Integrated Wellbore/Reservoir Model Predicts Flow Transients in Liquid-Loaded Gas Wells
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 110461, "Integrated Wellbore/Reservoir Model Predicts Flow Transients in Liquid-Loaded Gas Wells," by Gaël Chupin, SPE, Bin Hu, SPE, and Tor Haugset, SPT Group; Jan Sagen, IFE; and Magali Claudel, SPE, Gaz de France, prepared for the 2007 SPE Annual Technical Conference and Exhibition, Anaheim, California, 11–14 November.
An integrated wellbore/reservoir model was used to investigate liquid loading in a gas well. The well produces from a storage reservoir, and it experiences water coning from an aquifer. The integrated model showed how the water cone caused the gas-flow rate from each gas layer to decrease and the liquid holdup in the wellbore to increase. Depending on reservoir conditions, the well may enter into a mode of unsteady production, during which the gas-flow rate cycles over a period of several days. The simulation revealed the reason for this unsteady flow.

Introduction
Liquid loading in gas wells is a challenge in mature fields. Several techniques have been developed to address the issue. There is a growing demand for better simulation tools to optimize the operations.
A dynamic wellbore/reservoir integrated simulation is required when studying the transient liquid-loading processes in gas wells. A reservoir model is needed to simulate the changes in well productivity and phase mobility as fluid saturations gradually change in the near-wellbore region. A transient wellbore multiphase-flow model is required to predict the onset of loading and the flow transients resulting from liquid accumulation.
Further, the mechanisms of the many liquid-loading-mitigation techniques (e.g., plunger lift, compression, pumping, and gas lift) are dynamic processes in which the variables change and interact constantly. The complete system, from the reservoir to the receiving facility, must be considered rather than studying individual components separately.
A coupled model of the wellbore and near-wellbore reservoir is required to simulate the dynamics accurately, analyze the phenomena, and numerically test the remedial actions or control schemes. A dynamic well/reservoir model has been developed to simulate oil production from thin oil rims subject to gas and water coning. The model was used to develop gas- and water-coning-control schemes. However, the transient-wellbore-flow model was too simple to be used widely on complicated well flows. A reservoir simulator was linked with a commercial transient-multiphase-flow simulator to simulate the formation heading and liquid loading in a gas well. Both the reservoir and the well-bore model are comprehensive and have strong functionalities. Unfortunately, the link itself is explicit, which limits the simulation speed and numerical stability. An integrated dynamic wellbore/reservoir model is presented.

Wellbore/Reservoir Model
Near-Wellbore Reservoir Model. The model can simulate transient three-phase (gas/oil/water) flow in a porous medium. The flow equations can be solved in one, two, or three dimensions, yielding saturations and pressures varying in space and time.
No energy equation is solved; instead, the reservoir is assumed to be isothermal. The equations are solved by use of the Newton-Raphson iterative method at each timestep.