深层页岩储层现今地应力场特征及其对页岩储层改造的影响——以川南永川页岩气区块五峰—龙马溪组为例

The present-day in-situ stress filed characteristic of deep shale reservoirs and its effect on the reconstruction of shale reservoirs:A case study of the Yongchuan shale gas field in South Sichuan Basin

深层页岩储层现今地应力特征及其控制下的裂缝有效性和对水力压裂影响尚不明确.以川东南永川页岩气区块为例,综合水压致裂法、岩石声发射试验、测井资料以及数值模拟明确深层页岩储层应力特征及其对页岩储层改造的影响.研究表明:川东南深层页岩储层现今地应力场主要为拉张走滑和走滑态,部分地区属于挤压走滑态,两向应力差主要介于10~30 MPa[0.05~0.3 Sv(Sv:垂向主应力)],最大主应力方向总体以近EW向(92°~105°)为主,应力分布受断层和褶皱扰动明显;侧向应力系数(K)受埋深影响小,主要介于0.9~1.1,平均K值接近1.0.深层页岩的裂缝面摩擦系数较低(普遍小于0.2),倾角为中高角度(>45°±10°)以及与最大主应力走向介于15°~65°的天然裂缝在改造后具有较好的开启性;现今地应力对渗透率起到了一定的控制作用;随着水平应力差增大,裂缝网络复杂程度逐步降低,压裂效果较差;偏方位井导致的剪应力会诱导水力裂缝产生较大的扭转角(>40°),深层页岩因岩相差异会在纵向上形成应力屏障;拉张走滑态地区的改造效果相对更好;结果可为深层页岩储层地应力研究和深层页岩气的高效勘探开发提供科学依据.

The present-day in-situ stress characteristic of deep shale reservoirs and the impact of fracture efficacy and fracturing effect under their control is still unclear. Taking the Yongchuan shale gas play in southeastern Sichuan baisn as an example, the hydraulic fracturing method, rock acoustic emission experiment, logging data, and numerical simulation were used to clarify the stress characteristic of deep shale reservoirs and its effect on the reconstruction of shale reservoirs. Our findings demonstrate that the deep shale reservoirs in the southeast Sichuan basin are currently experiencing strike-slip and strike-slip with the tension, and some areas are experiencing strike-slip with the compression. Its difference of in-situ horizontal stresses mostly range from 10 to 30 MPa[0.05~0.3 S_v(S_v: Vertical principal stress)], a prevailing near EW(92°~105°) trending S_(Hmax) orientation, and obviously affected by faults and folds. The buried depth has little impact on the lateral stress coefficient(K), which typically ranges from 0.9 to 1.1 and is close to 1.0. Deep shale reservoirs have a low fracture friction coefficient(typically less than 0.2), and natural fractures with a medium-high dip angle(>45°±10°) and an angle between 15° and 65° with the maximum horizontal stress have favorable mechanical effectiveness following transformation. The present-day in-situ stress has a certain control effect on the permeability. As the difference of in-situ horizontal stresses increases, the complexity of the fracture network gradually decreases, and the fracturing impact deteriorates. The shear stress produced by the off-azimuth well induce the hydraulic fracture to produce a larger kinking angle(>40°). Deep shale forms a stress barrier in the longitudinal direction due to lithofacies differences. The region of strike-slip with tension has a better overall reconstruction impact. Our results can provide a reference for the study of in-situ stress in deep shale reservoirs and the efficient exploration and development of deep shale gas.