Hydrocarbon Recovery from Williston Basin Shale and Mudrock Cores with Supercritical CO2: Part 1. Method Validation and Recoveries from Cores Collected across the Basin

Rock core samples (51) from multiple lithofacies and depths were collected from 10 wells located throughout the Bakken Petroleum System. Each 11.2 mm diameter core was exposed to CO2 for 24 h at reservoir conditions of 34.5 MPa (5000 psi) and 110 °C in a pressurized apparatus designed to mimic the fracture-dominated flow expected to occur during a CO2 injection into hydraulically fractured tight unconventional formations. The oil recovered from the rock samples was collected hourly by slowly depressurizing the CO2 into a collection solvent, while maintaining both CO2 pressure and temperature constant in the extraction chamber. Recoveries of light and heavy oils were validated by comparing rock samples before and after CO2 exposure using the extended slow heating Rock-Eval analysis. Extractions of replicate core samples from Middle Bakken (MB) tight nonshale, Upper Bakken shale (UBS), and Lower Bakken shale (LBS) gave reproducible results, demonstrating that the 11.2 mm diameter cores represent the original 10.2 cm (4 in.) core, and that the extraction and associated analysis procedures are reproducible. Recoveries of oil from the Three Forks (TF) and all MB cores ranged from 65 to >99% after 7 h of exposure and exceeded 94% for all cores at 24 h, despite median pore throat radii of only about 13 nm (MB) to 26 nm (TF). Surprisingly, significant oil was obtained from UBS and LBS cores despite the median pore throat radii of only ca. 3.5 nm, sizes that approach molecular dimensions. Although all TF and MB reservoir rocks showed high oil recoveries, the oil obtained in 24 h from UBS and LBS source rocks varied greatly for different well locations and ranged from as low as 11% to as high as 80%. Data analysis of mineralogical components, including clays, carbonates, evaporates, feldspars, and pyrite, showed that these factors were not useful to predict oil recoveries. Both total organic carbon (4–15 wt % for shales and 0.1–0.4 wt % for TF and MB) and the pore throat radii appear to control oil recovery, though they were not predictive for individual UBS and LBS cores. Results from the 51 rock core samples demonstrate that CO2 is capable of penetrating oil-bearing pores and displacing crude oil from the UBS and LBS source rocks as well as the MB and TF reservoir rocks.