Two-Phase Flow Model of Coalbed Methane Extraction with Different Permeability Evolutions for Hydraulic Fractures and Coal Reservoirs

Abstract

Fluid flow modeling of coalbed methane (CBM) wells is effective in predicting gas production and designing appropriate depressurization schemes. Moreover, hydraulic fracturing is an important measure for improving the permeability of CBM reservoirs. In this work, we assumed the hydraulic fracture area near the main fracture to be rectangular. We developed a gas–water two-phase flow model considering the difference in stress sensitivity between the hydraulic fracturing area and the original coal reservoir. The numerical simulation results indicate that our model can accurately predict the production of CBM wells. In the early stage of CBM extraction, gas production increased with the increase of gas relative permeability. When the water saturation was below 0.7, the relative permeability of the gas phase was difficult to increase. At this time, the absolute permeability of the hydraulic fracturing area continued to decrease, leading to a decrease in gas production. Thus, if the difference of permeability evolution between the two areas is not considered, the production of CBM wells will be overestimated. In the process of CBM extraction, both reservoir pressure and water saturation decreased, but the distribution was inconsistent. The variation in reservoir pressure was mainly affected by absolute permeability and gas desorption, while the variation of water saturation was further affected by the distribution of relative permeability and initial water saturation. Therefore, the evolution of water saturation is generally more complex than that of reservoir pressure. In the hydraulic fracturing area, the effect of fracture compressibility on gas production was greater than the initial permeability. In the original coal reservoir, the larger initial permeability was more beneficial than the fracture compressibility for improving the production of CBM wells.

Coupling between Damage Evolution and Permeability Model with the Adsorption Effect for Coal under Gas Extraction and Coal Mining Conditions

“…At the same time, any decrease in pore pressure would lead to an increase in effective stress, and effective stress would close the coal fracture structure, thus preventing seepage behavior. 8 To predict the gas flow law during CBM extraction, a series of permeability prediction models have been put forward. Gray 9 considered the influence of matrix desorption shrinkage on fracture deformation for the first time in the process of establishing a permeability model.…”

“…In this process, the shrinkage of coal matrix caused by gas desorption has led to promote, significantly, fracture propagation, including the opening of effective gas seepage channels. At the same time, any decrease in pore pressure would lead to an increase in effective stress, and effective stress would close the coal fracture structure, thus preventing seepage behavior …”

Coalbed Methane Production Model Based on Random Forests Optimized by a Genetic Algorithm

“…Clarkson and Qanbari 1 introduced the CBM well prediction method based on the concept of dynamic drainage area (DDA) and established a new semianalytical DDA model. Zhu et al 20 established a gas–water two-phase flow model considering the difference in stress sensitivity between the hydraulic fracturing zone and the original coal reservoir. Considering the influence of temperature and gas slippage on CBM production, Li et al 21 established a permeability model of coal under stress–temperature coupling.…”

Response Characteristics of Coal Measure Strata Subjected to Hydraulic Fracturing: Insights from a Field Test

“…Liang et al analyzed borehole wall strain and acoustic emission response characteristics of coal during a laboratory HF experiment and divided HF into four stages. Currently, the research topic on HF mainly focuses on the seepage mechanism, the rock fracture mechanism, , proppant, fracture evolution characteristics, , and stress evolution characteristics . As for the experimental method, it primarily includes theoretical analysis, laboratory test, numerical simulation, , and field experiment. , Based on the achievements discussed above, although HF has been investigated in various aspects to some extent, the study on induced stress and strain evolution behaviors of coal and rock mass (CRM) caused by HF in engineering practice remains few and insufficient.…”

“…Liang et al 12 analyzed borehole wall strain and acoustic emission response characteristics of coal during a laboratory HF experiment and divided HF into four stages. Currently, the research topic on HF mainly focuses on the seepage mechanism, 13 the rock fracture mechanism, 14,15 proppant, 16 fracture evolution characteristics, 17,18 and stress evolution characteristics. 19 As for the experimental method, it primarily includes theoretical analysis, 20 laboratory test, 21 numerical simulation, 22,23 and field experiment.…”