Predicting fractured zones in the Culebra Dolomite

Fracturing in the Culebra Dolomite Member of the Permian Rustler Formation exhibits a high degree of spatial variability in the vicinity of the Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico. The WIPP is the U.S. Department of Energy's deep geological repository for transuranic and mixed wastes resulting from the nation's defense programs. The WIPP repository is located 655 m below ground surface in bedded halite of the Permian Salado Formation, which underlies the Rustler and Culebra. Culebra transmissivities (T's) in the vicinity of the WIPP vary over six orders of magnitude, with higher T's (log 10 T (m 2 /s) > -5,4) reflecting zones of well-interconnected fractures. We develop, test, and refine a conceptual model for predicting fracture zones within the Culebra. We define three regional-scale controls on Culebra fracturing, including the dissolution of salt from below the Culebra, the presence of halite above and below the Culebra, and overburden thickness. We also identify two local-scale controls on Culebra fracture zones including fracture-filling cements and localized deformation due to ductile flow of the mudstone that underlies the Culebra. The spatial distribution of the regional-scale controls is easily predicted. However, the influence of local controls can only be uniquely identified in hydraulic test data. A drilling program initiated in 2003 tests aspects of this conceptual model and leads to minor revisions of our conceptual understanding of the geologic controls on fracturing in the Culebra.