Shut-in time optimization after fracturing in shale oil reservoirs

Abstract

A multi-process (fracturing, shut-in and production) multi-phase flow model was derived considering the osmotic pressure, membrane effect, elastic energy and capillary force, to determine the optimal shut-in time after multi-cluster staged hydraulic fracturing in shale reservoirs for the maximum production. The accuracy of the model was verified by using production data and commercial software. Based on this model and method, a physical model was made based on the inversion of fracture parameters from fracturing pressure data, to simulate the dynamic changes of pore pressure and oil saturation during fracturing, soaking and production, examine effects of 7 factors on the optimal shut-in time, and find out the main factors affecting the optimal shut-in time through orthogonal experiments. With the increase of shut-in time, the increment of cumulative production increases rapidly first and then tended to a stable value, and the shut-in time corresponding to the inflection point of the change was the optimal shut-in time. The optimal shut-in time has a nonlinear negative correlation with matrix permeability, porosity, capillary pressure multiple and fracture length, a nonlinear positive correlation with the membrane efficiency and total volume of injected fluid, and a nearly linear positive correlation with displacement. The seven factors in descending order of influence degree on optimal shut-in time are total volume of injected fluid, capillary force multiple, matrix permeability, porosity, membrane efficiency, salinity of fracturing fluid, fracturing fluid displacement.