大宁—吉县地区5号煤层现今地应力预测及其对煤层气开发的影响

Prediction of Present-day in-situ Stress in No.5 Coal Seam of Daning-Jixian Area:Implications for Coalbed Methane Development

现今地应力场对煤储层渗透率具有极其重要的控制作用。中国鄂东大宁—吉县地区煤层气资源丰富,然而该区现今地应力研究程度较低,限制着该区煤层气的高效开发。为查明大宁—吉县地区主采煤层现今地应力场特征,利用三维有限元数值模拟方法对下二叠统山西组5号煤层现今地应力分布进行预测。结果表明,5号煤层现今地应力表现为正断型应力机制。鄂东大宁—吉县5号煤层最大水平主应力在15.4~21.6 MPa,最小水平主应力在9.8~14.4 MPa,构造变形的复杂性影响地应力分布。现今地应力控制压裂裂缝扩展,研究区现今地应力状态下的压裂缝沿NNW-SSE向垂直扩展。研究区5号煤层应力差大部分小于6 MPa,具备形成复杂裂缝网络的地应力基础。煤层渗透率随有效地应力的增大呈指数减小,有效地应力越大,煤储层的渗透性越差。研究区东南部远离断层,地应力值相对较低,有利于形成煤层气富集区。研究成果可为大宁—吉县地区煤层气效益开发提供地质基础与科学指导。

The present-day in-situ stress field has an extremely important controlling effect on the permeability of coal reservoirs.The Daning-Jixian area of eastern Ordos Basin is rich in coalbed methane(CBM)resources,however,the present-day in-situ stress in this area has been studied to a low degree,which is not conducive to the efficient development of CBM in this area.To investigate the current characteristics of the geostress field in the main coal seam of Daning-Jixian area,a three-dimensional finite element numerical simulation method was used to predict the present-day distribution of No.5 coal seam in the Shanxi Formation of the Lower Permian.The results show that the present-day in-situ stress in No.5 coal seam was characterized by a positive break-type stress mechanism.The maximum horizontal principal stresses in No.5 coal seam in the eastern Ordos Basin range from 15.4 MPa to 21.6 MPa,and the minimum horizontal principal stresses range from 9.8 MPa to 14.4 MPa,and the complexity of tectonic deformation affects the distribution of in-situ stresses.The present-day stress controls the expansion of fracturing cracks,and the fracturing cracks in the study area under the present-day stress state expand vertically along the NNW-SSE direction.Most of the stress differences in the No.5 coal seams in the study area are less than 6 MPa,which provides a foundation for the formation of complex fracture networks.The permeability of the coal seams decreases exponentially with the increase of effective ground stress,and the higher the effective ground stress,the worse the permeability of the coal reservoirs.The southeastern part of the study area is far away from the faults and is a stress reduction area,where coalbed methane is easy to gather,forming a rich area for coalbed methane storage.The research results can provide geological basis and scientific guidance for the beneficial development of coalbed methane in the study area.