Abstract
To solve the serious filtering problems of fracturing fluid during active hydraulic fracturing of coalbed methane, which blocks gas migration channels with residual fracturing fluid and results in a rapid decline in coalbed methane production, several preliminary experiment of thermal nitrogen injection was carried out. To explain the process and mechanism of thermal nitrogen injection at the nanoscale, an adsorption and diffusion model was established to study the adsorption and diffusion of methane, water and fracturing fluid during thermal nitrogen injection. The results showed that the self-diffusion coefficient of methane was increased by 23%, 45% and 32%, the self-diffusion coefficient of water molecule was increased by 108%, 185% and 138%, the self-diffusion coefficient of fracturing fluid molecule was increased by 169%, 275% and 181%, and the total number of atoms in the slit hole was decreased by 15%, 25% and 37%, respectively, when hot nitrogen was injected at 2, 4 and 6 Mpa, respectively. This shows that the numbers of methane, water and fracturing fluid molecules in the slit hole were significantly decreased by the thermal and displacement effects of hot nitrogen injection. The self-diffusion coefficient was significantly improved, and the effects of plug removal was remarkable.
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