Integrated Assessment of Interaction Between Supercritical CO2 and Shale During Thermal Maturation

Abstract

ABSTRACT: Recent studies have shown that shale reservoirs can be an attractive candidate as a CO2 storage reservoir or a caprock for geologic CO2 sequestration. Shales are source rocks, and thus, there is a continuous diagenetic process that can alter its properties during thermal maturity before it reaches maturity. However, there is a significant knowledge gap in the geochemical and other alterations in shales due to the interaction with supercritical CO2 (ScCO2) stored in the underlying reservoir unit during the diagenetic process. This study investigates the changes in mineralogical properties and Total Organic Carbon (TOC) of shales during thermal maturation when it is exposed to ScCO2 at in-situ conditions and assesses its implications for CO2 utilization and sequestration. Here, we used Bakken shale and Green River shale samples, and exposed the samples to ScCO2 for a specific period. This was followed by inducing the samples to thermal maturity for the pyrolysis process. Subsequently, we evaluated the mineralogical properties and TOC of the pre- and post-CO2 treatment of mature shale samples. Our results from this study provide novel insights for CO2 utilization and sequestration in the mature Bakken and Green River shale formations. 1. INTRODUCTION Carbon dioxide is injected into the subsurface in a supercritical phase. Supercritical CO2 (ScCO2) state is found above the critical point of temperature and pressure of 31.8 C and 7.38 MPa, respectively where CO2 behaves like a gas but with a density like that of a liquid (Span and Wagner, 1996). There are three recognized geological formations for CO2 storage: saline aquifers, depleted oil and gas fields, and coalbeds. Basically, injection of CO2 has two economic benefits (Baines and Worden, 2004): first, a financial incentive with the carbon tax credits by solely capturing and storing CO2 (e.g., saline aquifers) to reduce CO2 emissions in the atmosphere. Secondly, utilization of CO2 injection in enhanced oil recovery (EOR) or coalbed methane production. Once the CO2 percolates the reservoir, it migrates upward due to buoyancy reaching the top of the formation where it is contained by a cap-rock layer. Trapping mechanisms play a key role in preventing leakage risk (Fatah et al., 2020) and are not only structural and stratigraphic, but also related to the trio CO2-fluid-rock interactions. In residual or capillary trapping, CO2 droplets are immobile and trapped in pore space (Kang, 2011). Solubility trapping happens when CO2 dissolves in brine water or is trapped by mineral precipitation.