Abstract

This paper presents an analysis of the pump pressure while pumping gravel pack slurry down into pressure while pumping gravel pack slurry down into the well. Five field cases from the Troll field in the North Sea are evaluated. The rheological behaviour of the prepad and gravel pack slurries have been investigated. The pump pressures have been simulated by fluid mechanical analysis and are compared with measured values.

Introduction

During recent gravel pack bobs in gas wells at the Troll field in the North Sea, the observed frictional pressure losses were lower than those predicted by standard techniques. In one of the predicted by standard techniques. In one of the bobs a significantly higher pump pressure than expected was observed when the sand slurry was pumped down in to the well. As this was not pumped down in to the well. As this was not anticipated the bob was terminated. A reduction in pump pressure is expected while pumping the gravel, pump pressure is expected while pumping the gravel, after having pumped the prepad. This results from the additional hydrostatic head caused by the heavier sand slurry. In all except one, of these gravel pack bobs the pump pressures did not decrease after introduction of sand.

The pump pressure have been evaluated, given the prepad and sand slurry properties. The influences prepad and sand slurry properties. The influences of temperature, rheology, density differences and flow properties on the frictional pressure loss and hydrostatic head have been studied. The results of this study have been used to analyse five of the above mentioned gravel pack bobs. The case histories of these bobs are presented.

Recently, models have been presented which describe the flow while the gravel is set in place. Less attention has been given to the apparently simpler case of flow of prepad and sand slurry down the gravelpack pipe. The prepad pressure loss can in theory be calculated using analytical models. The rheological behaviour of the sand slurry is more difficult to evaluate. There are models for dilute particle suspensions like the Einstein equation. particle suspensions like the Einstein equation. Similarly, there exist rheological models for concentrated coal-fuel oil suspensions. Borghesani has presented a suspension viscosity function, as given by Equation 1.

The viscosity of the suspend in medium,, is dependent on temperature, T. Since the solids are chemically inert, no temperature dependence is expected in the concentration factor. The results are correlated using an exponential factor in which c is the particle concentration by weight and A is a numerical constant. For coal-fuel oil suspensions this model correlates well for concentrations up to at least 60% by weight. The particle sizes in gravel pack slurries are several magnitudes larger than the coal powder. It was therefore neccessary to check the validity of this equation for HEC-gravel suspensions. In field practice during gravel pack jobs, there are several models similar to pack jobs, there are several models similar to Equation 1. One of these was investigated and was found applicable.

WELL GEOMETRY AND SURFACE EQUIPMENT

All the investigated wells were approximately 1550 meter measured depth from rotary kelly bushing (RKB). RKB seabed depth was 325 m. The upper part of the well is a vertical section down to 460m RKB. Then two deviated sections are buildt up. First a section is build up with an average deviation of approximately 23.5 deg. and a maximum deviation of 300, ranging down to 1050 m Total Vertical Depth. This section is followed by a section with an average deviation of 14 deg. down to the bottom of the well.

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