Geomechanics Issues of Steam Assisted Gravity Drainage

Abstract

After successful testing and full scale implementation of the steam assisted gravity drainage (SAGO) process at AoSTRA's (now Department of Energy, Oil Sands and Research Division) Underground Test Facility (UTF), many operating companies are applying this recovery process ( or variations thereof) in bitumen and heavy oil projects. The thermal SAGO process with adjacent sets of well pairs provides a process geometry where the geomechanical response of an unconsolidated sand reservoir may have an impact on the efficiency of the recovery process. This paper describes a series of numerical analyses which illustrate how fluid pressure increases and thermal loading combine to create an optimum stress environment for shear failure of unconsolidated sand formations. Shear failure in dense unconsolidated sands results in a volumetric increase in sand pore volume due to dilation of the sand grains. To date, bitumen production rates for the SAGO process have been history matched for the UTF Phase A and Phase B tests by increasing the absolute permeability within the formation to a veryReferences and illustrations at end of paper high value; typically to the maximum value within a range of possible values. It will be shown that for the UTF geometry, absolute permeability increases resulting from dilation in advance of the growing steam chamber is likely a factor in the SAGO process. Due to the nature of SAGD, however, reservoir modelling for UTF scenarios are not be significantly effected since dilation in advance of the steam chamber would create the same "absolute permeability" effect as selecting an initially high absolute permeability value. For different reservoir conditions, such as stress conditions, or wellpair spacings then at the UTF, not including geomechanical phenomena in process simulations may adversely affect the results.

Evaluation of Shear Dilation Capability/Potential and Permeability Changes in Karamay Oil Sands under Water Injection

“…For the second method, the triaxial compression tests at a core scale (usually cylindrical standard cores of 25 mm × 50 mm, diameter × length) were widely conducted to study the shear dilation behavior [8,16,53,54]. At the field scale, the injectivity simulations and fluidstructure coupled analyses were mainly studied by the numerical approaches like the finite element method [13,14,32,34,36,41]. This paper used three scales to predict the dilation effects and obtained good results for the guidance of water injection.…”

“…Dilation refers to the process of pore volume expansion as neighboring sand grains attempt to move past each other under the action of shear stresses within the formation. The nearly pure quartzitic nature of the sand grains ensures that no grain crushing occurs during shear [13]. Pore pressure increases during water or steam injection, decreases the effective confining stress, and results in an unloading of the reservoir matrix.…”