Stratified Flow Model and Interpretation in Horizontal Wells
Authors: Theron B.E. , Unwin T. 
Published: December 2007

A mechanistic model of stratified oil-water flows in horizontal wells is presented and a new interfacial friction factor correlation is developed using flowloop data. In the field context, horizontal refers to well deviations in the range of about 15 degrees around the horizontal. Flowloop experiments show that for pipe deviations between 80 degrees and 91 degrees from the vertical, the deviation is a key parameter of the flow: at low fixed flowrates the holdup can vary up to more than 60% over this range of deviation; the slip velocity between the oil and water phases can also change significantly and reverse its sign.

Comparisons between predicted and measured holdups and slip velocities are made. The model reproduces very well the flow behaviour observed in the flowloop experiments when the deviation changes from 91 degrees to 89 degrees. Therefore an accurate measurement of the well deviation around the horizontal is necessary in order to make a correct interpretation.

The model can be used to interpret production logging data (spinner, markers, holdup, …) in horizontal wells when there is insufficient data to compute the oil and water flowrates directly. For example: the slip velocity can be evaluated from the model using only holdup and total flowrate measurements, and hence the flowrates of each phase can be computed.

The effect of a production logging tool string on the holdup is investigated using the model. It is found that the tool can change the holdup by up to 10%, and hence a tool body must be included in the model to interpret field data.

Introduction

Production logging in horizontal wells, which are being drilled more and more, is a new challenge when more than one fluid is produced. In the field context, horizontal refers to well deviations in the range of about 15 degrees around the horizontal. A very common situation is when water is produced in conjunction with oil. In such a situation, the flow structure in horizontal conditions is completely different from that in vertical or deviated (0 to 70 degrees deviation) wells. In order to interpret the data transmitted by production logging sensors in oil water horizontal wells or to design new sensors, it is essential to understand the behaviour of such flows.

In oil-water flows the two phases can be distributed in the well in different flow patterns depending on the flowrates, physical fluid properties and well geometry. For pipe deviations between 80 degrees and 91 degrees from the vertical, our flowloop experiments show that the two fluids are segregated with the oil flowing at the top of the well cross section and the water at the bottom. The interface between the oil and water phase is either flat, the flow is said to be stratified, waver or a composition of water droplets in oil or vice versa. Field logs also confirm these observations. The experiments also show that at low flowrates a key parameter governing the flow is the deviation, as shown in the Figure 1.

Although there has been some theoretical and experimental work on liquid-liquid stratified flows in horizontal pipes, there appears to have been no work on stratified liquid-liquid flows in slightly inclined pipes.

In this paper a mechanistic model of stratified oil-water flows in horizontal wells, following a classical approach, is presented and a new interfacial friction factor correlation is developed using flowloop data.

Comparisons between predicted and measured holdups and slip velocities are made.

A method, based on the model, for interpreting production logging data when there is insufficient data to compute the oil and water flowrates is proposed. An example of the method using flowloop data is shown.

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