Composition Swing Injection for CO2 Storage and EOR
Authors: Nazarian B. , Cavanagh A. , Ringrose P. S. , Paasch B. 
Published: December 2014

Increased emissions of anthropogenic carbon dioxide over the last two centuries correlate convincingly with increases in average global temperature to confirm that CO2 emissions are an important source for recent climate change (Petit et al. 1999). As well as being an important greenhouse gas, CO2 is an effective solvent for hydrocarbons making it an excellent fluid for miscible reservoir flooding (CO2 EOR). Geological storage of CO2, in saline aquifers and depleted hydrocarbon reservoirs, is considered an essential technology for realizing the 2°C limit scenario agreed in the Copenhagen Accord (IEA 2014). However, the business case for Carbone Capture and Storage (CCS) is currently stalled mainly due to a lack of agreement on a viable CO2 pricing policy. An alternative option called CCUS, that involves utilization of captured CO2 for EOR purposes, is therefore being considered a method to finance and stimulate large-scale CCS projects. If the CO2 source is anthropogenic, and injected CO2 is allowed to remain in the reservoir, and if the operation is monitored to insure permanent sequestration of injected CO2, CCUS can be considered as a valid strategy for reducing greenhouse gas emissions.

In the CCS process, CO2 is normally injected into brine-filled geological traps. CO2 displaces in-situ brine and segregates in the reservoir due to a strong density contrast between CO2 and brine; the resultant sweep efficiency, for cases where CO2 is injected from a limited perforated interval in a single well, is very poor as confirmed by seismic and gravity observations (Singh et al. 2010; Alnes et al. 2011). Supercritical CO2 has a gas-like viscosity when injected into an oil reservoir for EOR purposes. The viscosity difference between in-situ hydrocarbons and injected CO2 results in viscosity fingering and early breakthrough at the producers; this typically decreases both the vertical and areal sweep efficiencies.

Swing injection techniques have been proposed to control properties of the CO2 injection blend in order to improve sweep efficiency of the injected phase (Nazarian et al. 2013). CO2 Composition Swing Injection (CSI) is a novel injection technology that provides the operator of a CO2 injection project with some control over the CO2 front movement both in saline aquifer storage and in CO2 flood for hydrocarbon production. The composition of the injected stream is manipulated to reduce gravity override in reservoirs with a large gravity number, resulting in a more uniform gas-water front. The advantages of the CSI injection approach for CO2 storage operations are capacity enhancement and a reduced seepage risk. CSI injection, when applied to a CO2 EOR flood, reduces the unwanted segregation of the CO2-rich phase, increasing the sweep efficiency. The properties of the injected stream can be designed, at little cost, to lower the viscosity and density contrast with the in-situ reservoir fluids. This reduces the segregation and fingering of CO2, which results in a more uniform and controlled displacement front. Compared to conventional methods for front stabilization such as the use of viscosifiers and CO2 WAG, the CSI method can provide a more affordable alternative. The properties of the injected stream can also be controlled by changing the injection temperature (Nazarian et al. 2013) – a technique called Temperature Swing Injection (TSI). This technique gives the operator a relatively affordable method to control the properties of the injected blend by changing the injection temperature. This paper explains the concept behind the swing injection techniques and reports simulation results from realistic models that address the performance of these techniques for CCS and CO2 EOR.