Permeability is one of the most fundamental reservoir rock properties required for modeling hydrocarbon production. Many shale gas and ultralow permeability tight gas reservoirs can have matrix permeability values in the range of tens to hundreds of nano-darcies. 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, non-Darcy 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 the adsorption gas layer upon shale matrix apparent permeability during reservoir depletion.

In this article, we first present a thorough review on gas flow in shale nano-pore space and discuss the factors that 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 the adsorption gas layer into a single, coherent equation. In addition, a mathematical framework for an unconventional reservoir simulator that was developed for this study is also presented.

Different matrix apparent permeability models are implemented in our numerical simulator to examine how the various factors impact matrix apparent permeability within the Simulated Reservoir Volume (SRV). Finally, the impact of a natural fracture network on matrix apparent permeability evolution is investigated. The results indicate that even though the conductive fracture network plays a vital role in shale gas production, the matrix apparent permeability evolution during pressure depletion can't be neglected for accurate production modeling.

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