Mechanisms and Influencing Factors of High-Temperature and Pressure Huff-n-Puff Imbibition Process in Continental Shale Oil Reservoirs

Abstract

As continental shale oil is explored and developed on a larger scale, its high starting pressure gradient in the shale matrix poses challenges for flow. This paper investigates the effect of imbibition development and the mechanism of oil mobilization under high temperature and high pressure using self-developed dynamic huff-n-puff imbibition equipment based on micropore and percolation characteristics. Using six different injection fluids—slickwater, imbibition agent CY-IMNF-1, sand-carrying agent, guanidine gel breaking fluid, imbibition agent #G-1, and CO2—this study involved an experimental investigation of high-temperature and high-pressure huff-n-puff imbibition at a pressure of 30 MPa and a temperature of 110°C. The effect of injection fluids on imbibition development recovery is analyzed, and the contribution of different scale pores to the oil recovery of continental shale is quantified. It was found that slickwater huff-n-puff resulted in the highest imbibition oil recovery, followed by imbibition agent CY-IMNF-1. Shale oil production in nanopores and micropores was mainly achieved through slickwater huff-n-puff, and the pore and throat size limit for oil mobilization was found to be 4–7 nm. Based on a synergy perspective, optimizing the hybrid system of slickwater and CO2 is suggested. This study provides technical support for developing similar continental shale reservoirs with high-temperature and high-pressure huff-n-puff imbibition. Based on a self-made experimental device and two-dimensional (2D) nuclear magnetic resonance testing, the quantitative evaluation method of sensitivity of different shale pores to external fluids is established. The results show that the oil recovery rate of imbibition using sliding water is the highest, the oil in nanopores and micropores is mainly produced through huff-n-puff by slickwater, and the limit radius of oil mobility in pores and throats was found to be 4–7 nm. The research findings have important implications for huff-n-puff experiments and the formulation design of shale fracturing fluid systems.