The paper presents a comparison of a recently developed gel breaking model with field data in two extensively instrumented HPHT wells. The gel breaking model, which predicts pressure peaks when starting pumps after a static period while drilling a well, is described briefly. The authors are not aware of any other such model. The development of the local part of gel breaking model was based on published works and on a detailed laboratory study of fluids with properties that are similar to those of real drilling fluids. Gel breaking and thixotropic behaviour were first analysed in detail through rheometer measurements. The fluids showed a pronounced time dependent behaviour. Afterwards each fluid was placed in a flow loop where circulation was started after a static period. The time dependent signals were logged and analysed. An initial sharp pressure peak followed by a slow decrease in pressure was typical. The local model has been integrated in a transient drilling simulator which predicts the pressure peaks that follow when starting pumps after a static period. Gel strength is broken successively from the pump, down the drill string, and up the annulus. The model will help drilling engineers to determine whether extra care is required when starting pumpsThe field data were taken in two North Sea HPHT wells, one with water based and one with oil based drilling fluid. Gel breaking pressures were measured when starting pumps after static periods of different lengths. The effect of rotating the drill string prior to pumping was tested. The transient model reproduced measured data with reasonable values of gel model parameters. It is, however, not always clear how gel building time should be determined when the static time is interrupted by pipe movements. The comparison demonstrates that laboratory measurements with standard Fann viscometers and a gel breaking model can be used for predicting gel breaking peaks. The observed effects are expected to be of importance when drilling any critical well, which may be deep water, HPHT, or extended reach wells.

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