SAGD and Geomechanics
Authors: Carlson M. 
Published: March 2010

Introduction

During the recent AEUB Chard Leismer Gas Over Bitumen Hearing, there was a considerable amount of technical evidence presented on a wide variety of SAGD technology. One aspect that was different from the last such hearing (Surmont) was the introduction of geomechanics related to the SAGD process. The purpose of this article is to outline the underlying concepts behind geomechanics and then to demonstrate that geomechanics are necessary for a consistent description of SAGD design and actual field performance.

Outline

The principles behind geomechanics were developed primarily in soil mechanics, which is widely used in civil engineering-in particular, the design of foundations and earth works. Soils are comprised of grains of mineral and/or rock that are not cemented or are weakly cemented. Unfortunately, the latter is often called "unconsolidated," although cementing and consolidation (volume reduction due to loading) are not directly linked.

The fundamental areas where geomechanics plays a critical role include:Sampling procedures-in particular, coring;Evaluating conventional formation properties such as porosity and bitumen, water, and gas saturations;Determining in situ permeability;Understanding the mechanisms within the formation during SAGD; and,Determining operating conditions.

The above issues will be discussed in detail in the following, starting with a preliminary introduction to geomechanics.

Soil Mechanics

Interlocked Structure

The fundamental control of physical properties of bitumen sands is the structure of the sand grains. In typical bitumen deposits, the sands are fine grained with the long axes of the grains lying horizontally. Deep burial in the past has resulted in a matrix structure of interlocked grains(1) (note that glaciation was not a major load compared to overburden in the Athabasca).

When subjected to loading, possible intergranular behaviour includes grain:overrideshearingrotation (rolling)translation (sliding)elastic deformationcrushing.

Sands do not have any substantive tensile strength. Further, determining the bulk properties of granular material requires that the original structure of the grains not be disturbed. To date, no reliable methodology has been developed to reproduce the same sand grain structure once it has been disturbed. If great care is taken, it is possible that such effects can be minimized.

Material Testing

Two methods were developed in civil engineering to quantify the strength of sands. They are:the direct shear apparatus; andthe triaxial cell.

Basic depictions of these are shown in Figures 1 and 2(2). These tests can be run in two fundamentally different ways. In the first method, pore fluids are allowed to escape; this is termed a "drained test." In the second method, the pore fluids are not allowed to dissipate; this is termed an "undrained test." This has a profound effect on the behavior of the tested materials. In an undrained test, pore pressures will initially support the majority of the incremental load. In a drained test, sufficient time is allowed for the pore pressures to dissipate during loading. The speed of drainage is a function of permeability.