Investigating fracture propagation characteristics in shale using sc-CO2 and water with the aid of X-ray Computed Tomography

Abstract

The further development of unconventional resources is anticipated to increase the intensity of hydraulic fracturing operations. Injecting water-based fracturing fluids into shale formations, especially shales producing gas, may have adverse impacts of water blocking, scale formation, and water-sensitive clay minerals. Accordingly, nonaqueous candidates such as carbon dioxide (CO₂) need to be explored to avoid injection of millions of gallons of water and to increase the effectiveness of stimulation jobs. This study investigates fracturing behavior accompanying supercritical carbon dioxide (sc-CO₂) injection compared to water. A High Pressure High Temperature (HPHT) triaxial cell was utilized to conduct shale breakdown experiments under reservoir-like conditions. Furthermore, the experimental setup allows continuous monitoring of in situ details using X-ray Computed Tomography (CT). Here, CT images were utilized for the first time to investigate and confirm the breakdown pressure using Fast Iterative Digital Volume Correlation (FIDVC) that permits visualization of in situ deformation and strain. One inch diameter Green River shale samples were fractured under triaxial stress conditions. Results demonstrated a roughly 2 to 3 times greater breakdown pressure for sc-CO₂ compared to water for the samples studied. Compressive infiltration stress might explain such behavior, and mineralogy is believed to be a major contributor to such differences between injectants. Under isotropic horizontal stresses, sc-CO₂ induces fractures propagating almost independent of bedding planes. This is a promising finding for probable larger stimulated volume achieved by sc-CO₂. CT imaging to monitor fracture propagation demonstrated excellent performance in detecting fracture propagation. We anticipate this technique to help significantly when monitoring fracture propagation mechanisms for laboratory samples with slow pressurization as well as studies of slip on pre-existing fractures.