This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 96685, "Pioneering ‘Gate Valve Milling’ Operation Performed With Coiled Tubing, Recovers an Important Oil Producer Offshore Brunei," by N.M. Al Araimi, SPE, Brunei Shell Petroleum; B. Gedge, SPE, and K. Burns, Weatherford; and M. Mahajan, SPE, BJ Services, prepared for the 2005 SPE Annual Technical Conference and Exhibition, Dallas, 9-12 October. Brunei Shell Petroleum offshore Well A was shut in because the 3 1/8-in. master valve on the 5,000-psi monoblock tree was stuck in the closed position because of scale. Two options were considered for recovering the well with well intervention techniques rather than a complete workover and well-kill operation. Because the nearest gate-valve milling machine was in Europe, the valve was milled by use of a coiled-tubing (CT) unit that was on the platform performing a stimulation program on adjoining wells. This operation had been performed only once before, in the North Sea. Summary During routine wellhead maintenance, the lower master gate valve on Well A was found to be stuck in the closed position, most likely because of a broken stem related to scale buildup and attempts to open the valve. A number of attempts to free the seized valve by turning the wheel handle failed. The lost production caused by this well being offline was 1,260 BOPD, with a monetary loss of U.S. $1.9 million for every 30-day period. Calcium carbonate scaling tendencies were identified as a likely cause of both the valve seizure and the progressive production decline observed since completion. A hydrochloric acid (HCl) soak treatment was performed, pumping 20 to 25 bbl acid into the wellbore. This treatment did not return the valve to operable service. The HCl soak treatment was repeated. Because the gate valve was closed, this treatment consisted of a series of 1- to 1.5-bbl acid soak pills that were pumped and left to soak on top of the closed valve. After several hours, each spent pill was displaced through the flow wing outlet. This treatment also was not successful. An alternative approach was required, and its timing was driven by a compliance need to access and test the subsurface safety valve (SSSV) that was beneath the gate valve. Milling would allow throughbore access to test the SSSV and set plugs to allow tree repair. Considerations Use of a gate-valve milling machine has proved to be successful in the past in a number of locations in the world, particularly in geothermal operations, where tree valve seizure is a common occurrence because of the often high scale production and cooling at surface, causing scale precipitation. The problem for this option was that the only unit available was in Europe. To mobilize the unit to Brunei would be expensive and time consuming, and additional equipment like blowout preventers (BOPs) also would have to be mobilized, making this a very expensive option, even if it was sound technically.