Permeability is one of the most fundamental reservoir-rock properties required for modeling hydrocarbon production. However, the ultrafine pore structure of shale-gas reservoirs (pore-throat radii in the range of 1 -200 nm) cause the violation of the basic assumptions behind Darcy’s law. Depending on a combination of pressure-temperature conditions, pore structure and gas properties, non-Darcy flow mechanisms such as Knudsen diffusion, and/or gas-slippage effects will affect the matrix apparent permeability. Additionally, formation compaction and the release of the adsorption gas layer will affect the matrix apparent permeability during reservoir depletion.
The authors of this paper propose a unified matrix apparent-permeability model for shale-gas formations, which unifies non-Darcy flow / gas-slippage mechanisms, adsorption gas layer release, and geomechanical effects into a coherent global model, as shown below.
Mechanisms that alter shale-matrix apparent permeability during production
The fully coupled unconventional reservoir simulator developed in this study provides a comprehensive tool to investigate the combined real-gas nanoflow mechanisms and rock deformation effects on the matrix apparent-permeability, as well as the long-term productivity of hydraulically fractured shale-gas formations.
Conclusions from this research work are:
- Matrix apparent permeability in a shale formation is not only determined by the non-Darcy flow / gas-slippage behavior but also by the intrinsic permeability within the nanopore structure.
- Shale-matrix permeability derived from core samples at laboratory conditions needs to be correlated to reservoir conditions cautiously due to the extreme sensitivity of factors to the pore radius.
- While geomechanical effects (formation compaction) can temporarily dominate the matrix apparent-permeability evolution during the early-production stages, these will be offset by non-Darcy flow / gas-slippage behavior and gas-desorption effects and matrix apparent-permeability will start to increase due to pressure depletion.
- Impaired productivity in fractured shale formations during depletion is most-likely caused by reduction in fracture conductivity, rather than reduction in matrix permeability.
- The gradual release of the molecular adsorption layer has a significant impact on the matrix apparent-permeability evolution for small pore radii.
- Matrix apparent-permeability evolution during production can make a difference in well performance and long-term shale-gas production – even with the presence of a conductive natural-fracture network.
This summary, written by Special Projects Manager Nancy Musick, contains highlights of the following paper: Wang, H. Y. and Marongiu-Porocu, M. 2015. Impact of Shale-Gas Apparent Permeability on Production: Combined Effects of Non-Darcy Flow / Gas Slippage, Desorption, and Geomechanics. SPE Res Eval & Eng. SPE-173196-PA (in press; posted 7 October 2015).