Abstract

The development of the SAGD process has been facilitated by the ability to predict performance from theory. Analytical and numerical methods have given results similar to those obtained in the field and in laboratory scaled models. It was realized before any field projects were undertaken that horizontal wells would be required and that production rates of hundreds or even a thousand or more barrels per day of bitumen production were possible. There was also success in predicting the quantities of steam required.

In the early analyses non-condensible gas was ignored. Since then several authors have pointed out that when dissolved gas is included in their numerical simulation models it tends to accumulate in the steam chamber, particularly towards the top, and to inhibit the process by lowering the dew point of the steam. In some cases this appears to choke the process and severely limit production and recovery. On the other hand it has been appreciated that the accumulation of gas, and even its intentional addition to the steam, can be desirable because the lowering of the temperature of the steam chamber at the top reduces the heat, and hence the steam, requirement. The SOR is improved.

In this paper the role of gas is discussed and it is shown that gas can move relatively easily, in small fingers, through the reservoir beyond the steam chamber. This allows the purging of gas from the chamber and also the pressure support of the chamber by gas flowing from the exterior. The intrusion of gas into the region above a rising chamber raises the pressure and tends to push oil downwards-the "Steam and Gas Push". Varying the steam injection rate can control pressure and allow the optimization of the gas content of the chamber. Results from a new computer program, " HOTSTEAM" will be shown. Unlike its predecessor, " HOTWELL" the new program allows the injection rate of the steam to be scheduled and it also provides for the support of the chamber pressure by gas - either from the reservoir or from injection. The program includes a continuing analysis of the production well hydraulics and predicts the WHP as a function of time for natural lift.

Introduction

The Steam Assisted Gravity Drainage Process (SAGD)[1],[2] is finding increasing application for the in situ recovery of Canada's tar sand and bitumen deposits[3]. This paper describes new concepts and ideas for the optimization of the process.

The SAGD Process

In the SAGD process steam is injected, usually from a horizontal well, into a growing steam chamber. Oil drains, driven by gravity, from the heated region around the chamber to a horizontal production well placed low in the reservoir. The main mechanism is darcy flow for the oil drainage and conductive heating of the reservoir surrounding the chamber that reduces the viscosity of the oil and allows flow at practical rates. Production rates from horizontal SAGD well pairs are typically about 100m3/d and have been reported as high as 380m3/d, Rates of this order are predicted both by analytical equations and also by numerical simulation.

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