Shear strength reduction method and apparatus

Abstract

A method of imposing a simple harmonic wave of bidirectional vibration on an object stuck in sand in a well bore, where vibrations are imposed in close proximity to the stuck location, and where the harmonic wave has sufficient amplitude and frequency to create soil liquefaction at the surface of the stuck object, thereby reducing the amount of overpull necessary to free the object from the sand.

Description

CROSS REFERENCE TO RELATED APPLICATIONS [0001] Not Applicable STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT [0002] Not Applicable BACKGROUND OF THE INVENTION [0003] 1. Field of the Invention [0004] The present invention is in the field of downhole jarring devices used in oil and gas well drilling and downhole equipment recovery. More specifically, it is a method for loosening a tubular object stuck in sand, by imparting vibration to the tubular object. [0005] 2. Background Art [0006] In well operation, there is often a need for jarring, impact or vibration devices to move tubular objects that are stuck in a well bore, as a result of excessive friction with sand at a downhole location, often called “sticktion” or “stiction” force. The tubular object may be a work string or a production tube. It may be stuck in a sand formation in an open hole, or it may be stuck in a cased hole, where there is a gravel pack or completion sand between the tubular object and the casin...

AI Technical Summary

Benefits of technology

[0010] The method of the present invention includes a bi-directional application of vibration to the stuck tubular object, very near the stuck location, where the vibrations form a simple harmonic wave of

Problems solved by technology

Therefore, only a limited amount of energy can be delivered to a stuck object over a given period of time, with this type of tool.

This type of loading does not produce a favorable rate of buildup of pore pressure, in the sand in which the tubular object is stuck.

Therefore, the necessary reduction of soil strength in the area surrounding the stuck portion of the object, as a result of soil liquefaction, is not realized, and the stiction force must be overcome by a significant amount of overpull on the work string.

That is, a larger over-pull will deliver a harder blow to the stuck portion of the tubular object.

That is, this method results in a high degree of soil liquefaction and a high degree of friction force reduction, resulting in a comparatively low extraction force requirement.

However, in this method, the vibrations are imparted to the tubular object at the Earth's surface, and the tool has a limited ability to propagate the vibrations to great depths in the well bore.

So, a tubular object can not be extracted by this method, if it is stuck in sand at a greater depth than the depth to which the tool can propagate sufficient vibration energy.

Spacing the pulses more closely assists in pore pressure buildup, but the smaller pulse amplitude is generally not great enough to induce plastic strains in the soil.

As a result, the degree of soil liquefaction is only moderate, and the resultant reduction in the stiction force is only moderate.

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