Chelating agent-introduced unconventional compound acid for enhancing coal permeability

Abstract

The low permeability of coalbed methane (CBM) reservoirs can be attributed to cementing minerals, which make it difficult for water/gas to flow in the pores and fractures. Acid stimulation is considered a promising method to enhance coal permeability by dissolving these inorganic minerals. Laboratory experiments were carried out to investigate the interactions between acids and coal samples from the Qinshui Basin. XRD and SEM-EDS analyses showed that these coal samples contained a large amount of clay minerals, and some carbonates, pyrite, etc. Considering the complexity of the mineral concentration, three kinds of acids (organic acids, chelating agents, and fluoroboric acid) were optimized via static leaching tests, ICP-OES, etc. Based on the results, out of the four chelating agents, HEDTA showed the highest chelating amount for Ca2+ (380.38 mg/g) and Fe3+ (476.4 mg/g), and 1 wt% HEDTA (pH = 2) yielded the highest dissolution rate. For the organic acids, the dissolution rate decreased with an increase in acid concentration because of the common ion effect. The dissolution rate with 2.5 wt% OA-1 was 8.02% while that with 20w% HCl was only 7%. In addition, 639 mg/L Ca was detected in the 5 wt% OA-1 leachate, which was higher than that for the 5 wt% HCl (575 mg/L). Afterward, orthogonal methods were used to optimize the compound acid system which consisted of 1.5 wt% HEDTA +4 wt% OA-1 + 5 wt% HBF₄. According to the core-flooding test results, after acid injection for approximately 100 PV (pore volume), the porosity of coal core sample increased from 3.36% to 4.95%, and the permeability increased 3.29 times. In general, previous studies have been limited to the conventional acids (HCl and mud acid), yet the drawbacks such as clay breakage, migration, and precipitation cannot be neglected. Accordingly, this paper proposed a set of chelating agent-introduced compound acid and it shows great promise in the application of enhanced CBM recovery.