Numerical simulation of gas production from tight, ultratight and shale gas reservoirs: Flow regimes and geomechanical effects

Production from unconventional gas resources (UGRs) has received great attentions because of their large reserves as well as technical advances in developing these reservoirs. The fluid flow in ultralow permeability porous media cannot be considered in the range of conventional Darcy flow as it undergoes a transition from a Darcy regime to slip flow and free molecule flow regimes. Understanding fluid flow inside the matrix and how the matrix permeability evolves over depletion are among the major challenges to unconventional gas reservoirs characterization. Considering different flow regimes in UGRs and time dependent permeability during the production of reservoir, the applicability of the availabe numerical simulatior to predict the production from unconventional reservoirs is questionable. In this paper, a numerical approach is proposed for simulation of gas production of UGRs including geomechanical effect, slippage effect and non-Darcy flow. In this simulation, gas production is calculated using a pseudo-pressure integral for well inflow performance and material-balance for reservoir depletion. The numerical approach has been verified by comparing with the results of fine-grid compositional simulation for a typical conventional gas reservoir. The pseudo pressure-integral has been extended to include the geomechanical effect and time dependent matrix permeability. The flow regime is distinguished by Knudsen number for each regions of the reservoir during the reservoir depletion. According to the numerical results, the matrix permeability changes depending on the flow regime determined by Knudsen dimensionless number. Slip flow and Knudsen diffusion which are dependent on net pore pressure can play important roles in the gas production. Higher-than-expected matrix permeability becomes more highlighted when the permeability of the matrix decreases and dimensionless Knudsen number is higher than 0.1. This higher permeability enhances the gas production. On the other side, the matrix permeability decrease as the net overburden stress increases during the production life of the reservoirs. This decrease in matrix permeability clearly decreases the rate of gas production. The presented numerical simulation evaluates the significance of different flow regimes, time dependent permeability and geomechnical effect in production from UGRs. It also offers a rapid and simple tool for prediction of gas deliverability of UGRs well.