Propped fracture conductivity in shale oil reservoirs: Prediction model and influencing factors

Hydraulic fracturing is the main measure for stimulation of shale oil reservoirs, but the high content of clay minerals, well-developed bedding, and low mechanical strength of shale rock often result in a strong stress sensitivity of propped fractures associated with shale hydration expansion and proppant embedment in fracture wall. Especially under conditions of low sand laying concentration, the fracture conductivity can be greatly reduced. In this paper, a comprehensive prediction model of propped fracture conductivity in shale oil reservoirs was established, which considers the damage mechanisms of proppant compression, embedment, crushing, and shale hydration and expansion. The sensitivity analysis of factors affecting the fracture conductivity indicates that the proppant particle size, involved damage mechanisms, Kozeny-Carman coefficient, proppant layer number, and proppant density are the main factors to determine the fracture width and permeability and further affect the fracture conductivity. Most of the rest factors are related to the specific fracture damage mechanisms. The influence of shale hydration expansion is larger than that of proppant particle compression which is further larger than that of proppant crushing. In a real hydraulic fracture, the fracture width decreases from fracture heel to toe, caused by the non-uniform laying concentration of proppant. For the fracture near the wellbore usually with a large sand laying concentration, the influences of different factors are ranked as follows: proppant particle size > elastic modulus of proppant > fluid pressure > hydration expansion coefficient > filtration depth. For the front of the fracture with a low sand laying concentration, it is easy to close, which is sensitive to all the above factors. To achieve a high and stable fracture conductivity, the anti-swelling agent should be used to prevent shale hydration expansion. Large-size proppants with high elastic modulus should be selected to prop up the front of the fracture, and the decline of bottom-hole flow pressure should be controlled during the depressurized production process. The obtained results have a certain guiding significance for understanding the factors of shale fracture conductivity and the optimization of fracture parameters.