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. Introduction Most drilling fluids used in gas or oil wells are designed to build some degree of gel strength when not under shear flow, in order to prevent cuttings and mud weighting materials from precipitating out of the fluid during periods of stopped circulation. This is important in order to prevent slumping of cuttings and to maintain the pressure balance in the well. When circulation is resumed, sections of the well bore will experience a pulse of increased pressure during a short time until the gel strength has been broken. In addition, because of thixotropic effects in most gelled muds, the viscosity of the mud will stay enhanced for some time after the gel breaking. This can be a significant effect, especially in deep water, HPHT and extended reach drilling where gelling can have a big impact on pressure losses. Pressure effects due to gel braking have been investigated through a series of laboratory experiments. It has been shown that these effects partially can be predicted from measurements on the muds with laboratory equipment. Measurements with both a high precision CarriMed CSL 50 rheometer and a common Fann 35 instrument have been compared to gel breaking effects observed in an annulus equipped with pressure sensors along the outer tube. Such high-precision, loggable rheometers, like the CarriMed instrument, are seldom available on off-shore drilling installations, and scarce enough in on-shore laboratories. It is therefore of interest to determine to what extent the pressure pulse imposed on the well due to thixotropic effects can be predicted from measurements performed with a common Fann 35 viscometer.