Experiment and simulation study on dissolution widening of carbonate rock fracture

Previous researches show that the dissolution of carbonate rocks can be characterized by dissolution rate equation, but the form of characterization is quite different, which is mainly reflected in the undersaturated state far from equilibrium. In order to further determine the dissolution characteristics of carbonate rocks in the undersaturated state and the characterization form of dissolution rate, first of all, the fracture dissolution test of carbonate rocks was carried out, and the influence of CO₂ partial pressure and initial concentrations of Ca²⁺ on dissolution was discussed. Based on the test results, the dissolution rate equation in the undersaturated state was constructed. Then, the carbonate rock fracture seepage dissolution coupling model was used, calibrating and verifying the parameters of the quantitative model through numerical simulation. The results indicate that: (1) The participation of CO₂ accelerates the dissolution expansion of carbonate rocks, and the higher the initial Ca²⁺ concentration of the solution, the higher the degree of dissolution inhibition of carbonate rocks. (2) The average dissolution rate of carbonate rock fractures under the action of CO₂ increased by 1.82−2.29 times. (3) Under the same interval flow rate conditions for different initial Ca²⁺ solutions, the difference of Ca²⁺ concentration in distilled water decreased by 0.0915 mmol/L, and the initial Ca²⁺ concentration of 0.352, 0.476, 0.581 mmol/L decreased by 0.0742, 0.0536, 0.0474 mmol/L, respectively. (4) The dissolution kinetics is controlled by the linear rate law under the highly undersaturated state. With the increase of the concentration of Ca²⁺ in the solution, the dissolution kinetics becomes nonlinear, and the threshold of Ca²⁺ concentration between the two is 0.4 times the saturated of Ca²⁺ concentration. The study provides a reference for quantitative evaluation of karst development and evolution.