RIASSUNTO
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Abstract
An analytical model of the drillstring, using finite element techniques, has been constructed to study the effect during jarring of overpull, damping, collar and pipe dimensions, jar stroke, jar length, jar area changes and sticking force. The fishing string is modeled using truss elements with a bilinear element representing the hammer-anvil connectivity and the Newmark method is used for the direct integration of the resultant equations of motion. The force history at the stuck point is computed and trends in peak force and fish displacement are noted. Jar placement indications are consistent with field experience. Experimental verification of the analytical model is provided by comparison with results from jarring tests on a 267 foot test string.
Introduction
Recognition of the potential economic importance of an understanding of the jarring process was first evidenced in the study by Skeem, et al. A sufficiently detailed model can facilitate parameter studies to optimize the jarring process parameter studies to optimize the jarring process by judicious choosing of jar position, jar internal area changes, jar stroke length, etc., and can provide a tool for interpretation of measurements provide a tool for interpretation of measurements made anywhere in the fishing string.
The jarring process is one of Conversion of stored potential energy in the pipe, collars and booster potential energy in the pipe, collars and booster during overpull into kinetic energy after the jar releases. This kinetic energy is then used to do useful work on the ""fish"", that portion of the drillstring stuck in the borehole. The useful work may come in the form of Sliding motion of the stuck string past a keyseat or differentially stuck region, or a combination of sliding and rock fracture.
The total potential energy stood in the fishing string system during overpull is easily calculated. Calculation of the kinetics of conversion of this stored energy into motion after jar release and finally useful work after jar impact is not so straightforward. Two fishing strings of different configuration but equal stored potential energy will not produce the same useful work.