ABSTRACT:
Permeability enhancement of oil sands during SAGD, a gravity drainage process, is desirable to minimize start-up time and improve overall recovery efficiency. High pressure cold water injection may be used as a stimulation process where water is injected into a SAGD well pair at high pressure and limited volume to distort the sand texture and enhance permeability or break thin impermeable interbeds impeding the hot fluid movement in long-term SAGD operation.
In this study an iteratively coupled reservoir-geomechanics simulation is used to evaluate the extent of permanently stimulated and dilated volume as well as the efficacy of rupturing the impendent impermeable barriers. The geomechanical model incorporates a non-linear elasto-plastic constitutive model calibrated with the available McMurray sand public data. Estimates of the initial oilsand permeability and porosity were calibrated using the flow and shut-in periods of existing minifrac test data. The updated coupling parameters from the stress module in any time step enables the 3D thermal multi-phase reservoir model to sensitize various water injection scenarios and optimize the permeability enhancement affecting long-run performance of the SAGD recovery. The study reveals a minimum injection pressure about 15% larger than the initial vertical stress is required for an efficient dilation operation.
1. INTRODUCTION
Thermal recovery processes in oil sands typically rely quite heavily on gravity drainage as one of the primary drive mechanisms. Steam Assisted Gravity Drainage (SAGD) is one such process that requires gravity drainage and vertical communication between the steam injector and producer for an efficient start-up and long-term recovery. The oil sands, especially those in the Alberta are very densely packed and if they are failed in shear at low effective stress they will dilate and increase permeability. This enhancement of permeability can be used to accelerate SAGD start-up as well as increase the efficiency of long-term drainage. This paper presents a calibrated reservoir and geomechanical model illustrating the pressure requirements (or effective stress) for significant permeability increases that will positively affect SAGD performance.
A detailed study has been conducted to model oil sand dilation by means of cold water injection prior to a typical SAGD process. The goal was to demonstrate an increase in absolute and water relative permeability from a rate and volume limited cold water injection into the SAGD well pairs. An advanced coupled reservoir and geomechanical simulation technique has been implemented to conduct this study.
