Pore Structure and Mechanical Property Change on the Surface of the Tight Limestone After Acid-Rock Reaction

Due to low permeability, acid can hardly penetrate deep into the tight limestone, causing pore structure and mechanical property change on the surface of the rock. In this study, the porosity, permeability, and acoustic velocity of the tight limestone were first tested, followed by acid coreflood tests. Then, triaxial compression test and surface indentation hardness test were conducted. After mechanical tests, fragments of the broken core samples were collected to carry out SEM investigation, keeping the consistency of core samples. Experimental results indicated that the acid could not penetrate deep into the core with continuously high pumping pressure up to 8 MPa, showing that acid-rock reaction mainly occurred on the surface of the core. The fact that triaxial compressive strength after reaction had barely changed also confirmed that. However, there was an obvious reduction in Young’s modulus and surface indentation hardness, with the reduction extent jointly dominated by acid concentration and rock permeability. SEM images illustrated that there were many pores on the surface of the core after reaction. This study reflects the characteristics of pore structure and mechanical property change on the surface of the tight limestone, rather than the entire core sample, after acid-rock reaction.