页岩气藏水力压裂渗吸机理数值模拟研究

Imbibition Mechanism of Hydraulic Fracturing in Shale Gas Reservoir

针对页岩储层在水力压裂作业和生产中渗吸机理及作用规律不清的问题,开展了渗吸机理及其引起的地层伤害评估的研究。建立了考虑不同影响因素的页岩水力压裂渗吸数学模型,包括基质和裂缝流动,气体扩散和解吸,应力敏感效应和毛细管压力,然后,讨论了在压裂气藏和后续生产期间如何通过量化裂缝面表皮演变来评估由于渗吸机制导致的储层伤害现象。结果表明,(1)在试井以及生产阶段渗吸对储层特性有较大影响,极大的毛细管压力是导致渗吸现象和水力裂缝附近水封的主要原因;(2)对于实施了水力压裂增产措施的新井通过探测裂缝压力可以获得原始气体压力;(3)润湿相阻塞导致的储层伤害是影响致密气藏水力压裂井生产能力的主要来源之一。研究结果对于页岩气藏的渗流特性能够提供深刻的理解,尤其是为早期生产阶段降低由渗吸作用可能造成的储层伤害来优化生产提供理论依据。

To better understand the imbibition behavior in shale reservoirs during production and hydraulic fracturing operations,we investigated the imbibition mechanism and evaluated the formation damage resulting from imbibition.This paper firstpresents a hydro-mechanical model for a shale gas reservoir with consideration for multiple flow regimes,gas diffusion anddesorption,stress sensitive effect,and capillary pressure.Then the formation damage caused by the imbibition mechanismis evaluated by quantifying facture face skin evolution during fracture cleanup and subsequent production.The simulationresults indicate that(1)the imbibition has a huge influence on reservoir performance in well tests and production periods,anda high capillary pressure is the main cause behind the imbibition phenomenon and water blockage around hydraulic fractures;(2)it is possible to obtain the original gas pressure by detecting the fracture pressure of new wells with hydraulic fracturingstimulation;(3)formation damage caused by wetting phase trapping is one of the main causes impairing well productivityhydraulic fracturing of tight gas reservoirs,which should not be neglected.This research provides a theoretical foundation for abetter understanding of reservoir performance of shale gas,especially for optimizing production by reducing formation damagecaused by imbibition at an early period.