A study on the interaction of mud acid with rock for chemical stimulation in an enhanced geothermal system

In order to explore the thermal energy from the low-permeability hot dry rocks, both thermal and shear stimulations were employed to create connective fracture networks that allow fluid flow in the reservoir, which is the concept of enhanced geothermal system (EGS). However, strong hydraulic stimulation may lead to preferential short-circuiting flow paths where the fluid transports too fast to take in sufficient heat from the reservoir. To solve this problem, the chemical stimulation was proposed recently, which injects aqueous chemical agents such as acids into reservoir to enlarge the fractures by dissolving the minerals in the rocks. This study examines in the laboratory the effects of using mud acid to enhance the permeability of the rock cores that were recovered from Yingcheng Formation in Songliao Basin, China. The results suggest that the permeability in the core increases with the injection rate of the mud acid. The core scanning electron microscope analysis shows that the increase in permeability is mainly induced by the dissolution of K-feldspar and albite. Secondary amorphous silica was formed due to the acid–rock interaction. The chemical stimulation in the laboratory can be reproduced by reactive transport simulation. The experimental and numerical studies confirmed that the injection of mud acid allows the permeability enhancement during field chemical stimulation for an EGS reservoir.