Prediction Of Oil Sand Failure Due To Steam-Induced Stresses
Authors: Tortike W.S. , Ali S.M. Farouq 
Published: March 2010

A comprehensive numerical model of oil sand, consisting of steam/water-oil-gas flow in three dimensions plus the geomechanics of oil sand has been developed. In this manner, apart from fluid saturations and pressures and temperature, it is additionally possible to predict the stress distribution in the oil sand, and thus forecast the type of failure (shear and parting) which would occur. The mathematical description of the problem is achieved by means of discretization employing finite elements, which provide great flexibility in describing the physical system.

This particular study illustrates the use of one of the components in this model with output from a conventional numerical reservoir simulator, an approach which is more accessible to many engineers. The results given examine preliminary investigation of three steam injection cycles. The results show that the geomechanical response is dependent particularly on changes in temperature, on changes in pressure, and on the localized material response.

Introduction

The recovery of heavy oil and bitumen from unconsolidated siliclastic sediments is considerable in central and eastern Alberta, western Saskatchewan, and elsewhere, particularly in the U.S.A, and Venezuela. While production has been established from these sediments for many decades, the degree of understanding of the reservoir mechanic giving rise to the production has varied considerably. In certain instances, the ability to predict a reasonably accurate production response to a given injection strategy has been poor. Lack of confidence in predictive ability, in turn, leads to the downgrading or outright cancellation of oil recovery projects, where an economic return might instead have been possible.

One component missing from the conventional reservoir engineering analysis of these unconsolidated formations is the inclusion of geomechanical behavior to a greater degree than usually considered in parameters such as pore compressibility. In the particular instance of thermal recovery of bitumen from the Athabasca oil sand by steam injection, the economic return depends mainly on the rate of mobilization and subsequent recovery of bitumen in the sand. The rate of mobilization of bitumen is clearly a function of the areal distribution of the injected heat energy — the wider the better, provided sufficient heat is available. The recovery of mobilized bitumen is thereafter primarily dependent on gravity drainage, hence on the combined vertical and horizontal conductivities of the porous medium in the mobilized zone.

The injection of steam or another hot fluid into a formation saturated with essentially immobile bitumen can take place only through the parting or fracturing of the tar sand. This situation persists until the injected fluids reach a nearby well or a pressure sink such as an aquifer. The formation must be parted to inject steam in a classic cyclic steam stimulation process, unless communication to the nearby sink is established. In each injection cycle one would hope that the parting was made in a relatively cool bitumen zone, where the heating would be most beneficial. The parting orientation and position is a function of several parameters — local ones such as lithology, pore pressure, local concentrations in total stress, material properties, and the far field effects of the regional principal stresses.