Abstract

A triaxial experimental program has been conducted to evaluate volumetric strains and permeability variations in very dense, reconstituted water wet/bitumen sand specimens for a pore pressure injection stress. The triaxial cell is housed in an environmental chamber to simulate in situ reservoir boundary conditions in terms of stress state, temperature, and pore pressure. A series of experiment were designed and executed for both isotropic or anisotropic stress states. This paper presents an improved technique and method to prepare a reliable and representative wet/oil sands specimen for reservoir geomechanical testing. The packing technique allows high quality, repeatable (porosity, relative density, and dry density), very dense sands specimen to be created. Eight synthetic water wet/oil sands core specimens have been prepared with different fluid saturations. The preliminary results from three experiments show that the permeability variations are very sensitive to stress state evolution along the pore pressure injection stress path. For initial isotropic stress states, only temporary improvement on permeability was found. However, for initial anisotropic stress states, permanent deformation following pore pressure injection resulted in substantial increases the effective permeability to water and the porosity in the dilated zone.

1. INTRODUCTION

One enhanced SAGD (Steam-Assisted Gravity Drainage) start-up scheme [1] has recently been proposed and deployed in a pilot SAGD well pad to shorten SAGD start-up operation time. This accelerated start-up scheme is related to pore pressure injection, which is expected to create a dilation zone vertically connecting two horizontal wells. This dilation zone is expected to increase porosity, permeability, and water mobility in the interwell region. There have been several publications on permeability variation associated with shear induced in unconsolidated sands through a series of triaxial tests [2, 3, 4, 5, 6, 7, 8, and 9].

Most of these studies are conducted through triaxial tests with the following stress path: maintaining confining stress while increasing axial or vertical stress to induce shearing failure. In general, such tests are designed and intended to simulate the stress variation, such as triaxial compression and radial extension, during thermal recovery process. They may not fully represent the stress path and deformation that oil sands reservoirs undergo during fluid injection. Yale et al. [10] studied field oil sands samples with interbedded mudstone under fluid injection. The triaxial tests were carried out at room temperature under reservoir boundary condition with only modest levels of stress anisotropy (stress ratio 1.1).

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