页岩储层吸水特征及其对裂缝导流能力的影响

页岩储层体积压裂后普遍存在压裂液返排率低的问题,大量压裂液滞留在储层内不仅会对页岩气的流动及解吸产生影响,还会影响到压裂后的裂缝导流能力。文中以四川盆地某区块龙马溪组页岩储层为研究对象,在分析了页岩储层特征的基础上,开展了页岩储层吸水能力实验,并评价了页岩吸水对裂缝导流能力的影响。结果表明:目标区块页岩储层具有超低孔、低渗、低初始含水饱和度的特点,且黏土矿物质量分数较高,具备较强的吸水性。页岩岩心在蒸馏水中的吸水能力最强,在地层水中次之,在滑溜水压裂液中最弱。由于页岩的吸水作用明显降低了裂缝表面的强度,导致支撑剂滑动或者嵌入,从而明显降低了支撑裂缝的导流能力。页岩导流板通入蒸馏水时裂缝导流能力下降幅度最大(83.02%),而通入滑溜水压裂液时裂缝导流能力下降幅度最小(39.13%)。因此,可以通过调整压裂液体系组成来抑制页岩储层的吸水作用,从而提高压裂液的返排率,降低页岩储层吸水对裂缝导流能力的影响程度。

Water absorption characteristics of shale reservoir and its effect on fracture conductivity

The low rate of fracturing fluid backflow is common in shale reservoirs after volume fracturing.A large amount of fracturing fluid remaining in the reservoir will not only affect the flow and desorption of shale gas,but also affect the fracture conductivity after fracturing.Therefore,taking the Longmaxi Formation shale reservoir in a block of Sichuan Basin as the research object,based on the analysis of the characteristics of shale reservoir,the water absorption capacity experiment of shale reservoir was carried out,and the influence of shale water absorption on fracture conductivity was evaluated.The results show that the shale reservoir in the target block has the characteristics of ultra-low porosity,low permeability and low initial water saturation,and the clay mineral content is high,so it has strong water absorption.Shale core has the strongest water absorption ability in distilled water,followed by formation water and the weakest in slip water fracturing fluid.Because of the water absorption of shale,the strength of fracture surface is obviously reduced,which leads to the sliding or embedding of proppant,thus the conductivity of propping fracture is obviously reduced.When the shale diversion plate enters distilled water,the fracture conductivity decreases the most(83.02%)and when the slip water fracturing fluid enters,the fracture conductivity decreases the least(39.13%).Therefore,the water absorption of shale reservoir can be restrained by adjusting the composition of fracturing fluid system,so as to improve the return rate of fracturing fluid and reduce the influence of water absorption of shale reservoir on fracture conductivity.