Effect of Impurities in Carbon Dioxide Stream on Phase Behavior for Geological Storage of Carbon Dioxide in Low API Gravity Oil Reservoirs

Abstract

Abstract While previous studies investigate the effect of impure CO2 streams in high and medium API gravity oils, this work focus on reservoirs with low API. This analysis provides additional alternatives for CO2 storage due to the uneconomical exploration and the vast extension of heavy oil resources. CO2 streams (re)injected for storage may contain impurities such as nitrogen, water, and light hydrocarbons, particularly for flue gases and CO2-rich natural. These impurities greatly affect CO2 solubility trapping mechanisms and storage effectiveness. This work investigates the phase behavior of reservoir fluids during injection of CO2 with impurities for storage into two low API gravity oils; 12° (San Ardo heavy oil) to 8.2° (Athabasca extra-heavy bitumen). Numerical studies are conducted using the Peng-Robinson Equation-of-State with Volume-Translation and evaluating Vapor-Liquid and Vapor-Liquid-Liquid equilibria, with rigorously implementation of mutual solubility water-hydrocarbons-CO2. Simulations are conducted at typical reservoir conditions and at high pressure/high temperature synthetic scenarios. The effect of impurity concentration is represented constructing pseudo-ternary diagrams. Results show that C3-CO2 mixtures may achieve miscibility at low pressures. In the presence of N2 and C1 as impurities, miscibility with crude oil was found to be affected negatively. The water presence also affects the solubility. Introduction Carbon dioxide (CO2) storage in oil reservoirs is a well-known carbon capture and sequestration (CCS) alternative to reduce the environmental footprint of CO2 emissions [1, 2]. Though high purity CO2 injection is ideal to maximize storage. The industrial activities (e.g. flue gas and CO2-rich natural gas) or carbon capture plants are usually the sources of impure CO2, containing N2, H2, O2, H2S, water, and light hydrocarbons [2, 3, 4, 5, 6]. Such impurities present an economic challenge to CCS as current CO2 capturing technologies to achieve high-purity streams are expensive, costing 12–60 US$/tCO2 and accounting for up to two-thirds of the total CCS project cost [2, 7, 8]. Hence, the impure CO2-rich streams may be disregarded for storage and contribute even further to CO2 emissions if vented or flared. An alternative is storing these impure gases in their raw state in uneconomic unconventional oil reservoirs. The low API gravity crude oils account for approximately 70% of the world's oil reserves [9]. Mature heavy oil fields not subjected to thermal recovery methods could be a good potential for CCS [10].