Role of mineralogy in controlling fracture formation

Abstract:

In subsurface CO₂ sequestration systems, impermeable caprock layers are required to prevent leakage of the injected CO₂. However, leakage may occur through caprock fractures, enhanced or reduced by dissolution and precipitation reactions. These mineral reactions and reaction rates are controlled by interactions of the injected fluid with the exposed mineralogy, those present along the fracture walls. The goal of this work is to examine the relationship between mineralogy and fracture formation to better estimate what mineral phases will be present at the fracture surface and ultimately improve understanding and predictive capabilities of the evolution of fracture aperture and permeability. For this experiment, shale cores from the Mancos and Marcellus formations are subjected to unconfined compression until an initial fracture formed. The unaltered fracture surfaces are imaged using Scanning Electron Microscopy (SEM) in Backscattered Electron (BSE) and Energy Dispersive Spectroscopy (EDS) modes to determine their mineral compositions. This data is compared to averaged X-Ray Powder Diffraction (XRD) data to determine if the overall formation mineralogy is representative of the fracture surface mineralogy. The expected composition of the Marcellus formation from XRD data is 47.9% quartz and 27.1% calcite. However, the fracture surface contains about 97% calcite, where the fracture predominately occurred at calcite-calcite interfaces. This data suggests that predictions made using XRD data would greatly underestimate reaction rates within fractures in this formation. The Mancos formation is expected to contain 58.6% quartz, 2.3% kaolinite, and 4.3% calcite. In addition, the Mancos formation has two visually distinct strata: a darker, clay rich layer (71.0 to 79.7% kaolinite, 8.5 to 9.6% quartz, 3.6 to 6.8% calcite) and a lighter, quartz-calcite-clay layer (26.7 to 48.5% kaolinite, 25.1 to 40.0% quartz, 21.9 to 22.3% calcite). Optical imaging of the fracture surface shows that the dark and light layers make up an average of 73.8% and 26.2%, respectively, where the fracture occurred predominately at dark-dark interfaces. These results of the Mancos formation suggest that the fracture surface would be less reactive than predicted by XRD data alone.