U.S. patent application number 10/953237 was filed with the patent office on 2007-11-08 for method and apparatus for reducing a skin effect in a downhole environment.
Invention is credited to James R. Birchak, James W. Estep, Jeroen J. Groenenboom, Wei Han, Lyle V. Lehman, Ali Mese, Wes Ritter, Harry D. JR. Smith, William Trainor, Diederik van Batenburg, Frederick Van der Bas, Peter van der Sman, James J. Venditto, Sau-Wai Wong, Kwang Yoo, Pedro Zuiderwijk.
Application Number | 20070256828 10/953237 |
Document ID | / |
Family ID | 35134282 |
Filed Date | 2007-11-08 |
United States Patent
Application |
20070256828 |
Kind Code |
A1 |
Birchak; James R. ; et
al. |
November 8, 2007 |
Method and apparatus for reducing a skin effect in a downhole
environment
Abstract
A method for reducing a skin effect in a downhole environment is
provided, including the step of radiating vibrational waves at a
wellbore wall such that the vibrational waves have at least one
direction of greatest vibrational energy transfer directed toward
the wall, thereby reducing the skin effect. An apparatus for
reducing a skin effect in a downhole environment is also provided.
The apparatus includes at least one vibrational wave source having
at least one direction of greatest vibrational energy transfer, and
a means for positioning the vibrational wave source proximate a
wellbore wall.
Inventors: |
Birchak; James R.; (Spring,
TX) ; Wong; Sau-Wai; (Houston, TX) ; Estep;
James W.; (Houston, TX) ; Trainor; William;
(Houston, TX) ; Han; Wei; (Missouri City, TX)
; Ritter; Wes; (Katy, TX) ; Yoo; Kwang;
(Houston, TX) ; Lehman; Lyle V.; (Katy, TX)
; Venditto; James J.; (Hilliard, OH) ; Smith;
Harry D. JR.; (Montgomery, TX) ; van Batenburg;
Diederik; (Delft, NL) ; Mese; Ali; (Houston,
TX) ; Groenenboom; Jeroen J.; (Den Haag, NL) ;
Van der Bas; Frederick; (Vlaardingen, NL) ;
Zuiderwijk; Pedro; (Delft, NL) ; van der Sman;
Peter; (Heemstede, NL) |
Correspondence
Address: |
JOHN W. WUSTENBERG
P.O. BOX 1431
DUNCAN
OK
73536
US
|
Family ID: |
35134282 |
Appl. No.: |
10/953237 |
Filed: |
September 29, 2004 |
Current U.S.
Class: |
166/249 ;
166/177.6 |
Current CPC
Class: |
E21B 28/00 20130101;
E21B 43/003 20130101 |
Class at
Publication: |
166/249 ;
166/177.6 |
International
Class: |
E21B 28/00 20060101
E21B028/00; E21B 37/00 20060101 E21B037/00 |
Claims
1. A method of reducing a skin effect in a downhole environment,
comprising the step of radiating vibrational waves at a wellbore
wall such that the vibrational waves have at least one direction of
greatest vibrational energy transfer directed toward the wellbore
wall, thereby reducing the skin effect.
2. The method of claim 1 wherein the radiating step comprises the
step of positioning at least one vibrational wave source proximate
the wellbore wall, wherein the at least one vibrational wave source
has at least one direction of greatest energy transfer.
3. The method of claim 2 wherein the radiating step further
comprises the steps of: radiating vibrational waves from at least
one vibrational wave source at the wellbore wall; and flushing away
any particles from the wellbore wall and from any structures or
materials present at the wellbore wall with fluid flow.
4. The method of claim 3 wherein the step of radiating vibrational
waves from at least one vibrational wave source and the step of
flushing away any particles occur simultaneously.
5. The method of claim 2 wherein the radiating step further
comprises the steps of: radiating vibrational waves from a
plurality of vibrational wave sources at the wellbore wall; and
flushing away any particles from the wellbore wall and from any
structures or materials present at the wellbore wall with fluid
flow.
6. The method of claim 5 wherein the step of radiating vibrational
waves from a plurality of vibrational wave sources comprises the
step of radiating vibrational waves in succession from each
vibrational wave source.
