页岩气储层蠕变特性及其对页岩气开发的影响

页岩气储层渗透率极低,必须经过压裂改造才能形成有效产能,大量狭小自支撑裂缝在天然气解吸及流动中具有重要作用。页岩气储层生产周期长,人工裂缝导流能力对裂缝变形极其敏感。研究结果表明,页岩气储层具有一定的蠕变特性,并随着粘土含量的增加而增强。压裂改造后,储层会产生大量裂缝,裂缝闭合蠕变是蠕变形变的主要形式。裂缝的存在会使储层蠕变速率大幅度提高,并对裂缝导流能力产生不可忽视的影响。裂缝闭合蠕变速率与裂缝界面之间、裂缝界面与支撑剂间的相互作用有关,并与基质蠕变速率成正比。压裂改造形成的裂缝网络越发育、单裂缝宽度越小,蠕变对裂缝导流能力的影响越大。在页岩气数值模拟中,考虑裂缝闭合蠕变的累积产量与未考虑裂缝闭合蠕变的累积产量相差较大。同时,在地应力计算、储层可改造性、支撑剂和施工参数优选以及生产方式的选择等方面也应考虑蠕变的影响,保持流体压力有利于减小蠕变形变,维持裂缝导流能力,提高气井产能和采收率。

Gas shale creep and its influence on the shale gas development

Since the permeability of gas shale is extremely low, hydraulic fracturing is the main measure to obtain an economic shale gas productivity. Shale reservoirs have long production cycles and artificial fracture conductivity is extremely sensitive to the fracture deformation. A large number of narrow self-supporting fractures play an important role in the gas desorption and flow. Shale reservoir has a certain creep characteristic, its creep rate increases with the clay content. After fracturing, the reservoir generates a large number of cracks and crack-closure creep is the main part. Fracturing treatment greatly increases the reservoir creep rate and the impacts of creep on the fracture conductivity cannot be ignored. The fracture closure creep rate is related to the interactions between fracture interfaces and proppant. It is also proportional to the matrix creep rate. Complex fracture network and tiny fractures have larger creep rate and greater effects on flow conductivity. Therefore, the creep deformation of fractures should be taken into account in shale gas numerical simulation. And, in-situ stress calculation and proppant selection should also consider the effect of creep. Maintaining fluid pressure is helpful to enhance fracture conductivity and improve gas well productivity and recovery.