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.
Abstract. Fibre-optic sensing technology has recently become popular for oil
and gas extraction, mining, geotechnical engineering, and hydrogeology applications.
With a successful track record in many applications, distributed acoustic
sensing using straight fibre-optic cables has become a method of choice for
seismic studies. However, distributed acoustic sensing using straight fibre-optic cables cannot detect off-axial strain at high incident angles (the
angle between the ray and normal vector of the surface); hence, a helically
wound cable design was introduced to overcome this limitation. The helically
wound cable field data at the New Afton deposit in British Columbia, Canada,
showed that the quality of the data is highly dependent on the incident
angle and surrounding media. A 3D finite element model developed using
COMSOL Multiphysics quickly and efficiently assessed the effects of various
materials surrounding a helically wound cable for simple geometry for
scenarios corresponding to a real deployment of such cable underground at
the New Afton mine. The proposed numerical modelling workflow could be
applied to more complicated scenarios (e.g., non-linear material
constitutive behaviour and the effects of pore fluids). The results of this
paper can be used as a guideline for analyzing the impact of surrounding
media and incident angle on the response of helically wound cable,
optimizing the installation of helically wound cable in various conditions,
and validating boundary conditions of 3D numerical models built for
analyzing complex scenarios.
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