氧化溶蚀作用对页岩水相自吸的影响

页岩气藏水力压裂需将大量含溶解氧的水基压裂液泵入地层,但返排率却普遍低于30%,分析水相自吸机理对解释压裂液滤失分布行为具有重要作用.富有机质页岩沉积于缺氧还原环境,在富氧条件下易发生氧化反应,进而影响水相自吸行为.为此,选取四川盆地龙马溪组页岩露头样品,开展大尺度岩样先后自吸蒸馏水、氧化液实验,以及基块、裂缝柱塞岩样氧化前后水相自吸对比实验.氧化液加速微裂缝扩展和新裂缝产生,促进更多可溶盐沿着裂缝析出,水相分布范围较蒸馏水更大.氧化前平行、垂直层理基块柱塞岩样自吸量分别为0.4250和0.4461 g,而氧化后增加到0.4900和0.4978 g,对应的裂缝柱塞岩样自吸量分别为0.9912和0.9500 g,氧化后增加到1.0886和1.0669 g.氧化溶蚀作用使基块柱塞岩样自吸量增长11.6%~15.3%,自吸势提高2.32%~8.26%;裂缝柱塞岩样自吸量增长9.8%~12.3%,自吸势提高22.5%~33.3%,且自吸平衡时间缩短13.68%~20.23%.氧化溶蚀作用对页岩组分的物质净移除效应,扩大了水相赋存空间的同时改善了基质孔隙半径分布、增强裂缝面水润湿性、诱发裂缝萌生与扩展对应降低自吸前缘饱和度,协同作用加速了水相扩散分布并增加了自吸距离.

Effect of oxidative dissolution on water spontaneous imbibition in shale gas reservoirs

A large amount of water-based fracturing fluid containing dissolved oxygen needs to be pumped into the forma⁃tion for hydraulic fracturing in shale gas reservoirs,but the flowback rate of fracturing fluid is generally lower than 30%.The analysis on water imbibition mechanism plays an important role in interpreting the distribution behavior of fracturing fluid leak-off.The organic-rich shale deposited in the hypoxic reduction environment is prone to oxidation reaction under the oxygen-rich condition,however,the effect of oxidation dissolution on the imbibition behavior is still unclear.The shale outcrop samples from Longmaxi Formation in Sichuan Basin were selected in this study,then the experiments of spontane⁃ous imbibition of distilled water and oxidative fluid were carried out in large-scale shale samples successively.And the spontaneous imbibition of distilled water into shale plug samples were performed,exploring the change in water imbibition behaviors in the plugs before and after the treatment of oxidative fluid.The results showed that compared with distilled wa⁃ter,oxidative fluid accelerates the propagation of microfractures and generation of new fractures,promoting more soluble salt to separate out along the microfractures,and showing a larger distribution range of water phase.The imbibed water masses of shale plugs with parallel and perpendicular bedding are 0.4250 g and 0.4461 g before oxidation,and they are in⁃creased to 0.4900 g and 0.4978 g respectively after oxidation.Similarly,the imbibed water masses of shale plugs with frac⁃tures are 0.9912 g and 0.9500 g,and they are increased to 1.0886 g and 1.0669 g respectively after oxidation.The oxida⁃tion increases the imbibition capacity of shale plug with matrix by 11.6%-15.3%,and the imbibition potential is increased by 2.32%-8.26%.The imbibed water masses of shale plugs with fracture are increased by 9.8%-12.3%,and the imbibition balance time is decreased by 13.68%-20.23%.The results indicated that the net material removal effect of oxidative disso⁃lution on shale components enlarges the storage space of water phase,improves the porosity and permeability,enhances the water wettability of fracture surface,induces the generation and propagation of microfractures,and decreases the saturation of imbibition front,accelerating water diffusion and increasing the imbibition distance.