7. The method of claim 5 wherein the step of radiating vibrational
waves from a plurality of vibrational wave sources comprises the
step of radiating vibrational waves simultaneously.
8. The method of claim 5 wherein the step of radiating vibrational
waves from a plurality of vibrational wave sources comprises the
step of radiating vibrational waves simultaneously and
substantially continuously.
9. The method of claim 5 wherein the step of radiating vibrational
waves from a plurality of vibrational wave sources comprises the
step of radiating vibrational waves having at least two different
frequencies from the plurality of vibrational wave sources.
10. The method of claim 5 wherein the step of radiating vibrational
waves from a plurality of vibrational wave sources comprises
selecting an order of activation and one or more periods of
activation time for the plurality of vibrational wave sources to
optimize usage of available power.
11. The method of claim 2 wherein the at least one vibrational wave
source is an acoustic wave source having at least one direction of
greatest energy transfer.
12. The method of claim 11 wherein the acoustic wave source is an
oval-mode acoustic wave source having a plurality of directions of
greatest energy transfer.
13. The method of claim 2 further comprising the step of placing
the at least one vibrational wave source in the well.
14. The method of claim 2 further comprising the step of orienting
the at least one vibrational wave source such that at least one
direction of greatest vibrational energy transfer is directed
toward the wellbore wall.
15. The method of claim 2 further comprising the step of
maintaining a standoff distance between the vibrational wave source
and the wellbore wall.
16. The method of claim 2 further comprising the step of optimizing
reduction of the skin effect by creating a standing wave pattern
between the vibrational wave source and the wellbore wall.
17. The method of claim 2 further comprising the steps of:
detecting accretions of particles between the wellbore wall and the
at least one vibrational wave source; moving the at least one
vibrational wave source away from the wellbore wall when a
threshold level of accreted particles between the wellbore wall and
the at least one vibrational wave source is detected; radiating
vibrational waves at the accreted particles; and repositioning the
at least one vibrational wave source proximate the wellbore
wall.
18. The method of claim 2 further comprising the steps of:
monitoring whether the vibrational wave source transfers sufficient
vibrational energy to the wellbore wall to reduce the skin effect;
and repositioning the vibrational wave source to optimally decrease
the skin effect.
19. The method of claim 2 further comprising the steps of:
monitoring whether the vibrational wave source transfers sufficient
vibrational energy to the wellbore wall to reduce the skin effect;
and altering the vibrational waves radiated by the vibrational wave
source to optimally decrease the skin effect.
20. The method of claim 2 further comprising the step of
determining how much the skin effect in the downhole environment
has been reduced.
21. The method of claim 20 wherein the determining step comprises
the steps of: measuring a speed of sound for the downhole
environment; and comparing the measured speed of sound to a control
speed of sound for a previously-cleaned wellbore wall.
22. The method of claim 20 wherein the determining step comprises
the steps of: measuring a speed of sound for the downhole
environment; and comparing the measured speed of sound to a control
speed of sound measured before the vibrational waves were radiated
at the wellbore wall.
23. The method of claim 20 wherein the determining step comprises
the steps of: measuring an acoustic attenuation value for the
downhole environment; and comparing the acoustic attenuation value
to a control acoustic attenuation value for a previously-cleaned
wellbore wall.
24. An apparatus for reducing a skin effect in a downhole
environment, comprising: at least one vibrational wave source
having at least one direction of greatest vibrational energy
transfer; and a means for positioning the vibrational wave source
proximate a wellbore wall.
25. The apparatus of claim 24 wherein the vibrational wave source
comprises an oval-mode acoustic wave source
26. The apparatus of claim 24 further comprising a tool body,
wherein the tool body houses a control for the vibrational wave
source.
27. The apparatus of claim 24 further comprising a means for
placing the vibrational wave source in a well.
28. The apparatus of claim 27 wherein the means for placing the
vibrational wave source in the well comprises a wireline.
29. The apparatus of claim 27 wherein the means for placing the
vibrational wave source in the well comprises coiled tubing.
30. The apparatus of claim 27 wherein the means for placing the
vibrational wave source in the well comprises a well tractor.
31. The apparatus of claim 24 wherein the means for positioning the
vibrational wave source proximate the wellbore wall is a
decentralizer.
