Short-term interactions between Longmaxi shale and carbon dioxide-based fracturing fluids.

Area of Science:

• Geology and Petroleum Engineering
• Materials Science
• Environmental Engineering

Background:

• Shale gas extraction relies on fracturing rock formations for efficient gas release.
• Supercritical carbon dioxide (CO2) fracturing is an emerging, eco-friendly alternative to water-based methods.
• Understanding CO2-shale interactions is crucial for optimizing reservoir stimulation and CO2 sequestration.

Purpose of the Study:

• To investigate the effects of CO2-based fracturing fluids on shale microstructure and mechanical properties.
• To evaluate the influence of different CO2 fluid compositions (water, gaseous/supercritical CO2, brine mixtures) on shale behavior.
• To provide insights for improving shale gas extraction and CO2 sequestration strategies.

Main Methods:

• Simulation tests involving injection of various CO2-based fluids into shale reservoirs.
• Mechanical experiments including uniaxial compression and tensile tests with acoustic emission monitoring.
• Physical characterization using scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray fluorescence (XRF).

Main Results:

• All CO2-based fluid treatments reduced shale's uniaxial compressive/tensile strength, elastic modulus, and brittleness index.
• The addition of brine to CO2 fluids resulted in more significant decreases in mechanical parameters and brittleness.
• Water content was identified as a key factor influencing shale's elastic modulus; supercritical CO2 had a greater impact on mineral composition than gaseous CO2.

Conclusions:

• CO2-based fracturing fluids alter shale's mechanical properties and microstructure, with water and brine exacerbating these changes.
• Increased shale ductility and altered acoustic emission patterns were observed due to CO2 and brine coupling effects.
• Findings offer critical data for optimizing injection parameters and permeability transformation in shale reservoirs like Longmaxi.