An experimental investigation into the effect of pore size distribution on the acid-rock reaction in carbonate acidizing

Abstract

This study presents the correlation analysis between pore size distribution and diffusion coefficient based on the experimental results to evaluate the effect of pore size distribution on the acid-rock reaction in carbonate acidizing. The mercury injection capillary pressure (MICP) was used to analyze the pore size distribution of four limestone samples. Also, the acid-rock reaction experiments were conducted to measure diffusion coefficients using rotating disk apparatus (RDA). In the experimental results, micropore dominated rock samples have lower dissolution rate and diffusion coefficient than macropore dominated rock samples. It is obviously revealed that the dissolution rate and diffusion coefficient for limestone samples with similar mineral compositions are quite different due to various pore size distribution. In the result of correlation analysis, it is revealed that neither porosity nor permeability exhibits a strong correlation with diffusion coefficient compared with pore size distribution. Therefore, it can imply that the petro-physical properties such as porosity and permeability have a lower relevance to the acid-rock reaction kinetics than pore size distribution. To investigate the effect of pore size distribution on matrix acidizing, optimum injection rates were derived by the experimental results. As a result, the optimum injection rates increase with the mean and median pore size, and the micropore dominated rock samples show lower optimum injection rate than the macropore dominated rock samples. Although the pore size distribution shows a strong correlations with matrix acidizing, the optimum injection rate has been estimated without ample considerations of pore size distribution. Therefore, the effect of pore size distribution on diffusion coefficient should be considered to design the optimum injection conditions in carbonate acidizing job.