32. The apparatus of claim 31 wherein the decentralizer comprises a
bowed spring member that pushes against a first side of the
wellbore wall to position the vibrational wave source proximate a
second, opposing side of the wellbore wall.
33. The apparatus of claim 31 further comprising a means for
orienting the vibrational wave source such that at least one
direction of greatest vibrational energy transfer is directed
toward the wellbore wall.
34. The apparatus of claim 33 wherein the means for orienting the
vibrational wave source comprises a rotator-resolver, wherein the
rotator-resolver orients the vibrational wave source such that the
at least one direction of greatest energy transfer is directed
toward the wellbore wall.
35. The apparatus of claim 34 wherein the decentralizer comprises:
at least two articulated joints connecting the vibrational wave
source to the rotator-resolver; and at least one retractable arm,
wherein the at least one retractable arm positions the vibrational
wave source proximate the wellbore wall.
36. The apparatus of claim 34 wherein the decentralizer comprises:
a vibrational wave source pad attached to the rotator-resolver; and
at least one retractable arm, wherein the at least one retractable
arm positions the vibrational wave source proximate the wellbore
wall.
37. The apparatus of claim 24 further comprising at least one
standoff contactor, wherein the at least one standoff contactor
maintains a standoff distance between the vibrational wave source
and the wellbore wall.
38. The apparatus of claim 37 wherein the at least one standoff
contactor maintains a standoff distance chosen to enable creation
of a standing wave pattern between the vibrational wave source and
the wellbore wall.
39. The apparatus of claim 37 wherein the at least one standoff
contactor includes contact points that contact the wellbore
wall.
40. The apparatus of claim 39 further comprising an actuator that
moves the vibrational wave source relative to the contact points to
adjust the standoff distance.
41. The apparatus of claim 24 further comprising a means for
detecting accretions of particles between the vibrational wave
source and the wellbore wall.
42. The apparatus of claim 41 wherein the means for detecting
accretions of particles comprises: an accelerometer coupled to the
vibrational wave source, wherein the accelerometer produces an
electrical signal proportional to vibrations experienced by the
vibrational wave source; and a processing unit that monitors the
electrical signal, wherein the processing unit can detect a
signature vibration pattern indicating that particles have
accreted.
43. The apparatus of claim 24 further comprising a means for
monitoring energy transfer from the vibrational wave source to the
wellbore wall.
44. The apparatus of claim 43 wherein the means for monitoring
energy transfer comprises: a hydrophone suitable for use in
downhole environments, wherein the hydrophone converts vibrational
energy traveling through a fluid present near the wellbore wall
into an electrical signal; and a processing unit, which monitors
the electrical signal.
45. The apparatus of claim 43 wherein the means for monitoring
energy transfer comprises: an accelerometer connected to the
vibrational wave source, wherein the accelerometer produces an
electrical signal proportional to vibrations experienced by the
vibrational wave source; and a processing unit, which monitors the
electrical signal.
46. The apparatus of claim 43 wherein the means for monitoring
energy transfer from the vibrational wave source to the wellbore
wall comprises: an accelerometer that produces an electrical signal
proportional to vibrations experienced by the wellbore wall,
wherein the accelerometer is acoustically isolated from the
vibrational wave source; and a processing unit that measures the
electrical signal.
47. The apparatus of claim 24 wherein the apparatus for reducing a
skin effect in a downhole environment comprises a plurality of
vibrational wave sources.
48. The apparatus of claim 47 wherein the plurality of vibrational
wave sources are displaced axially with an axial gap between each
vibrational wave source.
49. The apparatus of claim 47 wherein the plurality of vibrational
wave sources are displaced circumferentially with a circumferential
gap between each vibrational wave source.
50. The apparatus of claim 24 further comprising a means for
determining how much the skin effect in the downhole environment
has been reduced.
51. The apparatus of claim 50 wherein the means for determining how
much the skin effect in the downhole environment has been reduced
comprises: an accelerometer contacting the wellbore wall, wherein
the accelerometer is acoustically isolated from the vibrational
wave source; and a processing unit coupled to the
accelerometer.
