Surge Pressures: Are Steady-State Models Adequate?
Authors: Mitchell R.F. 
Published: December 2007

SPE Member

It is generally accepted that pulling and running pipe causes pressure surges. The prediction of pressure surges is of economic importance in wells pressure surges is of economic importance in wells where the pressure must be maintained within narrow limits to prevent lost circulation and formation fluid influx. For well control in these situations, the drilling engineer needs an accurate method of calculating surge pressures.

Surge pressures are commonly calculated using steady state flow assumptions. Recently, fully dynamic surge calculations have become available. In this paper, sensitivity studies are used to quantify the effects of fully dynamic surge predictions. The cases used in the sensitivity studies are actual field tests with surge pressure measurements. The result of this sensitivity study is a set of guidelines for development of future steady flow surge models.

These guidelines are developed by investigation of the following factors not present in steady flow surge models:

*Fluid compressibility

*Fluid inertia

*Pipe longitudinal elasticity

*Pipe distance off bottom hole

The field test cases are studied with a dynamic surge model to show the contribution of each of these factors.

Introduction

Pressure surges have long been known to cause well control problems, e.g. Cannon in 1934 and Goins in 1951. In most wells, proper casing design and mud programs leave large enough surge pressure safety margins. In critical wells, however, pressure surges must be maintained within narrow pressure surges must be maintained within narrow limits. Also, some operations are particularly prone to large pressure surges, such as running low clearance liners in deep wells.

Pressure surges in critical wells are commonly determined using steady flow surge models, such as those proposed by Burkhardt, Fontenot and Clark, and Schuh. In these models, the drilling mud is perfectly displaced by the pipe motion. Fluid pressures are calculated to be consistent with non-Newtonian frictional pressure drops due to this fluid motion. These models neglect fluid inertia (Burkhardt includes an approximate inertia effect), the compressibility of the fluid and wellbore, and the axial elasticity of the pipe. The lack of fluid compressibility is considered a conservative assumption because it predicts a higher flow rate which generates a higher frictional pressure drop. The neglect of fluid inertia is not a conservative assumption because, for example, negative pressure surges may occur due to fluid backflow when the pipe is brought to rest. Axial elasticity of the pipe reduces the rate of fluid displacement, so rigid pipe displacement is considered conservative.

Fully dynamic surge models have been developed because of the uncertainty in the steady state model assumptions. The first fully dynamic surge pressure model was developed by Lubinski et. al. This model emphasized the importance of compressibility in pressure calculations. Lal has corrected a number of pressure calculations. Lal has corrected a number of deficiencies in the Lubinski model and begun an investigation of parameters affecting surge pressures. Both Lubinski and Lal assumed rigid pipe displacement. Mitchell added the effect of pipe axial elasticity to dynamic surge analysis.

Fully dynamic surge models are much more complex and require much more computer resources than steady state surge models.

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