Osmosis and Clay Swelling Effects in Gas Shale Formations under Stress

Abstract

Water-shale interactions are traditionally perceived as complex phenomena due to reactive nature of shale with water. However, the current trends in shale gas industry requires an advanced-level of understanding of these interactions and their impact on gas production. In this paper we investigate the invasion of fracturing water into the formation and the subsequent water-shale interactions. Objective of this work is to study osmosis and clay swelling effects of the invasion on the formation permeability. For this purpose, a new geomechanically-coupled reservoir flow simulator is developed, which accounts for water imbibition, osmosis and clay swelling effects on the formation permeability under stress. The simulation model considers the formation has a multi-scale pores consisting of microcracks, clay pores and organic pores. Water imbibition occurs in the water-wet inorganic part of the matrix in the microcracks. Osmosis and clay swelling effects develop in the clay pores acting as semi-permeable membrane to the imbibed water and changing the local stress in the formation. The simulation model includes aqueous and gaseous phases with three components: water, gas and salt. The simulation results show that the formation permeability is dynamically affected during the shut-in period by a combination of mechanisms including imbibition, capillarity, diffusion/osmosis, and total stress. Notably, a permeability impairment zone, rather a fracture skin, develops near the fracture. The permeability alteration is due to osmosis-related clay swelling and changing stresses in the formation. The magnitude of the permeability alteration is controlled mainly by the salt concentration difference between the fracturing fluid and the clay-bound water, the clay-membrane efficiency, the clay cation exchange capacity (CEC), the clay porosity, the stress and the duration of the shut-in time. We develop a fracture skin factor that can be used with the single-phase (gas) shale reservoir flow simulators that are typically run in the absence of water invasion at the scale of the stimulated reservoir volume (SRV) and in multidimensional geometries. Currently there is a clear need in the unconventional industry to better-understand and control the hydraulic fracturing fluid-shale interactions. This work is an important milestone considering the complexity of the problem and suggesting that the water chemistry and the formation lithology plays a significant role after the fracturing operations.