52. An apparatus for reducing a skin effect in a downhole
environment, comprising: a vibrational wave source having at least
one direction of greatest vibrational energy transfer; at least one
standoff contactor, wherein the at least one standoff contactor
maintains a standoff distance between the vibrational wave source
and a wellbore wall; a decentralizer, wherein the decentralizer
positions the vibrational wave source proximate a wellbore wall;
and a wireline, wherein the wireline may be used to place the
vibrational wave source in the well.
53. The apparatus of claim 52 further comprising a hydrophone.
54. The apparatus of claim 52 further comprising an
accelerometer.
55. An apparatus for reducing a skin effect in a downhole
environment, comprising: a vibrational wave source having at least
one direction of greatest vibrational energy transfer; at least one
standoff contactor, wherein the at least one standoff contactor
maintains a standoff distance between the vibrational wave source
and a wellbore wall; a rotator-resolver, wherein the
rotator-resolver orients the vibrational wave source such that the
at least one direction of greatest vibrational energy transfer is
directed toward the wellbore wall; at least two articulated joints
connecting the vibrational wave source to the rotator-resolver; and
at least one retractable arm, wherein the at least one retractable
arm positions the vibrational wave source proximate the wellbore
wall.
56. The apparatus of claim 55 further comprising a hydrophone.
57. The apparatus of claim 55 further comprising an
accelerometer.
58. An apparatus for reducing a skin effect in a downhole
environment, comprising: a vibrational wave source having at least
one direction of greatest vibrational energy transfer; at least one
standoff contactor, wherein the at least one standoff contactor
maintains a standoff distance between the vibrational wave source
and a wellbore wall; a rotator-resolver, wherein the
rotator-resolver orients the vibrational wave source such that the
at least one direction of greatest vibrational energy transfer is
directed toward the wellbore wall; a vibrational wave source pad
attached to the rotator-resolver; and at least one retractable arm,
wherein the at least one retractable arm positions the vibrational
wave source proximate the wellbore wall.
59. The apparatus of claim 58 further comprising a hydrophone.
60. The apparatus of claim 58 further comprising an
accelerometer.
61. An apparatus for reducing a skin effect in a downhole
environment, comprising: a plurality of vibrational wave sources
wherein each vibrational wave source has at least one direction of
greatest vibrational energy transfer; at least one standoff
contactor, wherein the at least one standoff contactor maintains a
standoff distance between the plurality of vibrational wave sources
and a wellbore wall; a rotator-resolver, wherein the
rotator-resolver orients the plurality of vibrational wave sources
such that the at least one direction of greatest vibrational energy
transfer is directed toward the wellbore wall; a vibrational wave
source pad attached to the rotator-resolver; and at least one
retractable arm, wherein the at least one retractable arm positions
the plurality of vibrational wave sources proximate the wellbore
wall.
62. The apparatus of claim 61 further comprising a hydrophone.
63. The apparatus of claim 61 further comprising an accelerometer.
Description
BACKGROUND
[0001] The present invention relates to apparatuses and methods for
treating a downhole environment, and more specifically, to
apparatuses and methods for reducing a skin effect in a downhole
environment.
[0002] In any typical hydrocarbon well, damage to the surrounding
formation can impede fluid flow and cause production levels to
drop. While many damage mechanisms plague wells, one of the most
pervasive problems is particles clogging the formation pores that
usually allow hydrocarbon flow. These clogging particles can also
obstruct fluid pathways in screens; preslotted, predrilled, or
cemented and perforated liners; and gravel packs that may line a
well. Clogging particles may even restrict fluid flow in open-hole
wells. Drilling mud, drilled solid invasion, or even the porous
formation medium itself may be sources for these particles. In
particular, in situ fines mobilized during production can lodge
themselves in the formation pores, preslotted liners, screens and
gravel packs, sealing them to fluid flow. Referred to as the "skin
effect," this damage is often unavoidable and can arise at any
stage in the life of a typical hydrocarbon well. The hydrocarbon
production industry has thus developed well-stimulation techniques
to repair affected wells or at least mitigate skin-effect
damage.
