TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractDifferential sticking is one of the major downhole complications encountered in drilling operations. Generally oil base spotting fluids are used to release the stuck ups. However, the results are not always good.Based on sticking severity in the laboratory experiments, differential sticking has been classified into two major categories viz.; normal and extreme differential sticking. Sticking of first category can be released by using conventional oil base spotting technique, however, it fails to release the extreme stuck ups which invariably results in operations like jarring, washover, side tracking etc. and leads to loss of considerable precious rig days, costly drill string and/ or loss of the well.In order to meet the challenges of such extreme stuck ups, a new acid based formulation (Formulation-A) has been developed. It reacts chemically with the cake instantaneously and destroys it. The chemical destruction of the cake leads to the release of both types i.e. extreme as well as normal differential sticking. It has no adverse effect on the properties of drilling fluid and is also non damaging to the drill string.The two case histories i.e. in the deep well no. GGG, TD-2900 m. with horizontal drift of 660 m. in the Geleki field and high drift well no. AAA, TD-1835 m. with horizontal drift 904 m. in Ahmedabad field prove the usefulness and effectiveness of this formulation in releasing more than 7 days old severe stuck ups. Said formulation designed in tune with the expected contact area exhibited excellent results with quick release of stuck ups. This technique has since been widely accepted for the use in extreme cases of stuck ups.

Differential sticking is one of the major downhole complications encountered in drilling operations. Generally oil base spotting fluids are used to release the stuck ups. However, the results are not always good. Based on sticking severity in the laboratory experiments, differential sticking has been classified into two major categories viz.; normal and extreme differential sticking. Sticking of first category can be released by using conventional oil base spotting technique, however, it fails to release the extreme stuck ups which invariably results in operations like jarring, washover, side tracking etc. and leads to loss of considerable precious rig days, costly drill string and/ or loss of the well. In order to meet the challenges of such extreme stuck ups, a new acid based formulation (Formulation-A) has been developed. It reacts chemically with the cake instantaneously and destroys it. The chemical destruction of the cake leads to the release of both types i.e.extreme as well as normal differential sticking. It has no adverse effect on the properties of drilling fluid and is also non damaging to the drill string. The two case histories i.e. in the deep well no. GGG, TD-2900 m. with horizontal drift of 660 m. in the Geleki field and high drift well no. AAA, TD-1835 m. with horizontal drift 904 m. in Ahmedabad field prove the usefulness and effectiveness of this formulation in releasing more than 7 days old severe stuck ups. Said formulation designed in tune with the expected contact area exhibited excellent results with quick release of stuck ups. This technique has since been widely accepted for the use in extreme cases of stuck ups. Introduction Drill string sticking is one of the commonest problems faced in drilling operations. It not only causes the loss of rig days in attempting to free the string, but also results in the loss of substantial portion of costly tubular, downhole equipment and tools as well as side tracks, and substantially increased cost. In some cases, it may even lead to the abandonment of the well. The problem is a predominant threat to drilling engineers especially in drilling high-tech wells like highly inclined/ ERD/ Horizontal/ Multilaterals/ LDST/ Re-entry/ Drainhole etc. In general, a stuck pipe may be of two types: Mechanical and Differential. Mechanical stuck ups are caused because of any 8 sub set of factors1,2 including adverse well geometry (doglegs etc.) or key-seating or caving of unstable lithological sequences such as shales/coals. It can even occur due to undergauging in swelling shales or plastic clays. Differential sticking is the sticking of the drill string against a permeable formation containing less pore fluid pressure than hydrostatic pressure exerted by the drilling fluid column and usually occurs when the drill string remains motionless for a period of time. These are caused in wells where either very high mud densities are used and/ or where formation pressures are greatly depleted and higher mud weights are desirable to stabilise comparably pressured upper shales. The high differential pressure pushes the pipe deep into the mud cake causing the stuck pipe. The mechanism has been studied by several authors in detail and the reader is referred to their work2,3 for detailed description. Differential sticking is identified by no impedance of drilling fluid flow in the annular space and the inability to move the pipe in either vertical direction. This problem is likely to continue to grow in frequency and severity as older reservoirs are produced and drawn down. These stuck ups are normally released by spotting Diesel, oil base spotting fluid or water base spotting fluid in the hole opposite the stuck interval2,4,5. The point of stuck pipe is located either by a pipe stretch method or by wireline logging. Exact volume requirements are worked out and placement of oil is done by an understanding of simple physical principles. The spotted oil, with the help of surfactants enters the mud cake and reaches at the interface for reduction of friction between cake and pipe through capillary action2 and weakens the mud cake.