合川地区须二段低各向异性储层现今地应力方向评价方法

Methods to Evaluate Present-Day In-situ Stress Direction for Low Anisotropic Reservoirs in the Second Member of the Xujiahe Formation in Hechuan Area

合川地区须二段致密砂岩气资源十分丰富,但该区复杂地质条件对目前油气资源勘探开发造成了巨大影响,尤其是现今地应力方向认识不清制约了后期水平井部署和压裂改造的持续推进。基于波速各向异性、差应变及古地磁等实验测试分析,并结合特殊测井与微地震监测资料,开展了各类地应力方向测试与解释方法在致密砂岩储层中应用的适用性评价研究,查明了合川地区现今地应力方向的分布特征。结果表明合川地区须二段最大水平主应力方向主要分布在N103.1°EN134.3°E,平均最大水平主应力的方位角为N117.4°E,即合川地区须家河组砂岩现今最大水平主应力方向为NWW-SEE。平面上,地应力方向变化不明显,其较小的地应力偏转主要受沉积结构影响,而合川地区须二段致密砂岩均质性相对较强,各向异性偏弱,波速各向异性测试方法在该区无法适用。通过微地震监测和井壁成像测井评价地应力方向,认为差应变联合古地磁的实验测试结果与上述评价结果具有较高的一致性,故该测试方法更加适用于该区均质且各向异性较弱的砂岩地层。并结合地应力方向和裂缝优势走向等,建议水平井的部署方位为N40°E-N55°E。

The Hechuan area is rich in dense sandstone gas resources in the second member of the Xujiahe formation,and the complex geological conditions in this area have a great impact on the current exploration and development of oil and gas resources.One of the issues resulting from this complexity is an unclear understanding of how present-day in-situ stress direction restricts the later horizontal well deployment and the promotion of fracture transformation.Based on an analysis of wave velocity anisotropy,differential strain,and paleomagnetism,combined with special logging and microseismic monitoring data,the applicability of various types of in situ stress direction testing frameworks in dense sandstone reservoirs was evaluated,and the distribution of present-day in situ stress directions in the Hechuan area were determined.The results show that the maximum principal stress orientation of the second member of the Xujiahe Formation in the Hechuan area is mainly distributed between N103.1°E-N134.3°E,and the azimuth angle of the average maximum principal stress is N117.4°E;that is,the present-day maximum principal stress orientation of the sandstone in this area is NWWSEE.The change in the direction of the in situ stress in the plane is not significant,and its weaker in situ stress deflection is mainly influenced by the sedimentary structure.In the Hechuan area,the dense sandstone in the second member of the Xujiahe Formation is relatively homogeneous,and the anisotropy is weak;therefore,the wave velocity anisotropy test cannot be applied in this area.The experimental test results of differential strain combined with paleomagnetism are highly congruent with the results above.Therefore,the test more applicable to sandstone formations in this area,specifically the ones with a homogeneous and weak anisotropy,is microseismic monitoring and well wall image logging to evaluate the in-situ stress direction.Combined with the direction of the in situ stress and fracture development,the recommended orientation for horizontal well deployment is N40°E-N55°E.