Abstract

Adequate lifting of produced fluids is an important issue for SAGD producers. It is often necessary to evaluate the natural lift capability of the SAGD process for given producer well design and modify the design if self-flowing capability of the well is a requirement.

This paper describes the methodology for calculating pressure, temperature and fractional vaporization of water profiles along SAGD producers, leading to the development of a computer program RISEWELL to perform such calculations. Sample calculations using the program indicate unstable flow regimes for low fluid flow rates. The implications of this instability for lift gas injection and steam production into the well to enhance lifting capability are discussed in relation to the example used for the sample calculations.

Introduction

The analysis of fluid flow in producing wells of the Steam-Assisted Gravity Drainage (SAGD) process is necessary for evaluating the natural lift capability of the process and for well design. The lifting of produced fluids at an adequate rate is of prime importance for the successful operation of the SAGD process. In many SAGD projects, the early performance of the process has been severely limited by inadequate lifting. When this occurs the produced oil tends to be replaced by water rather than gas and steam, and a steam chamber either does not form or is confined to only the upper part of the reservoir.

If the steam chamber pressure is sufficiently high in relation to the depth of the reservoir, it may be possible to achieve natural lifting of produced fluids in a SAGD project without the use of a pump. While this mode of operation is very convenient, it is clearly not feasible for deep reservoirs - the high operating pressure required to produce natural lift would then result in poor oil-steam ratio making SAGD not viable for such reservoirs. Even for shallow reservoirs, the producer has to be designed appropriately to render it self-flowing and it is necessary to perform pressure drop calculations to aid well design.

This paper describes the development of a computer program RISEWELL for the flow analyses of SAGD producers. In this program, pressures, temperatures and fractional water vaporization profiles along the well are calculated using momentum and energy balance principles combined with heat transfer equations and experimental correlations for pressure drop in multi-phase pipe flow. Sample calculations are presented to illustrate the use of the program for the design of producers in SAGD projects.

BALANCE OF MOMENTUM AND ENERGY

The calculations are based on the fundamental principles of balance of momentum and energy for pipe flow. Consider the multi-phase flow of water, oil and gas through a deviated producer well of constant cross-sectional area (not necessarily circular) as shown in Figure 1. Let s denote the arc length measured along the centre line of the well, increasing along the fluid flow direction.

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