沁水盆地南部煤储层赋存环境条件及其对渗透率的影响

Geological conditions of coal reservoir occurrence in the Southern Qinshui Basin and its impact on permeability

煤层气赋存与产出受控于煤储层地应力、压力和地温等赋存环境条件,正确分析煤储层赋存环境条件及其对渗透率的影响是煤层气有效开发所关注的关键问题。采用沁水盆地南部煤层气井63个测试资料,系统分析了研究区煤储层地应力、压力和温度条件,揭示了煤储层应力、压力和温度随埋藏深度的变化规律,建立最小水平主应力与垂直主应力和煤储层压力之间关系模型。采用三轴渗流试验系统,开展了不同应力、压力和温度条件下煤层气渗流试验研究,揭示了不同温度、应力和压力条件下煤样渗透率变化规律及其控制机理。研究结果表明,研究区煤储层最大、最小水平主应力分别为6.62~42.06和3.30~26.40 MPa,其梯度分别为1.20~5.26和0.99~2.95 MPa/hm;煤储层压力及其梯度分别为0.99~12.63 MPa和0.23~1.18 MPa/hm;煤储层温度及其梯度为19.36~38.84℃和1.98℃/hm;且煤储层应力、压力和温度均随深度的增加呈线性增大的规律。随着有效应力的增加,煤储层渗透率不断降低,在初始加压阶段,渗透率下降幅度较大,随着有效应力的增加,下降幅度变缓。在相同的应力条件下,温度的增加使得煤样渗透率不断降低,渗透率的下降速率随温度的升高而减小。随着有效应力和温度的增加,煤储层渗透率按负指数函数规律降低。随着孔隙压力的降低,有效应力增加,煤储层渗透率不断降低。在初始降压阶段,煤储层渗透率急剧下降,随着孔隙压力的降低,渗透率下降速率逐渐变缓;当孔隙压力小于0.6 MPa后,煤储层渗透率随孔隙压力的降低而升高。在高孔隙压力条件下,渗透率随温度的升高呈负指数函数降低,在低孔隙压力条件下,煤储层渗透率随温度的升高呈线性降低。在此基础上,建立了煤储层渗透率与应力、压力和温度之间的关系模型,揭示了煤储层渗透率随应力、压力和温度应力的增加按负指数函数降低的规律和控制机理。

The occurrence and output of coalbed methane(CBM)are controlled by the occurrence geological conditions of coal reservoirs,such as stress,pressure,and temperature.The correct analysis of the occurrence geological conditions of coal reservoirs and their impact on permeability is a key issue of concern for an effective development of CBM.Based on the test data of 63 CBM wells in the southern part of the Qinshui Basin,the ground stress,pressure and temperature conditions of coal reservoirs in the study area are systematically analyzed,the variation law of coal reservoir stress,pressure and temperature with burial depth is revealed,and the relationship between the minimum horizontal principal stress and the vertical principal stress and the pressure of coal reservoir is established.Using the triaxial seepage test system,the experiment of CBM seepage under different stress,pressure and temperature conditions is carried out,and the variation law and control mechanism of coal sample permeability under different temperature,stress and pressure conditions are revealed.The results show that the maximum and minimum horizontal principal stresses of the coal reservoirs in the study area are 6.62−42.06 MPa and 3.30−26.40 MPa,respectively,with the gradients of 1.20−5.26 MPa/hm and 0.99−2.95 MPa/hm,respectively.The coal reservoir pressures and their gradients are 0.99−12.63 MPa and 0.23−1.18 MPa/hm;the coal reservoir temperatures and their gradients are 19.36−38.84℃and 1.98℃/hm,respectively.The coal reservoir stress,pressure and temperature increase linearly with the increase of depth.With the increase of effective stress,the permeability of the coal reservoir decreases continuously,the permeability decreases greatly in the initial pressurization stage,but decrease slows down with the increase of effective stress.Under the same stress conditions,the permeability of coal samples and the decrease rate of permeability decrease continuously with the increase of temperature.With the increase of effective stress and temperature,the permeability of coal reservoir decreases according to the law of negative exponential function.With the decrease of pore pressure,the effective stress increases,but the permeability of coal reservoir decreases.In the initial depressurization stage,the permeability of the coal reservoir decreases sharply,and with the reduction of pore pressure,the decrease rate of permeability gradually slows down.When the pore pressure is less than 0.6 MPa,the permeability of the coal reservoir increases with the decrease of pore pressure.Under the condition of high pore pressure,the permeability decreases with the increase of temperature in a negative exponential function,while under the condition of low pore pressure,the permeability of coal reservoir decreases linearly with the increase of temperature.Based on the above results,the relationship model between coal reservoir permeability and stress,pressure and temperature is established.Also,the law and control mechanism of coal reservoir permeability decrease according to negative exponential function with the increase of stress,pressure and temperature stress are expounded.