RIASSUNTO
SPE Members
Abstract
Two stuck pipe databases have been developed with data from more than 1000 wells drilled in the Gulf of Mexico and the North Sea. Statistical techniques were used to develop stuck pipe predictive models. When classifying in three categories - differentially, mechanically or not stuck - an accuracy of 75 % is achieved, rising to 80% for two class (stuck/not stuck) prediction. Using these models, graphical tracking of the probability of stuck pipe occurring may be undertaken, to monitor drilling operations for stuck pipe avoidance. A similar statistical model was developed to predict the probability of freeing pipe once stuck. Comparisons are also made with two published pipe-freeing models. The data were further analysed to identify any trends which may provide operational guidelines for stuck pipe avoidance or freeing operations.
Introduction
Stuck pipe is a major drilling trouble cost in the petroleum industry. Historically, one out of three wells drilled in the Gulf of Mexico and the North Sea experience stuck pipe problems, at a total estimated cost of $250 million annually.
Between 1988 and 1992, two joint industry projects were undertaken to assemble and analyse data relating to stuck pipe. A total of 20 oil operating companies were involved, several of them in both projects. Two databases were assembled, one relating to wells drilled in the Gulf of Mexico area, the other relating to wells drilled in the European North Sea.
Database assembly
The data collection strategy was defined after a number of meetings with the project participants. Each company collected about 50 data sets. For the Gulf of Mexico each data set contains three days of information, which was extended to five days for the North Sea database. Each company was to submit half stuck wells and half not stuck wells. The database was to include wells drilled in the previous five years. The variables to be included were selected during discussions with participants, following a literature review and consultation with acknowledged experts. Special attention was directed toward the twenty variables suggested by Kingsborough et al 51 in 1985.
It was observed that some of the smaller Companies may not have 25 stuck wells or 50 total wells drilled since 1983. The larger companies were asked to submit a higher proportion of stuck wells, say 35-40 stuck and 10-15 not stuck. This would allow the final database to have the required mix of half stuck and half not stuck. Selection of wells was requested in reverse chronological order, ie. the most recently drilled wells reported first.
`Stuck' was defined as the pipe unable to be moved and for which special operations (eg. jarring, spotting fluid placement) had commenced. For stuck wells, the ""Day 1 "" data was from the last morning report prior to becoming stuck, and ""Days 2-5"" the four days prior to that for a total of five consecutive days recorded (for the North Sea - three days for the Gulf of Mexico). The other wells were selected at random and included 3 or 5 drilling days (not necessarily consecutive), selected below a depth of 3500 ft. It is possible that a not stuck well was used in more than one data set if a participant lacked sufficient wells; the actual days are not duplicated.
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