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 Long- maxi 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 dis- tribution 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 dis- solved 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.