Tight oil reservoirs are characterized by ultralow permeability, causing hydrocarbons to be trapped in tiny pores and hinder the crude oil flow. CO2 injection technology has been paid more attention in low-permeability reservoirs. In this work, the long core experiments of CO2 flooding are conducted to study the displacement pressure and enhanced oil recovery, and CT scanning method is used to obtain the oil saturation profiles along the samples. The results show that the reservoirs pressure increases initially and decreases afterwards during CO2 miscible displacement. The final reservoirs pressure is still much larger than the initial state, suggesting high energy supplement efficiency of CO2 miscible displacement. At the initial stage (20 min), the pressure increases gradually, but oil saturation does not change very much. The initial oil flow is related to oil expansion induced by oil injection. After 20 min, the oil saturation at the inlet falls quickly. The oil saturation begins to decrease uniformly in the whole range of sections until the displacement front breakthrough. The sections near the inlet and outlet has larger residual oil saturation. The research is significant for understanding the microscopic mechanism of CO2 flooding and enhancing oil recovery in tight oil reservoirs.

1. Introduction

In contrast to conventional oil reservoirs, the tight oil reservoirs are characterized by more heterogeneous matrix, much lower matrix permeability and more complicated pore structure (Gao and Li, 2015). Horizontal-well drilling and multistage fracturing can result in network systems to justify economic production (Tian et al., 2015). The fast production decline is one of the most-intractable problems during tight oil development. The natural inability of oil flow refers to lower matrix permeability (<1.0 mD) and crude oil trapped by tiny pores. Globally, CO2 flooding has been successfully used to recover crude from conventional oil reservoirs, but few investigations have examined tight oil reservoirs (Ben Salem et al., 2013). CO2 can reduce interfacial tension, oil viscosity and residual oil saturation to recovers oil (Dong et al., 2012). In tight oil reservoirs, the oil and gas diffusion intensified by CO2 is an important oil-recovery mechanism. In this paper, macro pressure distribution and micro oil saturation profiles are obtained to clarify the mechanism of CO2 flooding in tight oil reservoirs.

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