Wellbore Evaluation of the Basal Cambrian System

The Plains CO2 Reduction (PCOR) Partnership performed a wellbore integrity assessment to evaluate the relative leakage potential of 826 wells penetrating the basal Cambrian system in the United States, drilled between 1921 and 2010. The basal Cambrian system is a deep saline reservoir that has been identified by the U.S. Department of Energy as a potential carbon dioxide (CO2) storage site. The ability of the basal Cambrian system to retain injected CO2 over an extended period of time is, in part, dependent on the integrity of wellbores that penetrate the target reservoir. Wellbore integrity is the ability of a well to maintain hydraulic isolation of geologic formations and prevent the vertical migration of fluids (Zhang and Bachu, 2011; Crow and others, 2010). This study’s evaluation of wellbore integrity involves analyzing wellbore characteristics (i.e., cement types, cement additives, completion techniques, well depths, and well casing) to derive a relative leakage potential score using methods similar to Bachu and others (2012). Wells were assigned a classification of minimal, lower, moderate, or higher based on their relative leakage potential. This study provides a screening-level evaluation to compare and rank wells for further detailed evaluation. Site-specific risk analysis within these target areas would trigger a more detailed assessment of those wells identified for further investigation. Potentially leaking or high-risk wells could be addressed using established remediation programs employing current well mitigation technologies or appropriate monitoring during CO2 injection. The results of this regional screening-level evaluation determined that 15% of the wells assessed were classified as moderate or higher potential for deep well leakage, and 6.0% of the wells were classified moderate or higher for shallow well leakage. Of the wells assessed, 3.4% exhibited moderate or higher potential for both shallow and deep leakage. The majority of the moderate- or higher-potential wells are located in western North Dakota and eastern Montana in areas of intensive oil and gas exploration and production. The practice of producing oil and gas from these wells has increased the relative well leakage potential (based on the available data and methods utilized). The ranking of the relative leakage potential provides a mechanism to screen wells for detailed evaluation in areas targeted for CO2 injection.