Fully compositional dual permeability numeric reservoir simulation was used to model flow from a Duvernay horizontal well after fracture stimulation. Water saturation was increased in the fracture systems and reservoir matrix where microseismic events occurred before production matching to simulate the effects of leakoff from slickwater fracturing treatments.
A geologic model based on well log data referenced to universal transverse mercator (UTM) coordinates was constructed and imported as the grid for the simulator. Wellbore trajectories and microseismic data from the fracturing treatments conducted along the wellbore were then imported using UTM coordinates. Microseismic data was used to determine which grid blocks to include in a stimulated reservoir volume (SRV) for each fracturing treatment. Within each SRV, the permeability and fracture width of the primary and secondary fracture systems were defined. By using a fully compositional simulator, an equation of state was developed and applied to properly simulate phase behavior under dynamic downhole conditions. This is essential to properly model multiphase flow effects on relative permeabilities and resulting production.
An underlying assumption in most, if not all, Duvernay well stimulations is that creation of a secondary fracture system is essential to economic production. This is normally achieved through use of friction reduced water stages with lower, finer mesh proppant concentrations. Initial slickwater stages are, in some designs, followed by stages of a viscous fluid with higher proppant concentrations, normally of coarser mesh. The objective of viscous slurry stages is to generate a primary higher conductivity biwing fracture, which serves as a connection between the created secondary fracture system and the wellbore. This combination is often referred to as a "hybrid fracturing fluid system."
The impact of the injected water on production can be very significant in a dual permeability system, which relies on flow from a secondary fracture system to provide economic production rates from a very low permeability matrix. Water addition will result in single phase gas flow becoming two phase flow with resulting reduction in production rates attributed to associated relative permeability effects. This, in turn, can result in additional reservoir pressure drawdown, which can drop reservoir pressure below the dew point of the produced fluid system. Resulting hydrocarbon liquids dropout from the gas phase can now cause a transition to three phase flow with further reduction in production rates attributed to additional relative permeability effects.
The use of a compositional reservoir simulator allows one to study the impact of water saturation changes by incorporating water saturation changes in the model before production matching then reducing the water saturations in subsequent runs to assess the impact on production and associated economic return. This could then provide the necessary economic justifications to include changes in the design of future fracturing treatments including:
Use of high quality water based foams or hydrocarbon based fracturing fluids to reduce the volume of injected water and facilitate fracturing fluid recovery.
Use of effective surfactant systems to reduce interfacial tensions and minimize relative permeability effects.
Use of 200-mesh ceramic microproppant to more effectively prop the smaller secondary fractures, thereby providing improved conductivity to reduce the impact of multiphase flow on production.