EFFECT OF FLUIDS CHEMISTRY AND DIAGENESIS ON FRACTURE PROCESSES IN A FAULT ZONE

The creation of opening-mode fractures is fundamental to the formation of faults and mechanical failure in the upper crust. One example of fault-controlled CO₂-brine-rock interaction over geologic time scales is the Little Grand Wash Fault/Crystal Geyser field site near Green River, Utah. There, the main impact of fault-fluid interactions is highly elevated calcite content across all lithologies in proximity to the fault due to degassing of CO₂-charged brine. This has measurably and significantly impacted the flow and mechanical properties of these rocks, including resistance to fracturing, which may affect further deformation and fault evolution.

Fracture mechanics testing of sandstones with enhanced CO₂-related calcite cementation show more than 2x greater fracture toughness and subcritical index, whereas bleaching by reducing CO₂-rich fluids in a reddish hematite-cemented sandstone unit has lowered fracture toughness by around 40%. Fracture toughness and subcritical index of other sandstone, siltstone, and shale lithologies from the field site are also similarly impacted by CO₂-brine-rock interactions. Additionally, fracture mechanics data obtained using the double torsion technique on reservoir and seal rocks under controlled environments demonstrates that fracture toughness and subcritical index are sensitive to pH, salinity, temperature, and the presence or absence of water, and that these sensitivities vary depending on rock composition. For example, low pH fluids measurably weaken rocks rich in carbonate cements and/or grains to fracturing. In contrast, fracture mechanics parameters of rocks rich in clays tend to show a greater sensitivity to fluid salinity. Also, the aforementioned CO₂-bleached sandstone shows greater sensitivity to chemical environment than its unaltered equivalent. Combined, this evidence indicates that fluid-rock interactions, including both the immediate chemical environment and its long-term consequences, may impact fracturing and failure behavior in and around faults.

Fracturing and faulting in the crust may be subject to multiple coupled processes and fluid-involved feedbacks, and this has implications for fault development, subsurface injection of fluids (e.g. CO₂), nuclear waste storage, and geothermal energy development.