POROSITY AND RESERVOIR HETEROGENEITY OF A CO2 SEQUESTRATION RESERVOIR: EXAMPLE FROM THE DONOVAN SAND, CITRONELLE FIELD

The Citronelle Field, located in the Mobile County area of Alabama, has been a longstanding (since 1955) oil and gas producing basin (537MMbbl oil in place, 169MMbbl oil produced) and more recently, a carbon sequestration target. Located overtop of a salt-cored anticline, main production in the Citronelle Field is from the Donovan Sand of the Cretaceous Rodessa Formation. Overall, the Donovan Sand is characterized by discontinuous fine- to medium-grained fluvial sandstone, with inclusions of pebble-sized mud rip-up clasts as well as some feldspathic grains, interbedded with mottled to fissile mudstone. The Donovan Sand is currently being injected with supercritical-CO₂ in hopes of enhancing oil recovery as well as serving as a pilot for long-term geologic carbon sequestration. Estimated enhancement of reserves is approximately 20%, and there are about 20 wells (on about 1 mile spacing) that will be considered for this study within a 20 square mile area. Porosities and sedimentary lithofacies within the Donovan Sand are highly variable. The goal of this work is to measure porosity from thin-sectioned samples taken from pre-injection cores of Donovan Sand reservoir rock to document heterogeneity of the reservoir matrix and interpret fluid flow through the reservoir. Approximately 30 thin sections from the injection well (Permit #3232) core will be used as reference for other wells in the area. Thin sections taken from this well have been compared to depth-equivalent well log curves in an attempt to discern the reason behind porosity differences (as much as 4% between some intervals) in the vertical direction. Log characters for these different porosity intervals are almost identical, suggesting that no relationship can be shown between these varying porosities and the log character.