[0003] The two classic stimulation techniques for formation damage,
matrix acidizing and hydraulic fracturing, suffer from limitations
that often make them impractical. Both techniques require the
operator to pump customized fluids into the well, a process that is
expensive, invasive and difficult to control. In matrix acidizing,
pumps inject thousands of gallons of acid into the well to dissolve
away precipitates, fines, or scale on the inside of tubulars, in
the pores of a screen or gravel pack, or inside the formation. Any
tool, screen, liner or casing that comes into contact with the acid
must be protected from its corrosive effects. A corrosion inhibitor
must be used to prevent tubulars from corrosion. Also, the acid
must be removed from the well. Often, the well must also be flushed
with pre- and post-acid solutions. Aside from the difficulties of
determining the proper chemical composition for these fluids and
pumping them down the well, the environmental costs of matrix
acidizing can render the process undesirable. Screens, preslotted
liners and gravel packs may also be flushed with a brine solution
to remove solid particles. While this brine treatment is cheap and
relatively easy to complete, it offers only a temporary and
localized respite from the skin effect. Moreover, frequent flushing
can damage the formation and further decrease production. In
hydraulic fracturing, a customized fluid is ejected at extremely
high pressure against the wellbore walls to force the surrounding
formation to fracture. The customized gel-based fluid contains a
proppant to hold the fractures open to fluid flow. While this
procedure is highly effective at overcoming near-borehole skin
effects, it requires both specialized equipment and specialized
fluids and therefore can be costly. Fracturing can also result in
particle deposition in the formation because the gels involved may
leave residue in the vicinity of the fractures.
[0004] The hydrocarbon production industry developed acoustic
stimulation as an alternative to the classic stimulation
techniques. In acoustic stimulation used for near-borehole
cleaning, high-intensity, high-frequency acoustic waves transfer
vibrational energy to the solid particles clogging formation pores.
The ensuing vibrations of the solid particles loosen them from the
pores. Fluid flow, including production-fluid flow out of the
formation or injection-fluid flow into the formation from the well,
may cause the particles to migrate out of the pores, clearing the
way for greater fluid flow. Acoustic stimulation may also be used
to clean preslotted liners, screens and gravel packs. Near-wellbore
cleaning by acoustic stimulation has shown great promise in
laboratory experiments, and the industry has developed several
tools using this technique for use in real-world wells.
[0005] Acoustic stimulation tools require a compact source of
acoustic waves that may be used downhole. Current tools, however,
often radiate acoustic waves over 360 degrees or in an uncontrolled
direction in an attempt to reduce the skin effect along the
circumference of a wellbore wall at a given depth all at one time.
These tools must consume large quantities of energy to radiate
waves of sufficient intensity to vibrate the solid particles along
the circumference of the wellbore wall. Supplying this energy
downhole to create the necessary high-intensity acoustic waves is
no easy feat, and thus current tools are poorly suited for removing
particles from the formation.
SUMMARY
[0006] The present invention relates to apparatuses and methods for
treating a downhole environment, and more specifically, to
apparatuses and methods for reducing a skin effect in a downhole
environment. An example method for reducing a skin effect in a
downhole environment may comprise the step of radiating vibrational
waves at a wellbore wall such that the vibrational waves have at
least one direction of greatest vibrational energy transfer
directed toward the wall, thereby reducing the skin effect.
[0007] An example apparatus for reducing a skin effect in a
downhole environment may comprise at least one vibrational wave
source having at least one direction of greatest vibrational energy
transfer and a means for positioning the vibrational wave source
proximate a wellbore wall. Some example apparatuses for reducing a
skin effect in a downhole environment may comprise a vibrational
wave source having at least one direction of greatest vibrational
energy transfer and at least one standoff contactor, wherein the at
least one standoff contactor maintains a standoff distance between
the vibrational wave source and a wellbore wall. These example
apparatuses may also include a decentralizer, wherein the
decentralizer positions the vibrational wave source proximate the
wellbore wall and a wireline, wherein the wireline may be used to
place the vibrational wave source in the well.
[0008] Other example apparatuses for reducing a skin effect in a
downhole environment may also comprise a vibrational wave source
having at least one direction of greatest vibrational energy
transfer and at least one standoff contactor, wherein the at least
one standoff contactor maintains a standoff distance between the
vibrational wave source and a wellbore wall. These example
apparatuses may include a rotator-resolver, wherein the
rotator-resolver orients the vibrational wave source such that the
at least one direction of greatest vibrational energy transfer is
directed toward the wellbore wall. These example apparatuses may
also include at least two articulated joints connecting the
vibrational wave source to the rotator-resolver and at least one
retractable arm, wherein the at least one retractable arm positions
the vibrational wave source proximate the wellbore wall.
