页岩气保存机制探讨

Preservation mechanism of Fuling shale gas

常规天然气与页岩气在储集空间上的本质差别就是孔径或孔隙结构的差异。根据四川盆地涪陵焦石坝地区页岩埋藏史、含气量、生产数据及解吸气同位素分馏这4方面资料,指出页岩气是以单个或多个有机质碎片所形成的连通孔隙网络为单元被保存在页岩中,毛管压力与静水压力是页岩气孔隙压力的保存机制,因此页岩的富气程度不受构造形态或圈闭的控制。构造活动对页岩气保存条件的破坏主要决定于构造升降对静水压力破坏及地质应力对页岩页理面的破坏程度,水平挤压条件下层理面张开所引起的毛管封闭能力下降,导致页岩气散失。而断层往往是应力释放区域,使页理面不会受到进一步的破坏,相反可能对下盘起到保护作用,成为页岩气的有利勘探目标。根据毛管力原理可进一步估算页岩气的可采收率,从而为勘探开发提供指导。

The essential difference between conventional gas and shale gas in reservoir space is the difference in pore size or pore structure.The shale burial history,gas content,production data and isotope fractionation of desorbed gas in the Jiaoshiba area of Fuling,Sichuan Province indicate that shale gas was stored in the inter-connected pore units formed by single or multiple organic matter types,and its preservation mechanism was attributed to capillary pressure and static water pressure.Therefore,tectonic structure or trap shape has little impact on shale gas enrichment.The destruction of shale gas preservation conditions by tectonic activity was mainly determined by the decrease of static water pressure caused by tectonic movements,the destruction of lamination caused by geologic stress,and the decrease of capillary sealing ability caused by the opening of lamellae under horizontal compression conditions.Faults releasing tectonic force would not cause further destruction of lamination.On the contrary,they might be helpful for the preservation of a footwall which could be the optimum shale gas target.The recovery ratio of shale gas could be derived from the theory of capillary pressure,which would supply guidance for production.