Selective dissolution of tight sandstone in an acidic environment to enhance gas flow capacity

The exploitation of tight sandstone gas reservoirs is characterized by high fracturing costs and construction difficulties due to their deep burial, complex seepage space, multi-scale pore structure, and strong stress sensitivity. The selective acid dissolution of minerals may be a viable way to enhance the gas transfer capacity in the multi-scale pore structure. To quantitatively evaluate the efficiency of acid dissolution and permeability enhancement, experiments were conducted using hydrochloric acid and acetic acid at different concentrations on tight sandstone particles and cores. The results showed that hydrochloric acid was more effective than acetic acid in dissolving minerals, with the dissolution rate ranging from 8.74% to 43.36%. The dissolution is selective, in which calcite and potassium feldspar are mainly dissolved. The permeability increased by 2.61-4.90 times and 2.21-2.54 times after the dissolution of 8% hydrochloric acid and 10% acetic acid, respectively. The permeability enhancement ability of hydrochloric acid was stronger than that of acetic acid. The permeability of fracture and matrix samples increased on average by 3.17 times and 2.50 times, respectively. The dissolution effect of fracture is stronger than that of matrix. Selective acid dissolution significantly weakened the stress sensitivity of tight sandstone. Acetic acid was found to be more effective in weakening the stress sensitivity than hydrochloric acid. The mechanism of acid dissolution and permeability enhancement in tight sandstone is analyzed in terms of chemistry and mechanics. The results of this study could provide an effective way for the economical and efficient exploitation of gas resources in tight sandstone reservoirs.