页岩气钻井过程中的储层保护

Reservoir protection in the process of shale gas drilling

页岩气储层通常渗透率极低,钻井过程中的固液相浸入,使其易受损害。为此,就川渝地区上三叠统须家河组页岩露头,开展了页岩储层的孔隙结构、矿物组成、流体赋存方式、压力特征分析,指出了可能引起储层潜在损害的因素,即黏土水化膨胀、应力敏感及水锁效应。进而采用仿真模拟页岩裂缝储层在压差作用下的液相浸入过程,在正压差下,水相以渗流和渗透方式通过裂缝表面向孔隙基块内浸入,随时间延长,裂缝表面渗透、吸水带范围扩大,直至含水平衡饱和,导致页岩气被水相伤害带封闭在孔隙基块内,造成水锁损害。最后建议页岩气藏储层保护应采用全过程欠平衡钻井技术或气体水平井钻井技术。

Shale gas reservoirs have such an ultra-low permeability that they are susceptible to damage by solid or liquid invasion in the process of drilling.In view of this,a case study was conducted on the shale outcrops in the Upper Triassic Xujiahe Formation in the Sichuan and Chongqing areas.First,through the analysis of pore structures,mineral components,occurrence modes of formation fluids and stress features,the possible formation damage factors were pointed out for shale gas reservoirs,i.e.hydration swelling of clays,stress sensitivity and water-locking effect.Then,the invasion process was simulated of liquids into the fractured shale formation under differential pressure.It was found that under positive differential pressure,liquids like water would seep in or penetrate through the fracture surface into matrix pores;over time,the seepage or penetration range extended until the differential pressure was balanced,which would result in the water-locking damage.Because of this damage,shale gas would be sealed within matrix pores.Finally,it is suggested that the whole-process under-balanced drilling and gas drilling should be the effective solutions to shale gas reservoir protection.