Abstract

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

This paper presents a new model that is derived based on the unsteady-state two-phase flow phenomena and the conservation of mass taking into consideration for the slippage effect of the two phases. In this model, the fluid flow is divided into a number of segments at which pressure drop and gasliquid distribution under the unsteady-state flowing conditions can be calculated interactively utilizing the measured data at the well head. In this case, the bottom-hole pressure and water inflow rate change with time can be obtained and therefore, the selection of an optimized artificial lift can be achieved.

The new model has been successfully tested. In this paper, a field case is presented. The validity of the new method was illustrated by comparing the results obtained from the existing steady -state model and the new unsteady-state model.

Introduction

The total amount of gas a wet well can produce largely depends on its ability to lift water. Sometimes, it is necessary to engage artificial lift to de-water in wet wells. Hence, it is important to understand pressure losses when forecasting gas deliverability, as well as to recognize the effects of gas and water inflow for designing appropriate artificial lift method for these wells.

The commonly used two-phase flowing correlations based on steady state flows have been successfully applied to the gas wells in homogeneous reservoirs. But, the existing techniques often fail in reservoirs with high degree of heterogeneity, such as fractures, due to the unsteady-state flow behaviors from the reservoir to wellbore.

This model offers the ability to estimate descriptive engineering parameters, such as sandface pressures and sandface flow rates, using wellhead measurements, which are used to design artificial lift system to manage water loading problems and improves production and ultimate recovery based on those parameters. A gas lift design is induced to the paper as an example of the application in the model.

Theoretical Development

Wet gas seeps intermittently in a fractured formation, hence wet gas inflow from the formation to the sandface and the wet gas flow in tubing is unsteady and fluctuating. By studying wet gas unsteady-state flow in the tubing, we may be able to capture fluctuating gas and water inflow behaviours at the sandface.

Once the flow of wet gas into the wellbore begins to fluctuate, an unsteady-state flow develops. The production of wet gas at the wellhead is not in synchronization with sandface. That is the water rate at wellhead is not the same as that at the sandface. This is mainly due to the effect of storage inside the wellbore.

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