Effect of Oil Sands Core Disturbance Induced by Gas Exsolution on Geotechnical and Hydraulic Properties Measurements
Authors: Wong R.C.K. 
Published: March 2010

Oil sand cores tend to expand when they 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. In core tubes with perforated holes, the core dilation depends on the clearance gap between the inner diameters of the core barrel and core tube. This paper proposes a recompaction method to restore these dilated cores to a state close to its initial porosity (some of locked structure may not be recovered). 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 the presence of dissolved gas in the pore fluid, core disturbance is unavoidable if conventional coring techniques are used. Core disturbance disrupts the locked structure affecting not only the geotechnical, but also the hydraulic properties of the oil sand matrix. This paper examines the mechanism of core disturbance encountered during coring, retrieval, and handling. Based on the mechanism, a recompaction method is developed to restore some of the oil sand fabric inside the disturbed cores. In addition, this paper assesses how the core disturbance affects the measured geotechnical and hydraulic properties of oil sand based on the results of tests on intact, recompacted, and reconstituted specimens.

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

Dusseault(3) and Wong et al.(4) demonstrated that core disturbance resulting from gas exsolution was significant in gassy unconsolidated oil sands. Dusseault and van Domselaar(5) developed an index to quantify the degree of core disturbance for oil sands. The index, termed as "Index of Disturbance," ID is defined as:

Equation (Available In Full Paper)

where Φe is the porosity of the test specimen and Φ?is the in situ porosity 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, does not reflect the changes in fabric due to core disturbance. It is of practical importance to quantify the degree of core disturbance that has resulted from different stages in coring and handling.

Under in situ conditions (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, thereby causing a decrease in the pressure in pore fluid.