Gas/CO2 Mobility under 2-Phase and 3-Phase Flow in Carbonate for CO2-EOR and CCS Projects

CO2 EOR is a mature technology which has been applied either as continuous CO2 injection or CO2 WAG since the early 1970s. The number of CO2 EOR projects are increasing with time and a significant increase in the number of carbon capture and storage (CCS) projects has been observed in the last few years. CCS is a key enabler to reduce carbon emissions, recognized to play a key role in the journey to net zero, delivering low carbon energy and featuring prominently in many countries’ climate action plans.

A comprehensive experimental program was designed to investigate parameters that affect gas/CO2 mobility under 2-phase and 3-phase flow conditions. These measurements provide key input parameters for CCS and/or EOR projects. The core flood experiments were performed under reservoir conditions using live crude oil and carbonate core samples of up to 1 ft long and 2 in. diameter. The core wettability was restored by ageing the core in crude oil for several weeks, except for the two-phase gas-water experiments. All gas injection experiments were performed using vertically oriented cores, with gas injection from the top. The experimental results show that: 1- Gas injection conducted under miscible/near miscible conditions in the presence of immobile connate water has the highest mobility and Krg(Sorm)follows a linear curve as expected for miscible floods, 2-During 3-phase flow experiments, the presence of mobile water has significant impact on gas/ CO2 mobility, where CO2 mobility decreases as the mobile water saturation increased at the start of CO2 injection, 3- Mobile water saturation—whether from alternating injection cycles or high initial water saturation in transition zones—has similar effect on gas mobility and hence reduces Krg(Sorm), 4- Gas relative permeability end point Krg(Sorm) showed no cyclic dependent for all gas injection cycles starting at mobile water saturation, 5- The lowest gas mobility is observed in two-phase gas-water displacements, highlighting the critical role of water in blocking gas flow, and 6- The gas/CO2 relative permeability end points are not dependent on their own saturation alone as assumed in three-phase models, significant variation in the relative permeability end points was measured at the same saturation depending on the presence of mobile water.

The results of this study have important implications for the design and performance predictions of CO2/WAG EOR and CCS projects. For example, the presence of mobile water at the beginning of CO2 injection significantly reduces CO2 mobility and hence improving sweep efficiency. However, the reduction of gas mobility in the presence of mobile water reduces gas injectivity.