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.
