| The main focus of this work was to study formation damage during shut-in time and production following hydraulic fracturing a gas shale formation. More specifically, this work investigated numerically the intricate effects of water imbibition, osmosis and clay swelling on shale permeability under stress. For this purpose a new conceptual petrophysical model was proposed for the shale matrix containing slit pores (microcracks), clay pores and organic pores, and a new geo-mechanically coupled reservoir flow simulator was developed, which dynamically accounts for the impact of these mechanisms on the permeability.;Simulation case studies were conducted to investigate the nature of the formation damage. It is observed that an altered permeability zone, rather a 'fracture skin', is developed during the shut-in following the fracturing. The permeability changes due to osmosis-related clay swelling and stress. The magnitude of permeability alterations is controlled mainly by the salt concentration difference between the injected fracturing fluid and the clay-bound water, the clay-membrane efficiency, the cation exchange capacity (CEC), the salt type, the clay porosity, the overburden stress and the duration of the shut-in time.;Clay pore pressure buildup due to osmosis increases the local stresses in the altered zone during the shut-in time and the production. However, it is predicted that the increased stress is insufficient to induce new microcracks and that significantly higher pore pressure is required to reach the tensile failure limit of the formation. The overpressure required for the tensile failure is dependent on the depth and the tensile strength of the formation, and on the geometry of the pores. |
