Abstract

DrillingintheBrentfieldbegantoexperienceseverelost circulation problems after the start of depressurisation in 1998. The losses occur in the reservoir sections which are penetrated by extended reach and near-horizontal wells. In contrast, wells drilledbeforetheonsetofdepressurisationexperiencedfew losses.

Thelostcirculationproblemsresultfromanarrowingof the drilling operating window as reservoir depletion gradually reduces the fracture gradient. Numerous interbedded reservoir shales, which are sub-seismic in thickness, require a minimum mudweighttomaintainstabilityespeciallyinthesub- horizontal sections. Problems become acute when the fracture gradient is reduced to a level below the equivalent circulating density required to successfully drill the shales.

The rate of decline in the fracture gradient has been much lowerthanwouldhavebeenpredictedfromaconventional geomechanicsestimation, andthishasextendedtheduration of the Brent drilling programme during depressurisation.

TheBrentin-filldrillingprogrammecontinuesvery successfully, throughaccuratelyestimatingthefracture gradientdecline which accompaniesdepletionusingthefield validated depletion constant model, andthroughateam wide Equivalent Circulating Density (ECD) managementapproach to drilling. The key objectives of this paper are, a) to explain severaloftheBrentmudlossincidentswhichaccompany depletion, andb)tooutlinetheapproachusedtodevelopa reliabledepletionconstantmodelinsupportofcontinuing drillingoperationsandtheenhancedECDmanagement strategy.

Introduction

In-filldrillingofdepletedreservoirsisrecognisedtobean increasinglyproblematicissueforthepetroleumindustry which has been the subject of increasing interest. This reflectstheongoingfocusinmaturedepletedareasandthe needtorealisethe maximumeconomicrecoverablereserves throughin-filldrillingcampaigns.Recentlylostcirculation problemshavebeenexperiencedduringin-filldrillingofthe Brentfield, whichhavebeenlinkedtothefield depressurisation.

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