Impact of thermally reactivated micro-natural fractures on well productivity in shale reservoirs, a numerical study

Abstract

Recovered core samples and extensive outcrops studies have proved the existence of natural fractures in many tight formations. These natural fractures are likely filled with digenetic materials such as clays, quartz or calcite. In this numerical study, we suggest that small cemented natural fractures, exposed to the surface of the hydraulic fracture, can be opened by the induced tensile stress due to the temperature difference between the cold fracturing fluid and hot formation. Cohesive zone model (CZM) is utilized here to simulate these natural fractures. Tensile strength of digenetic cements, temperature difference between the fracturing fluid and formation, fractures spacing, and rock conductivity are the parameters controlling the opening and length of reactivated micro-fractures. Reactivated natural fractures may improve the rock permeability in the vicinity of the hydraulic fracture; however, the amount of permeability enhancement depends on the density and width of reactivated fractures. Contribution of these micro natural fractures to cumulative gas production from a shale reservoir is investigated by modifying the transmissibility coefficient in the dual porosity and dual permeability model. Transmissibility coefficient is then modified accordingly to estimate increase in gas production from the reactivated natural fractures in the reservoir simulator. Reservoir simulation results suggest that reactivated natural fractures in the tight formations at early stages can improve gas production up to 25% which may improves net present value of the project; however, their effect significantly reduces to 3% in long term cumulative production.