Abstract

SAGD (steam-assisted gravity drainage) is a robust thermal process that has revolutionized the economic recovery of heavy oil and bitumen from the immense oil sands deposits in western Canada, which have 1.6 to 2.5 trillion barrels of oil in place. With steam injection, reservoir pressures and temperatures are raised. These elevated pressure and temperatures alter the rock stresses sufficiently to cause shear failure within and beyond the growing steam chamber. The associated increases in porosity, permeability, and water transmissibility accelerate the process. Pressures ahead of the steam chamber are substantially increased, which promote future growth of the steam chamber. A methodology for determining the optimum injection pressure for geomechanical enhancement is presented, which allows operators to custom-tailor steam pressures to their reservoirs.

In response, these geomechanical enhancements of porosity, permeability, and mobility alter the growth pattern of the steam chamber. The stresses in the rock will determine the directionality of the steam chamber growth, and these are largely a function of the reservoir depth and tectonic loading. By anticipating the SAGD growth pattern, operators can optimize on the orientation and spacing of their wells.

Monitoring of the SAGD process is central to understanding where the process has been successful. Methods of monitoring the steam chamber are presented, including the use of satellite radar interferometry. Monitoring is particularly important to ensure caprock integrity, as it is paramount that SAGD operations be contained within the reservoir.

There are several quarter-billion dollar SAGD projects in western Canada that are currently in the design stage. It is essential that these designs use a fuller understanding of the SAGD process in order to optimize on well placement and facilities design. Only by including the interaction of SAGD and geomechanics can we achieve a more complete understanding of the process.

Introduction

Geomechanics examines the engineering behaviour of rock formations under existing and imposed stress conditions.SAGD imposes elevated pressures and temperatures on the reservoir, which then has a geomechanical response. Typically, the SAGD process is used in unconsolidated sandstone reservoirs with very heavy oil or bitumen. In situ viscosities can exceed 5,000,000 mPa•s (mPa•s ? cP) under reservoir conditions.

These bituminous unconsolidated sandstones, or "oilsands" are unique engineering materials for two reasons:firstly, the bitumen is essentially a solid under virgin conditions; and secondly, the sands themselves are not loosely packed beach sands. Instead, they have a dense, interlocked structure that developed as a result of deeper burial and elevated temperatures over geological time. In western Canada, the silica pressure dissolution and re-deposition over 120 million years developed numerous concavo-convex grain contacts[1,2,] in response to the additional rock overburden and elevated temperatures. As such, these oilsands are at a density far in excess of that expected under current or previous overburden stresses. Furthermore, once oilsands are disturbed, the grain rotations and dislocations preclude any return to their undisturbed state.

Oilsands, by definition, have little to no cementation. As such, their strength is entirely dependent upon grain-to-grain contacts, which are considerable in their undisturbed state. These contacts are maintained by the effective confining stress. Any reduction in the effective confining stress will result in a reduction in strength. Since the SAGD process increases the formation fluid pressure, it reduces the effective stresses and weakens the oilsand.

Disturbance.

Once the individual sand grains rotate and translate, there is an increase in bulk volume ("dilation") due to an increase in porosity. The associated increase in absolute permeability can be a factor of 10. It is this remarkable behaviour of oilsand that makes geomechanics so important to the SAGD process.

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