酸化作用下龙马溪组页岩孔隙结构演化实验研究

富有机质页岩致密、低渗，孔径多为纳米级，增产措施是页岩气商业化生产所必须的手段。四川龙马溪组页岩富含碳酸盐矿物及黏土矿物，酸化处理可以溶蚀页岩的矿物成分，改善页岩致密的孔隙结构，最终达到提高产量的目的。以四川龙马溪组页岩为研究对象，对所选页岩进行了两种酸化处理:在30 ℃的恒温条件下，浸泡质量分数10%HCl和3%HF的混合酸及先浸泡质量分数10%HCl再浸泡质量分数3%HF的顺序酸，酸浸泡时长最长为24 h。利用超景深光学显微镜研究了页岩酸化前后细观结构的演化特征，利用扫描电子显微镜研究了酸化对页岩微观结构的影响;基于EDS能谱仪的元素分布结果分析了页岩酸化前后矿物组分的变化特征;通过压汞测试研究了酸化前后页岩孔径的分布特征及孔隙直径、体积和比表面积的变化特点。结果表明:① 酸液可以溶解页岩中的矿物，在矿物被溶解的位置产生孔隙，但混合酸浸泡会产生大量白色沉淀堵塞酸蚀产生的孔隙，沉淀以Ca2+,Al3+的化合物为主，顺序酸化几乎无沉淀生成;② 酸化改变了页岩原有的致密结构，酸化后0.050~6 μm及25~575 μm的孔隙显著发育，酸化使得页岩的累积孔隙体积提高2个数量级;③ 未经处理页岩试样的孔隙体积为0.006 9 mL/g，经混合酸和顺序酸处理24 h后分别增至0.14,0.17 mL/g，顺序酸的酸蚀作用可以在页岩内产生更多的孔隙。

Experimental study on pore evolution of Longmaxi shale under acidification

The organic-rich shale has is the characteristics of tight and low-permeability.Well stimulation is nec-essary for commercial production of shale gas.The Longmaxi Formation shale in Sichuan Basin is rich in carbonate minerals and clay minerals.Some mineral composition of shale can be dissolved by acidification,finally,the pore structure of shale is improved and gas production is increased.Two acidification schemes were used in this experiment for Longmaxi shale:10% HCl and 3% HF mixed acid soaking and sequential acid soaking (10% HCl soaking first and then 3% HF soaking)at 30 ℃.The acid soaking time was up to 24 h.The ultra-depth optical microscope was used to study the evolution characteristics of mesostructure before and after acidification treatment.The effect of acidification on the microstructure of shale was studied by scanning electron microscopy.Based on the element distribution results of EDS spectrometer,the variation characteristics of mineral components before and after acidification were analyzed.The distribution characteristics of shale pore size and the variation in pore diameter,volume and specific surface area of shale before and after acidification were studied by mercury intrusion.The results show that the minerals in shale can be dissolved by acid and pores can be created where minerals are dissolved.A large amount of white precipitates appears and clog the pores after acidification of mixed acid.The precipitates are mainly Ca2+ and Al3+ compounds.But the amount of white precipitate formed during the sequential acidification is small.The natural tight structure of shale is altered by acidification.After acidification,the pores of 0.050-6 μm and 25-575 μm are developed significantly.The cumulative pore volume of shale increases by two orders of magnitude.The pore volume of the untreated shale sample was 0.006 9 mL/g,which was increased to 0.14 and 0.17 mL/g,respectively,after 24 h of mixed acid and sequential acid treatment.Thus,sequential acidification is more suitable for the improvement of pore structure of Longmaxi shale.