A technical assessment of CO2 Interim Storage in deep saline aquifers

Abstract

CO2 Interim Storage (CIS) involves storing carbon dioxide in subsurface reservoirs for a finite period of time to be subsequently withdrawn and utilized in enhanced oil recovery (EOR) or other industrial processes. Through its potential role in matching CO2 supply and demand and buffering any variability in each, CIS could facilitate the expansion of EOR operations in a number of small and dispersed oil fields, and it could reduce the cost of carbon capture and storage (CCS) by allowing increased flexibility in CO2 capture and economies of scale in transportation infrastructure. This study identifies and assesses the technical challenges and energy requirements of CO2 Interim Storage by examining two scenarios simulating different patterns of variable CO2 injection and production in an underground saline aquifer. The results from reservoir modeling show that the pressure buildup and CO2 plume associated with variable injection are similar to those of constant injection, and the overall variability in pressure transients reduces away from the injection site and as injection proceeds with time. The position of injection and production zones along the well plays a significant role in controlling CO2 plume migration; injection throughout the entire reservoir thickness can prevent early water invasion into the well. Furthermore, CIS presents some unique tradeoffs. On the downside, water vaporization by injected CO2 leads to salt accumulation in the aquifer after every production-then-injection sequence, which is not commonly experienced in underground natural gas storage. High and rapidly fluctuating injection and production rates accelerate salt buildup and may block the flow near the well. On the upside, the same water vaporization phenomenon facilitates the formation of a dry-out zone near the well, which, under relatively high injection and low production rates, allows the recovery of dry CO2 while preventing the undesirable liquid-water production. Still, a clear compromise exists between produced CO2 purity and overall CO2 recovery. In the well, lower water-cut leads to lower pressure drop during CO2 production, thus reducing the overall energy penalty for interim storage. The energy needed to dehydrate and recompress the produced CO2 is estimated to be around 88.6kJ/kg; compared to CO2 capture and compression, the energy costs for interim storage are small but not insignificant.