Permeability, mineral and pore characteristics of coals response to acid treatment by NMR and QEMSCAN: Insights into acid sensitivity mechanism

The sensitivity of coal reservoir is an essential reason decreasing the coalbed methane (CBM) production during the CBM development. Whether the acid sensitivity is a negative or positive effect on the coal reservoir needs further discussion. In this work, the core flooding experiments with different acids treatment were conducted to investigate the acid sensitivity mechanism, combining with X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICP-MS), nuclear magnetic resonance (NMR) and quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN) methods. The results of core flooding show that the permeability damage is about 13%, 66% and 58% after 3% hydrochloric acid (HCL) treatment, 3% fluoric acid (HF) treatment and 3%HCL+3%HF treatment, respectively. The mineral distributions before and after 3%HCL+3%HF treatment obtained by QEMSCAN reveal the dissolution of carbonates, the generation of new precipitations and the mobilization of coal particles. The effect of acid on pore structure obtained by NMR indicates that the pore size between 7.5 nm and 110 nm decreases after 3%HCL treatment, while the pore size over 18 nm decreases after 3% HF treatment. The porosity analyzed by QEMSCAN increases at the inlet side and decreases at the outlet side, suggesting the pore structure at the outlet of core is more vulnerable. Moreover, the potential chemical reactions between minerals and acids are deduced by analyzing the results of XRD and the variation of ions' concentration. The siderite, ankerite, dolomite, chlorite in coals are dissolved by HCL or HF and some of the quartz and kaolinite are dissolved by HF. The fluorite and sellaite were generated after HF treatment. In short, evaluating the effect of acid on the coal reservoir needs to consider the dissolution of minerals, generation of new precipitates and migration of particles.

Automated summary

The sensitivity of the coal reservoir to acids can impact the production of coalbed methane. Core flooding experiments with acid treatments were conducted to investigate the mechanism, revealing permeability damage and changes in mineral distribution and pore structure. The dissolution of minerals, generation of new precipitates, and migration of particles were observed during the experiments.