CO_(2)-水作用对层状页岩拉伸破坏影响的微观损伤研究

Effect of microscopic damage on tensile failure of laminated shale after CO_(2)-H_(2)O treatment

CO_(2)压裂页岩储层时CO_(2)−水作用损伤页岩微观结构,改变拉伸破坏特征与裂缝扩展模式。为研究CO_(2)−水影响层状页岩拉伸破坏的微观损伤原因,设计了矿物含量测定、扫描电镜观察与巴西劈裂试验,对CO_(2)−水作用前后龙马溪与长7段页岩试件的微观损伤、破坏特征与裂缝扩展进行研究。结果表明:CO_(2)−水对层理的微观损伤作用较基质显著,层理黏土矿物脱水而体积减小,有机质被萃取收缩,纹层内产生沿层理分布的大尺寸(长度为10~30μm,开度为1~5μm)微裂缝,基质碳酸盐、长石等矿物被溶蚀,产生随机分布的小尺寸(长度小于1μm,开度小于0.5μm)微裂纹;CO_(2)−水作用后页岩抗拉强度降低且各向异性增强,破坏模式变为拉剪混合破坏,垂直层理加载试件的剪切作用更强;CO_(2)−水作用后层理对裂缝扩展的影响更大,垂直层理加载裂缝延伸被层理限制,裂缝沿层理方向水平扩展,平行加载裂缝扩展轨迹受层理约束,裂缝仅在层理扩展。

CO_(2)-H_(2)O can damage the rock microstructure and change the tensile failure characteristics and fracture propagation mode during CO_(2) fracturing in shale reservoirs.X-ray diffraction(XRD)tests,scanning electron microscope(SEM)observation,and Brazilian tests are conducted to investigate the microscopic damage and failure characteristics,and fracture propagation mode of Longmaxi and Chang-7 shale specimens after CO_(2)-H_(2)O treatment.The results show that the microscopic damage of bedding after CO_(2)-H_(2)O treatment is more significant than that of the matrix.The volume of bedding clay minerals is reduced due to dehydration,the organic matter is extracted and contracted,and the large-size microfractures(10−30μm in length and 1−5μm in width)are generated in the laminae distributed along the bedding.Carbonate and feldspar minerals in the matrix are dissolved and induce randomly distributed small-size microcracks(<1μm in length and<0.5μm in width).After CO_(2)-H_(2)O treatment,the tensile strength of shale decreases,and the anisotropy increases.The failure mode of shale changes from tensile failure to mixed tension-shear failure,and the shear action of specimens loaded vertically to the bedding is stronger.Fracture propagation is restricted by the bedding for specimens loaded vertically to the bedding,leading to fracture propagation along the bedding;for specimens loaded horizontally to the bedding,the bedding exerts stronger constraints on fracture propagation,resulting in fracture propagation merely within the bedding.