Drilling in the Brent field began to experience severe lost 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 drilled before the onset of depressurisation experienced few losses.

The lost circulation problems result from a narrowing of 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 mud weight to maintain stability especially in the sub-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 lower than would have been predicted from a conventional geomechanics estimation, and this has extended the duration of the Brent drilling programme during depressurisation.

The Brent in-fill drilling programme continues very successfully, through accurately estimating the fracture gradient decline which accompanies depletion using the field validated depletion constant model, and through a team wide Equivalent Circulating Density (ECD) management approach to drilling. The key objectives of this paper are, a) to explain several of the Brent mud loss incidents which accompany depletion, and b) to outline the approach used to develop a reliable depletion constant model in support of continuing drilling operations and the enhanced ECD management strategy.

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