Abstract

Steam injection is used to recover bitumen from oil sand reservoirs in the Cold Lake region. The overburden contains the fluids injected, but responds to this injection by upheaval of the ground surface. The heave magnitude, distribution, and rate of dissipation away from the injection areas must be known to be related to the safety of civic and industrial facilities on the ground surface and to assess local environmental issues sensitive to the local topography such as stream flow.

Surface heave is influenced by many factors such as overburden structure and properties (Young's Modulus, Poisson's Ratio), depth of injection; and volume and distribution of the fluid injection. However, normalized surface heave, as a fraction of the formation expansion, dissipates away from an area undergoing injection in a manner that depends only on normalized distance, as a fraction of the injection depth, away from the steamed area. This relationship has been investigated by using the Finite Element Program ABAQUS to demonstrate it is approximately independent of all other parameters including overburden structure and properties and distribution of the fluid injection. Example field heave measurements and analytical solutions are consistent with these results.

Introduction

Hydrocarbon recovery operations have been observed to generate changes in surface elevation1. In particular, the Cyclic Steam Stimulation process in the Cold Lake region has resulted in surface uplift or heave. Knowledge of the mechanisms causing surface heave is important for the design of safe operations compatible with the environment.

MECHANISM

In all cases of heave or subsidence associated with injection or production, the root cause of the surface motion is a change in the pore system of the reservoir oil sands. Before recovery operations this pore system is filled with an intertwined volume of water and hydrocarbons at some pressure roughly correlated with the depth of burial. Recovery operations change this pore pressure by injection or withdrawal of fluids from wells penetrating the reservoir.

The burial history and the mineral composition of the grains determine the strength of the sand structure and how it deforms as the pore pressure is changed from current day, pre-development values. In all cases in which subsidence or significant heave occurs, the reservoirs are made up of high porosity unconsolidated sands, which means that the grains are not cemented together, but are in contact with each other in a manner very similar to the way they were originally laid down as sand bars or beaches.

In the northern Alberta area, several periods of glaciations have occurred in recent geological time which have put ice sheets of a couple of kilometers thickness on top of these reservoirs2. These have loaded the sands equivalently to being buried at least twice as deep as which they are currently. This means these sandstones are very insensitive to decreases in pore pressure, because the sand grains have already once been subjected to higher grain stresses than would be generated by decreasing the pore pressure to zero.

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