A hybrid model of production prediction for CBM wells with heterogeneous coal cleats and stress sensitivity effect

The accuracy of predictions regarding the productivity of coalbed methane (CBM) wells is significantly influenced by fracturing parameters and the complex nonlinear dynamics of fluid flow within the coal matrix and cleats. However, traditional models frequently overlook the intricacies of heterogeneous cleats and the effects of stress sensitivity. Moreover, their oversimplified gas-water two-phase seepage flow models can lead to considerable errors in dynamic analysis. To address these limitations, this paper introduces a hybrid model that thoroughly considers the nonlinear flow mechanisms in the coal matrix, as well as the heterogeneity and stress sensitivity of coal cleats. The proposed model utilizes an enhanced tri-linear representation to capture the complex characteristics of the hydraulic fracture network. A semi-analytical solution is derived in the Laplace domain, with dynamic parameters being iteratively updated through the coupling of material balance equations. The results indicate that the hybrid model provides accurate predictions for two-phase flow scenarios while maintaining high computational efficiency. Importantly, the development and scale of coal cleats have a significant impact on the initial productivity of CBM wells. More developed cleats allow for quicker pressure drop propagation, thus enhancing gas desorption within the matrix. Larger cleat apertures and higher linear densities correspond to increased initial productivity and cumulative gas output from wells. Additionally, critical seepage parameters associated with hydraulic fractures greatly affect the production dynamics of gas-water two-phase flow. When the length of the horizontal well modification section is constant, the number of fractured sections, the half-length of artificial fractures, and the conductivity of these fractures emerge as key factors determining productivity. Furthermore, stress sensitivity results in reduced permeability and increased fluid flow resistance. Ignoring stress sensitivity can lead to an overestimation of the true gas production capacity of the reservoir. As pressure decreases, stress sensitivity becomes unavoidable, underscoring the need to manage production pressure differences to lessen its negative effects. The findings of this study contribute to a better understanding of gas-water two phase flow behavior and production performance of multiple-fractured CBM wells.