The effect of thermal maturity on geomechanical properties in shale reservoirs: An example from the Upper Devonian Duvernay Formation, Western Canada Sedimentary Basin

Abstract

Abstract Shale reservoirs are characterized by low matrix permeability and therefore require effective models of geomechanical properties to optimize drilling and hydraulic fracturing strategies. Both initial rock composition and thermal maturity are potentially critical controls on geomechanical properties. We investigate the Upper Devonian Duvernay Formation, Western Canada Sedimentary Basin, a major shale gas target that spans a wide range in rock compositions and thermal maturity to identify relationships between these parameters and geomechanical properties. Core hardness measurements and dipole sonic and density log data were used to characterize the geomechanical properties. Major element chemical analysis, X-ray diffraction analysis and LECO combustion were used to determine mineralogy, bulk rock chemistry and total organic carbon (TOC) content and to distinguish biogenic from detrital silica. Scanning electron microscopy (SEM) images with complementary energy-dispersive spectroscopy (EDS) maps were obtained for representative samples to document the rock fabric and distribution of organic matter and minerals. Hardness and Al2O3 concentrations are strongly negatively correlated in all cores, regardless of thermal maturity, suggesting that clay minerals are the most significant factor controlling geomechanical properties. Biogenic silica is positively correlated to hardness. Detrital silica is negatively correlated to hardness, an artifact of the positive correlation between detrital clay minerals and detrital quartz. The positive correlations between CaO content and hardness in all cores suggest the brittle behavior of carbonate minerals. Increased thermal maturity from immature to oil window results in greater hardness for rocks of similar geochemical compositions. We propose that this results from: (1) enhanced mechanical compaction; (2) carbonate cementation; (3) increased stiffness of kerogen; and (4) partial conversion of kerogen into expelled hydrocarbons that reduces the load-bearing function of the organic matter. At maturities greater than oil window, thermal maturity does not exert a major control on the rock strength; thus, there are no significant changes in hardness between samples from oil window and dry gas window. Geomechanical properties can be related to mineralogy, which exerts a major impact on the geomechanical properties. Quartz cements sourced from biogenic silica rather than smectite-to-illite transition are the primary contributor to the rock strength in the Duvernay Formation. However, it is noteworthy that thermal maturity should also be considered as an important factor when predicting geomechanical properties from mineralogy.