The Need to Understand the Dynamic Interaction Between Wellbore and Reservoir in Liquid Loaded Gas Wells

Abstract

Liquid loading in gas wells is a phenomenon where the liquid content of the well is sufficient to create a back pressure (usually dominated by gravitational pressure changes) which restricts, and in some cases even stops, the flow of gas from the reservoir. Liquid loading is an all too common problem in mature gas fields around the world. It is estimated that in the U.S.A. alone, at least 90% of the producing gas wells are operating in liquid loading regime. The phenomenon is more detrimental in tight wells than in prolific wells and it poses a serious problem in subsea tie-backs, where back pressure effects through the risers and the flowlines may have an important role. Such is the importance of liquid loading; the oil and gas industry has devoted a lot of attention to the alleviation of the problem using various measures. However, the fundamental understanding of the associated phenomena is still surprisingly weak. This applies not only to the flows in the wells, but also to how these flows interact with those in the reservoir. It is this latter dynamic interaction that has received the least attention by the industry. Reliable predictive models to link the well dynamics with the intermittent response of a reservoir, that is typical of liquid loading in gas wells, remain unavailable. This paper introduces the complexity of liquid loading and critically reviews recent attempts to model liquid loading and the dynamic interactions between reservoir and wellbore. The paper then illustrates the need for a better understanding of the transient flow phenomena taking place in the near-wellbore region of the reservoir. This includes re-injection of the heavier phase, a phenomenon that has yet to be proven by fluid mechanics.