Fred S. Keny, Patrick O. Oyugi
Geothermal Development Company
P.O Box 100746-00101, Nairobi-Kenya
A stuck drill string occurs when the static force required to move the string exceeds the drilling rig capabilities or the tensile strength of the drill pipe. This may be due to several causes such as inadequate removal of drilled cuttings from the annulus, wellbore instabilities, squeezing (creeping) and key seating. A drill string can either be mechanically or differentially stuck. Differential sticking occurs when the hydrostatic pressure of the drilling fluid is greater than the pore pressure of a permeable formation (Costo 2012). This overbalance presses the drill string against the well bore and is in contact with a permeable formation or a thick mud cake. Mechanical sticking on the other hand occurs is caused by
physical obstruction or restriction (Costo 2012). Freeing a mechanically stuck drill string depends upon the cause. In the case of cuttings build in the annulus, rotating and reciprocating the drill string and increasing flow rate of the drilling fluid without exceeding the maximum allowed equivalent circulating density (ECD) is a possible remedy for freeing the pipe. For hole narrowing as a result of plastic shales and salts, increasing the mud weight and circulating fresh water may free the drill string. However if the drill string is stuck because of key-seating the most successful solution is backing off below the key seat. Other operations such as tripping in with a fishing bottom hole assembly (BHA) or plugging back and side tracking to continue drilling the well to the target depth may be carried out. Mechanical backing off is the last resort to recover the drill string as the exact backing off point cannot be precisely determined. This paper looks at how the accuracy of the mechanical backoff can be improved by taking into account parameters such as the neutral point weight, maximum overpull, make up torque, pick up weight, weight of string and slack off weight. Data from MW24; a vertical exploration well in the Menengai Geothermal Project is used in the computations and determining the best approach in conducting a mechanical backoff procedure.
Key words: Stuck drill string, Mechanical backoff, Menengai Geothermal Project.