On Subcool Control in Steam-Assisted-Gravity-Drainage Producers— Part I: Stability Envelopes

Abstract

Summary Steam-assisted-gravity-drainage (SAGD) industry experience indicates that the majority of producer workovers occur because of liners or electrical submersible pumps (ESPs), and both failures appear to result from inefficient “steam-trap control.” Thermodynamic steam-trap control, also termed “subcool control,” is a typical operation strategy for most SAGD wells. Simply, subcool (or reservoir subcool vs. pump subcool) is the temperature difference between the steam chamber (or injected steam) and the produced fluid. The main objective is to keep subcool higher than a set value that varies between 0 to 40° and even higher values. This study presents a method to calculate the liquid-pool level from the temperature profile in observation wells, and liquid-pool shrinkage as a function of time. Unfortunately, it is not practical to monitor the liquid level by having observation wells for every SAGD well pair. For this reason, the algebraic equation for liquid-pool depletion on the basis of wellbore-drawdown, subcool, and emulsion productivity is generated. By use of this equation, the envelopes are suggested to differentiate three different regimes: “stable production,” “liquid-pool depletion,” and “steam-breakthrough limit.” Gas lift operations such as the MacKay River thermal project suggested that envelopes for constant wellbore drawdown are not practical. Therefore, the steam-breakthrough limit is defined for constant rate, which is more consistent in gas lift operations. In this study, the steam-breakthrough limit is validated for operation data from the MacKay River. This study provides a new insight into how factors such as production rate and wellbore drawdown can compromise subcool control and cause steam breakthrough, and how liquid-pool depletion may result in uncontrolled steam coning at long time. As a part of this study, a minimum-subcool concept (or target reservoir subcool) is presented as a function of skin and pressure drawdown. It is shown that the minimum subcool is highly dependent on the maturity of steam-chamber and underburden heat loss especially for zero-skin producers. The results of this work emphasize that the target subcool on the producer should increase slightly with chamber maturity, considering that the skin is nonzero for most SAGD producers.