Geomechanical characterization of the Montney shale, northwest Alberta and northeast British Columbia, Canada

Abstract

Unconventional reservoirs require hydraulic stimulation to be commercially productive. Recently, distinctions have been made between reservoir quality vs. completion quality (Cipolla et al. 2012), emphasizing the importance of both elements for production. There are many sources of variability in reservoir quality; in this thesis I examine several fundamental reservoir properties in detail and combine them in a new way: the Rock Quality Index (RQI). Through the definition of a geomechanical model and corresponding mechanical stratigraphy, those factors having a substantial effect on reservoir quality became apparent. Two fundamental categories; compositional variation and fabric variation, are used to characterize overall reservoir variation. Burial, compaction, hydrocarbon generation, diagenesis, and tectonics all affect the mechanical character and in-situ stress state of the reservoir. The Rock Quality Index (RQI) is an effort to understand how composition and fabric relate to stress anisotropy, fracturing, and rock properties, and ultimately aid in defining the best zones for exploitation. Therefore, this Rock Quality Index (RQI) is vital for the defining the second element of unconventional reservoir success; completion quality. Without a reservoir framework to drive the completion design, high completion quality will be harder to achieve. The original mechanical stratigraphy definition is in turn used as a framework for relating Rock Quality Index (RQI) variations to the factors which caused them. The comparison between Rock Quality Index (RQI) and mechanical stratigraphy shows that zones traditionally thought of as desirable for hydraulic completion (brittle) are also zones of high internal heterogeneity. Formation heterogeneity may be detrimental to hydraulic fracture growth. Using several other data types (multicomponent time-lapse seismic, microseismic, and reservoir engineering tests) in conjunction with the Rock Quality Index (RQI), it is observed that there is a strong formation influence on the progression of hydraulic fractures. The locations of interfaces between changes in rock properties and/or stress state are locations where the hydraulic fracture character will also change. It was found that energy is dissipated in heterogeneous/brittle zones, while hydraulic growth occurs in homogenous zones. However, at the intersection of a homogenous zone with a brittle zone, both hydraulic fracture growth and energy dissipation is possible. Here relatively higher production is observed. Stress shadowing amplifies the effects of energy dissipation in brittle zones. Understanding the geological factors that have the greatest influence on stimulation has proven to be a useful method of predicting productivity and efficiency in shale reservoirs. The results of this geomechanical study are calibrated with diagnostic fracture injection tests, microseismic, spinner gas data, and time-lapse multicomponent seismic to corroborate the predictions of reservoir performance in the Montney Shale.