Engineering Analysis of the Recovered SAFOD Borehole Instrumentation

The basic data set was acquired during 18 instrumentation setting tests conducted from 2004 through 2007 as part of SAFOD. Active hydrocarbon-rich water invaded the borehole because the borehole had not sealed at the bottom. For this reason, the water in the casing reached the cable head and caused a malfunction. Attempts were made to use better O rings, Krytox oil, and epoxy seal, but all failed. In most of these deployments, standard wire-line was used and the average failure time was two weeks. Based on design review, many improvements such as stainless tube encapsulate electric cables, all-metal seals, and Swageloks were applied. Setting of improved downhole instrument packages was carried out in the autumn of 2008. Still, these instruments could not endure the high temperature environment of 120 degrees Celsius for a long period of time, and all were offline in less than a month. A SAFOD engineering subcommittee was established in 2010 to investigate the problems caused through an examination of the removed instruments. It became clear that fluid including active hydrocarbons had invaded the stainless tubing from the seal part and caused all instruments to fail. Based on the above-mentioned experience, recommendations for the future are as follows. - Perform strict oversight of subcontractors. - Perform robust risk analysis and prepare a risk mitigation plan. - Multiple checks should be performed on points of failure by different people. - If possible, avoid the use of active electronics in environments like those in SAFOD. - Components must be rated for use in the temperature range of the environment in which they are to be used. - If a failure occurs, remove the instrumentation in a timely manner. - The SAFOD POD design was not inherently flawed but poorly implemented. (author)