Impact of Geologic Diversity on Static Earth Models in a CO2-EOR Reef Complex

The Northern Pinnacle Reef Trend (NPRT) is composed of over 700 Silurian-age (Niagaran) reefs. Many reefs have been used for CO₂-EOR and provide an excellent opportunity to evaluate the geologic variability in complex carbonate reservoirs and its impact on CO₂ storage configurations. The reefs have complex internal architecture, lithology, and diagenetic changes which strongly affect the storage capacity and pressure response, and ultimately the reservoir performance of each individual field. A detailed understanding of the geologic controls is a foundational requirement for effective site-selection, permitting, development, operation, monitoring, and verification of carbon dioxide (CO₂) storage in saline or oil-bearing reservoirs. Static Earth Models (SEM) integrate all available geologic and geophysical information into a single framework that can be used to conceptualize CO­₂ injection, migration, and retention in the subsurface. SEMs also provide the basis for incorporating geologic information into dynamic models for the reservoirs.

A series of SEMs were developed to determine an efficient methodology that would capture the heterogeneity of the geology and predict reservoir characteristics by correlating changes in whole core with wireline log responses. Five reefs were selected that represented the diversity of the geology including lithology, salt plugging, multiple connected reefs, and single compartmentalized reefs. These reefs also represent different stages of EOR development; one reef has undergone CO₂ injection in the past, three are currently being injected with CO₂ as part of an active EOR program, and one reef has not had any CO₂ injection. Additionally, new wells were drilled in two reefs and wireline log, whole core, and well test data was collected. New data was used to update the SEMs and validate geologic interpretations.

The resulting SEMs demonstrate the efficacy of a new, integrated approach to reservoir evaluation using a simplified workflow, as well as illustrating the diversity of the geology and diagenesis of the Niagaran–Lower Salina reef complexes, and how this complexity influences reservoir potential and behaviour. This study also illustrates the high quality of models that can be built with limited to robust datasets. Finally, the geologic modelling approaches developed for the CO₂-EOR fields will be generalized to implement an assessment of CO₂ storage strategies and potential across the regional reef complex.

The study is part of the Midwestern Regional Carbon Sequestration Partnership (MRCSP) Michigan Basin Large-Scale Injection Project, which is part of a larger national carbon storage research program headed by the United States Department of Energy. Since injection operations began in February 2013, MRCSP has successfully injected and monitored the net storage of more than 700,000 metric tons of CO₂ into Niagaran reefs. The goal of the project is to store one million metric tons of CO₂ into depleted oil and gas fields during a span of roughly six years. MRCSP (www.MRCSP.org) is funded under DOE/NETL Cooperative Agreement # DE-FC26-0NT42589 with co-funding by Core Energy, LLC, and several other partners.