页岩气藏水相圈闭损害实验研究及控制对策——以四川盆地东部龙马溪组露头页岩为例

水平井分段水力压裂是开发页岩气藏的主要技术,但多数页岩气井压裂后压裂液返排率仅为10%~50%,潜在水相圈闭损害严重,需加大对页岩气藏水相圈闭损害的认识。以四川盆地东部龙马溪组露头页岩为研究对象,模拟压裂作业过程,利用裂缝和基质岩样开展了压裂液滤失与自吸实验,观察了页岩水相返排现象,评价了水相圈闭损害程度。实验结果表明:压裂液滤失与自吸作用将使基质含水饱和度显著增加,且在气藏压力下返排困难,从而引起基质渗透率、扩散系数以及气体压力传递能力大幅下降。分析认为,纳米孔隙发育、亲水性粘土矿物含量高以及超低含水饱和度现象普遍存在是页岩气藏水相圈闭损害严重的主因;加强返排机理研究,选择合适的表面活性剂,并采用非水基压裂液和高温热处理技术是解除或缓解水相圈闭损害的根本途径,也是页岩气井增产改造的重要发展方向。

Laboratory investigation of water phase trapping damage in shale gas reservoir-a case of Longmaxi shale in the eastern Sichuan Basin

Although multi-stage fracturing of horizontal wells has been used as the main method to develop shale gas, a long time for fluid recovery and the small amount of fluid recovered with only 10% to 50% of total pumped have led to concerns that phase trapping of fracture fluids may do serious damage to gas reservoir. The experiments of fracturing fluid leakoff and spontaneous imbibitions have been investigated with Longmaxi shale in the eastern Sichuan Basin, and the process of water flowback was surveyed, and then evaluated the impact of rising water saturation on gas-transport and propagation ability of pressure. Experiments results reveal that water saturation in shale is more than 40% after fracturing, but water amount and rising rate are much higher in fractured shale. Much of the loadwater that is not recovered from pores is effectively blocking gas flow path, and decreased permeability, diffusivity and propagation speed of pressure, and then form water phase trapping. It is concluded that water phase trapping damage is mainly caused by nanopores, abundant hydrophilic clay, and ultra-low water saturation. Furthermore, the load water has led to more gas-transport resistance,and decreased the supply capacity of gas flowing from matrix to fractures. Therefore, it is a promising and significant work to investigate the fluid flow-back mechanism, optimization of surfactant, and the application of non-aqueous fracturing technologies and heat treatment for alleviating or eliminating water phase trapping damage in shale gas reservoirs.