Investigation of the Geomechanical Controls on Hydraulic Fracturing in the Bakken Formation, Southeast Saskatchewan

Abstract

This thesis presents the results of a research project pertaining to the hydraulic fracturing of oil reservoirs hosted in the siltstones and fine-grained sandstones of the Devonian to Carboniferous aged Bakken Formation in southeast Saskatchewan, Canada. The Bakken Formation contains significant volumes of hydrocarbon, but large-scale hydraulic fracturing is required to make the production economic. The most challenging issue in hydraulic fracturing in this formation is fracture containment. As the fracture propagates out of zone, water production can increase dramatically as the reservoir becomes hydraulically connected to the overlying/underlying aquifers of the Lodgepole and Torquay formations. To analyze fracture containment in this formation, mechanical properties and in-situ stresses were interpreted from core measurements, wireline logs, and diagnostic fracture injection tests. Next, multistage hydraulic fracturing of horizontal production wells was simulated using the commercial hydraulic fracture simulator Gohfer©, using actual treatment schedules as input and using pressure data to verify the model. This study also reviewed methods for estimating changes to the in-situ stress field (stress shadow) resulting from mechanical effects (fracture opening), poro-elastic effects, and thermo-elastic effects associated with fluid injection for hydraulic fracturing. The application of this method was illustrated for a multi-stage hydraulic fracturing operation to predict principal horizontal stress magnitudes and orientations at each stage. Microseismic events area were also predicted by evaluating shear failure of natural fractures around hydraulic fracture and was verified by microseismic field data. Results showed that shales may not be highly effective for mitigating against out-of-zone height growth of fractures. Thin barriers near to the top and base of the Middle were shown to exist, in intervals that have relatively high carbonate (especially dolomite) contents and relatively low quartz contents. These barriers are effective at minimizing fracture height growth up to a certain point; however, these barriers are thinner and less effective than the shales that were generally assumed to serve as barriers prior to this work. Comparison of simulation results against field observations (water production data) supports the fracture barrier model developed in this work. A sensitivity analysis was conducted and showed that fracture attributes are related closely to operational parameters (such as proppant mass, fluid type, pad volume), to mechanical properties (specifically Poisson’s ratio and reservoir pressure), and to geological properties (specifically existence of barriers). The results of this study suggest that hydraulic fracture treatments using 4 tonnes of proppant (and appropriate fluid properties and slurry injection rates) should achieve a favourable balance between fracture containment and fracture conductivity in the Bakken Formation. Stress shadow analysis showed that thermo and poro-elastic stresses were negligible for hydraulic fracturing in the Bakken Formation of southeast Saskatchewan. A mechanical stress shadow formulation was developed for analyzing multistage hydraulic fracture treatments. Predicted shear failure on natural fractures with strike and dip, logged from cores, compared favourable with field microseismic events area for an assumed natural fracture strike of 115° and dip of 65°.