Experimental investigation on low-velocity seepage characteristics and influencing factors in a shale oil reservoir

The seepage behavior in shale oil reservoir is very complicated. In this study, the characteristics and influencing factors of low-velocity seepage in brittle shale oil reservoirs were investigated by high precision experimental methods. The velocity used in this study was as low as 10−4 ml/min, and at least eight different flow rates were measured for each experiment. During the operation, the pressure difference between the inlet and outlet was controlled within 0.35 MPa to eliminate the influence of stress sensitivity. The effects of core permeability, fluid viscosity, pore pressure, and propped artificial fracture on the seepage behavior were analyzed. The results show that the seepage flow rate and pressure gradient in shale core samples presents a convex relationship, which is entirely different from the results of a concave relationship in tight sandstone oil reservoirs. The slippage effect and boundary layer effect in the flow channels were investigated to explain the convex flow regime theoretically. The convex shape flow regime is caused by the heterogeneous of shale core samples associated with the completed developed micro-fractures in this area. The seepage flow divides into two stages: the non-linear stage when the applied pressure gradient is lower than 2 MPa/m and the linear stage when it is higher than 2 MPa/m. The non-linear seepage characteristics become more obvious with a higher viscosity crude oil. With the increase of pore pressure, micro-fractures in shale core samples gradually close, leading to the reduction of flowing channel and the significant decrease of apparent permeability. The seepage characteristics are similar to those of tight sandstone oil, and the flow rate and pressure gradient curve presents a concave shape. With a propped artificial fracture, the seepage flow behavior follows Darcy's law. This work provides a feasible and reliable method and valuable experiment data for the study of low-velocity seepage in shale reservoirs.