[0009] An example apparatus for reducing a skin effect in a
downhole environment may comprise a vibrational wave source having
at least one direction of greatest vibrational energy transfer and
at least one standoff contactor, wherein the at least one standoff
contactor maintains a standoff distance between the vibrational
wave source and a wellbore wall. These apparatuses may also include
a rotator-resolver, wherein the rotator-resolver orients the
vibrational wave source such that the at least one direction of
greatest vibrational energy transfer is directed toward the
wellbore wall. The example apparatuses may include a vibrational
wave source pad attached to the rotator-resolver and at least one
retractable arm, wherein the at least one retractable arm positions
the vibrational wave source proximate the wellbore wall.
[0010] Another example apparatus for reducing a skin effect in a
downhole environment may comprise a plurality of vibrational wave
sources wherein each vibrational wave source has at least one
direction of greatest vibrational energy transfer. The example
apparatuses may also include at least one standoff contactor,
wherein the at least one standoff contactor maintains a standoff
distance between the plurality of vibrational wave sources and a
wellbore wall and a rotator-resolver, wherein the rotator-resolver
orients the plurality of vibrational wave sources such that the at
least one direction of greatest vibrational energy transfer is
directed toward the wellbore wall. These apparatuses may also
include a vibrational wave source pad attached to the
rotator-resolver and at least one retractable arm, wherein the at
least one retractable arm positions the plurality of vibrational
wave sources proximate the wellbore wall.
[0011] The features and advantages of the present invention will be
readily apparent to those skilled in the art upon a reading of the
description of the exemplary embodiments, which follows.
BRIEF DESCRIPTION OF DRAWINGS
[0012] A more complete understanding of the present disclosure and
advantages thereof may be acquired by referring to the following
description taken in conjunction with the accompanying drawings,
wherein:
[0013] FIG. 1 illustrates a cross-sectional view of an example of
an oval-mode acoustic wave source in a well.
[0014] FIG. 2 illustrates the vibratory mode for an example of an
oval-mode acoustic wave source.
[0015] FIG. 3 illustrates a cross-sectional view of an example of a
vibrational wave source in a well.
[0016] FIG. 4 illustrates an example apparatus for reducing a skin
effect including a wireline in a downhole environment.
[0017] FIG. 5 illustrates an example apparatus including coiled
tubing in a downhole environment.
[0018] FIG. 6 illustrates an example apparatus including coiled
tubing in a downhole environment.
[0019] FIG. 7 illustrates an enlarged view of an example apparatus
in a downhole environment generating a standing wave pattern.
[0020] FIG. 8 illustrates an example apparatus including an
actuator to adjust the standoff distance between the vibrational
wave source and the wellbore wall, a means for detecting accretions
of particles, and a means for monitoring energy transfer from the
vibrational wave source to the wall.
[0021] FIG. 9 illustrates an example apparatus including an
accelerometer to monitor energy transfer from the vibrational wave
source to the wellbore wall.
[0022] FIG. 10 illustrates an example apparatus including multiple
vibrational wave sources in a downhole environment.
[0023] FIG. 11A illustrates a cross-sectional view of example
apparatus including multiple vibrational wave sources in a downhole
environment.
[0024] FIG. 11B illustrates a cross-sectional view of example
apparatus including multiple vibrational wave sources in a downhole
environment.
[0025] FIG. 11C illustrates an example apparatus including multiple
vibrational wave sources in a downhole environment.
[0026] FIG. 12 illustrates an example apparatus including multiple
vibrational wave sources and a means for determining by how much
the skin effect has been reduced.
[0027] While the present invention is susceptible to various
modifications and alternative forms, specific exemplary embodiments
thereof have been shown by way of example in the drawings and are
herein described in detail. It should be understood, however, that
the description herein of specific embodiments is not intended to
limit the invention to the particular forms disclosed, but on the
contrary, the intention is to cover all modifications, equivalents
and alternatives falling within the spirit and scope of the
invention as defined by the appended claims.
