Unconventional gas resources from tight sand and shale gas reservoirs have received great attention in the past decade around the world, because of their large reserves as well as technical advances in developing these resources. As a result of improved horizontal drilling and hydraulic fracturing technologies, the progresses are being made towards commercial gas production from such reservoirs, as demonstrated in the US. However, understandings and technologies needed for effective development of unconventional reservoirs are far behind the industry needs, e.g., gas recovery rates from those unconventional resources remain very low. There are some efforts in the literature on how to model gas flow in shale gas reservoirs using various approaches from modified commercial simulators to simplified analytical solutions, leading to limited success. Compared with conventional reservoirs, gas flow in ultra-low permeability unconventional reservoirs is subject to more nonlinear, coupled processes, including nonlinear adsorption/desorption, non-Darcy flow (at high flow rate and low flow rate), and strong rock-fluid interaction, and rock deformation within nano-pores or micro-fractures, coexisting with complex flow geometry and multi-scaled heterogeneity. Therefore, quantifying flow in unconventional gas reservoirs has been a significant challenge and traditional REV-based Darcy law, for example, may not be in general applicable.

In this paper, we will discuss a generalized mathematical model and numerical approach for unconventional gas reservoir simulation. We will present a unified framework model able to incorporate all known mechanisms and processes for two-phase gas flow and transport in shale gas or tight gas formations. The model and numerical scheme are based on generalized flow models using unstructured grids. We will discuss the numerical implementation of the mathematical model and show results of our model verification effort. Specifically, we discuss a multi-domain, multi-continuum concept for handling multi-scaled heterogeneity and fractures, i.e., using hybrid modeling approaches to describe different types and scales of fractures from explicitly modeling of hydraulic fractures and fracture network in simulated reservoir volume (SRV) to distributed naturally fractures, microfractures, and tight matrix. We will demonstrate model application to quantify hydraulic fractures and transient flow behavior in shale gas reservoirs.

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