Tertiary oil recovery and CO2 storage from laboratory injection of CO2 or water-saturated CO2 into a sandstone core

Abstract

Water-saturated CO2 (wsCO2) injection can improve CO2 sweep efficiency during CO2 enhanced oil recovery (CO2-EOR). Previous studies investigated wsCO2 injection only as a secondary oil recovery method, CO2 injection begins at connate water saturation - the rest bring oil. However, CO2 injection usually follows water injection, in which case the injection is referred to as a tertiary recovery method.In this study, we inject pure CO2 and wsCO2 into a heterogeneous, Hutton Sandstone core that is subjected to 116 °C, 15.5 MPa, represents immiscible conditions. The core is saturated with brine followed by injecting 40 pore volumes (PVs) of oil, reaching the irreducible water saturation (Swi) – a state analogous to connate water. Afterwards, 40 PVs of brine are injected into the core, analogous to secondary oil recovery, thereby reaching residual oil saturation after water injection (Sorw). Subsequently, 40 PVs of pure CO2 or wsCO2 are injected, analogous to tertiary oil recovery.The oil and water are injected with the core horizontal (that is, with its ends facing sideways). However, to receive the CO2 or wsCO2, a given core is held in one of two orientations: vertical with CO2 or wsCO2 injected at the top, or horizontal. Pressure difference across the core, oil produced, and CO2 stored are recorded. For both vertical and horizontal orientation, pressure difference during wsCO2 injection is found to exhibit local maxima 2.2 to 4.3 times the normal level, indicating a reduction in wsCO2 mobility and therefor reduced relative permeability to CO2. Vertical orientation is found to yield similar oil production and CO2 storage for pure CO2 and wsCO2 injection, which is consistent with the expectation of 100% sweep efficiency. However, for horizontal orientation, the ultimate oil recovery where wsCO2 is injected is found to be 3.2% PVs higher than that where pure CO2 is injected, and CO2 storage where wsCO2 is injected is found to be 10% PVs higher than that where pure CO2 is injected. These results for horizontal orientation are attributed to low CO2 sweep efficiency, which is due to the gravity effect (which causes the CO2 to congregate at the top of the rock, thereby bypassing a large quantity of oil at the bottom of the rock). WsCO2 injection is superior to pure CO2 injection in terms of oil recovery and CO2 storage. However, wsCO2 has more profound effect on CO2 storage compared to oil recovery because the injected wsCO2 enhances the production of both water and oil, leaving more pore space for CO2 storage.