DETAILED DESCRIPTION
[0028] The present invention relates to apparatuses and methods for
treating a downhole environment, and more specifically, to
apparatuses and methods for reducing a skin effect in a downhole
environment. We provide a system for positioning a vibrational wave
source downhole to optimally reduce a skin effect. The system may
treat a variety of structures and materials located downhole,
including, but not limited to, porous materials such as screens,
gravel packs, frac packs or geologic formations. Moreover, the
system may also treat openhole wells and wells with cemented and
perforated casings or slotted liners.
[0029] An exemplary apparatus for reducing a skin effect in a
downhole environment includes at least one vibrational wave source
and a means for positioning the vibrational wave source proximate a
wellbore wall. The vibrational wave source has at least one
direction of greatest energy transfer. FIG. 1 shows a well 300
containing an example apparatus 1000 including a vibrational wave
source 100. As persons of ordinary skill in the art will realize,
the void in well 300 around vibrational wave source 100 may be
filled with fluids such as completion fluids, hydrocarbons, and
other formation fluids. For some example apparatuses, the column of
fluid in the well should be lighter than the reservoir pressure in
order to allow fluid inflow from the reservoir. However, in many
drilling situations that are more conducive to forming mud cake and
accreting fines, borehole pressure can exceed formation pressure,
as in overbalanced drilling. In FIG. 1, apparatus 1000 uses an
oval-mode acoustic wave source 104 as a vibrational wave source.
U.S. Pat. No. 6,412,354 B1, assigned to the assignee of this
disclosure, discloses an oval-mode acoustic wave source that may be
modified for use in apparatus 1000. Other acoustic wave sources,
such as pistons, tuning forks, cantilever bars and wobble plates,
may alternatively serve as vibrational wave sources. Oval-mode
acoustic wave source 104 has been selected only for the purpose of
describing the apparatus for reducing a skin effect in a downhole
environment.
[0030] Oval-mode acoustic wave source 104 includes a housing 101
and a vibratory mechanism, denoted generally by the numeral 102.
The vibratory mechanism 102 causes housing 101 to expand and
contract at acoustic frequencies. As a result, housing 101 produces
acoustic waves that propagate radially outward from an outer
surface 103 of housing 101. Housing 101 may be cylindrical, and
vibratory mechanism 102 may have four piezoelectric transducers
105, 106, 107, and 108 spaced equally around the circumference of
housing 101. When subject to a changing voltage at opposite ends,
piezoelectric transducers 105, 106, 107, and 108 may expand and
contract, causing vibrations in housing 101 that create the desired
vibrational waves. Arrows 109, 109', 110, 110', 111, 111', 112, and
112' in FIG. 1 indicate the directions of expansion and contraction
of piezoelectric transducers 105, 106, 107, and 108. This radial
configuration of the piezoelectric transducers represents only one
configuration out of many possible choices. For example, an axial
configuration of the piezoelectric transducer with a moment arm
coupled to housing 101 may be preferred for some example
apparatuses 1000.
[0031] FIGS. 2A through 2F include a time series of diagrams
showing the motion of housing 101. These diagrams illustrate an
example vibratory mode for oval-mode acoustic wave source 104. The
progression in time of the vibratory mode can be seen when the
figures are viewed in succession in a clockwise direction, starting
with FIG. 2A. Vibratory mechanism 102 for this oval-mode acoustic
wave source 104 may include a first and second pair of opposing
piezoelectric transducers 201 and 202. The first pair of opposing
piezoelectric transducers 201 expands and contracts along the
directions indicated by arrows 203 in phase. The second pair of
opposing piezoelectric transducers 202 expands and contracts in
phase along the directions indicated by arrows 204 but 180 degrees
out of phase with the first pair of opposing piezoelectric
transducers 201. The motion of housing 100 begins with FIG. 2A. The
first pair of opposing piezoelectric transducers 201 expands,
pulling housing 101 in the directions indicated by arrows 203.
Simultaneously, the second pair of opposing piezoelectric
transducers 202 contracts, pulling housing 101 in the directions
indicated by arrows 204. The result of this pulling can be seen in
FIG. 2B: housing 101 assumes an oval shape, with the elongated axis
of the oval along the directions of arrows 203.
