Long term conductivity of narrow fractures filled with a proppant monolayer in shale gas reservoirs

Abstract

The primary goal of hydraulic fracturing is to create a high conductive pathway. Gas shale is mainly fractured by slick-water. A complex network of narrow secondary fractures is created in slick water fracturing. These narrow fractures without proppants maintain low conductivity. A partial monolayer of proppant can be used to enhance the conductivity of these fractures and then improve the production. Due to the interaction between proppants and fracture surface under confining stress, the proppants will embed into the formations, which results in a decrease in fracture width and conductivity. Researches available in literature have addressed the problem. However, the shale reveals varying amounts of creep deformation in response to applied stress, which will continuously enhance the proppant embedment and reduce fracture width. The influence of this time dependent effect on the long term conductivity of partial monolayer proppant is not well understood. Study the characteristics and controlling factors of the long term change in conductivity can benefit to the production analysis and hydraulic fracturing optimization. Therefore, models combined numerical and analytical methods are developed in this paper. A finite element model is developed to simulate the long term change in fracture width. Then a simplified model based on Carman-Kozeny equation is used to calculate the long term conductivity. Simulation results show that after considering long term creep effects, there is still an optimal proppant concentration, which remains the maximum residual conductivity after proppant embedment. And the calculated optimal proppant concentration becomes larger than that without considering the long term effect. The simulation results also indicate that the optimal concentration depends on stress, rock mechanical properties, proppant mechanical properties and time.