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With increasing emphasis on low-carbon environmental protection, CO2 enhanced coalbed methane production and methane reuse in abandoned mines (rich in N2) have gradually become one of the future development directions. These scenarios involve the coordinated migration of different gases such as CO2, CH4, and N2, and the differences in properties of different gases that affect the flow process. Previous studies often focused on the adsorption differences between gases, neglecting the differences during desorption process. In view of this, the current work conducted experiments and finite element numerical analysis on the desorption process of CO2, CH4, and N2, clarified the differences and influencing factors of desorption among the gases, and analyzed the flow change rules under different permeability and diffusion capabilities. The results indicated that the main differences among CO2, CH4, and N2 during desorption are reflected in the parameters of Langmuir volume, permeability, and diffusion coefficient. These parameters showed that CO2 has the highest value during desorption, while N2 has the lowest. The factors affecting the magnitude of differences between CO2, CH4, and N2 are mainly their compositions. Specifically, ash content significantly affects the difference in adsorption capacity, while moisture content influences permeability and diffusion coefficient. During desorption, permeability plays a continuous role throughout the whole process, while diffusion coefficient is exhibited mainly in the initial stage of desorption. Different gases have varying sensitivities to permeability and diffusion coefficients during desorption. Changes in permeability and diffusion coefficient significantly affect the CO2 desorption process. N2, on the other hand, is the least sensitive, especially to changes in diffusion coefficient. During gas flow, when reservoir permeability is less than 0.01 mD (= 9.869233 × 10−18 m2), permeability becomes the main factor that affects flow. When the diffusion coefficient is less than 5 × 10−9 m2/s, increasing the diffusion coefficient is necessary to effectively promote gas outflow. To effectively increase gas production, it is necessary to comprehensively consider the magnitudes of permeability and diffusion coefficient. [ABSTRACT FROM AUTHOR]

Multi-layer superposition of coal seams and tight sandstone layers is common in Chinese major coal-bearing basins, which possesses huge resource potential. Co-production of coalbed methane (CBM) and tight sandstone gas reservoirs is still in the stage of engineering test in China; the control factors towards the co-production performance remain to be clarified. Based on discussion on the key problems involved in co-production of coal seams and tight sandstone layers, the main control factors governing the co-production performance of CBM and tight sandstone gas are analyzed. Taking well X in Ordos Basin as an example, a co-production numerical model is established. The co-production characteristics of well X are analyzed. The optimization of co-production sequence and hydraulic fracturing parameters of tight sandstone formation is carried out. The results show that the early drainage in coal seams and the reasonable control of bottom-hole flowing pressure are the keys to achieve good development benefits. The co-production is mainly controlled by reservoir permeability, water saturation of sandstone formation, formation-desorption pressure difference of coal seam, reservoir pressure gradient, and surrounding rock properties. Under the optimized co-production sequence of perforating coal seam first and the tight sandstone layer, the tight sandstone layer has a low recovery degree, which may be targeted for later reservoir reconstruction. The recovery degree of tight sandstone layer can be improved by optimizing the hydraulic fracturing parameters such as the fracture half-length and the fracture conductivity. For well X, the fracture half-length and the fracture conductivity are optimized to be 150 m and 300 mD∙m. The findings of this study can be applicable to evaluating the adaptability of geological conditions for multi-layer co-production in coal-bearing basins. Also, the findings can provide theoretical guides for the optimization of production sequence and hydraulic fracturing to improve the co-production benefits. [ABSTRACT FROM AUTHOR]

Adsorption and desorption of coalbed methane are generally at a dynamic equilibrium state under the undisturbed coal reservoir. However, as the reservoir pressure drops to a certain value during the extraction of coalbed methane, the equilibrium state is destroyed and thus more coalbed methane desorbs and escapes from coal to wellbore. Here the corresponding bottom-hole fluid pressure is called initial gas production pressure (IGPP) in the development practice of coalbed methane wells. This paper, which has taken Changzhi-Anze block in the central-southern part of Qinshui basin as the study object, addresses the distribution characteristic and control factors of IGPP of coalbed methane wells and then explores the key factors affecting IGPP using grey correlation analysis theory. The results indicate that IGPP varies from 1.09 MPa to 6.57 MPa, showing a distribution law with high in the middle and low in the west and east of the study area, which presents a similar distribution characteristic with burial depth, thickness, coal rank, gas content, original reservoir pressure, and in-situ stress. Further, through grey correlation analysis, it concludes that the correlation degrees of control factors affecting IGPP of coalbed methane wells in the descending order are decline rate of working fluid level, water yield before gas production, reservoir pressure, coal thickness, coal rank, minimum horizontal principal stress, burial depth, and gas content. Among these factors, engineering factors, including decline rate of working fluid level and water yield before gas production, present a key controlling effect, because they can determine the smooth migration pathway directly during initial water production. And another key factor, original reservoir pressure also builds strong and positive correlation with IGPP under the interaction of other geology and reservoir factors, revealing the capability of gas desorption and the transmission of pressure drops. [ABSTRACT FROM AUTHOR]