龙马溪组页岩应力敏感性实验评价

X射线衍射和全应力-应变结果表明龙马溪组页岩具有明显的硬脆性特征,破裂前曲线均呈现明显的线弹性变形。在5 MPa和10 MPa围压下,平行层理方向与垂直层理方向岩芯力学性质表现明显不同,平行层理方向的微裂缝由于受到张力作用而导致页岩突然破裂而完全失去承载能力;当围压达到20 MPa时,由于层理微裂缝的闭合,页岩的力学性质表现相似。氮气脉冲压力延迟测试结果表明:平行层理方向取芯页岩的应力敏感性更强,当有效应力增至10 MPa时,垂直层理方向岩芯渗透率下降1~2个数量级,而平行于页岩层理方向岩芯渗透率下降2~3个数量级;方解石充填的天然裂缝可提高岩芯整体渗透率2~3个数量级,且闭合型页岩天然裂缝应力敏感性强于张开型天然裂缝岩芯。

Experimental Study of Stress Sensitivity of Shales in Longmaxi Formation

Both X-ray diffraction and stress-strain result show the shale in Longmaxi has obvious hard brittle characteristics and the stress-strain curve presents obvious linear elastic deformation before rupture. At 5MPa and 10MPa confining pressure, there is significant difference in mechanical property between core of parallel bedding direction and vertical direction, and micro-crack of parallel bedding direction causes sudden rupture of shale due to tensile force so as to completely lose the load capacity; when at 20MPa confining pressure, mechanical properties of shale is similar because of closed bedding micro-crack. Nitrogen pulse-decay permeability measurement shows that stress sensitivity of shale in parallel bedding direction is stronger; when the effective stress is up to 10MPa, permeability rate of core in parallel bedding direction will decrease by one~2 orders of magnitude, while in vertical direction will decrease by 2~3 orders of magnitude; natural fracture filled with calcite can increase the overall permeability of core by 2~3 orders of magnitude, and stress sensitivity of shale with closed natural fracture is stronger than that of core with open natural fracture.