Permeability Evolution of Shear Failing Chalk Cores under Thermochemical Influence

Area of Science:

• Petroleum Geoscience
• Rock Mechanics
• Geochemistry

Background:

• Petroleum reservoir development causes stress changes, deforming rock and altering petrophysical properties.
• Chalk reservoirs are sensitive to fluid composition and temperature, affecting mineralogy and texture.
• Understanding coupled mechanical, chemical, and thermal effects is crucial for predicting permeability evolution.

Purpose of the Study:

• To investigate the impact of stress, temperature, and fluid chemistry on chalk core mechanical behavior and permeability.
• To simulate in situ conditions relevant to North Sea chalk reservoirs.
• To determine how different brine compositions influence permeability changes during reservoir operations.

Main Methods:

• Mechanical testing of outcrop chalk cores (Niobrara Formation) in triaxial cells.
• Applying systematic combinations of deviatoric stress, temperature (50 and 130 °C), and injecting fluids (NaCl, Na2SO4, synthetic seawater).
• Monitoring core deformation, fracture development, and permeability changes.

Main Results:

• Shear failure under deviatoric loading increased permeability, irrespective of brine type.
• Subsequent loading cycles had a lesser impact on permeability.
• Sodium sulfate brines largely preserved permeability gains post-fracturing compared to NaCl and synthetic seawater.
• Synthetic seawater caused the most significant permeability loss at 130 °C.

Conclusions:

• Sulfate adsorption generates repulsive forces that help maintain fracture permeability in chalk.
• Fluid chemistry significantly influences permeability evolution in stressed chalk reservoirs.
• Results provide insights for improved reservoir management and enhanced oil recovery strategies.