A New Approach to Evaluate and Predict the Reservoir Performance of a Two-Phase Flow Coalbed Methane Well with Fully Coupled Time-Dependent Desorption and Pressure-Dependent Diffusivity Effects

Abstract

In coalbed methane (CBM) reservoirs, permeability changes dynamically throughout the life of the reservoir as the pressure depletes due to production. The dynamic variation of coal permeability mainly depends on the combined effect of effective stresses and coal matrix shrinkage caused by gas desorption. Under the conditions of pressure-dependent diffusivity in coal reservoirs, the flow mechanism in CBM reservoirs complicates the solution of the two-phase diffusivity equation. In this study, the authors present a new approach to describe the CBM reservoir diffusivity equation in a form that fully couples the pressure-dependent diffusivity and time-dependent desorption effects in the transport mechanism of CBM reservoirs. This paper presents a semi-analytical solution of the fully coupled diffusivity equation under pseudo-steady state flow conditions. The proposed solution is then leveraged to predict the performance of a two-phase CBM well using an approach that is alternative to the conventional method for CBM reservoirs. The proposed semi-analytical model presents a computational method not only to predict the performance but also to evaluate the performance of a producing CBM well. The model presented in this study has been used to investigate the impact of pressure-dependent diffusivity and time-dependent desorption on the production behavior of CBM wells producing under pseudo-steady state conditions. The results of the semi-analytical solution have been verified using numerical simulator CMG-GEM as well as actual in-field data.

A Novel Alternative Method of Compositional Simulation: A Semi-Analytical Approach for Predicting and Evaluating Reservoir Performance of a Multicomponent Coalbed Methane (CBM) Well

“…Later on, to account for the heterogeneity of the cleat network, Upadhyay and Laik , and Neelu et al incorporated the n th exponent relationship between cleat permeability and porosity. Here, k i and φ i represent the initial cleat permeability and porosity, respectively, and α is a constant given by . The changes in cleat porosity and permeability with pressure depletion as established by eqs and , which lead to pressure-dependent diffusivity, can be accounted for in CBM flow equations by substituting k = αφ n in eq . Neelu et al presented a new pressure-transform, , in the form of stress-dependent adjusted pressure (SDAP) that reduces eq to a single-phase diffusivity equation. The SDAP transform includes the combined effect of two-phase flow and pressure-dependent diffusivity.…”

“…Several studies have established the relationship between coal cleat porosity and pressure by combining coal geomechanics with sorption-induced coal swelling/shrinkage. ,− In this study, the Palmer and Mansoori model is used to relate cleat porosity with pressure. Using matchstick geometry model, a cubic relation can be established between cleat permeability and porosity . Later on, to account for the heterogeneity of the cleat network, Upadhyay and Laik , and Neelu et al incorporated the n th exponent relationship between cleat permeability and porosity. Here, k i and φ i represent the initial cleat permeability and porosity, respectively, and α is a constant given by . The changes in cleat porosity and permeability with pressure depletion as established by eqs and , which lead to pressure-dependent diffusivity, can be accounted for in CBM flow equations by substituting k = αφ n in eq . Neelu et al presented a new pressure-transform, , in the form of stress-dependent adjusted pressure (SDAP) that reduces eq to a single-phase diffusivity equation.…”

“…However, these models are limited to capturing single-phase flow conditions and instantaneous desorption effects only. Taking inspiration from the work of Perrine, 22 who defined modified a pseudopressure function for capturing multiphase flow effects in conventional hydrocarbon wells, Neelu et al 23 proposed a fully coupled semi-analytical model that describes two-phase flow effects in CBM wells along with time-dependent desorption and pressure-dependent diffusivity effects. However, the mentioned semi-analytical model is valid for evaluating and predicting the performance of CBM wells having composition limited to a single component (CH 4 ) only.…”

Characterization of Coal Particle Methane Desorption and Optimization of Desorption Model Based on Desorption Damage

“…26 Neelu et al described the gas desorption equation and explained the desorption effect of diffusion rate and time. 27 Liu et al conducted gas adsorption and desorption tests under various pressure boundary circumstances, proposing a new desorption model. 28 Yang et al artificially changed the microstructure of coal by adding surfactants, improved the efficiency of gas extraction, and avoided the abrupt emission of mass gas.…”

“…Yan et al researched the effect of temperature on the gas desorption characteristics of anthracite, finding that higher temperatures led to increased desorption amount and diffusion coefficient . Neelu et al described the gas desorption equation and explained the desorption effect of diffusion rate and time . Liu et al conducted gas adsorption and desorption tests under various pressure boundary circumstances, proposing a new desorption model .…”