A New Method Using Wellhead Measurement to Approximate Unsteady- State Gas-Water Two-Phase Flow in Wellbore to Calculate Inflow Performance
Authors: Lin Q. , Zhang L. , Lin Y.P. , Xie N.X. 
Published: March 2010

An accurate analysis of the pressure and water inflow at the bottomhole of a gas well with free water influx is important for determining an optimized artificial lift method. The commonly used two-phase flow correlations based on steady-state flow have been successfully applied to gas wells in homogeneous reservoirs. However, for reservoirs with a high degree of heterogeneity, such as fractures, the existing techniques often fail due to the unsteady-state flow behaviour from reservoir to wellbore.

This paper presents a new method that is derived based on the unsteady-state two-phase flow phenomena and the conservation of mass, taking into consideration the slippage effect of the two phases. In this method, the fluid flow is divided into a number of segments at which pressure drop and gas-liquid distribution under unsteady-state flow conditions can be calculated utilizing the measured data at the wellhead. In this case, more representative bottomhole pressures and water inflow rate changes with time can be calculated, permitting the selection of an optimum artificial lift method.

The new method has been successfully tested. A field case is presented involving a gas well in the Sichuan Field in China. The results of the new calculation method and the steady-state calculations were compared with field measurements to determine the accuracy of the method.

Introduction

The total amount of gas that can be produced from a gas well with free water influx largely depends on its ability to lift water. Sometimes, it is necessary to engage artificial lift to dewater the well. Accurate forecasting of gas well deliverability requires accurate predictions of pressure loss and gas and liquid inflow performance to select and design the appropriate artificial lift method.

Most often, two-phase flow correlations are employed to determine flow rate and pressure loss in gas wells. Those methods are generally adequate for wells completed in homogeneous reservoirs that exhibit steady-state flow. But these correlations typically fail in reservoirs with a high degree of heterogeneity, such as fractures, due to the unsteady-state flow behaviour from the reservoir to the wellbore.

The modelling calculation presented in this paper outlines a method using wellhead measurements to calculate sandface pressure and flow rate in order to design artificial lift systems to manage water loading problems and improve production and ultimate recovery. An example of the application of this method for a gas lift design is included in this paper.

Theoretical Development

Gas flows intermittently with water from a fractured formation with free water influx. The intermittent flow is maintained in the reservoir into the wellbore. By studying gas-water unsteady-state flow in the tubing, it is possible to estimate fluctuations along the entire flow path from the sandface to the wellhead.

The production of gas and water at the wellhead is not synchronized with the sandface. This is primarily due to wellbore storage effects.

The slippage between water and gas under unsteady-state flow conditions causes time dependent flow rate changes to the gas and liquid phases as the gas arrives at the wellhead.