The Devonian Duvernay Formation in Alberta is a carbonate-siliceous unconventional source rock that produces prolific oil, gas condensate liquids, and gas. In the southern part of the basin and the East Shale Basin, high frequency interbeds of limestone and organic-rich calcareous shales are common at spacing of inches to several feet. Prolific gas liquids are trapped within the shale/mudstone units while the highly cemented and tight carbonate units show a lack of hydrocarbons. Even though most wellbores completed to date have primarily landed in shale units, still, due to geo-steering and other considerations penetration through limestone unit along the lateral cannot be avoided. High contrast in mechanical properties of shale and limestone units along with anisotropy in geomechanical properties of the same rock unit makes optimal hydraulic fracturing breakthrough and placement often challenging.
This study reviews examples of high resolution mechanical Duvernay stratigraphy combined with log and core analyses in order to better understand the implications of hydraulic fracture development through such complex reservoirs. Integrated petrophysical well log and core data indicate that there are strong heterogeneities on lithology, and geomechanical properties (hardness, brittleness, and elastic properties) on various scales. The shale units generally have lower Young's modulus and Poisson's ratio, whereas the carbonate units have high Young's modulus and Poisson's ratio. Core data also indicate strong vertical transverse isotropy in the shale units. Combined with occurrence of various types of naturally-occurred sub-vertical fractures and sub-horizontal polished slip faces, weak bedding plane strength, and strong anisotropy in in-situ stresses, and high heterogeneities in lithology and geomechanical properties likely result in complex and poorly constrained and un-optimized hydraulic fractures throughout the Duvernay Formation.
The results indicate that high Young's modulus of carbonates causes a narrow fracture width which makes fracture initiation and placement and in particular proppant transport a challenge. Normal and strike slip faulting regimes are observed in some areas of East Basin. The lower anisotropy in horizontal stresses estimated for the shale zones than the carbonate zone may suggest that the complex fractures is more likely to occur in the shale zones than the carbonate zones.
In Duvernay minifrac tests, longer fall off data collection (> 2 weeks) is critical for a successful determination of closure pressure and pore pressure estimation. During fracture stimulations, tight cluster spacing, i.e. ≤10m apart, might limit fracture geometry growth on the inner clusters due to higher induced stress. Similarly, due to higher stress shadow effects, tighter stage spacing might affect the fracture growth on the subsequent stages.