高煤级煤储层渗透性与应力耦合模型及控制机理

煤储层应力敏感性是影响煤层气井产能的地质因素,以鄂尔多斯盆地东南缘高煤级煤储层为对象,通过煤样的应力敏感性实验和现场测试,建立了高煤级煤储层渗透性与应力之间的相关关系和模型;探讨了渗透性变化的控制机理.研究结果表明, 煤储层渗透率随应力的增加按负指数函数规律降低.在应力小于5 MPa时,煤储层渗透率随应力增加快速下降,应力敏感性最强;应力在5~10 MPa时,渗透率随应力增加而较快下降,应力敏感性较强;而当应力大于10 MPa后,渗透率随应力的增加下降速度减缓,应力敏感性减弱.与沁水盆地南部高煤级煤样实验结果对比认为,鄂尔多斯盆地东南缘山西组2煤层应力敏感性要小于沁水盆地南部山西组3煤层的应力敏感性.煤储层渗透性是在应力作用下煤储层中裂隙产生压缩(压密)变形,裂隙开度急剧减小的结果.因此在煤层气开发过程中控制排采速度,尤其是排采早期降液速度,对于防止煤储层应力敏感性,提高采收率具有实际意义.

Coupling model of stress-dependent permeability in high-rank coal reservoir and its control mechanism

Stress sensitivity of the coal reservoir is one of the geological factors affecting the production of CBM well. Targeting on the high-rank coal reservoirs in Southeast margin of Ordos Basin, based on field measured data and through the stress sensitivity experiment of coal samples, the correlations and their models between permeability and stress of high-rank coal reservoir were established, and the control mechanism of permeability change was studied. Research results show that the permeability decreases according to negative exponential function with the increase of stress. When stress is less than 5 MPa, the coal reservoir permeability decreases rapidly with the increase of stress, the stress sensitivity of the coal samples is the highest; when stress is between 5 MPa and 10 MPa, the stress sensitivity is medium; when stress bigger than 10 MPa, the decline of the permeability slows down, the stress sensitivity is weak. Compared with the experimental results of high-rank coal in Southern Qinshui Basin, the stress sensitivity of the NO.2 coal seam of Shanxi formation in the research area is smaller than the NO.3 coal seam's in Southern Qinshui Basin. The compressive deformation of fracture induced by stress in coal reservoir leads to the sharp decrease of fracture aperture then resulted in stress sensitivity of coal reservoir. Controlling the production rate especially the drop liquid rate in early stage has practical significance to prevent the stress sensitivity in coal reservoir and improve recovery.