Practical Metrics for Monitoring and Assessing the Performance of CO₂-EOR Floods and CO₂ Storage in Mature Reservoirs

Monitoring an appropriate set of performance metrics is an essential part of a reservoir management strategy that seeks to maximize the efficiency of CO₂-EOR floods, and benchmark the performance of the reservoir to other fields in the region. While a number of studies have outlined various options for evaluating the performance of the reservoir, many are not applicable to depleted oilfields that are commonly encumbered with limited data availability. As interest in CO₂-EOR grows beyond giving new life to old fields, into a means of reducing anthropogenic CO₂ emissions, metrics that enable the analyst to assess the storage capacity of the reservoir in addition to oil recovery efficiency have also become increasingly desirable. This paper will present a streamlined set of metrics that meet these objectives, along with their implementation in a dashboard-style presentation. These metrics provide complementary and non-redundant information, and crucially, are still applicable where a paucity of pressure data exists. The analyst is equipped with four groups of critical information. These are (A) flow rate and cumulative production data; (B) EOR performance trends (incremental oil recovery efficiency, CO₂ storage efficiency, and net CO₂ utilization factor); (C) operational performance trends (voidage replacement ratio and producing CO₂-oil ratio); and (D) storage performance trends (incremental oil recovery factor vs. CO₂ storage and CO₂ storage capacity vs. voidage). To facilitate comparison, all metrics are normalized to percentage of original hydrocarbon pore volume (HCPV) injected.

The utility of the selected set of metrics is demonstrated with data obtained from CO₂-EOR activities in depleted oilfields of the Midwest Regional Carbon Sequestration Partnership (MRCSP) region. First, a nine-panel dashboard is populated using the metrics discussed for the D-33 reservoir, and will be shown to be useful in: quantifying CO₂ storage capacity, highlighting reservoir performance parameters (CO₂ breakthrough, oil recovery, voidage-related pressure changes during EOR etc.) and identifying CO₂-flood maturity. Second, a four-panel dashboard is displayed as a comparative tool across all reservoirs in the region in order to highlight best and worst performing reservoirs and establish representative ranges for reservoir performance. Both dashboards will be useful when comparing performance against averages published in literature.

This work's novelty is attributed to presenting an ideal set of performance metrics by streamlining the numerous options available. A unique dashboard-style presentation is suggested as a means of rapidly assessing oil recovery and CO₂-storage performance, and collectively yielding useful insights.