Geological Association of Canada [GA]

 CC:Hall E  Wednesday  1400h

Integrated and Multiscale Studies in Petrophysics and Applied Geophysics II Posters

Presiding:  L Cheng, University of Quebec in Abitibi Temiscamingue; P Glover, Université Laval


Numerical Modeling of Seismicity Induced by Hydraulic Fracturing in Naturally Fractured Reservoirs

* Zhao, X (, University of Toronto, 170 College Street, Toronto, ON M5S3E3, Canada
Young, P (, University of Toronto, 170 College Street, Toronto, ON M5S3E3, Canada

The problem of the interaction between hydraulic and natural fractures is of great interest for the oil and gas industry since natural fractures can significantly influence the overall geometry and effectiveness of hydraulic fractures. Based on the tri-axial fracturing lab experiments (Zhou et al., 2008) and fluid stimulation in Bonner sand at the Dowdy Ranch field, U.S. (Sharma et al., 2004), a fully dynamic 2D distinct element model is validated to simulate fluid injection into a reservoir containing a natural fracture. The capability of replicating not only damage and failure in rock under stress, but also the associated seismicity from the numerical model enables us to validate the model. Comparison of the geometry of hydraulic fractures and seismic source information (locations, magnitudes, and mechanisms) produced by the model with actual recorded fractures and induced seismicity has been performed, elucidating our understanding the mechanics behind the seismicity related to natural fracture arrests, bends or changes in the propagation regime of a hydraulic fracture. This could possibly deliver extra information on areas of aseismic deformation that might be occurring and help in understanding the relationship between the seismicity, stress/damage and the fluid front. At the lab scale, the numerical model simulated a series of fracturing experiments on rock blocks with pre- fractures with different angles of approach. The model mainly captured the geometry of hydraulic fractures, and three interaction types (crossing, dilating and arresting) between induced fractures and pre-existing fractures. Furthermore, seismic mechanisms obtained from the model which is not reported in the laboratory data confirmed that hydraulic fractures were arrested by shear slippage of the pre-existing fracture. In the field scale, the calibrated model simulated the stimulation conducted in the tight gas reservoir. The model produced realistic seismic source parameters and geometries of hydraulic fractures. Therefore, the model is valid and effective and can help examine in detail the micro mechanism behind the failure, and the relationship between the natural fracture, induced seismicity, and the fluid front through direct observation of the model.


How are Grain Size and Pore Size Related? A Transformation Based on Electro-kinetic Theory

* Glover, P W (, Université Laval, Département de géologie et de génie géologique, Québec, QC G1V 0A6, Canada
Walker, E (, Université Laval, Département de géologie et de génie géologique, Québec, QC G1V 0A6, Canada

Most permeability models use effective grain size or effective pore size as an input parameter. Until now, an efficacious way of converting between the two has not been available. We propose a simple conversion method for effective grain diameter and effective pore radius using a relationship derived by comparing two independent equations for permeability, based on the electro-kinetic properties of porous media. The relationship, which we call the theta function, is not dependent upon a particular geometry and implicitly allows for the widely varying style of microstructures exhibited by porous media by using porosity, cementation exponent, formation factor, and a packing constant. The method is validated using 22 glass bead packs, for which the effective grain diameter is known accurately, and a set of 188 samples from a sand-shale sequence in the North Sea. This validation uses measurements of effective grain size from image analysis, pore size from mercury injection capillary pressure (MICP) measurements, and effective pore radius calculated from permeability experiments, all of which are independent. Validation tests agree that the technique accurately converts an effective grain diameter into an effective pore radius. Furthermore, for the clastic data set, there exists a power law relationship in porosity between effective grain size and effective pore size. The theta function also can be used to predict the fluid permeability of a sample, based on effective pore radius. The result is extremely good predictions over seven orders of magnitude.