Experimental Investigation of Oil Displacement Caused by Fracturing Fluid Imbibition in Hydraulically Fractured Oil-Wet Shale Reservoirs

Abstract

Spontaneous imbibition (SI) has proven to be an effective method for enhancing oil recovery in water-wet shale reservoirs due to the capillary force. There are few articles that focus on the water imbibition and oil displacement for oil-wet reservoirs. However, we believe that the potential of oil displacement caused by water imbibition should not be neglected, since field observations from dozens of wells show strong positive relevance between initial oil production and post-fracturing well shut-in time in the oil-wet shale oil reservoir of Junggar Basin, China. In this paper, several experiments and numerical simulations are designed to shed light on the mechanism of oil displacement by fracturing fluid imbibition, including the driving force and the oil recovery of different pores. The real field fracturing fluid which is prepared by guar gel, anti-emulsifiers, anti-swelling, cleanup additives and gel breakers is used as the test fluid. Also, the oil-wet shale cores saturated with crude oil are used as the samples. Firstly, contact angle after different contact times and interfacial tension are measured to figure out the petrophysical properties of fracturing fluid and crude oil. Secondly, the SI experiments using the fracturing fluid made up of deuterium oxide as the imbibed fluid are conducted to discover the oil recovery of different sizes of pores with shale samples. The low-field nuclear magnetic resonance(LF-NMR) is used to describe the relative content of crude oil under different pores and different SI times. Finally, an experimental core model is established based on the above experimental results. The results show that wettability alteration appears in the oil-wet the core sample. As the contact time increases, the contact angle decreases continuously. The interfacial tension decreases from 72(water and crude oil) mN/m to 1.2(fracturing fluid and crude oil) mN/m. These two phenomena effectively explain the driving force of imbibition and displacement in oil-wet samples. The imbibition oil recovery is about 18.4% in the oil-wet core samples, which indicates that oil-wet cores have a certain oil displacement effect. The fracturing fluid enters the mesopore first because of the driving force produced by wettability alteration and the low frictional resistance, and the imbibition oil recovery in the mesopore is 35%, which is higher than that in micropore and small mesopore in oil-wet samples. This phenomenon and result show a sharp contrast with that of water-wet rock cores proved by previous studies. Finally, a new capillary force curve accounting for the effect of wettability alteration is fitted to characterize the oil displacement in oil-wet samples by the experimental core model simulation. This study aims to demonstrate the SI characteristics of oil-wet shale and helps to provide crucial theoretical foundations for developing oil-wet shale reservoirs.