致密储集层压裂液与致密砂岩相互作用研究

致密油水平井多级分段压裂对压裂液的用量很大，低返排率造成压裂施工后仍有大量压裂液滞留在裂缝中。通过岩石表面润湿性评价实验和压裂液静滤失评价实验，对压裂液与致密砂岩基质相互作用进行描述和阐释。润湿性测量结果表明，在滤失实验后，致密砂岩表面润湿性由强亲水性向弱亲水性甚至中性润湿转变，这能够降低压裂液向致密砂岩基质中的滤失（渗吸）速率，减少压裂液对致密储层伤害的同时节约压裂液整体用量。静滤失评价实验结果表明，特低渗透砂岩的压裂液滤失曲线可表现出完整的三段式滤失规律，而致密砂岩仅能呈现稳定滤失阶段；岩心渗透率越低，压裂液滤失量越少，达到稳定滤失状态所需的时间越长，且采出液中聚合物浓度明显较低；不同滤失压差时达到滤失稳定时刻的采出液聚合物浓度均在200 mg/L左右，且滤失速率基本相同，均表明大滤失压差所造成的渗滤能力增强与其所导致的岩心伤害基本相抵消。

Interaction between non-recovered fracturing fluid and tight sandstone in tight formation

Large amount of fracturing fluid is required for multi-stage hydraulic fracturing in tight oil. Low recovery of fracturing fluid leaves much chemical residual in microfractures, which is closely related to relative permeability near fracture, flow back, production rate, etc. In order to investigate the interaction of non-recovered fracturing fluid and tight sandstone, wettability alteration and fracturing fluid leakoff property in tight sandstone were investigated in this paper. Experimental results showed that water-wet tight sandstone became weak hydrophilicity or even intermediately-wet due to fracturing fluid leakoff. This might impair the leakoff or imbibition behavior of fracturing fluid in tight sandstone. Thus, formation damage related to fracturing fluid leakoff could be reduced to some extent and the fracturing fluid volume could be also saved. The three-stage leakoff characteristics were presented for ultra-low permeability sandstone, while tight sandstone only exhibited stable leakoff stage. The leakoff volume was reduced and more time was required to achieve stable leakoff stage as rock permeability decreased. Moreover, polymer concentration was reduced in effluent of fracturing fluid filtration. An interesting finding was that effluent polymer concentration was kept at about 200 mg/L when stable leakoff stage was achieved at different leakoff pressure drops, which indicated displacement force associated with high pressure drop offset the rock damage it caused.