Abstract

Oil sands cores tend to expand when these cores are recovered from deep heavy oil formations. Computer tomography scans of these cores show that discrete tensile fractures are induced within the cores. Formation of these fractures is attributed to the gas nucleation and exsolution process in the viscous heavy oil under the overburden stress relief in coring and retrieval. The core dilation depends on the clearance gap between the inner diameters of the core barrel and core liner. This paper proposes a recompaction process to restore these fractured (disturbed) cores to its in situ state. Geotechnical and hydraulic tests along with computer tomography imaging technique were conducted on these recompacted core specimens. Test results of the recompacted specimens were compared to those of the intact and reconstituted specimens to illustrate the effectiveness of this recompaction method.

Introduction

Oil sand is a very dense uncemented granular material with locked fabric 1,2. The main mineral composition is dominated by quartz. Its in situ porosity varies from 0.30 to 0.33. Bitumen filling the intergranular spaces does not contribute to the geotechnical properties of the material. Because of its uncemented nature and presence of dissolved gas in pore fluid, sample disturbance is unavoidable if conventional coring techniques are used. Sample disturbance disrupts the locked structure affecting not only the geotechnical, but also the hydraulic properties of oil sand matrix. This paper examines the mechanism of sample disturbance encountered during coring, retrieval, and handling. Based on the mechanism, a recompaction method is developed to restore the disturbance. In addition, this paper assesses how the sample disturbance affects the measured geotechnical and hydraulic properties of oil sand based on results of tests on intact, recompacted and reconstituted specimens.

Responses (Sample Disturbance) of Oil Sand Cores in Drilling, Retrieval and Preservation

Dusseault3 and Wong et al.4 demonstrated that sample disturbance resulted from gas exsolution was significant in gassy unconsolidated oil sands. Dusseault and van Domselaar5 developed an index to quantify the degree of sample disturbance for oil sands. The index, ID is defined as:

Equation (1) (Available in full paper)

where Øe is the porosity of the test specimen and Ø is the in situporosity of the specimen (determined from downhole petrophysical logs or from an undisturbed core with zero gas saturation under in situ stresses). This index, a scalar parameter, doe not reflect the changes in fabric due to sample disturbance. It is of practical importance to quantify the degree of sample disturbance resulted from different stages in coring and handling.

Under in situ condition (Figure 1a), the oil sand formation is subjected to its overburden total stress (σ) and reservoir pore pressure (pL). Coring induces a reduction in total stress due to the difference between the formation and drilling fluid unit weights. This decrease in total stress causes an equal drop in the pore pressure if the oil sand matrix is more compressible than the pore fluid. Total relief in the overburden stress could induce an negative pore pressure maintaining no change in effective stress or no core dilation.

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