Improved Numerical Simulation for Shale Gas Reservoirs
Producing gas from shale strata has become an increasingly important factor to secure energy over recent years for the considerable volume of natural gas stored. Unlike conventional gas reservoirs, gas transports in shale reservoir is a complex process. In the organic nano pores, slippage effect, gas diffusion along the wall, viscous flow due to pressure gradient, and desorption from Kerogen coexist; while in the micro fractures, there exists viscous flow and slippage. In addition, there are two other factors that can play significant roles under certain circumstances and have not received enough attention in previous models. During pressure depletion, gas viscosity will decrease, and reservoir porosity will increase when the absorbed gas desorbs from the pore wall. It is important to construct a unified model considering all known mechanisms for shale gas reservoir performance study and analyze their importance at varied conditions. This paper proposed a unified mathematical model for shale gas reservoirs. The proposed model was constructed based on the dual porosity continuum media model; mass balance equations for both matrix and fracture systems were constructed using the dusty gas model. In the matrix, Knudsen diffusion, gas desorption, and viscous flow were considered. Gas desorption was characterized by the Langmuir isothermal equation. In the fracture, viscous flow and non-darcy slip permeability were considered. The increase of pore radius due to gas desorption was calculated from gas desorption volume from the pore wall. Gas viscosity was characterized as a function of Knudsen number. Weak form for the system has been derived based on constant pressure boundary and solved using COMSOL. A comprehensive sensitivity analysis was conducted, and detailed investigation was done for their impact on the shale gas reservoir performance. The results show that considering gas viscosity change will greatly increase gas production under given reservoir conditions and slow down the production decline curve. Considering pore radius increase due to gas desorption from the pore wall will obtain higher production, but the effect is not very significant under the given reservoir condition. In shale reservoirs, both the matrix and fracture permeability increase with time during production. Ignoring one of these factors such as Knudsen diffusion, slippage effect, desorption, viscosity decrease and porosity increase will lead to less cumulative production. Therefore, for more accurate shale gas production prediction, it is crucial to incorporate these factors into the model.
C. Guo et al., "Improved Numerical Simulation for Shale Gas Reservoirs," Proceedings of the Offshore Technology Conference Asia: Meeting the Challenges for Asia's Growth (2014, Kuala Lumpur, Malaysia), vol. 3, pp. 2083-2099, Offshore Technology Conference, Mar 2014.
The definitive version is available at http://dx.doi.org/10.4043/24913-MS
Offshore Technology Conference Asia: Meeting the Challenges for Asia's Growth (2014: Mar. 25-28, Kuala Lumpur, Malaysia)
Geosciences and Geological and Petroleum Engineering
Mathematics and Statistics
Keywords and Phrases
Dual Porosity Media; Dusty Gas Model; Production Prediction; Shale Gas; Slippage Effect; Desorption; Fracture; Gas Dynamics; Gases; Mathematical Models; Offshore Gas Fields; Petroleum Reservoir Evaluation; Petroleum Reservoirs; Pore Pressure; Porosity; Quay Walls; Viscous Flow; Cumulative Production
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