ABSTRACT

Brunei Shell Petroleum offshore well A was offline and shut in due to the 3 1/8” 5K lower master valve on the monoblock tree, being seized in the closed position, due to scale production. The subsequent loss of production was 200 m3 (1260 BOPD), with a monetary value of US$1.5 million per month at $40 per barrel.

Two options were considered for recovering the well using well intervention techniques rather than a complete workover and well killing operation.

1. Mobilize a “Gate Valve Milling Machine” the nearest being available was in Europe.

2. Mill the Gate Valve with a coiled tubing unit, which was already on the platform, performing a stimulation programme on adjoining wells.

Option one was expensive due to high mobilization costs, some HSE issues, well control but this was a tried and test method of remedying the problem. Option two was a somewhat untried operation, which had only been performed as far as BSP and the contractors involved knew, once before in the North Sea.

Given the coiled tubing unit was already on the platform, and the specialist low speed high torque motors with an anti stall device and specialist mills were availed in the time frame required. The decision process based on HSE, engineering review (SWOP), and cost considerations, chose the coiled tubing option.

The paper will examine the problem with the wellhead master valve, the HSE, engineering and cost decision process leading up to the decision to go with the coiled tubing option. The milling operation from programme inception to successful execution of the milling operation, and placing the well back on line, will review in detail.

Other operators will be able to benefit from this safe, low cost and time saving method of recovering production from wells with similar problems, which are many around the world.

SUMMARY

In Brunei Shell Petroleum Offshore during routine wellhead maintenance the LMGV on Well-A was found to be seized. Given the well was online and producing then it was assumed that the valve had seized in the fully open position though no immediate means to confirm same were available. A number of attempts were made to free the seized valve by moving / turning the wheel handle but without success.

Calcium carbonate scaling tendencies were identified as a likely cause of both the valve seizure and the progressive production decline observed since completion. As a result an HCL acid (15% pre-mix) soak treatment was performed pumping a total of 20-25 bbls acid to the wellbore. This wasn’t successful in regaining movement of the valve and returning it to fully operable service.

An HCL acid (15% pre-mix) soak treatment was repeated. Given the closed nature of the LMGV then the treatment consisted on this occasion of a series of smaller volume 1-1.5 bbls acid soak pills, which were pumped and left to soak on the top of the closed valve. Each spent pill was displaced after several hours via the flow wing outlet. The treatment was not successful in regaining movement of the valve.