RIASSUNTO
Member SPE
Abstract
This paper presents information on a computer program currently in field use for determining optimum program currently in field use for determining optimum drilling jar positioning in Bottom Hole Assemblies. A finite element program models the BHA and predicts the force at the stuck point with the jar at predicts the force at the stuck point with the jar at various locations and tripping loads. Comparison of predicted impacts shows which position will produce predicted impacts shows which position will produce the best response at the struck point increasing the chances of recovery.
Introduction
The benefits for placing a drilling jar in the drill string when sticking problems are anticipated has long been an accepted drilling practice. However, since the introduction of drilling jars proper use has depended more on experience and proper use has depended more on experience and common practice than on engineering analysis. With the advent of tapered strings, directional drilling and the use of heavywall drill pipe experience may not be enough. Experience tends to be based on surface reactions which may not reflect what is happening downhole. With the use of a finite element computer modeling program the relative effectiveness of the jarring operation at different jar locations can be investigated. The objective of this paper is not to present information on the finite element method but to demonstrate how information from the model in conjunction with static calculations and field experience is used to recommend jar placement and trip settings. The information provided by this program is presented in three parts: Placement program is presented in three parts: Placement Recommendations, Trip Setting Recommendations, and Bottom Hole Assembly design information.
PLACEMENT RECOMMENDATIONS PLACEMENT RECOMMENDATIONS The description of the Bottom Hole Assembly, including pipe O.D.'s, I.D.'s and lengths, mud weight, inclination angle, weight on bit, and overpull allowance above string weight is entered into the program. A jar position is also entered. The first step in the analysis is to see which positions in the BHA meet the five placement positions in the BHA meet the five placement conditions. These five conditions are:
1. Jar location at least 60 feet from a stabilizer or section change.
2. Maximum xx000 lbs. slacked off above jar while drilling.
3. Minimum 5000 lbs. on trip mechanism while drilling.
4. Maximum stiffness ratio of 3.0 at the jar.
5. Minimum 5000 lbs. available weight above the jar.
The reasons for these placement conditions are:
1. Jar locations close to section changes (e.g. a change in drill collar size or where the heavyweight starts) are more highly stressed due to the tendency for bending stresses to concentrate in these areas. Drilling jars are inherently more limber than other drill string components of the same O.D. due to having an exposed mandrel and working parts inside. This causes bending stresses to concentrate in the smaller sections of the tool. These stresses are more uniformly distributed and less damaging in components of constant cross section.
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