Prediction of gas production rate from shale gas reservoirs using a micro–macro analysis

Abstract

Shale gas has become one of the important contributors to the global energy supply. The declining pattern of the gas production rate with time from an unconventional gas reservoir is due to the depletion of shale gas stored in the nanovoids of the shale formation. However, there are only limited ways to predict the variation of the gas production rate with time from an unconventional gas reservoir. This is due to the multiple transport mechanisms of gas in nano-scale pores and changes in shale gas permeability with pressures in nano-scale pores, which is impacted by the pore structure of the shale. In this study, the permeability-pressure (K-p) relationship for different shales (Eagle Ford, Haynesville, Longmaxi and Opalinus) were determined using an equivalent anisotropic pore network model (PNM). This PNM has REV-scale shale gas flow in randomly generated nanovoids and their connection in the shale matrix, and the multiphase flow of shale gas including viscous flow, slip flow and Knudsen diffusion. These predicted K-p correlations were then used in a finite element model (FEM) to predict the variation of the gas production rate with time (flux-time curves) at the macroscale. The simulation results show that the flux-time curves can be simplified to two linear segments in logarithmic coordinates, which are influenced by the fracture length and initial gas pressure. The predicted results using the PNM-FEM were validated by comparing them with the reported field test data. The method described in this study can be used to upscale the gas transport process from micro- to macroscale, which can provide a predictive tool for the gas production in shales.

Hydraulic Fracturing and Enhanced Recovery in Shale Reservoirs: Theoretical Analysis to Engineering Applications

“…A set of field data (including both geological and engineering parameters) is prepared and used as input in Comsol Multiphysics to investigate the impacts of kerogen pore volume, kerogen permeability, inorganic matrix permeability, fracture permeability, effective stress, and Langmuir parameters on shale gas reservoir production performance. − Most simulation approaches first establish a base case model, which is an extract from the field-scale model, upon which sensitivity analysis is performed after verifying model accuracy with the field data or analytical data. − ,− For example, Zhang et al used only the gray portion (a quarter of the entire domain) of the production area to simulate and analyze, because of the symmetry of the production area, as shown in Figure a. Tian et al extended the base case model to a field-scale model by 10 times to quantitatively classify the domain contribution, as shown in Figure b.…”