Correlations Between a Mudstone Heterogeneity Index and Micromechanical Properties in the Lower Mancos Shale

Abstract

Examining the interplay between sedimentary heterogeneity and fracture propagation in mudstone is crucial to assessment of the potential of low permeability reservoirs as an unconventional resource. A large body of experimental research has demonstrated that micromechanical processes play a role in the failure of brittle rocks. Several experimental studies have also concluded that heterogeneities and discontinuities in a rock mass influence the micromechanical processes of microcracking and strain localization. These micromechanical processes initiate fracture development and evolve to form macroscopic fractures. While these studies have concluded that heterogeneity influences fracture development, there remains a lack of understanding of the complete fracture process, partly due to a failure to quantify heterogeneity. Also lacking in the understanding of micromechanical fracture development are ties between depositional geologic processes and the heterogeneous nature of micromechanical properties in mudstone. Lending understanding to the way that induced fractures propagate in mudstone, this study tests the hypothesis that there is a correlation between sedimentary heterogeneity and the manner in which energy is released as mudstone is fractured. Samples from three cores of the Lower Mancos Shale have been described utilizing lithofacies and chemofacies analyses. The Lower Mancos Shale encompasses the most organic rich portion of the formation, the Juana Lopez Member, and covers a range of heterogeneity due to complex sedimentology within a single cored section. Facies within the core were constrained using core description, XRF analysis, XRD analysis, and SEM and thin section microscopy. From the completed facies analysis, a database of sedimentary heterogeneity referred to as the mudstone heterogeneity index was developed from which to select a broad range of sample variation for geomechanical testing. Results from indirect tensile tests, unconfined compressive strength tests, triaxial shear tests, and fracture toughness tests are utilized to determine correlations between the mudstone heterogeneity index and the manner in which energy is released as mudstone is fractured. Insight into the micromechanical heterogeneity of mudstone will create a link between sedimentary facies and the distribution of mechanical properties in the subsurface in the low permeability Mancos Shale.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016