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TitleMechanisms of CO2-EOR in Bakken shale
AuthorBizhani, MORCID logo; Ardakani, O HORCID logo; Hawthorne, SORCID logo; Kurz, B; Cesar, JORCID logo; Percival, J BORCID logo
Source2022 Core Conference; 2022 p. 1-2
Alt SeriesNatural Resources Canada, Contribution Series 20220196
PublisherCanadian Society of Petroleum Geologists
MeetingCanadian Society of Petroleum Geologists 2022 Core Conference; Calgary, AB; CA; June 23-24, 2022
File formatpdf
Subjectsfossil fuels; Science and Technology; Nature and Environment; hydrocarbons; hydrocarbon recovery; shales; fracturing
ProgramEnergy Geoscience Clean Energy Resources - Decreasing Environmental Risk
Released2022 06 23
AbstractThe need for new hydrocarbon resources has led to the unconventional boom in the early 2000s. Horizontal drilling and multi-stage hydraulic fracturing enabled production from shales. On the other hand, the recovery factor remains low in these deposits, typically 5-10%. Low recovery factors have prompted researchers and the industry to explore the possibility of Enhanced Oil Recovery (EOR) methods in shales. Simultaneously, the need to manage greenhouse gas emissions and decarbonization of the energy industry are driving the development of various carbon management strategies, including Carbon Capture, Utilization, and Storage (CCUS). CCUS refers to a number of technologies that aim to capture CO2 from processes (e.g. fossil-fueled power plants), pressurize it, and eventually either naturalize it through added-value products or store it underground. Application of CO2-EOR has been identified as one example of CO2 utilization. Combining CO2-EOR with CCUS is referred to as CO2-EOR+. In this work, we study the extraction of hydrocarbons from organic-rich Bakken shales using supercritical CO2. A full range of characterization tests is performed to analyze the response of the samples to CO2 exposure. The goal of the study is to gain insight into the mechanism by which CO2 extracts hydrocarbon from the samples and develop an understanding of the controlling factors for CO2-EOR in shales. The CO2-hydrocarbon extraction experiments were performed on crushed 1-4mm sized shale samples at 5000psi (34.4 MPa) and 110°C for 24 hrs. The samples were immersed in a CO2 chamber, resembling the huff-n-puff recovery scheme in a fractured medium. A total number of 7 cycles of injection and production are performed. We further characterize the samples through Mercury Intrusion Capillary Pressure (MICP), X-ray powder diffraction (XRD), programmed pyrolysis, and Gas Chromatography-Mass Spectrometry (GC-MS) of original hydrocarbon in the samples. Programmed pyrolysis is performed on samples before and after CO2 extraction to gain insight into the changes to organic contents due to CO2 exposure. The results show soaking is an effective strategy for CO2-EOR. Most tight shale samples show a recovery of over 50% after 24hrs. Owing to higher solubility in CO2 and permeation rate in the tight rock pores, lighter hydrocarbons are recovered faster than heavier components. Samples from the Lower Bakken (LB) formation show a higher recovery factor after 24 hours of exposure to CO2 than Upper Bakken samples (UB). However, we only test two sets of samples for each formation, and the results may not be statistically meaningful. Programmed pyrolysis data place the samples in the immature oil zone, with type II kerogen. All 4 shale samples have high TOC values (>12%). Pore throat size distribution measured via Mercury Intrusion Capillary Pressure (MICP) shows both sets of samples are similar, with a median pore throat size of 7.5nm. XRD analyses indicate shale samples are rich in illite with no detectable mixed-layer or swelling clays. Overall, except for TOC, the shale samples are broadly similar. However, the CO2-EOR performance is different in the LB and UB samples. In terms of recovery mechanism, both lower Bakken shale samples tested in this study conform to a simple diffusion-driven recovery profile. An error function type profile, with a tuning parameter that contains the length scales and diffusion coefficients, perfectly fits the recovery profiles of the lower Bakken samples. On the other hand, upper Bakken samples do not fit a diffusion dominant profile. The reduced recovery factor in upper Bakken samples may have its root in the deviation of recovery profile from a diffusive regime. Higher TOC content of the upper Bakken samples, coupled with low maturity of the samples are believed to be the main contributing factors to this observation.
Summary(Plain Language Summary, not published)
Shales are abundant and comprise a huge portion of proven oil and gas reserves. While hydraulic fracturing and horizontal drilling have unlocked these resources 2 decades ago, the recovery rates are much lower than in conventional reservoirs. Concurrently, the movement toward decarbonization of the energy industry is pushing the energy industry for innovative approaches to reduce its carbon footprint. One of these solutions is to capture and inject anthropogenic CO2 into hydrocodone-bearing shale formations for improving recovery as well as sequestering the CO2 in the process. It is estimated that a barrel of oil produced in this scheme can have as much as 63% less carbon footprint. Additionally, the additional recovered hydrocarbon can be used for hydrogen production. Technical challenges, however, have hindered the large-scale deployment of such an approach to date. One of these problems is the lack of knowledge about mechanisms by which CO2 penetrates the ultra-tight matrix of shales. In this study, we look at this using an experimental approach, targeting the Bakken Formation.

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