Abstract

Geotechnical engineering in oil sands development involves studying the stresses and deformations that occur in a formation caused by changes in stress, pore fluid pressure and temperature. The geotechnical response of an oil sands reservoir to fluid pressure increase or to in situ heating results in stresses and deformations which affect hydraulic fracture propagation, formation shearing, well casing performance, the stability of underground openings (uncased wells, tunnels and shafts), and the magnitude of surface heave. The analytical procedures used to model the above mechanisms require a number of parameters whJ.ch are derived from the mechanical, thermal, and physical properties measured under temperatures and pressures appropriate to the in situ bitumen production technique. These geotechnical properties of oil sands as well as those of the overburden, underburden and interbedded strata are being measured and analyzed in the laboratories of he geotechnical group in the Department of Civil Engineering at the University of Alberta.

The importance of following similar stress paths in laboratory testing as the stress paths that will occur during in situ processes is emphasized in this paper. Test results for stress-strain moduli, shear strength, Poisson's ratio and compressibility are given and discussed to show the effect of stress path on granular material properties.

The laboratory measurements have shown that the geomechanical characteristics of uncemented sands also depend on the sand grain mineralogy, geological environment of deposition and geological history. As a result these properties vary from deposit to deposit and, in some cases, within the deposit as the grain composition and arrangement and geological stress history change. Meaningful measurements of strength, compressibility and deformation modulus can only be obtained by testing specimens in which the integrity of the sand matrix has been preserved. Test measurements show that smal1 disruptions of the sand matrix caused by gas evolving from the bitumen during coring and sample preparation has a major affect on these mechanical properties. The bitumen also has a controlling role in the rate of pore pressure dissipation and therefore on the magnitude of shear deformations. In addition, gas evolution has a major influence on maintaining pore fluid pressures as a decrease in confining stress or increase in temperature allows as bubbles to form which retard the dissipation of are pressures.

The experimental result show that the geotechnical properties of Cold Lake Cleanwater formation oil sands differ significantly from those effective stress will be an increase in stress. Such a stress path is shown in Figure 1b. As the stress conditions are approaching the shear failure envelope, shear deformation will take place. If the stress changes are sufficient, shear failure will be reaches.

TEST PROCEDURES

During core sampling from boreholes, in situ confining stresses and pore pressures are reduced rapidly and gas evolution from the bitumen and water may occur. This formation of gas bubbles combining with a lack of cementation and therefore, cohesive strength between sand grains, results in expansion of the oil sand microfabric. The sample disturbance caused by core expansion significantly affects the geotechnical properties of oil sands.

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