Predicting long-term reservoir performance with realistic wellbore models is fraught with uncertainty owing to the complexity of two-phase flow. That is because even a calibrated two-phase-flow model departs from its expected performance trend when changes in flow conditions occur. These inevitable changes include gas/liquid ratio, wellhead pressure, and flowline pressure with time, among others. Influx of water further exacerbates the prediction problem.
This study explores the possibility of using simplified approaches to compute bottomhole pressure (BHP) from wellhead pressure (WHP), measured rates, gravity of producing fluids, and tubular dimensions. BHP computations on three independent data sets comprising 167 gas/condensate-well tests suggest that the no-slip homogeneous model applies quite well. Statistical results show the homogeneous model compares quite favorably with mechanistic two-phase-flow models. However, the main advantage of the simplified model is that its recalibration with field data is not required because the gas/oil ratio increases with time, thereby making the model increasingly reliable.
Most field data sets suggest random error in BHP calculations; uncertainty in rate measurements appears to be the most probable cause. High gas/liquid ratio (GLR) systems can tolerate large errors in rate measurements, but low-GLR wells demand greater accuracy because of increasing importance of the hydrostatic head.