Modeling Impact of Gas Well Deliquification Measures on Capacity and Reserves

Kees Veeken

NAM – Shell The Netherlands

Abstract

Gas well deliquification is gaining importance outside onshore USA and Canada where operating conditions are typically more stringent (subsurface safety valve) and more costly (offshore). This justifies improved production forecasting to better assess the incremental capacity and reserves associated with different deliquification techniques.

A simple analytical forecasting tool is presented in which the reservoir is described by a tank model, the inflow performance by the Forcheimer Darcy equation and the outflow performance by the Cullender-Smith model and a modified Turner equation. The impact of deliquification is captured by modifying the Forcheimer, Cullender-Smith and/or Turner constants.

Example production forecasts are presented which illustrate that the benefit of deliquification measures critically depends on the inflow performance i.e. that the best deliquification strategy for prolific gas wells is very different from the best strategy for tight gas wells.

Limitations of the current simple model are highlighted. At the same time ongoing efforts to further improve production forecasting are discussed. Key aspects that need further development are:

  • The classical Turner equation carries significant uncertainty. Dynamic multiphase wellbore flow modeling is pursued to better predict the moment that liquid loading starts. Liquid loading field data are collected and analyzed for calibration and/or validation.
  • In some cases gas wells will produce at stable rates below the liquid loading rate. This so-called meta-stable rate will reduce the incremental value of deliquification. Field data are being collected and analyzed.
  • In many cases simple tank, inflow and outflow models do not reliably describe the interaction between reservoir and wellbore. Especially in case of tight gas (low permeability) or slow gas (compartmentalization, dual porosity system) gas wells can operate in cyclic mode for a long period thereby reducing the incremental value of deliquification measures.

Likewise the simple model does not accurately capture the benefit of most deliquification techniques. For example plunger lift escapes an easy description and is very much reservoir dependent. Alternatively, foam injection has been reported as highly beneficial in wells producing above the liquid loading rate. Coupled dynamic modeling of reservoir flow and multiphase wellbore flow is pursued to better predict the benefits of deliquification.