Thermally-Induced Secondary Fracture Development in Shale Formations During Hydraulic Fracture Water Invasion and Clay Swelling

摘要

Current trends in shale gas industry require an advanced-level understanding of fracturing water invasion into formation and the subsequent water-shale interactions.Previously,we studied osmosis and clay swelling effects on the permeability of the shale formation.Shale,with an average 50% clay content,could hold large cation-exchange-capacity and significantly improved membrane efficiency,which may promote swelling and changes in the stress.In addition,large temperature-gradient effects due to cold water contacting the formation has not been investigated in detail.A new geomechanically-coupled reservoir flow simulator is developed,which accounts for cold freshwater imbibition,osmosis and clay-swelling effects on the formation permeability under stress.The model includes aqueous and gaseous phases with three components: water,gas and salt.Governing geomechanical equation includes pore-pressure as well as temperature gradients.Volumetric strain (porosity changes) is calculated as a function of the mean normal stress,pore pressure and temperature.Imbibition occurs in water-wet inorganic part of the matrix,in the micro-cracks.Osmosis and clay swelling effects develop when the imbibed water in the micro-cracks interacts with the saline water in clay pores,which acts as a semi-permeable membrane to the water and experiences pore (osmotic) pressure changes and swelling of the clay in the formation.The effect of temperature is pronounced early during the shut-in when imbibition of cold water takes place rapidly.Cold water introduces a low-stress region near the fracture due to thermal expansion effect and pore pressure buildup.We used a criterion and discuss the potential for fracturing.It is anticipated that the fracturing develops during forced imbibition of cold water given that a large difference exists between the injected water and the formation temperatures.