Abstract

An investigation was made into the efficacy of various post-communication development processes in an inverted [our-spot pattern in an Athabasca oil sand deposit. The recovery mechanism is the gravity drainage of mobilized bitumen into a previously established basal communication zone in the reservoir. Several different operating procedures were considered using two-dimensional (cross-sectional) thermal reservoir simulations. Five cases were considered:

  • a direct steam flood of dead bitumen;

  • pressure cycling dead bitumen;

  • pressure cycling dead bitumen with CO2, added during the pressure build up cycle, using CO2, concentrations of 3 mol%. and

  • 5 mol%.;

  • pressure cycling live bitumen.

The efficiency of each process was compared using the cumulative oil-steam ratio (OSR) achieved. The results show that the addition of 3 mol%. CO2, yielded the highest OSR, while the addition of 5 mol% was slightly less effective. The total recoveries at the cut off time were actually highest when no steam additive was used, but the process efficiency measured by the OSR was poorest A study was also undertaken to investigate the sensitivity of the gravity drainage process to anisotropy in the permeability, grid refinement, and relative permeabilities.

Introduction

The study is of an Athabasca oil-sand deposit in which 26m of pay were identified, of which the bottom 4-6m contain a low bitumen saturation. The proposed recovery scheme required the development of communication between the injection and production wells in the low bitumen saturation basal zone. Once communication had been established the bitumen was expected to be recovered through the mechanism of gravity drainage, enhanced by steam pressure cycling and steam additives.

A numerical study can provide insight into combinations of operating procedures with particular emphasis on individual mechanisms. In this work pressure cycling a dead bitumen was compared to a direct steam drive. Subsequently, the addition of carbon dioxide to the steam was considered during the reservoir pressure build up portions of the pressure cycles. Then the study was extended to the effect of a solution gas-oil ratio (GOR) in the bitumen. The final study was of parametric sensitivity, to provide a level of confidence in the results generated.

The simulations were performed using an element of symmetry in an inverted four-spot pattern. A 5 × 10 grid was chosen to represent the reservoir for silJ1ulation, comprising five areal blocks of irregular widths and ten vertical blocks.

Gravity Drainage

Butler and Stevens1 have fully described the gravity drainage process in bitumen, where vertical and lateral growth of a steam chamber occurs. Closmann and Smith,2 discussed "ceiling drainage" into a horizontal fracture, where the steam front only expands vertically, heat being applied al an interface covering the entire base of the tar sand. This latter form of gravity drainage could occur where an initial communication path was present, such as a basal water sand. For these two cases the essential mechanics of recovery are the same.

Pressure Cycling

One complete pressure cycle consists of raising the reservoir pressure to a given maximum followed by blow down of the reservoir.

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