A Miscibility Scoping Study for Gas Injection into a High-Temperature Volatile Oil Reservoir in the Cooper Basin, Australia

Abstract

Abstract As part of the Moomba Carbon Storage Project, a scoping study for the potential of miscible gas injection into Tirrawarra, an Australian onshore high temperature volatile oil reservoir, has been completed. Twenty-six injectants with varying compositions of CO2, CH4, C2H6, n-C5H12 and N2 are considered in the study. The volatile oil under study has a CO2 content of approximately 20 mol%. High temperature reservoirs and volatile oils are two extremes of a miscible gas injection system that have not commonly been studied in previous literature. The minimum miscibility pressure (MMP) between oil and an injectant is a key parameter in a miscible gas injection project, directly affecting project design. MMP is highly dependant on reservoir temperature as well as oil and injectant compositions. Commonly, MMP calculations involve one of the following three methods:thermodynamic miscibility modeling,laboratory-based miscibility tests and simulations,use of correlations in the literature. In this study an Equation-of-State is used to facilitate 1-D slim tube simulations, through which the MMP between the volatile oil and various injectants is determined. Applying commonly used MMP correlations to our study reservoir provided poor results - with some correlations predicting MMP values at up to approximately 140% of those determined through 1-D slim tube simulation. This was due to the inability of many correlations to account for the effect of volatile oil composition and high reservoir temperatures. Our results show that MMP correlations can give misleading results if applied to reservoir data which is dissimilar to that with which the correlation was developed. The MMP with 100% CO2 injectant was calculated to be 2680 psig at 285°F. The highest MMP was obtained with pure N2 injectant at greater than 3600 psig. Variation of injectant composition had clear effects on the MMP. The increase of both N2 and CH4 concentrations in the injectant raised MMP by 25.6 psi/mol% and 15.2 psi/mol%, respectively. Increasing the C2H6 and n-C5H12 concentrations decreased MMP by 6.4 psi/mol% and 34.4 psi/mol%, respectively. The results of this study will help in developing an optimized injection strategy for the candidate reservoir, in addition to providing insights into key design parameters for surface facilities. Introduction The primary aim of the project was to assess the potential of Tirrawarra oil to achieve miscibility with injection gases of varying composition. Of specific interest was the determination of the minimum pressure at which the Tirrawarra oil and the injection gases would become miscible at reservoir temperature (i.e. the minimum miscibility pressure or MMP) and the effect of different common injection gas components on the MMP.