The in-hole failure rate and survey accuracy are key areas when judging the performance of Measurement While Drilling (MWD) tools. Shell UK Exploration and Production (Shell Expro) extensive use of two major MWD companies has enabled the underlying trends to be identified.

The mean time between the failure rates of both directional-only and directional plus Gamma Ray tools, is presented by year from 1981 until April, 1987. Over this period, the mean time between failure (MTBF) of directional-only tools has risen from 225 hrs in 1981 to 360 hrs in 1986 with an average of 260 in-hole circulating hours between failures over the 6.3 year period. The average MTBF rate for Gamma Ray tools for 1983 until the end of 1986 is 160 hrs.

Survey accuracy has been established by comparing the horizontal difference between the 13 3/8 inch casing shoe position calculated from the MWD surveys and the positions measured by the Ferranti FINDS survey tool. 125 wells were analysed; 95% of these wells exhibit errors of less than 7, 9, 12, and 14 ft/1,000 ft of along hole depth surveyed for averaged inclinations of 0 – 15, 15 – 25, 25 – 35, 35 – 50 degrees respectively.

The data does not reveal any strong trend towards lower accuracy in East/West wells, but does indicate that magnetic disturbance is a major error source.

You can access this article if you purchase or spend a download.