Interactions of CO2–H2O-coal and its impact on micro mechanical strength of coal

Abstract

High temperature flue gas enhanced coalbed methane (CBM) recovery is a potential technique for improving CBM recovery efficiency and carbon sequestration, and it is of significance to carbon emission reduction in coal mine area. Previous research has demonstrated that the injection of CO2 could markedly alter the mechanical properties of coal seam, which further changes the gas flow and long-term safety of CO2 storage. Aiming at this issue, we studied the coupling effect of CO2–H2O on micro mechanical properties of coal with nanoindentation method. And the pore-fracture structure and minerals of the target sample was systematically characterized with LF-NMR and SEM to explore the influence mechanism of CO2–H2O on coal. The results show that (1) the micro strength of coal is obviously weakened after the treatment of CO2–H2O, and with an increase of the time, both the hardness and reduced modulus decreases, while the indentation depth shows an opposite trend. (2) Compared with the control group, the volumes of micropore, minipore, mesopore and macropore/microfracture in coal treated by CO2–H2O all increase in varying degrees. The increase of micropore volume is generally less than 20%. While the increases of the volumes of mesopore and macropore/microfractures are significant, with the maximum increase of 44.13% for the mesopore, and 299.58% for the macropore/microfracture. (3) Ulan coal contains a large number of calcareous minerals, which undergo chemical reaction after encountering the acid solution formed by CO2–H2O, resulting in dissolution of part of the minerals, which can be evidenced by the corrosion pits observed on the coal surface. (4) The injection of CO2 results in nonuniform swelling of the coal, which further leads to the formation of new cracks and the resultant coal strength deterioration. In addition, the acid solution formed by CO2–H2O reacts with the minerals in coal, resulting in dissolution of carbonate and other minerals in the coal, thus weakening the coal strength. The research results in this work provide basic support for the evaluation of the effectiveness of high temperature flue gas enhanced CBM and the safety of long-term CO2 storage.