Analysis of superheated steam performance in offshore concentric dual-tubing wells

Superheated steam (SHS) injection in concentric dual-tubing wells (CDTW) is a new technology for offshore heavy oil recovery. Firstly, compared with conventional saturated steam injection in single-tubing wells, the key advantages of SHS injection in CDTW for heavy oil recovery are introduced. Secondly, a novel mathematical model is proposed to analysis the heat transfer characteristics of SHS flow in offshore CDTW. Then, based upon the validated model, type curves of SHS flow in offshore CDTW are analyzed in detail. The results show that: (1). The heat exchange between the integral joint tubing (IJT) and annuli has a significant influence on SHS temperature and superheat degree in each tubing. (2). More heat get lost when the seawater starts to flow, and the heat loss rate in the sea section of the wellbores is larger than that in the formation section of the wellbores. Moreover, in order to flexibly use the model in practice, the key intrinsic flow characteristics of SHS in offshore CDTW are unraveled. It is found that: (1). The SHS temperature increases at first but turns to decrease with the continuous increase of injection rate due to the fact that the drop of SHS pressure gradually replaces heat losses as the dominant factor on temperature change. (2). The superheat degree at well bottom always increases with the increase of injection rate. This paper presents a basic reference for engineers in heat loss evaluations as well as performance estimations of SHS flow in offshore CDTW.