The effect of impurity on miscible CO2displacement mechanism

Erhui Luo,Zifei Fan,Shuhui Dai,Liangtao Ma,Jianjun Wang,Qingying Hou,Yongle Hu

doi:10.2516/ogst/2019056

Abstract

The CO2displacement is one of the gasflooding Enhanced Oil Recovery (EOR) methods. The application from volatile oil to black oil is popular mainly because CO2requires a relatively low miscibility pressure, which is suitable to most reservoir conditions. However, CO2always contains some impurity, such as CH4, H2S and N2, leading to the change of phase behavior and flooding efficiency. Whether the gasflooding achieves successfully miscible displacement depends on the reservoir pressure and temperature, injected solvent and crude oil compositions. So three different types of oil samples from the real field are selected and mixtures of CH4, H2S and N2with various CO2concentrations as the solvent are considered. After a series of experimental data are excellently matched, three nine-pseudocomponent models are generated based on the thermodynamic Equation-of-State (EoS), which are capable of accurately predicting the complicated phase behavior. Three common tools of pressure–temperature (P–T), pressure–composition (P–X) and pseudoternary diagrams are used to display and analyze the alteration of phase behavior and types of displacement mechanism. Simulation results show that H2S is favorable to attain miscibility while CH4and N2are adverse, and the former can reduce the Multiple Contact Miscibility (MCM) pressure by the maximum level of 1.675 MPa per 0.1 mol. In addition, the phase envelope of the mixtures CO2/H2S displacing the reservoir oil on the pseudoternary diagram behaves a triangle shape, indicating the condensing-dominated process. While most phase envelopes of CO2/CH4and CO2/N2exhibit the trump and bell shapes, revealing the MCM of vaporization.

Highlights

The CO2 flooding has achieved many successful applications of field projects around the world (Babadagli, 2007; Luo et al, 2013a, b; McGuire et al, 2017)
Because the displacement process that CO2 is injected into the subsurface can Enhance Oil Recovery (EOR) and storage CO2 in order to reduce greenhouse gases discharge
H2S and CO2 existing in crude oil compositions are beneficial to achieve miscibility with reduced First-Contact Miscibility (FCM) pressures

Summary

Introduction

The CO2 flooding has achieved many successful applications of field projects around the world (Babadagli, 2007; Luo et al, 2013a, b; McGuire et al, 2017). The CO2 drive is one of the effective gasflooding methods that attain higher recovery than waterflooding, in particular the miscible displacement condition of approaching 100% oil recovery in the swept area. The reason why CO2 is preferably selected as an EOR injectant is that the crude oil would be swollen and promoted a reduction of viscosity, with requirement of relatively low miscible pressure (Abdurrahman et al, 2015; Han et al, 2016; Hand and Pinczewskl, 1990; Simon and Graue, 1964; Rahimi et al, 2017; Stalkup, 1987; Tsau et al, 2010; Yang et al, 2009). Three types of displacement processes in a CO2 injection process can be defined as shown in Figure 1a: immiscible, near miscible and miscible. The miscible drive is further classified as First-Contact Miscibility (FCM)

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