Abstract
Coal matrix deformation is one of the main controlling factors for coal reservoir permeability changes in nitrogen foam fracturing. The characteristics and mechanism of coal matrix deformation during the process of adsorption/desorption were studied by isothermal adsorption/desorption experiments with methane and nitrogen. Based on the free-energy theories, the Langmuir equation, and elastic mechanics, mathematical models of coal matrix deformation were developed and the deformation characteristics in adsorption/desorption processes were examined. From the study, we deduced that the coal matrix swelling, caused by methane adsorption, was a Langmuir-type relationship with the gas pressure, and exponentially increased as the adsorption quantity increased. Then, the deformation rate and amplitude of the coal matrix decreased gradually with the increase of the pressure. At the following stage, where nitrogen replaces methane, the coal matrix swelling continued but the deformation amplitude decreased, which was only 19.60% of the methane adsorption stage. At the mixed gas desorption stage, the coal matrix shrank with the reduction of pressure and the shrinkage amount changed logarithmically with the pressure, which had the hysteresis effect when compared with the swelling in adsorption. The mechanism of coal matrix deformation was discussed through a comparison of the change of micropores, mesopores, and also part macropores in the adsorption process.
Highlights
A coal seam is a kind of typical heterogeneous unconventional natural gas reservoir, developed abundantly in pores and fractures [1,2]
When the high-pressure nitrogen is injected, injected, the effective stress of the coal reservoir can be reduced slowly, which is advantageous in the effective stress of the coal reservoir can be reduced slowly, which is advantageous in maintaining maintaining the permeability of the coal reservoir [52,53]
This positive effect can counteract or even exceed the exceed the negative effect of mesopore and macropore shrinkage caused by gas adsorption. These negative effect of mesopore and macropore shrinkage caused by gas adsorption. These results show results show that coal reservoir permeability increases, but their relationship needs further work
Summary
A coal seam is a kind of typical heterogeneous unconventional natural gas reservoir, developed abundantly in pores and fractures [1,2]. Due to the differences in the experimental apparatus, measuring methods, selection of samples, and simulation calculation models, there are differences in the relationship between coal matrix deformation and the pressure or adsorption quantity of gas [27,28,29,30,31]. Studies on the continuous deformation characteristics of the coal matrix during the process of methane replacement by nitrogen injection, which involves two kinds of gas adsorption/desorption and their interactions, are relatively few in number. This paper studied the coal matrix deformation models at different stages by performing isothermal adsorption/desorption experiments of methane replacement by nitrogen injection. The deformation law of the coal matrix and the relationship between the coal matrix and its adsorption quantity or pressure was studied, which can provide technical support and a theoretical basis for tuning injection parameters in nitrogen foam fracturing and predicting coal reservoir permeability. The results revealed whether the output and migration of CBM was of benefit
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