页岩水化及水锁解除机制

Mechanisms of shale hydration and water block removal

选取四川盆地长宁地区龙马溪组页岩样品,开展电镜扫描、CT扫描、高压压汞、低温N_(2)吸附、水化自吸等实验,对比蒙脱石、伊利石水化特征,分析页岩水锁解除能力的主控因素,揭示页岩水化过程中孔隙结构的演变机制。研究表明:页岩水化特征与黏土组成密切相关,伊利石含量高的页岩不易水化膨胀,改善储集层孔隙结构的能力有限;蒙脱石含量高则易水化膨胀,改善储集层孔隙结构的潜力较大;页岩伊利石含量高,初期自吸作用相对较强,但扩散能力不足,易形成水锁;页岩蒙脱石含量高,初期自吸作用相对较弱,扩散能力较好,具有一定的水锁解除能力;页岩存在最佳水化时间,此时储集层物性最好,水化时间过长,则对储集层物性形成伤害,伊利石含量高的页岩最佳水化时间较短;无机阳离子对黏土矿物吸水膨胀具有抑制作用,对伊利石的抑制作用更强,K^(+)抑制效果较好,蒙脱石含量高的储集层可以通过降低压裂液阳离子含量促进黏土水化作用;伊利石含量高的储集层可注入高K^(+)含量的压裂液抑制水化作用。

Shale samples of Longmaxi Formation in the Changning area of the Sichuan Basin were selected to carry out scanning electron microscopy,CT imaging,high-pressure mercury injection,low-temperature nitrogen adsorption and imbibition experiments to compare the hydration characteristics of montmorillonite and illite,analyze the main factors affecting the water block removal of shale,and reveal the mechanisms of pore structure evolution during shale hydration.The hydration characteristics of shale are closely related to the composition of clay minerals,the shale with high illite content is not susceptible to hydration and thus has limited room for pore structure improvement;the shale with high montmorillonite is susceptible to hydration expansion and thus has higher potential of pore structure improvement by stimulation;the shale with high illite content has stronger imbibition in the initial stage,but insufficient diffusion ability,and thus is likely to have water block;the shale with high montmorillonite content has weaker imbibition in the initial stage but better water diffusion,so water blocking in this kind of shale can be removed to some degree;the shale reservoir has an optimal hydration time,when it is best in physical properties,but hydration time too long would cause damage to the reservoir,and the shale with high illite content has a shorter optimal hydration time;inorganic cations can inhibit the hydration of clay minerals and have stronger inhibition to illite expansion,especially K^(+);for the reservoir with high content of montmorillonite,the cation content of fracturing fluid can be lowered to promote the shale hydration;fracturing fluid with high K^(+)content can be injected into reservoirs with high illite content to suppress hydration.