Predicting Start-Up Liquid Loading in a Mature Oil & Gas Field: A Case Study

Abstract

Liquid loading of oil and gas wells occurs when the reservoir fails to deliver hydrocarbons to the surface due to accumulation of liquid in the well. In Oil reservoirs with gas cap and aquifer, liquid loading post shutdown revival of oil wells with high gas liquid ratio (GLR) is poorly understood and documented. The mechanism and prediction of onset of such liquid loading has been studied and discussed in this paper. The field observation of loading of high GLR oil wells could not be explained with steady state analysis in a mature carbonate offshore field. Transient analysis successfully revealed liquid loading as observed in field. However, correct set up of initial conditions is imperative for prediction using transient analysis method which is discussed in this paper. A workflow has been developed to predict onset of start-up liquid loading and to build in risking using a risk matrix incorporating relevant well parameters. The methodology was used to estimate the gas lift requirement and associated risked reserves. The liquid loading of high GLR oil wells after a brief period of shutdown has been successfully modelled using transient method by representing correct initial conditions. However, ability of well to re-deliver gas at the steady state GLR value during the start-up is an uncertainty. Hence, an element of risking has been captured to predict the most likely time of start-up loading using a risking criterion based on individual well's flow parameters. The results from the above methodology of prediction and risking show that wells are susceptible to liquid loading much in advance as predicted from steady state workflows which matches with the field observed well performance. This has a significant impact on the lift gas flowrate and top side infrastructure required to sustain flow from these wells. The paper also discusses a strong business case to evaluate liquid loading from predicted parameters at field development planning stage using the proposed workflow to secure future well production and save on additional costs of retrofitting gas lift infrastructure.