Experimental investigation on the effects of different fracturing fluids on shale surface morphology

With the widely use of hydraulic fracturing technique, large volumes of fracturing fluids are injected into shale reservoirs. The physical and chemical reactions between fracturing fluids and shale will change the surface roughness of shale fractures, which further impacts the production efficiency of hydrocarbons. To quantify the impacts of fracturing fluids on shale fracture roughness, a series of imbibition tests were conducted on shale samples soaked in different fracturing fluids (acid (pH = 6), neutral (pH = 7) and alkaline (pH = 8)) with different time (short-term (0–3 months) and long-term (3–12 months)). The surface morphology, microstructure, mineral composition, and element changes of shale samples before and after imbibition were determined. The results showed that: (1) pyrite and kaolinite were the main minerals that dissolved in the short-term of imbibition. The dissolution of pyrite caused the rapid decreasing of pH values for all fracturing fluids (decreased by 43%–51%). In the long-term of imbibition, clay minerals (illite-montmorillonite) and feldspar were the major dissolved minerals, resulting in the rise of pH values (increased by 10%–19%). (2) the dissolution, hydrolysis, and hydration process during imbibition resulted in the increase of fractal dimension (increased by 9%–30%). Alkaline fracturing fluid had higher effect on the roughness of shale than that of acid and neutral fracturing fluids. (3) the joint homogeneity on the shale fracture surface which obtained from the joint rose diagram revealed that acidic and neutral fracturing fluids enhanced the joint homogeneity (increased by 64% and 74.6%), whereas alkaline fracturing fluid weakened the joint homogeneity, which decreased by 43.8%. In this study, the impact of fluid-rock interaction on shale fracture surface roughness is explored for the first time, a variety of methods are used to evaluate, and the differences of different methods are compared. The results show that different pH fracturing fluids have significant changes in the surface roughness of shale fractures. For shale formation with high pyrite content, alkaline fracturing fluid has stronger reconstruction ability. Above conclusions may have significant impact on the efficiency of hydraulic fracturing.