Effect of Supercritical CO2 on Permeability and Surface Characteristics of Fractures in Shales

Abstract

ABSTRACT: The impacts of supercritical carbon dioxide (scCO2) on fracture permeability and fracture surface characteristics of shale samples with various mineral compositions were studied in this paper. We measured permeability and fracture normal displacement (FND) at different confining pressures and pore pressures using both argon gas and scCO2. Our results indicate that mechanical compaction and fine particles migration have opposite impacts on permeability and FND, especially in clay-rich shales. It appears that fine particle migration creates a self-propping aperture that results a permeability increase in the subsequent cycle. However, inelastic compaction associated with loading/unloading cycles reduce the permeability. In addition, an increase in permeability (and associated fracture surface degradation) were observed after more than 3 days of exposure to scCO2 due to carbonate dissolution. The rates of permeability increase in saw cut fractures are larger than FND suggesting that new flow pathways were created because of carbonate dissolution and that controls the transport characteristics of the scCO2-interacted fracture surface. 1. INTRODUCTION Geological carbon capture and storage (CCS) is one of the techniques to reduce green gas emissions and storage of CO2 in unconventional reservoirs has been the focus of many studies in the last few years (Goodman et al. 2020; Kim et al. 2017; Kolawole et al. 2020). Methane is found as a free phase in pores and fractures and also as adsorbed gas on clay or organic matter in shale gas plays. After the reservoir is depleted, CO2 could be sequestrated by similar mechanisms as methane in two populations. Supercritical CO2 (scCO2) has been also proposed as an alternative fracturing fluid for hydraulic fracturing operations in unconventional reservoirs (Ishida et al. 2012; Jia et al. 2019; Zhang et al. 2017). The phase diagram for carbon dioxide shows that CO2 behaves as a supercritical fluid that adopts properties somewhere between a gas and a liquid above the critical temperature and critical pressure of 31.1 °C and 7.37 MPa, respectively. Compared to aqueous-based fluids, scCO2 can develop more complicated fracture networks, improve shale gas recovery by preferential adsorption behaviors of CO2 over methane, reduce flow blockages and reduce water consumption (Zhou et al. 2020). For instance, a study on Yan-2011 shale gas by Li and Kang, (2018) showed that after scCO2 fracturing, the CO2-retention rate was 39.5% and the shale gas production rate was increased 1.5 times. Nevertheless, scCO2 has a lower viscosity than water in the reservoir temperature and pressure and that reduces its efficiency of carrying proppants into the fracture networks.