The Effect of a Variable Aperture System On the Two-phase Flow Response In an Oil Sand Fracture
Authors: Walters D.A. , Wong R.C.K. 
Published: March 2010

An experimental and numerical study was conducted to investigate the effect of the variable aperture system of an oil sand fracture on its two-phase flow response. An oil sand fracture was induced in core samples along the axis of the sample using the Brazilian tensile test. A Computer Assisted Tomography (CAT) scanning analysis was performed to determine the fracture geometry. The fracture was shown to be a variable aperture system which, when coupled with capillary dominated conditions, governed the occupancy of each phase in a two-phase system. A numerical program was used to investigate the occupancy and two-phase flow response of the derived aperture distribution. Phase interference was found to be significant with the interface between phases showing an unsteady nature. The impact of this unsteadiness may affect the assumption of relative permeability being a function of saturation alone.

Introduction

Heavy oil and bitumen are a major energy resource in Canada, and have become a major focus for the oil and gas industry. The fact that the heavy oil deposits of Canada contain 30% of the world's recoverable oil indicate the potential is certainly present(1). The challenge in developing this resource is the difficulty in recovering these highly viscous substances. Thermal recovery has become an accepted method with steam-based recovery drawing much attention.

Steam-based recovery methods involve the injection of steam into the reservoir to heat up the reservoir, lowering the viscosity of the bitumen, which increases production rates. Cyclic steambased recovery processes have become common practice in the thermal operations of the Cold Lake heavy oil reservoirs(2). The steam injection approach used at Cold Lake encourages fracturing of the reservoir each steam cycle, exploiting the permeability of the fracture instead of relying on a layer of bitumen depleted, permeable oil sand. The high hydraulic conductivity of the fracture enables heat transfer to take place further away from the well, thus, accessing more undisturbed reservoir material instead of depositing most of the heat in a previously heated and bitumen depleted area of the reservoir. In order to optimize this steaming approach an understanding of the medium used to carry the heating agent, the fracture, is essential.

The geometry of the fracture, characterized by surface roughness and aperture distribution plays a key role in the permeability of the fracture as well as the flow channels available to the injected mass. A detailed picture of an oil sand fracture, its surface characteristics and their relation to the two-phase flow is needed to better understand and possibly exploit the flow phenomena taking place during and after the hydraulic fracturing process of steam stimulation.

According to the authors' knowledge, no work is currently available characterizing an oil sand fracture or proving that fractures in this material can be characterized in a similar manner as to what has been previously studied in rock fractures(3,4,5,6). The following study presents an experimental, physical characterization of an oil sand fracture using a Computer-Assisted Tomography (CAT or CT) scanning analysis to visualize the fracture geometry.