Abstract
Classification of the subsurface using a rock mass framework provides insight into reservoir behaviour because the combined characteristics of intact rock and the natural fracture system can be quantified. Rock mass classification is useful for reservoir characterisation because the primary factors affecting deformation and flow in a reservoir are included in the classification scheme, which can be applied across a range of rock formations. In this dissertation, heterogeneous low-permeability (‘tight’) reservoirs are classified according to their geological strength index value. This is achieved through outcrop mapping, microseismic analysis, core logging, wireline log analysis and geomechanical testing. These methods are applied to the prolific tight gas/oil Duvernay Shale reservoir in Western Canada. The rock mass classification framework is then used to assist in understanding how fluid propagates in the reservoir during hydraulic fracturing. Rather than a static diffusive process, the movement is hypothesised to occur episodically and dynamically in much the same way as the natural earthquake cycle moves fluid through the rock mass. Evidence of this behaviour is provided from microseismic observations during hydraulic fracturing treatments and through a novel technique to analyse pressure fluctuations during the stimulation treatment in the time-frequency domain. This method provides insight into fluid flow mechanisms occurring in fractured shale and tight rock reservoirs, and can be used to constrain before-closure flow-regime interpretations. A pressure-dependent rock mass Biot’s coefficient is derived from previously published empirical methods and verified through distinct element modelling techniques. Results from this study indicate that a critical fluid pressure can be achieved where the fracture system is fully connected and rock mass mobilization occurs. Under lower fluid pressures, pressure diffusion is more likely to occur rather than localized flow. The stiffness of the intact rock is observed to play an important role on how fluid becomes segmented within the reservoir. This research advances the concept of rock mass characterisation in low-permeability petroleum reservoirs and demonstrates how it can be used to understand the dynamic geomechanical system.
