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Numerical Simulation of Low Permeability Unconventional Gas Reservoirs

Unconventional gas resources from tight sand and shale gas reservoirs have received great attention in the past decade and become the focus of the petroleum industry as well as energy resources worldwide, because of their large reserves as well as technical advances in developing unconventional resources. Compared to conventional reservoirs, gas production in ultra-lowpermeability unconventional reservoirs is driven by highly non-linear flow equations and involves many coexisting processes due to the presence of multi-scale fracture networks, and to the heterogeneous nature of a porous/fractured and stress-sensitive rock. Therefore, quantifying flow in unconventional gas reservoirs remains a significant challenge. In this paper, we discuss a mathematical model and a numerical approach for simulating the production of unconventional gas reservoirs, in order to assess well performance and understand the critical parameters that affect gas recovery. Specifically, we consider the flow behavior in a stimulated reservoir volume (SRV) including a tight matrix and multi-scale fracture networks, namely primary hydraulic fractures, induced secondary fractures and micro-fractures. The feasibility and the limits in the use of single-porosity or dual-porosity reservoir models to simulate gas flow in such a system are discussed, and a multiporosity approach is evaluated. The impacts of various physics related to unconventional gas reservoirs, such as adsorption/desorption, Klinkenberg and geomechanical effects, are quantified. This work helps to improve simulation technologies for low-permeability unconventional gas reservoirs. An appropriate modeling approach actually underlies effective simulation tools for quantitative studies of unconventional reservoir dynamics and performance, taking into account multi-scale fracture impacts on gas production, well and stimulation design, and optimal production schedules in field development.