Fracture behavior across interfaces

Faults and fracture networks at depth are important fluid pathways, especially in fine-grained, low permeability seal lithologies. Discontinues in sealing lithologies can create seal bypass systems, leading to the failure of CO₂ geosequestration sites or hydrocarbon traps. We characterize the occurrence of and changes in discontinuity patterns and the associated changes in elastic moduli across sedimentologic interfaces to document the importance of these discontinuities for fluid management in the subsurface and potential for re-activation in high-pressure injection scenarios. We evaluate well-exposed, fine-grained, low-permeability Mesozoic and Paleozoic units that are seals of potential CO₂ repositories on the Colorado Plateau and show evidence for open fractures and fluid flow in the subsurface. Field observations document changes in fracture distributions across lithologic boundaries allowing us to identify mechano-stratigraphic units and focus on the effect of lithologic interfaces on fracture distribution. An interface marks the boundary between facies in a seal and in this study the fractures are shown to deflect or arrest at the interface. In outcrop fracture intensity varies in from 1 to 18 fractures per meter and fracture apertures range from mm to cm. The mineralized fractures often have associated alteration halos along their boundaries; their general orientation follows that of discontinuities within the underlying reservoir facies or adjacent faults. The recognition of these changes in fracture distribution is important for forward modeling of fluid flow and risk management. Studying the occurrence of and changes in fracture patterns from outcrops and scaling it up for use in modeling at a field scale is difficult due to the lack of direct correlation between outcrop observations and subsurface data. Due to the size and amount of data needed to model fluid flow at the field scale the meso-scale (cm to m) variability of rock properties is often neglected. We evaluate this meso-scale variability in elastic moduli, where possible. We combine mechano-stratigraphic outcrop observations with elastic moduli calculated from publically available wire line log data to evaluate the variability in rock strength within the heterolithic top seal. Relationships between changes in Young's modulus to resulting fracture distribution can then be observed. The outcome of this analysis can be used for modeling the effectiveness of seal for storage of CO₂ in the underlying reservoirs. Digitized publically available wire line well log data were used to calculate Poisson's ratio and Young's modulus over the Carmel Formation and upper most 3 m of the underlying Navajo Sandstone. Our calculations show that Young's Modulus can range between 15 to 34 GPa across 60 cm of the intra-seal interfaces, and an average difference of 5 GPa across the reservoir seal interface. These variations will affect fracture distributions and fluid behavior in the subsurface. These data provide a means to closely tie outcrop observations to derived estimates of subsurface rock strength. The characterization of rock strength variability is especially important for modeling the response of seals to increased pressure, due to CO₂ injection, and will allow for better site screening and fluid management once injection projects are underway.