Numerical and Analytical Modelling of the Gas Well Liquid Loading Process

Abstract

Summary Liquid loading is a serious problem in areas where gas fields are maturing. This paper presents an analysis of the production behavior of liquid-loaded wells over time. This clearly shows that these wells can operate at two different rates—a stable rate, at which full production is taking place, and at a lower metastable rate, at which liquid-loading effects play a role. A model has been constructed that enhances the understanding of the process of water buildup and drainage in gas wells. It assumes a single gas- and water-coproduction point and a single water reinjection point. As expected, a water column is built up in the well as soon as production takes place below the critical rate. As observed in the field, for good inflow performance, a metastable flow rate can be observed. At this state, the water-reinjection and water-coproduction rate are equal to one another, and the water-column height stabilizes. A sensitivity analysis has been carried out to determine how well parameters influence the metastable flow rate, the time required to reach this metastable rate, the corresponding water-column height, and the shut-in time required to drain this water column. The results of the analysis indicate that significant metastable flow rates occur in wells that have good inflow performance, a low water/gas ratio, and a large distance between injection and production point. Furthermore, a steady-state analytical solution has been derived for the metastable rate and stabilized water-column height confirming the numerical-analysis results. Introduction Many of the mature offshore gas fields in the southern North Sea have already experienced considerable pressure depletion resulting in significantly reduced gas flow rates and, eventually, liquid loading. Insight into the flow characteristics of these liquid-loaded wells will help manage tail-end production (i.e., will help define the most effective means of accelerating and maximizing their ultimate recovery.