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Regional Characterization of an Oil-Bearing Reef Complex for Factors Affecting Assessment of Associated CO2 Storage

Abstract

There are over 800 identified Silurian-aged (Niagaran) Pinnacle Reefs in the northern Trend (NPRT) in the Michigan Basin. The complex internal architecture, production history, lithology, and diagenetic changes in these reefs strongly affect the storage capacity, pressure response, and ultimately the reservoir performance of each individual field. Therefore, these fields provide an excellent opportunity to evaluate the geologic variability in complex carbonate reservoirs and its impact on CO2 storage configurations. The reefs have historically been studied on an individual scale which does not represent the potential of the entire trend. Under this project, workflows were established to characterize reefs from an individual scale to a regional scale, capturing the entire NPRT. The reefs are being analyzed in three parts: 1) geologic characterization using wireline logs, whole core, and production records, 2) assessment of EOR/CCUS feasibility and comparison between the reefs by constructing representative static earth models, dynamic models, and geomechanics models, and 3) estimates of CO2 resources across the NPRT using three methods (fluid substitution, material balance, and dynamic modeling).

Results show the geologic variability in the reefs impacts the storage feasibility across the trend. Significant factors include lithology, diagenesis, and reef connectivity. A comprehensive dataset collected on this project successfully identified trends in the NPRT and characterized variability while also providing a resource and guide for future CCUS activities. New methodologies and techniques were developed which were used consistently and efficiently to analyze hundreds of fields. Some of the highlights are listed below:

1) Image analysis techniques were developed to statistically derive porosity indicators and to quantify diagenetic features in whole core using CT scan data

2) Petrophysical techniques were developed for complex carbonates and identified reefs better suited for CCUS

3) Reservoir characteristics changed across reef types and lithofacies, however the diagenetic overprint primarily controlled the reservoir

4) Reefs were ranked by CCUS feasibility using the results from dynamic modeling and storage resource calculations

5) Assessment of caprocks and geomechanical models showed the effectiveness of CCUS in reefs

6) An interactive database was established to quickly provide assessments of individual reefs as well as summaries across the entire NPRT