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

Not Applicable

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

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.

2. Background Art

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 casing. The terms “sand” and “soil” are used interchangeably herein, and other similar substances such as gravel are intended to be included. One method employed to loosen such stuck objects is the use of the impact jar. These are typically included in a pipe or work string, near the depth at which the object is stuck in the sand, to provide large amplitude, unidirectional pulses or impacts of very short duration. The amplitude of the pulse is typically between 6 and 8 inches, and the duration of the pulse is typically in the range of 10 to 100 milliseconds. Impact jars are also usually single impact devices which must be recocked each time before operation, so they typically impart pulses 1 or 2 minutes apart, or in the frequency range of about 0.02 to 0.03 Hz. 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.

Some of the known impact tools require the operator to pull up on the work string with a force sufficient to pre-stress the work string, thereby providing the motive force for an impact. The impact is typically initiated when some type of valve or other triggering device in the tool triggers an action which applies the energy stored in the pre-stressed work string in the form of an impact delivered to the stuck tubular object. The force of the impact delivered by such a tool depends upon how much energy is stored in the pre-stressed work string. That is, a larger over-pull will deliver a harder blow to the stuck portion of the tubular object. Because of the aforementioned limitations of this type of tool in reducing the friction force on a tubular object stuck at a deep location, the energy put into the system in the form of overpull has to be very large, in order to overcome the stiction force between the stuck object and the sand, and in order to mechanically break the interface bond between the tubular object and the sand.

A second method for loosening a tubular object stuck in sand is the application of bi-directional, simple harmonic, vibrations of a sufficient amplitude and frequency to induce soil liquefaction, which in turn reduces the stiction force between the sand and the tubular object. The vibration amplitude is in the range of about 0.6 inch to 0.8 inch. Rather than being discrete pulses, the vibrations are continuously applied, at a frequency of up to about 60 Hz. As compared to the use of the impact jar, this method requires much less overpull; in fact, the required overpull may be only a fraction of the original stiction force on the stuck object. Where sufficient vibration energy can be applied at the stuck location, this method can be very effective. 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.

A third method employed to extract a tubular object stuck in sand is delivering energy to the soil mass in the form of uni-directional pulses similar to those delivered by the impact jar, except that the pulse amplitude is much smaller, and the pulses are more closely spaced. The pulses are delivered to the tubular object near the stuck location. The pulse amplitude in this method is typically about 0.06 inch to 0.08 inch, the pulse duration is typically about 0.003 seconds, and the pulse frequency is typically about 10 to 20 Hz. 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. Because of the location of this type of tool near the stuck depth, the depth range of this method can be great, but the amount of extraction force required can still be appreciable.

BRIEF SUMMARY OF THE INVENTION

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 sufficient amplitude to induce soil liquefaction at the stuck location. The method may be performed by any apparatus capable of imposing the required bi-directional vibrations on the stuck tubular object.

The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The FIGURE is a schematic of an apparatus in which the method of the present invention can be performed.

DETAILED DESCRIPTION OF THE INVENTION

The Figure shows a tubular assembly 10 which has become stuck in sand S at a location downhole in a well bore WB. The assembly 10 includes a tubular such as a work string 12, along with a vibratory apparatus 14,18,20, attached to the stuck object or fish 16. The well bore is illustrated as being a cased hole, but it may be either open hole or cased hole, and the sand in which the fish 16 is stuck may be a sand formation, completion sand, gravel pack, or other similar substance. The location at which the fish 16 is stuck is commonly referred to as the stuck point SP. The vibratory apparatus 14,18,20 which will be used to perform the method of the present invention may have been incorporated into the tubular assembly 10 before its initial tripping into the well bore, or it may be lowered on the work string 12 and attached to the fish 16 after the fish becomes stuck. In either case, the vibratory apparatus should be installed at or very near the stuck point on the fish, and the vibratory apparatus 14,18,20 is adapted to deliver its pulses at or very near the stuck point SP.

The vibratory apparatus itself, by way of example only and without limtation, can include a valving arrangement 14, a cycling mass 18, and a bi-directional accelerator 20. As is known in the art, fluid can be pumped downhole through the work string 12 and through the valving arrangement 14. Operation of the valving arrangement 14 can be used to cause the fluid flow to alternatingly load the accelerator 20 in the uphole and downhole directions, then to release the accelerator 20 to act against the cycling mass 18 and deliver vibrations to the fish 16 in alternating uphole and downhole directions.

Typically, the accelerator 20 would include one or more biasing elements such as springs. Other energy storing devices, such as fluid accumulators, could be used. The cycling mass 18 could be moved by the fluid flow to load the accelerator 20 in the uphole direction, for instance, then the accelerator 20 would be released to move the cycling mass 18 and deliver a pulse in the downhole direction, followed immediately by hydraulic movement of the mass 18 in the downhole direction and subsequent release and delivery of a pulse in the uphole direction. By repetition of this process, alternating pulses of substantially equal magnitude are delivered by cycling the mass 18 in the uphole and downhole directions to create bi-directional vibrations. Continuous movement of the cycling mass 18 is preferred. The energy comes downhole in the form of the fluid flow; it is repetitively stored in the accelerator 20 and released, to repetitively accelerate the cycling mass 18 in alternating directions. This imposes a bi-directional simple harmonic wave on the tubular assembly 10, with the vibrations being applied at or very near the stuck point SP on the fish 16. Other mechanisms for generating excitations in alternating directions could also be used, such as the directing of fluid in alternating directions.

