Natural and Gas-lift in SAGD Production Wells

Abstract

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 designs 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 and a discussion of control methods for self-flowing SAGD production wells are included. 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 less attractive for very deep reservoirs-the high operating pressure required to produce natural lift would then result in poor oil-steam limiting the viability of SAGD for such reservoirs. Even for more shallower reservoirs, the production well has to be designed so that it is self-flowing. Therefore it is necessary to perform pressure drop calculations to aid in the well design. This paper describes the development of a computer program RISEWELL for the analysis of flow in 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. Although the fluid flow in SAGD wells is not strictly steady, owing to slow changes in the temperatures, pressures, and flow rates with time, the assumption of steady flow (particularly, constant mass flow rate in every cross-section at a given time) will be made to simplify the equations.