Abstract

Well control can be an unpleasant experience. In its initial stages the problem often appears unconquerable and weeks can pass without progress.

This paper is about the successful abandonment of such an episode of well control, in deepwater (i.e. greater than 300 m water depth), that was initially suspended with a closed-in subsea BOP. The BOP held in place a sheared 5-in. drillpipe that had been intermittently blowing dry gas and formation cuttings to the rig floor for 20 days.

The objective of the well-recovery operations was to re-enter the well and properly abandon it without creating another uncontrolled situation. All operations were to be conducted in compliance with HSE protocols.

It was paramount to have total control of the well at all times necessitating two BOP stacks. This contingency resulted in the suspension of a heavy load on the seabed, subject to excessive bending moments during bad weather. The well head arrangement was essential but the time loss caused by weather conditions was becoming intolerable. New and innovative plans made during the well-control operations cut 3 to 4 weeks of uninterrupted operation down to 5 days of operation. Cementing under pressure was the key abandoning solution. Several scenarios were drawn up for this eventuality and this paper elaborates on the decisions, choices, and solutions made during the final abandonment operation.

Introduction
Pre-Intervention Status of Well

A blowout occurred on the exploration Well A while drilling the 12 ΒΌ-in. section, after having set the 13 3/8-in. intermediate casing. The rig crew was safely evacuated. After the initial blowout incident the well flowed up the annulus for 4 days before the remotely operated vehicle (ROV) activated closure of the lower pipe variable bore rams (VBR) on the seabed (Fig. 1). Flow of dry gas and formation continued intermittently up the drillstring for a period of 20 days until the ROV activated the blind-shear ram (BSR). The well was left alone thereafter for several months while a recovery plan was put in place. During this time the seabed was monitored regularly and revealed gas leaking from the closed BSR.

It was estimated that over 10,000 bbl of equivalent mass (solids) exited from the drilled hole section during the blowout. Therefore, whether to expect a large gas-filled void downhole or a collapsed wellbore owing to the unconsolidation of the formation caused by the large amount of rock removal was uncertain.

Because the status of the openhole section was unknown and gas was possibly breaching out of the 13 3/8-in. casing shoe, simply bullheading kill-weight fluid down the wellbore to control the well might not have been effective. Not only would vast quantities of mud be required to fill the void, but removal of such a large volume of gas would be an arduous and time-consuming operation with no guarantee of success. The well-intervention philosophy was therefore focused on achieving isolation within the 13 3/8-in. casing shoe. Kill-weight mud would first be pumped into the formation to assess whether it could at least control the wellhead pressures.

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