Regardless of the type of vibratory apparatus used, the frequency of the vibratory tool can be tuned to match the natural or fundamental frequency of the tubular assembly 10, in order to set up a simple harmonic wave in the tubular assembly 10. Alternatively, the frequency of the vibratory tool can be tuned to match a whole number multiple of the fundamental frequency of the tubular assembly 10. The amplitude of the wave, the amount of cycling mass 18, and the magnitude of the energy repetitively stored and released by the accelerator 20, are selected to introduce sufficient energy into the tubular assembly 10 and the surrounding sand S to generate soil liquefaction at the interface between the fish 16 and the sand S. Attachment of the vibratory apparatus at or very near the fish 16 limits the attenuation of the vibratory energy by the tubular assembly 10 itself, and insures the application of the greatest possible fraction of this energy at the fish/soil interface. When soil liquefaction is induced, the amount of overpull necessary to pull the fish 16 free from the sand S is greatly reduced.

Because the size, thickness, shape, and materials of the tubular assembly 10 will vary greatly from one application to another, the frequency and amplitude of the vibration will necessarily be varied. However, the vibration must always be bi-directional, it must always be a simple harmonic wave, and it must always be applied at or very near the stuck point SP. In typical types and sizes of tubulars used in oil and gas well drilling and production, excitation amplitudes of at least one 1 inch are anticipated, at frequencies in the range of 10 to 20 hertz, with the bidirectional movement of the cycling mass being essentially continuous.

While the particular invention as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages hereinbefore stated, it is to be understood that this disclosure is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended other than as described in the appended claims.

Claims

1. A method for extracting an object stuck in sand, comprising:

attaching a vibratory tool to a stuck object, at a well bore depth at which said stuck object is stuck by friction with sand;

generating continuous bidirectional vibration in a simple harmonic wave form with said vibratory tool; and

imparting said continuous harmonic vibration to said stuck object, at said well bore depth at which said stuck object is stuck.

2. The method recited in claim 1, further comprising generating said continuous harmonic vibration with a sufficient amplitude to induce soil liquefaction in said sand.

3. The method recited in claim 2, wherein said continuous harmonic vibration is generated with an amplitude of at least 1 inch.

4. The method recited in claim 1, further comprising generating said continuous harmonic vibration with a sufficient frequency to impart mechanical energy at a rate sufficient to induce soil liquefaction in said sand.

5. The method recited in claim 4, wherein said continuous harmonic vibration is generated with a frequency between about 10 Hz and about 20 Hz.

6. The method recited in claim 1, further comprising generating said continuous harmonic vibration at the fundamental frequency of said stuck object.

7. The method recited in claim 1, further comprising generating said continuous harmonic vibration at a whole number multiple of the fundamental frequency of said stuck object.

8. A method for extracting an object stuck in sand in a well bore, comprising:

attaching a vibratory tool to a stuck object at a well bore depth at which said stuck object is stuck by friction with sand;

generating continuous bidirectional vibration in a simple harmonic wave form with said vibratory tool, said continuous harmonic vibration having sufficient amplitude and frequency to induce soil liquefaction in said sand; and

imparting said continuous harmonic vibration to said stuck object, at said well bore depth at which said stuck object is stuck.

9. The method recited in claim 8, wherein said continuous harmonic vibration is generated with an amplitude of at least 1 inch.

10. The method recited in claim 8, wherein said continuous harmonic vibration is generated with a frequency between about 10 Hz and about 20 Hz.

11. The method recited in claim 8, further comprising generating said continuous harmonic vibration at the fundamental frequency of said stuck object.

12. The method recited in claim 8, further comprising generating said continuous harmonic vibration at a whole number multiple of the fundamental frequency of said stuck object.

13. An apparatus for extracting an object stuck in sand in a well bore, comprising:

a vibratory tool adapted to generate continuous bidirectional vibration in a simple harmonic wave form;

a mechanical connection adapted to attach said vibratory tool to a stuck object substantially at a well bore depth at which said stuck object is stuck by friction with sand, said connection being adapted to transmit said continuous harmonic vibration to said stuck object, substantially at said well bore depth at which said stuck object is stuck; and

a work string adapted to position said vibratory tool substantially at said well bore depth at which said stuck object is stuck.

14. The apparatus recited in claim 13, wherein said vibratory tool is further adapted to generate said continuous harmonic vibration with a sufficient amplitude to induce soil liquefaction in said sand.

15. The apparatus recited in claim 14, wherein said vibratory tool is further adapted to generate said continuous harmonic vibration with an amplitude of at least 1 inch.

16. The apparatus recited in claim 13, wherein said vibratory tool is further adapted to generate said continuous harmonic vibration with a sufficient frequency to impart mechanical energy at a rate sufficient to induce soil liquefaction in said sand.

17. The apparatus recited in claim 16, wherein said vibratory tool is further adapted to generate said continuous harmonic vibration with a frequency between about 10 Hz and about 20 Hz.

18. The apparatus recited in claim 13, wherein said vibratory tool is further adapted to generate said continuous harmonic vibration at the fundamental frequency of said stuck object.

19. The apparatus recited in claim 13, wherein said vibratory tool is further adapted to generate said continuous harmonic vibration at a whole number multiple of the fundamental frequency of said stuck object.