Reservoir Dynamic Response And Interlayer Interference Mechanisms Of CBM Extraction In Superposed Gas-bearing Systems

Coalbed methane(CBM)is a type of unconventional hydrocarbon natural gas with methane as its main component.Its calorific value is approximately 2–5 times that of standard coal,and its combustion rarely causes pollution.Eastern Yunnan and Western Guizhou provinces possess abundant CBM resources.However,the geological conditions in the area are extremely complex,with characteristics such as the development of a large number of thin coal seams and superimposition of gas bearing systems.Therefore,the production of CBM faces many challenges.Accelerating the pace of CBM development in this area is of great value for expanding China’s CBM industrial bases,optimizing the energy structure,and upgrading the industrial scale.With an engineering background of CBM development in superposed gas-bearing systems,we used the self-developed large-scale multi-field coupling test system for CBM extraction in superposed gas-bearing systems as the main means.We also adopted the research approach of combining indoor physical simulation,theoretical research,numerical analysis,and practical engineering application.First,the characteristics of reservoir response and the evolution of the gas flow process in the CBM extraction in superposed gas-bearing systems were analyzed in vertical and horizontal wells.Second,to achieve a deeper understanding of the interlayer interference effects of superposed gas-bearing systems,experiments were conducted on interlayer interference mechanisms under various factors including different relationships between adjacent layers,different reservoir depths,different interlayer spacings,and different numbers of layers under CBM extraction.Next,we propose the calculation method for interlayer interference factors,construct the mathematical model of CBM migration in superposed gas-bearing systems,and explore the linkage relationship among reservoir pressure,gas-bearing property,and permeability.Through the above research,the following research results are obtained:(1)Based on the gas deposit characteristics of superposed gas-bearing systems and the current CBM extraction technology,we independently designed and developed the large-scale multi-field coupling test system for CBM extraction in superposed gasbearing systems.The coexistence of the indoor simulation of superposed gas-bearing systems,which are gas reservoirs with multiple sets of pressure systems,was achieved.The system can acquire the real-time status parameters of multiple physical fields in the reservoirs and wellbore pipelines during CBM extraction;this provides a new means for the safe and efficient CBM extraction in superposed gas-bearing systems.(2)Physical simulation experiments were carried out on vertical well for CBM extraction in superposed gas-bearing systems under different reservoir occurrence environments and different modes.The spatiotemporal evolution laws of reservoir pressure,in-situ stress,reservoir deformation,reservoir temperature,production,and other physical field parameters in the process of CBM extraction in superposed gasbearing systems are clarified.The dynamic response characteristics of reservoir physical parameters and the gas migration characteristics in the process of vertical well extraction are revealed,and the disturbance mechanism of CBM extraction is expounded.In addition,the study investigated the influences of reservoir occurrence environments and modes on the evolution of reservoir physical parameters and gas migration characteristics during vertical well extraction.We also investigated the differences in disturbance mechanism under different modes.(3)Physical simulation experiments were further performed on horizontal well for CBM extraction in superposed gas-bearing systems under different reservoir occurrence environments and different modes.The spatiotemporal evolution laws of multi-physical field parameters under horizontal well conditions were obtained.Furthermore,the study reveals the dynamic response characteristics of reservoir physical parameters and gas migration characteristics in the process of horizontal well extraction.Moreover,we expound on the influences of reservoir occurrence environments and modes on the evolution of reservoir physical parameters and gas migration characteristics in this process.The pressure compatibility coefficient,production loss coefficient,and production compatibility coefficient were used to quantitatively analyze the disturbance degree.Accordingly,the difference in the disturbance mechanism in horizontal wells under different reservoir occurrence environments and different modes are analyzed in depth.(4)Interlayer interference experiments were carried out under conditions including different relationships between adjacent layers,different reservoir depths,different interlayer spacing,and different numbers of layers under extraction.The spatiotemporal evolution laws and dynamic response characteristics of multiphysical field parameters caused by CBM extraction in the superposed gas-bearing systems are expounded.The interlayer interference mechanisms of CBM extraction in superposed gas-bearing systems are revealed.A quantitative method was conducted to clarify the degree of interlayer interference based on the characteristic parameters of reservoir pressure drop under different influencing factors,and a mathematical model of coalbed methane transportation in superimposed gas bearing system considering interlayer interference effect is constructed.In addition,by combining the actual on-site situations of CBM development projects in the Eastern Yunnan and Western Guizhou regions,a numerical simulation analysis was carried out on CBM production in superposed gas-bearing systems under different initial reservoir pressures,different gas-bearing properties,different permeability,etc.Consequently,the linkage relationship among reservoir pressure,gas-bearing property,and permeability of CBM extraction in superposed gasbearing systems was revealed.