The Clair field could be described as an ‘unconventional’ conventional reservoir. The rock matrix permeability places reservoir into the conventional category, for which conventional fracturing design in terms of high proppant concentration and fracture conductivity are required for production uplift. However, the presence of natural fractures brings the Clair field a similarity to unconventional reservoirs where impact and contribution of natural fractures must be taken into the equation.
This paper describes the integrated fracturing and production optimization study that was conducted to optimize multistage hydraulic fracturing design in the presence of natural fractures of various density in the Clair field. The production uplift of hydraulic fracturing in conventional reservoirs is well understood. However, the presence of natural fractures adds an unconventional twist of complexity and uncertainty to fracturing design and even more so to production uplift estimates.
To reduce the uncertainty of hydraulic fracturing uplift in the presence of natural fractures, specialized software was used to explicitly model cases with a range of density discrete fracture networks (DFNs) and the interaction with hydraulic fractures. Then the resulting fracture geometries were input into production modelling software to estimate uplift and calibrated back to producers in the segment. This process was repeated for several reservoir scenarios and fracturing designs to establish the production uplift range and ultimately inform optimal hydraulic fracturing design recommendations.
One of the most valuable, yet not most intuitive observations was that the natural fractures and the hydraulic fractures can have a synergistic effect on production. All DFN cases modelled showed benefit from using hydraulic fracturing including high density DFNs. Even when natural fractures are already present, hydraulic fractures will help in connecting the natural fractures to the well and increase production. Higher numbers of hydraulic fractures were associated with the best uplift predictions. The described work has been instrumental in changing how hydraulic fracturing is being considered for naturally fractured reservoirs in general and for the Clair field in particular. Hydraulic fracturing had originally just been seen as a mitigation to a poorly fractured (low/no DFN) outcome. With the results of this study however it is evident that hydraulic fracturing is also an enabler for increased production in a wide range of DFN cases. Several practical recommendations have resulted from this study such as multistage fracture spacing, number of fractures, optimized proppant placement between stages and fracture geometry. The impact of fracture vs wellbore orientation and overflush were also modelled.
This is the first time such a workflow has been applied for a conventional yet naturally fractured reservoir. The proposed modelling workflow allows for optimization and robust fracturing design in environment of reservoir and geological uncertainties.