Dynamic Characteristics Of Reservoir Depressurization During Coalbed Methane Reservoir Depletion And Its Influences On Gas Output In The Qinshui Basin,Shanxi Province

As a kind of organic rock,the micro-pore of coal is well developed with strong sorption property.The transfer law and dynamic variation of coalbed methane(CBM)reservoir during primary depletion is dramatically different from which of the conventional natural gas reservoir.The reservoir pressure is the energy in the process that CBM desorbs from the matrix surface and moves to the wellbore through the fracture networks.The determination of the dynamic characteristics of reservoir pressure is essential to evaluate the fluid seepage,effective desorption area,interwell interference,and multi-layer production,which is significant to the CBM productivity.There is theoretical function on understanding the gas desorption in the matrix,the flow laws in the fracture networks and the expanding of effective desorption area if the transfer laws of reservoir pressure is investigated.Furthermore,it can provide a reference for the controlling and optimization of bottom-hole pressure to realize the high and stable production CBM.In this study,the transfer characteristics of reservoir pressure depletion and its influences on CBM production are analyzed with a case study in the Qinshui Basin,Shanxi Province.During the production of CBM,the reservoir pressure varies in both spacial and temporal dimensions.Firstly,the natural fracture system is introduced,including the visible fracture system and micro-fractures.The visible fracture systems consist of exogenetic fractures,gas-expanding fractures,and cleats.The exogenetic fractures is the mainly passageway for the transfer of reservoir pressure,and they are formed under the tectonic stress,while the secondary fractures are gas-expanding fractures.Around the exogenetic fractures and gas-expanding fractures,there are many cleats containing face cleats and butt cleats.The micro-fractures are the bridge to connect the cleats and pores in the coal matrix.According to the characteristics of gas and water production rate,the production process of undersaturated CBM reservoirs can be divided into four stages,including single-phase water production stage,gas-water two-phase production stage,stable gas production stage,and declining gas production stage.Additionally,with consideration of the relationship between reservoir pressure and critical desorption pressure(CDR),the production stage consists of single-phase water production stage and gas production stage.In the single-phase water production stage,the reservoir pressure is higher than the CDR,and when reservoir pressure falls below CDR,it is gas production.Secondly,based on the transfer process of reservoir press in the fracture networks and coal matrix respectively,the basic transfer modes of reservoir pressure depletion are proposed,including water transfer and gas transfer.On the basis of the two basic transfer modes,a conceptual model of reservoir pressure depletion in the fractures and matrix are established respectively.The results show that in the visible fracture system,the main transfer mode is water transfer.While in the coal matrix,gas transfer is the main transfer mode.In the initial dewatering,water transfer is prior to gas transfer and after the production arrives into the stable gas production stage,gas transfer will be the main transfer mode for reservoir depressurization.Moreover,in view of direct deduction,the influencing factors of reservoir pressure dynamics are investigated,containing permeability,water capacity,gas saturation,feature of desorption/sorption,and bottom-hole pressure.The pressure transmission in the visible fracture systems is related to the permeability,water capacity,and bottom-hole pressure,while in the matrix it is mainly influenced by the gas saturation and feature of desorption/sorption.In addition,based on the study on the propagation of pressure perturbation area and gas desorption area in the stage of above and below the critical desorption pressure respectively,a novel mathematical approach is proposed to characterize the dynamic behavior of reservoir pressure,aiming at the undersaturated reservoirs and saturated reservoirs respectively.In this basis,the variation laws of reservoir pressure depletion with time and space in the Qinshui Basin,Shanxi Province,are analyzed and summarized.The definition of drainage boundary is the precondition to characterize the average reservoir pressure as different area has different average reservoir pressure.For the undersaturated CBM reservoirs,in the single-phase water production stage,the average reservoir pressure decreases linearly with the constant water production rate while in the gas production stage,there is similar variation behavior between the average reservoir pressure and bottom-hole pressure,both showing a fast to slow pattern.Meanwhile,the the radius of gas desorption area increases fast in the beginding and ranges slowly and slowly.In the spacial dimension,when the location is further away from the wellbore,the pressure gradient progressively decreases and the rate of pressure change at a fixed point shows a fast to slow pattern.For the saturated reservoirs,the reservoir pressure falls slowly in the initial dewatering stage while it decreases fast in the stable gas production.After the production arrives into the gas declining stage the decreasing rate of depressurization will return to be slow as the there is almost no water produced from the wellbore and the gas production rate is low.According to the comparison of calculation results between this model and a presented tank-type model,it verifies the reasonability of this study.In the end,on the basis of calculation and analysis of the reservoir pressure depletion,the changing laws of reservoir pressure,gas desorption area,and gas content in the Qinshui Basin are summarized and some optimization suggestions on the drainage are proposed.On one hand,the geology parameters influencing the reservoir depressurization should be tested before dewatering,such as fracture development,reservoir boundary condition,well-test parameters,CDR,sorption isothermal parameters,gas saturation,water capacity;on the other hand,during CBM production the bottom-hole pressure must be controlled strictly to follow the objective law of reservoir pressure depletion and the relationships between the influencing factors of depressurization should be considered fully to reduce the reservoir damage.The qualitative characterization of reservoir pressure dynamics is an exploratory study on the controlling fundamental of CBM production.It is meant to provide a theoretical reference for the fining management or optimization of CBM drainage under different geological conditions.