Abstract
This paper presents selected results of a broader research project pertaining to the hydraulic fracturing of oil reservoirs hosted in the siltstones and fine grained sandstones of the Bakken Formation in southeast Saskatchewan, Canada. The Bakken Formation contains significant volumes of hydrocarbon, but large-scale hydraulic fracturing is required to achieve economic production rates. The performance of hydraulic fractures is strongly dependent on fracture attributes such as length and width, which in turn are dependent on in-situ stresses. This paper reviews 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 is illustrated for a multi-stage hydraulic fracturing operation, to predict principal horizontal stress magnitudes and orientations at each stage. A methodology is also presented for using stress shadow models to assess the potential for inducing shear failure on natural fractures. The results obtained in this work suggest that thermo and poro-elastic stresses are negligible for hydraulic fracturing in the Bakken Formation of southeast Saskatchewan, hence a mechanical stress shadow formulation is used for analyzing multistage hydraulic fracture treatments. This formulation (and a simplified version of the formulation) predicts an increase in instantaneous shut-in pressure (ISIP) that is consistent with field observations (i.e., ISIP increasing from roughly 21.6 MPa to values slightly greater than 26 MPa) for a 30-stage fracture treatment. The size of predicted zones of shear failure on natural fractures are comparable with the event clouds observed in microseismic monitoring when assumed values of 115°/65° are used for natural fracture strike/dip; however, more data on natural fracture attributes and more microseismic monitoring data for the area are required before rigorous assessment of the model is possible.
Highlights
During fracturing operations, the changes in principal stress magnitudes and orientations induced in the reservoir during a given fracture stage may alter the conditions for the subsequent stages, giving rise to the so-called “stress shadow” effect
15 This paper reviews methods for estimating changes to the in-situ stress field resulting from mechanical effects, poro-elastic effects, and thermo-elastic effects associated with fluid injection for hydraulic fracturing
5 Discussion 5.1 Assessment of Stress Shadow Effect Based on Field Data To assess the validity of the stress shadow modeling workflow developed in this research, the instantaneous 480 shut-in pressure (ISIP) values measured for stages 2 through 30 during field operations at well S were plotted and compared against the model-predicted values of the minimum horizontal in-situ stress (Figure 25)
Summary
The changes in principal stress magnitudes and orientations induced in the reservoir during a given fracture stage may alter the conditions for the subsequent stages, giving rise to the so-called “stress shadow” effect (seeFigure 1). The changes in principal stress magnitudes and orientations induced in the reservoir during a given fracture stage may alter the conditions for the subsequent stages, giving rise to the so-called “stress shadow” effect Stress shadows can impact the effectiveness of fracturing operations by increasing the injection pressures required to create and propagate fractures, reducing fracture width, and potentially altering fracture trajectory (Nagel et al, 2013; 35 Zangeneh et al, 2015; Gorjian & Hawkes, 2017; Patterson, 2017; Roussel, 2017). The design of effective fracture stimulation treatments in a reservoir requires analysis of the geomechanical attributes of the reservoir and the prediction of stress shadow effects (Smith & Montgomery, 2015; Suppachoknirun and Tutuncu, 2017). Analytical solutions exist for modeling stress shadows under idealized conditions (e.g., homogeneous and isotropic rock properties; linear elastic material behaviour; 2-dimensiontal, plane strain geometry).
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