Drillstring Dynamics During Jar Operation
Authors: Skeem Marcus R. , Friedman Morton B. , Walker Bruce H. 
Published: December 2007

An analytical model, based on plane wave propagation through a distributed mass representation of the drillstring, has been developed to investigate jarring. Overpull, jar stroke, jar placement, and collar and pipe dimensions are related to predict impact time, speed, and force, along with stress history at the stuck point and impulse delivered to the stuck point.

Introduction

A drillstring stuck in a borehole can lead to an expensive recovery or sidetracking operation. If the string is stuck because of caving, plastic extrusion of the formation wall into the borehole, junk in the hole, or differential sticking, a drilling jar in the collar string can be activated in an attempt to free the drillstring by jarring. A modern drilling jar can be run routinely in the drillstring, so it is immediately available to jar the string free. Normally the jarring process is done several times. It may require 10 to 50 process is done several times. It may require 10 to 50 or more cycles to free the fish. About 10 to 15% of the time, the jar is unable to free the fish even after repeated jarring. In this case it is necessary to sidetrack. The location of the jar in the string usually is determined to maximize the life of the tool. In this study we show how different jar placements affect the stress history at the stuck point. Jarring provides a method for dynamically transferring strain energy from the drillstring above the jar to the stuck point of the collars below the jar.The jar contains a mechanism for internally releasing. and then re-engaging the tensile strained drillstring. Sufficient force is applied at the surface to bring the tension at the jar to the triggering value called "overpull." The amount of overpull must be below the maximum tensile force the drillpipe can withstand. After triggering, the released ends (called the hammer and anvil) are free to move toward each other. When their combined displacements are equal to the stroke of the jar, the hammer strikes the anvil. In the period between release and re-engagement, the collars above the jar are accelerated by converting strain energy from the drillpipe to kinetic energy of the upper collars. On re-engagement, some of this energy is transmitted to the stuck collars. The resultant force at the stuck point is a tensile force exceeding the initial overpull. If this force is great enough, the stuck string will slide during the period of the impulse and can be freed after a sufficient number of jar cycles.There are a number of parameters that influence the force and energy transmitted to the region of sticking. Some of these, such as collar size, pipe size, and maximum safe overpull are generally determined by the drillstring program and by material limitations. The collar string length and the position of the jar in the collar string are more flexible and offer a method to affect the jarring process. Also, jar stroke length can be evaluated to determine optimum values for new or improved jar designs. Although jars are widely employed, there appear to be no published studies that attempt to relate these or other published studies that attempt to relate these or other parameters to jarring effectiveness. Prior work in the parameters to jarring effectiveness. Prior work in the U.S. on jars has been concerned with the internal mechanics of the jar itself. Some analytical work has been done in Russia by Fershter. Of course the success of a jarring operation is dependent upon the nature of the sticking forces, which are largely unknown.

JPT

P. 1381