| الوصف: |
ABSTRACT: We used core samples from the Wolfcamp formation in the Permian Basin to conduct creep experiments with argon and scCO2 as the pore fluids. We conducted the tests at a temperature of ~ 42 °C, confining pressures of 40 and 70 MPa, pore pressures of 10 MPa, and various levels of differential stress. Creep stages lasted for either 24 or 48 hours and prior to creep the Young’s modulus was estimated based on the loading history. When CO2 results are compared to the argon results, the Young’s modulus of the carbonate-rich sample increases, while the Young’s modulus of the clay-rich sample decreases. The viscoplastic response with scCO2 as the pore fluid generally indicates more significant deformation. This is attributed to the aggregate effects of carbonate dissolution and swelling strain of clays and kerogen. A power-law model has been implemented to predict the longer-term creep response of the samples. The fitting parameters are satisfactory; however, additional investigation is needed for the scCO2 case, due to time-dependent chemical interactions with the sample. 1. INTRODUCTION Geological storage of carbon dioxide (GCS) is one of the relatively short-term feasible solutions to climate change (Bouckaert et al., 2021). While the storage of CO2 is likely to occur in a porous sandstone reservoir, the target formation needs to be sealed by a low permeability caprock such as shale. Employing CO2 as the stimulation fluid of unconventional reservoirs is another method of sequestering carbon dioxide into the subsurface. In both applications, the shale formation is exposed to CO2 over prolonged periods of time. Therefore, it is necessary to investigate the alterations in the mechanical/transport characteristics of shale reservoirs when they are exposed/interacted to/with CO2. Shales typically possess ultra-low permeabilities (usually below 1 μD) with very complex micro-structure, and is mainly composed of quartz, carbonates, clays, and organic matter (Zoback and Kohli, 2019). Interaction between shales and carbon dioxide in the presence of in-situ water results in (i) adsorption-induced swelling of clays and organic matter, (ii) dissolution of carbonate minerals due to interaction with carbonic acid, and (iii) mineral/salt precipitation (Linder et al., 2016; Kamali-Asl et al., 2021). Some past research has been dedicated to the changes in the micro-structure of shales as they are exposed to dry CO2 or CO2-saturated brine (Sanguinito et al., 2018; Hadian and Rezaee, 2020), while others have focused on changes of porosity/permeability (Wu et al., 2017; van Noort and Yarushina, 2019). |
