Long-Term Effect of Fault-Controlled CO2 Alteration on the Weakening and Strengthening of Reservoir and Seal Lithologies at Crystal Geyser, Green River, Utah

Abstract:

An understanding of the coupled chemical and mechanical properties and behavior of reservoir and seal rocks is critical for assessing both the short and long term security of sequestered CO₂. A combined structural diagenesis approach using observations from natural analogs has great advantages for understanding these properties over longer time scales than is possible using laboratory or numerical experiments. Current numerical models evaluating failure of reservoirs and seals during and after CO₂ injection in the subsurface are just beginning to account for such coupled processes. Well-characterized field studies of natural analogs such as Crystal Geyser, Utah, are essential for providing realistic input parameters, calibration, and testing of numerical models across a range of spatial and temporal scales.

Fracture mechanics testing was performed on a suite of naturally altered and unaltered reservoir and seal rocks exposed at the Crystal Geyser field site. These samples represent end-products of CO₂-related alteration over geologic (>10³ yr) time scales. Both the double torsion and short rod test methods yield comparable results on the same samples. Tests demonstrate that CO₂-related alteration has weakened one reservoir sandstone lithology by approximately 50%, but the subcritical index is not significantly affected. An altered siltstone sample also shows a reduction in fracture toughness values and lowered subcritical index in comparison to unaltered siltstone. In contrast, elevated calcite content in shales due to CO₂ alteration has increased fracture toughness. Similarly, fracture toughness was increased in what is otherwise a weak, poorly cemented sandstone unit due to increased calcite cement. Combined, these results demonstrate that CO₂-related alteration generally weakens rock to fracturing (i.e. lowers fracture toughness), except in cases where calcite cementation is significantly increased. The natural system at Crystal Geyser demonstrates that water-CO₂-rock interaction driven by changes in the geochemical environment have measurably altered rock geomechanical properties and that some rock units may become more prone to failure, ultimately leading to fracturing and leakage of subsurface reservoirs. These results also have application for CO₂-based enhanced oil recovery.