Shifting Turner's Paradigm: Gas Wells Can Produce in Slug Flow

Abstract

Abstract This paper aims at improving the understanding of gas wells stability and to shift Turner's paradigm, i.e. to demonstrate that, under specific circumstances, stable production below the liquid loading rate in the slug flow regime is feasible. Field examples are presented together with modelling results that highlight how stable slug flow gas production can be sustained and promoted. The new approach to evaluate well stability is based on conventional nodal analysis, but makes use of a total pressure drop curve rather than only looking at the well. This curve includes the wellbore, the choke and – in very tight reservoirs – the pressure drop over the near-wellbore reservoir. The minimum in this combined pressure drop curve yields the minimum flow rate for well stability. This method was applied and tested on several gas wells. Examples include two gas wells with drawdown limitations, one to avoid halite precipitation and one to reduce sand production, and a very tight gas well. Both steady state nodal analysis and dynamic simulators were used. Whereas a conventional nodal analysis indicated that stable production was not possible since the inflow performance curve and tubing performance curve did not intersect right of the minimum, the total pressure drop curve correctly predicted that these wells could flow continuously below their conventional critical rate. For the wells with drawdown limitations, this meant being able to further choke back the wells to mitigate halite precipitation and sand production, de facto extending the well production life and reducing operational intensity. For the tight gas well, this meant improving forecasting of a well for which intermittent production was deemed necessary. In conclusion, in cases where a choke or tight reservoir adds significant pressure drop, well stability is found to be dictated by the total pressure drop curve. This improved analysis can yield more accurate production forecasts and prevent unnecessary interventions. The innovative total pressure drop curve presented in this paper shifts a paradigm set by Turner in 1969: gas wells can now produce in slug flow. Furthermore, the fact that this methodology uses the existing and well known nodal analysis will simplify its wide diffusion and implementation.