An Integrated Approach to Identifying Residual Oil Zones in the Cypress Sandstone in the Illinois Basin for Nonconventional CO2-EOR and Storage
The central portion of the Illinois Basin has a fairway of incised valley fill Cypress Sandstone deposits that exceeds 150 feet in thickness. Where local structural features are present, the Cypress may contain a relatively thin oil reservoir above a significant aquifer. Because of the thickness of the aquifer, nonconventional carbon dioxide (CO2) enhanced oil recovery (EOR) is expected to require more CO2 compared to a conventional CO2 EOR for each barrel of incremental oil produced due to CO2 migrating into the underlying aquifer. For this study, the entire incised valley part of the Cypress is being investigated for the presence of residual oil zones (ROZs), regardless of the presence of an overlying oil reservoir.
In a case study of Noble Field, western Richland County, Illinois, identification of ROZs in the Cypress Sandstone has required an integrated, multidisciplinary approach of detailed geologic characterization, petrography, and petrophysics. Geologic characterization suggested that the Cypress Sandstone occurs as multistory fluvial sandstones that become estuarine upward and were deposited in an accommodation limited setting. A tilted producing oil water contact (OWC), an indicator of an ROZ, was identified and mapped. A zone of dense diagenetic calcite cement occurs along the OWC, with a second similar zone 20 ft below, is an indication of a relict OWC.
Water saturation determined with Archie's Equation was found to be anomalously high for several wells, suggesting the presence excessive conductivity, e.g. from water-saturated clay micropores. Clay mineral morphologies were identified, volume of clay was determined, and the percent-volume of clay micropores were quantified to correct water saturation estimates. The depths of the oil saturated, brine saturated, and ROZ within several wells was used to identify areas of the field that had the greatest potential to have an ROZ.
Integrating water-oil saturation into a detailed, representative geocellular model of the reservoir architecture will reduce CO2-EOR and storage uncertainty in the results of reservoir simulations. A summary of how each discipline was used to better understand the ROZ and preliminary findings of this integrated study will be presented.
AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016