碳酸盐岩超临界二氧化碳前置酸蚀裂缝导流能力实验

Experimental Study on Etched Fracture Conductivity in Carbonate Rocks for Supercritical CO_(2)Prepad Acid Fracturing

延安气田马家沟组碳酸盐岩储层前期采用常规酸压改造,酸蚀范围有限,改造后单井产量低,需开展酸压工艺优化。选取马家沟组五段储层露头,采用酸蚀裂缝导流仪及三维激光扫描装置开展了超临界CO_(2)(SC-CO_(2))前置酸蚀、常规酸蚀及导流能力实验,研究SC-CO_(2)前置和不同SC-CO_(2)闷井时间对酸蚀形态及裂缝导流能力的影响。实验结果表明,相比于常规酸蚀,SC-CO_(2)前置酸蚀开启了天然裂缝,增强了酸液进入微裂缝和扩散的能力,提高了酸蚀效果。SC-CO_(2)闷井12 h、24 h后的酸蚀形态以沟槽状为主,总体刻蚀程度和类型未发生较大改变;闷井48 h后酸蚀形态以桥墩状刻蚀为主;闷井72 h后酸蚀形态以均匀刻蚀为主。常规酸蚀和SC-CO_(2)前置酸蚀裂缝导流能力在闭合压力小于30 MPa时相差不大,超过30 MPa后导流能力出现明显的差距。SC-CO_(2)前置酸蚀的裂缝导流能力在整个闭合压力范围内都保持较高,始终高于常规酸蚀的裂缝导流能力。在低闭合压力下闷井时间与酸蚀裂缝导流能力呈正比,高闭合压力下过长的闷井时间会抑制裂缝导流能力,推荐闷井时间48 h。研究结果可为碳酸盐岩储层复合酸化压裂方案的制定提供理论支撑。

Previously,the carbonate reservoir of Majiagou Formation in Yan'an Gas Field adopted traditional acid fracturing treatments,which ended up with a limited acid-affected range and low post-frac well production.This necessitates the optimization of the acid fracturing practice.The reservoir outcrop samples of the Fifth Member of Majiagou Formation were collected for experiments,and the etched-fracture conductivity meter and 3D laser scanning technology were used for acid etching experiments and fracture conductivity measurement in the cases of supercritical CO_(2)(SC-CO_(2))prepad acid fracturing and conventional acid fracturing.The effects of prepad SC-CO_(2)and SC-CO_(2)soaking time on etched fracture morphology and conductivity were investigated.The results demonstrate that acid fracturing with prepad SC-CO_(2),compared with the conventional method,opens natural fractures,enhances the penetration and diffusion of acid solutions into micro-fractures,and thereby improves acidizing effectiveness.After 12 h and 24 h of SC-CO_(2)Soaking,the dominant acid etching pattern resembles grooves,with no significant changes in overall etching severity or types.The 48 h soaking ething is associated with the predominant bridge-pier-like etching marks,while the uniform etching is found dominant after 72 h soaking.The difference of fracture conductivity between conventional acid fracturing and SC-CO_(2)prepad acid fracturing is minor under the closure pressure below 30 MPa,and yet,a substantial disparity emerges beyond this threshold.The fracture conductivity of SC-CO_(2)prepad acid fracturing consistently exceeds that of conventional acid fracturing across the closure pressure range of the experiments.In cases of low closure pressure,the soaking duration is positively correlated with the etched fracture conductivity,whereas excessively prolonged soaking periods under high closure pressure can suppress fracture conductivity.Therefore,48 h well shut-in is recommended.The findings of this re-search provide theoretical support for the scheme development of compound acid fracturing in carbonate reservoirs.