Impact of Shale-Gas Apparent Permeability on Production: Combined Effects of Non-Darcy Flow/Gas Slippage, Desorption, and Geomechanics

Abstract

SummaryMany shale gas and ultralow permeability tight gas reservoirs can have matrix permeability values in the range of tens to hundreds of nanodarcies. The ultrafine pore structure of these rocks can cause violation of the basic assumptions behind Darcy's law. Depending on a combination of pressure-temperature (P-T) conditions, pore structure and gas properties, NonDarcy flow mechanisms such as Knudsen diffusion and/or Gas-Slippage effects will impact the matrix apparent permeability. Even though numerous theoretical and empirical models have been proposed to describe the increasing apparent permeability due to Non-Darcy flow/Gas-Slippage behavior in nano-pore space, few literature have investigated the impact of formation compaction and the release of adsorption layer on shale matrix apparent permeability, which can be coupled together with the Non-Darcy flow/Gas-Slippage mechanism, during reservoir depletion.In this article, we first presented a thoroughly review on gas flow in shale nano-pore space and discussed what factors can impact shale matrix apparent permeability, besides the well-studied Non-Darcy flow/Gas-Slippage behavior. Then, a unified shale matrix apparent permeability model is proposed to bridge the effects of Non-Darcy flow/Gas-Slippage, geomechanics (formation compaction) and the release of adsorption layer into a single, coherent equation. In addition, a mathematical framework for an unconventional reservoir simulator that developed in this study is also presented.Different matrix apparent permeability models are implemented in the numerical simulator to examine what factors impact the matrix apparent permeability most. Furthermore, how these factors influence the average apparent permeability in the Simulated Reservoir Volume (SRV) and long-term production are also discussed. Finally, the impact of natural fracture networks on matrix apparent permeability evolution is investigated. The results indicate that even though the conductive fracture networks play a vital role in shale gas production, the matrix apparent permeability evolution during pressure depletion can still make a difference.

A Numerical Study of Thermal-Hydraulic-Mechanical Simulation With Application of Thermal Recovery in Fractured Shale-Gas Reservoirs

“…Depending on a combination of pressure-temperature conditions, pore structure and gas properties, Non-Darcy flow mechanisms such as Knudsen diffusion and Gas-Slippage effects will impact the matrix apparent permeability (Fathi et al 2012;Michel et al 2011;Swami et al 2012;Sakhaee-Pour and Bryant, 2012). In addition, constant decreasing pore pressure during production (transient flow and pseudo-steady-state flow) can lead to reduction of thickness in gas adsorption layer and increase in the effective stress, which in turn, can impact the formation matrix microstructure and effective pore radius (Wang and Marongiu-Porcu, 2015). So the overall matrix apparent permeability in shale gas reservoir is dynamic and pressure dependent, as shown in Fig.2.…”

“…In the context of hydraulic fractured shale gas formations, the local matrix permeability in the Stimulated Reservoir Volume (SRV) is space and time dependent during production. (Wang and Marongiu-Porcu, 2015) Besides the matrix permeability, the surface area of natural fracture networks that connected to the main hydraulic fracture and its ability to sustain conductivity are also critical for predicting long-term production in shale gas formations (Ghassemi and Suarez-Rivera, 2012). It is recommended that Brinell Hardness Test (BHN) and Unpropped Fracture Conductivity Test (UFCT) should be done in shales in order to determine fracture treatment types and estimate the relationship between fracture conductivity and confining stress (Ramurthy et al 2011).…”

“…So in this article, molecule diffusion will not be discussed in the following derivations for matrix apparent permeability. Wang and Marongiu-Porcu (2015) conducted a comprehensive literature review on gas flow behavior in shale nano-pore space and proposed a unified matrix apparent permeability model, which bridges the effects of geomechanics, non-Darcy flow and gas adsorption layer into a single mode, by considering the microstructure changes in nano-pore space. In a general porous media, the loss in cross-section area is equivalent to the loss of porosity,…”

“…Different shale formations may have different nano-pore structure typology, which leads to different values of . In this study, equals 2 by assuming the overall all intrinsic permeability resembles fluid flow in a capillary tube ( Compared to the apparent matrix permeability model proposed by Wang and Marongiu-Porcu (2015), Eq. ( . )…”

Analytical modeling of coupled flow and geomechanics for vertical fractured well in tight gas reservoirs

“…Zheng proposed a two-part Hooke's model to consider the geomechanical effects [11]. Wang incorporated the effects of geomechanics into permeability by considering its impact on pore throats [12]. This approach makes it easy to understand reservoir variations, and is useful for industry application.…”