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Investigating the Effect of Thermal Maturity on Geomechanical Properties in Shale Reservoirs: An Example in Upper Devonian Duvernay Formation, Western Canada Sedimentary Basin

Abstract

Shale reservoirs, characterized by fine-grained nature and low matrix permeability, require good understanding of geomechanical properties to optimize drilling and fracturing strategies. A comprehensive characterization of geomechanical properties in shale reservoirs should be based on an integration of geochemical, geomechanical and petrographic methods. We test this approach on five long cores from the Upper Devonian Duvernay Formation, Western Canada Sedimentary Basin, representing a wide range in rock compositions and thermal maturity. The Upper Devonian Duvernay Formation is a major shale gas target in Western Canada Sedimentary Basin, receiving increasing interests. The cores were sampled at high resolution to investigate the impact of geochemical composition and thermal maturity on geomechanical properties. Core hardness measurements, Young's modulus, Poisson's ratio and brittleness calculated from dipole sonic and density log data were used to characterize the geomechanical properties. Geochemical composition analysis were carried out to determine the concentrations of major components and distinguish biogenic silica from detrital silica. In addition, scanning electron microscopy (SEM) images with complementary EDS maps were performed on representative samples to document the mineral distribution, fabric, texture and especially quartz cementation. The strongest relationship between hardness and geochemical components is a strong negative correlation between hardness and Al2O3 concentration in all cores, regardless of thermal maturity. This indicates that clay minerals are the most significant factor controlling geomechanical properties. Quartz content is also important, with detrital silica negatively correlated to hardness whereas biogenic silica is positively correlated to hardness. Increased thermal maturity results in greater hardness for rocks of similar geochemical compositions, a result of (1) changes in the physical properties of kerogen, (2) partial conversion of kerogen into migrated hydrocarbon that reduces the load-bearing function of the organic matter, and (3) excess silica sourced from smectite to illite reactions that forms stiff framework. Basic bulk mineralogical analysis, which by itself does not account for thermal maturity and does not differentiate biogenic from detrital silica, therefore provides insufficient information for the effective prediction of geomechanical properties.