Managing Drilling Risk in a Mature North Sea Field
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 124666, ’Managing Drilling Risk in a Mature North Sea Field,’ by Brett McIntyre, SPE, Ted Hibbert, SPE, Donald Keir, and Rachel Dixon, Apache North Sea Ltd., and Tom ORourke, SPE, Farid Mohammed, Adam Donald, Liu Chang, Anzar Syed, and Valerie Biran, SPE, Schlumberger, originally prepared for the 2009 SPE Offshore Europe Oil and Gas Conference and Exhibition, Aberdeen, 8-11 September. The paper has not been peer reviewed.
The likelihood of losses increases as reservoir pressures decline while higher mud weights are needed to prevent collapse of overburden shales as targets are pushed farther from the platform. Drilling parameters for the Forties field have become fairly well established after years of experience, yet 65% of the wells drilled between 2002 and 2007 experienced incidents attributed to instability. Through a better understanding of the field geomechanics and past drilling events, the drilling team has implemented fit-for-purpose drilling procedures that have improved drilling efficiency significantly.
Introduction
Discovered in 1970, the Forties field has been developed and produced from five platforms in the UK sector of the central North Sea since 1974. There are now more than 300 boreholes in the area. In recent years, the drilling program has pursued infill targets from donor wells sidetracked near the base of the 95/8-in. casing or step-out targets with sidetracks higher up existing wells.
Of the 94 boreholes drilled over a 5-year period (2002–07), 45% were lost because of drilling or completion problems. A bottomhole assembly (BHA) was lost in 40% of these failures. Even the successful wells encountered various drilling problems, and when this is taken into account, 65% of all the wells drilled during this period experienced some degree of instability. As field production declined, economic viability demanded a step change in performance.
Drilling parameters for the Forties field have become fairly well established after years of experience. Over the period, mud weights tended to creep up in response to cavings and packoffs and then were lowered when losses became of greater concern. Where these events became more than a minor problem, drilling practices frequently were responsible. Fragile zones often are stable if undisturbed, but mechanical or hydraulic disturbance will generate debris, sometimes in considerable volumes.
Were the holdups, stuck pipe, pickoffs, and loss incidents in the Forties field caused by hole instability as a result of incorrect mud weights or caused by drilling practices? Is the debris in the borehole fresh cavings or unremoved cuttings? If the cause of these problems can be identified correctly, appropriate drilling and operational contingencies can be defined before hazards are encountered. When hole problems do develop, appropriate remedial actions can be taken on the basis of an understanding of what is occurring downhole. Incorrect diagnosis can lead to an inappropriate solution, exacerbating the problem and leading to further damage.