The Influence of Organics on Supercritical CO2 Migration in Organic-Rich Shales

• DOI: 10.15530/urtec-2018-2902743
• Publication Date: 2018-08-14T18:55:01Z
• Authors: Bethany A. Kurz; James A. Sorensen; Steven B. Hawthorne; Steven Smith; Hamed Sanei; Omid Ardakani; Joel D. Walls; Lu Jin; Shane Butler; Christopher Beddoe; Blaise Mibeck
• Keywords: 13. Climate action; 02 engineering and technology; 0204 chemical engineering; 6. Clean water

The Bakken petroleum system is a world-class oil play with oil-in-place estimates in the hundreds of billions of barrels. Despite the resource potential, oil recovery factors are low, typically less than 10%. Efforts to evaluate mechanisms to increase oil recovery have focused on the use of supercritical carbon dioxide (CO2) for enhanced oil recovery (EOR) with the added benefit of associated geologic storage of CO2. This work describes a series of laboratory-based tests to evaluate the efficacy of CO2 for EOR and to better understand the controlling mechanisms of CO2 permeation in Bakken rock samples. Static extraction tests were performed on rock samples representative of the Bakken reservoir (siltstone) and source rocks (Upper and Lower Bakken shales). A dynamic CO2 injectivity test was also performed on an unfractured Bakken shale sample that was confined at reservoir pressure. In conjunction with the extraction tests, a suite of standard and advanced characterization techniques were used to better understand the rock fabric, pore and fracture networks, and the organic matter (OM) content of the samples, including field emission scanning electron microscopy (FESEM) imagery, focused ion beam SEM (FIBSEM), and extended slow heating (ESH) Rock-Eval analysis. The extraction test results demonstrated recovery of residual oil in the tight siltstone samples comprising the reservoir (recovery values approaching 100%) and also in the shale source rocks, in which oil recovery from 11-mm-diameter rods ranged from 12% to 65% after 24 hours. Analysis of samples pre- and post-CO2 extraction using the Rock-Eval ESH method confirmed the results of the CO2 extraction tests. FESEM imagery of the Bakken shale samples revealed the presence of fractures located within the OM, many of which appear to be connected and could provide a mechanism for CO2 transport into the OM and subsequent extraction of crude oil. Image-based analysis of the FESEM and FIBSEM imagery suggested that the majority of the porosity in the shales was OM-hosted and that much of the pore networks were connected. The results of this work revealed that CO2 is able to permeate samples of unconventional reservoir and shale source rocks much more readily than expected, resulting in significant recovery of residual oil. The mechanism of permeation within the organic-rich shales appears to be within OM-hosted fracture networks and possibly through nano-scale porosity associated within the samples’ solid bitumen. The implications of this work suggest that organic-rich source rocks may be a viable target for CO2-based EOR, with the added benefit of long-term CO2 storage via adsorption and absorption.