RIASSUNTO
Summary
A drilling or a fishing jar is used in the bottomhole assembly (BHA) of a drillstring to free the string should it become stuck during the drilling operation. Because many expensive sensors are now incorporated in downhole tools, it is necessary to predict the damaging effect of jarring on these downhole sensors accurately. In addition, BHA's are becoming increasingly complex and are difficult to analyze by closed-form methods. For such reasons, the finite-element method (FEM) was used in this study to predict the dynamic response of a drillstring to jarring. predict the dynamic response of a drillstring to jarring. Introduction
Drillstring response during jarring has become important because many expensive sensors are now incorporated into downhole tools, which are typically mounted close to the bit in the BHA of the drillstring (Fig. 1). For these tools to provide a reliable and economical service, they must be designed to survive the harsh vibratory environment during drilling, as well as the maximum shock transmitted during jarring. A limited amount of downhole measurement data available from a hard-wire telemetry system and separately from a downhole recording system has served as a guide in defining the vibration environment during drilling for these downhole tools. In case of jarring, the closed-form analysis by Skeem et al. provides a convenient method for computing the impact force and provides a convenient method for computing the impact force and impulse delivered to the stuck point. Although this work is very valuable, it is based on many simplifying assumptions that render the results somewhat unreliable, especially in the case of the complex BHA's often used in present-day drilling. In addition, Skeem et al.'s work fails to predict jarring acceleration (g levels) at various locations in the drillstring. The knowledge of acceleration values is essential to the design of any equipment integral to the drillstring. A complete discussion of the limitations of Skeem et al.'s work is included in the next section. This paper presents the results of an FEM analysis performed on an actual jar configuration that was successfully used in the field. The configuration consisted of a fishing jar, an intensifier jar, collars, and heavyweight drillpipes. A nonlinear, transient dynamic analysis was performed with a commercial general-purpose finite-element code to track the stress and displacement waves through the drillstring. Suitable assumptions of damping in the drill collars and mud and of the sticking force were made. Detailed time histories of displacement, velocities, accelerations, and impact forces were obtained at various locations of interest along the drillstring. The results of the FEM analysis are explained with the help of simple expressions from wave propagation theory, along with the differences between the two. Also, the role of the impact force at the stuck point in freeing the drillstring is explained. The methodology presented in this paper can be used in serving the following objectives: (1) to verify the adequacy of a drilling jar included in the drillstring for the purpose of freeing the string should it become stuck during drilling; (2) in case the drilling jar is deemed inadequate, to determine the optimum configuration of the fishing jar that could free the stuck drillstring; and (3) to determine design acceleration (g levels) at the location of downhole equipment caused by the stipulated jarring operation. The scope of this paper is limited to the FEM determination of time histories of displacement, velocity, acceleration, and impact force at various locations of interest along the drillstring. Equipment design for dynamic loads is adequately covered in standard references.