A Generalized Framework Model for the Simulation of Gas Production in Unconventional Gas Reservoirs

Abstract

Summary 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 and technical advances in developing these resources. As a result of improved horizontal-drilling and hydraulic-fracturing technologies, progress is being made toward commercial gas production from such reservoirs, as demonstrated in the US. However, understandings and technologies needed for the 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 by use of various approaches—from modified commercial simulators to simplified analytical solutions—leading to limited success. Compared with conventional reservoirs, gas flow in ultralow-permeability unconventional reservoirs is subject to more nonlinear, coupled processes, including nonlinear adsorption/desorption, non-Darcy flow (at both high flow rate and low flow rate), strong rock/fluid interaction, and rock deformation within nanopores or microfractures, coexisting with complex flow geometry and multiscaled heterogeneity. Therefore, quantifying flow in unconventional gas reservoirs has been a significant challenge, and the traditional representative-elementary-volume- (REV) based Darcy's law, for example, may not be generally applicable. In this paper, we discuss a generalized mathematical framework model and numerical approach for unconventional-gas-reservoir simulation. We present a unified framework model able to incorporate 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 with unstructured grids. We discuss the numerical implementation of the mathematical model and show results of our model-verification effort. Specifically, we discuss a multidomain, multicontinuum concept for handling multiscaled heterogeneity and fractures [i.e., the use of hybrid modeling approaches to describe different types and scales of fractures or heterogeneous pores—from the explicit modeling of hydraulic fractures and the fracture network in stimulated reservoir volume (SRV) to distributed natural fractures, microfractures, and tight matrix]. We demonstrate model application to quantify hydraulic fractures and transient flow behavior in shale gas reservoirs.

Numerical Simulation of Natural Gas Flow in Anisotropic Shale Reservoirs

“…Unlike conventional gas reservoirs, the gas flow in shale formations involves complex flow processes with many different mechanisms act at the same time. These processes include mechanisms such as Knudsen molecular diffusion, gas-slippage effect (Klinkenberg effect), adsorption and desorption, strong rock-fluid interaction, rock deformation, etc (Shi et al 2013;Guo et al 2014;Wu et al 2014). Shale formations are characterized by the very tiny pores and extremely low permeability in which different shale types will have different characteristics although the porosity, stress, and pore pressure are similar .…”

Temperature effect on low permeability porous media filled with water at high pressures

“…If the fluid pressure inside a pore increases beyond a given limit, it can fracture the The theoretical study is performed within an abstract thermodynamic framework that can be extended to other kinds of compressible fluids such as shale gas or oil. Shale formations are characterized by extremely low permeability from subnanodarcies to microdarcies [27].…”

An Analysis for the Influences of Fracture Network System on Multi-Stage Fractured Horizontal Well Productivity in Shale Gas Reservoirs

“…The existence of kerogen and higher contents of clay minerals in shale has undoubtedly caused the larger surface of nanopores [6,7]. In contrast to conventional reservoirs, the content of gas adsorbed on the surfaces might be up to 85%, while the other 15% is held as free gas in macropore spaces [8].…”