Transient Gel Breaking Model for Critical Wells Applications with Field Data Verification

Abstract

Abstract The paper addresses the problem of predicting pressure peaks when starting pumps after a static period while drilling a well. It demonstrates that laboratory measurements with standard Fann viscometers and a gel breaking model can be used for predicting such peaks. Fluids with properties that are similar to those of real drilling fluids have been studied in detail in the laboratory. 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. A local model based on rheometer measurements that predicts gel breaking pressures versus time is presented. The authors are not aware of any other such model. 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 pumps Controlled field measurements have been made 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. The observed effects are expected to be of importance when drilling any critical well, which may be deep water, HPHT, or extended reach wells.