a fracture are different. The reliability of the simulation results is verified by the comparison of experimental parameters and field investigation. The results prove the feasibility of combined seam fracturing in eastern Yunnan. The high Young’s modulus and thickness of the coal seam make the fracture geometry longer, but the fracture height is smaller. The low Young’s modulus, high Poisson’s ratio, and thickness of the No. 13 coal seam result in an increase in the length and height of the No. 9 coal seam. The increase in Young’s modulus of interlayer inhibits the propagation of fractures, while the high thickness and low Poisson’s ratio of interlayers facilitate the extension of the length and inhibit the extension of the height. This work provides a case reference for combined seam fracturing of coal reservoirs and has practical significance for the development of CBM characterized by multiple coal seams in eastern Yunnan.

Analysis of low production coalbed methane wells and application of secondary reconstruction technologies

Analysis of low production coalbed methane wells and application of secondary reconstruction technologies

Investigation on effective stress coefficients and stress sensitivity of different water-saturated coals using the response surface method

The relative magnitude of confining pressure and pore pressure, a key parameter for stress sensitivity of coal permeability, can be characterized by the effective stress coefficient (ESC). In this study, the stress sensitivity experiments of different water-saturated coals were carried out according to the dewatering of coalbed methane (CBM) wells. The ESC was calculated based on the response surface method (RSM), revealing the effect of water saturation on the ESC and stress sensitivity of high-rank coal. The results showed that the stress sensitivity coefficient of these high-rank coals ranged from 0.8136 to 1.5631, indicating a strong stress sensitivity. With growing net pressure and water saturation, the gas permeability of coals changed in a negative power exponential form. Compared with the binary linear regression (BLR), the effective stress calculated by the ESC using the RSM could better reflect the stress sensitivity of permeability. In addition, the ESC increased significantly along with water saturation. Finally, a mathematical model considering ESC and reservoir pressure was established to calculate the “turning point” of dynamic permeability, where the controlling factors of permeability transform from effective stress to matrix shrinkage.

Numerical Simulation Study of the Upper Aquifer Impact on the Coalbed Methane Production

The coalbed methane (CBM) resources in the Qinshui Basin are abundant with exceptional production potential. Coalbed methane wells usually need to be hydraulically fractured to build economical production. However, the exploration and production experiences have shown that the penetration distances of hydraulic fractures in the vertical direction of coal seams are uncertain, which may lead to fracture communication of the layers above and beneath aquifer layers, resulting in divergent production dynamics among coalbed methane producers. It has been observed that some of the CBM wells in Block M produced significantly more water than other wells in the surrounding area. It is suspected that the hydraulic fracturing among these producers may have communicated to the sandstone aquifers. To address the above challenge, this study investigated the geological characteristics of the aforementioned aquifer and coalbed layers, established a three-dimensional geological model, applied numerical simulation to quantify the influence of the aquifer on the production of the CBM wells, and developed a method to improve the history matching of CBM production simulation with the influence of the aquifer considered. This method has significantly improved the accuracy of the history matching performance and the reliability of the remaining gas study. The results from this study laid the foundation for the subsequent development strategy optimization.

Geological and engineering controls on the differential productivity of CBM wells in the Linfen block, southeastern Ordos Basin, China: Insights from geochemical analysis

In view of the differential productivity of coalbed methane (CBM) wells in the Linfen block on the southeastern margin of the Ordos basin, geochemical analysis is performed to determine the hydrodynamic condition of coal reservoirs, identify the water and gas source of commingled wells, reveal different pressure drop mechanisms, and finally clarify the geological and engineering controls on the production performance. The positive correlations between average daily gas production and the concentrations of Na+, Cl‾, TDS, Sr, and Ba imply that strong hydraulic closure is the premise of high productivity. There is no obvious relationship between water and gas production. Commingled (5 + 8#) wells tend to produce more water than single-layered (5#) wells, because the No. 8 coal seam has more active hydrodynamic condition, which is proven by the lighter δ2H and δ18O and smaller Sr/Ba ratio of waters from commingled wells. The difficulty in pressure drop funnel expansion is the primary constrains for the single-layered wells, which is only resolved by the horizontal wells located in low structural positions at present characterized by increasing trends of TDS, Na+ and Cl‾ contents during drainage. The faster the ion concentration rises, the faster the pressure drop funnel expands and a high peak daily gas production can be quickly achieved. The L-shaped horizontal well is thought to be the most suitable well type due to its high gas yield, low time investment and engineering cost. Due to hydrodynamic heterogeneity, commingled wells often show strong interlayer interference and a change in the water-producing horizon from 5# to 8# coal, which is manifested in the tendency of Cl‾, Na+, and TDS to increase first and then decrease. By selecting superior lithologic assemblages and avoiding strong aquifers, relatively continuous and stable production of commingled wells can also be achieved. This study provides a basis for the further development of CBM in the Linfen block and enriches engineering geochemistry theory.

Drainage Type Classification and Key Controlling Factors of Productivity for CBM Wells in the Zheng Zhuang Area, Southern Qinshui Basin, North China.

The production of coalbed methane (CBM) wells varies greatly in the Qinshui Basin, North China. Analyzing the primary factors controlling the CBM well productivity is essential to improve their development efficiency. Based on the geological conditions and production data of CBM wells in the Zheng zhuang area, the principal component analysis (PCA) method was used to classify the drainage types and screen the key factors influencing the production of gas and water. The drainage types of the CBM wells in the study area can be divided into four categories. The gas production shows an increasing trend with the increase of the comprehensive score of the PCA. The key controlling factors of productivity for CBM wells can be summarized by the gas-bearing property, permeability, groundwater fluid potential, and burial depth. The impact of burial depth on CBM well productivity is manifested in its control of gas content and permeability. The groundwater flows to a low fluid potential area, which leads to a high water production and a small pressure drop. The gas production shows a positive correlation with post-fracturing permeability. The gas content is a key factor for controlling the critical desorption pressure, critical gas production pressure, and pressure drop at the gas breakthrough point. High gas content is a prerequisite for the high productivity of CBM wells.