--> Experimental Geomechanics on Heterogeneous Mudstone—Developing Predictive Relationships Between Facies, Reservoir Quality, and Fracture Propagation

AAPG ACE 2018

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Experimental Geomechanics on Heterogeneous Mudstone—Developing Predictive Relationships Between Facies, Reservoir Quality, and Fracture Propagation

Abstract

The interplay between sedimentary heterogeneity and tensile and shear fracture propagation in mudstone is crucial to assess low permeability rocks as unconventional reservoirs. Discontinuities in a rock mass influence micromechanical processes such as microcracking and strain localization, which evolves into or propagates macroscopic fractures. Previous experimental research has demonstrated a relationship between heterogeneity and fracture of brittle rocks, yet understanding of the physical controls on this process is lacking. Our study tests the hypothesis that there is a correlation between sedimentary heterogeneity and fracture propagation in mudstone. Mudstone samples from a range of formations within prospective shale plays that represent a broad range of sedimentary geologic heterogeneity were utilized. Samples were categorized using facies analysis, which consisted of: visual sedimentary description, x-ray fluorescence and x-ray diffraction, scanning electron and thin section microscopy, as well as reservoir quality analysis. Reservoir quality analysis included total organic carbon content, porosity, permeability, and saturations (on moisture preserved samples or induced via KCl brine bath) measurements. Indirect tensile measurements, uniaxial and triaxial compression, and fracture toughness tests have been performed on a range of facies. These measurements were made on samples taken parallel and perpendicular to bedding and at various confining pressures to represent subsurface conditions. Preliminary conclusions reveal that the highest strength values result from cemented Ca-enriched samples regardless of other heterogeneities, medial to high strength samples have approximately equivalent proportions of Al-Ca-Si compositions, whereas Al-rich, homogeneous samples have consistently low strength. Increased saturation in the samples (as tested herein) is negatively correlated with strength and predictive relationships have been quantified for several facies. In addition, moisture preserved samples failed in a step-like pattern as opposed to discrete, continuous fractures that crosscut dry samples. We conclude that sedimentary heterogeneity plays a greater role in fracture propagation in moisture preserved samples, which are more indicative of in-situ conditions. Results also reveal that based on composition, orientation, and saturation there is a maximum confining pressure at which geologic heterogeneity controls fracture geometry.