[0032] Once the first pair of opposing piezoelectric transducers
201 has expanded to its maximum size, and the second pair of
opposing piezoelectric transducers 202 has contracted to its
minimum size, housing 101 has experienced its maximum overall
distortion in this direction. First pair of opposing piezoelectric
transducers 201 then begins to contract, and second pair of
opposing piezoelectric transducers 202 begins to expand, as shown
in FIG. 2C. Housing 101 returns to its original circular shape, as
shown in FIG. 2D. Then first pair of opposing piezoelectric
transducers 201 contracts, and second pair of opposing
piezoelectric transducers 202 expands, distorting housing 101
again. Housing 101 may then assume another oval shape, this time
with the elongated axis of the oval along the directions of arrows
204, as shown in FIG. 2E. Once housing 101 has experienced a second
maximum overall distortion, first pair of opposing piezoelectric
transducers 201 will begin to expand, and second pair of opposing
piezoelectric transducers 202 will begin to contract, as FIG. 2F
illustrates. Housing 101 will then return to the circular shape
shown in FIG. 2A. As a person of ordinary skill in the art will
realize, a driver located outside housing 101 may power and control
the first and second pairs of opposing piezoelectric
transducers.
[0033] The housing motion repeats with the repeated motion of the
first and second pair of opposing piezoelectric transducers,
causing the oval-mode vibrations that drive the desired vibrational
waves. Returning to FIG. 1, housing 101 will experience the
greatest localized distortion at the locations of each of the
piezoelectric transducers 105, 106, 107, and 108. The maximum
amplitude of the vibrational waves produced by the oval-mode
acoustic wave source 104 thus propagates radially away from the
outer surface 103 of housing 101 along the directions indicated by
arrows 109, 110, 111, and 112. Oval-mode acoustic wave source 104
will accordingly transmit the greatest vibrational energy possible
along the directions indicated by arrows 109, 110, 111, and 112.
Each of the arrows 109, 110, 111, and 112 thus points in a
direction of greatest vibrational energy transfer. Again, other
vibrational wave sources may be substituted for the oval-mode
acoustic wave source. These vibrational wave sources should have at
least one direction of greatest vibrational energy transfer.
[0034] FIG. 1 also illustrates an exemplary position for
vibrational wave source 100 within well 300. The means for
positioning, discussed in detail later in this disclosure, allows
vibrational wave source 100 to achieve this position. The direction
of greatest vibrational energy transfer in the direction along
arrow 109 may be directed toward a portion 301 of the wellbore wall
302. If vibrational wave source 100 is cylindrical, as in certain
example apparatuses, portion 301 will take a rectangular "strip"
shape. In operation according to an example method, vibrational
wave source 100 may direct its greatest vibrational energy on the
particles blocking the pores of the formation at portion 301. If a
preslotted liner, screen, gravel pack or other intervening
structure or material is present at portion 301, the vibrational
wave source 100 may direct its greatest vibrational energy on
blocking particles in or on that structure or material as well as
at portion 301. The vibrational energy transfers from vibrational
wave source 100 to the blocking particles and causes them to
vibrate. This vibration may loosen the blocking particles from
their positions clogging fluid pathways out of the formation or
intervening structure or material. Fluid flow, such as
production-fluid flow out of the formation or injection-fluid flow
into the formation, may flush away these particles, and
fluid-production levels may then increase. The flushing fluid may
flow during radiation of acoustic waves or after, as desired.
[0035] This embodiment of a method according to the present
invention may be repeated at a second portion 303 of wellbore wall
302 to expand the width of the cleaned area along the circumference
of the well, as shown in FIG. 3. The apparatus may comprise a means
for orienting the vibrational wave source in the well, as discussed
in detail later in this disclosure. If the apparatus does include
an orienting means, vibrational wave source 100 should first be
reoriented so that at least one direction of greatest vibrational
energy transfer is directed toward the second portion 303. Then,
vibrational wave source 100 must be repositioned at second portion
303. Alternatively, the apparatus may move vibrational wave source
100 continuously through well 300 such that vibrational wave source
100 radiates vibrational waves at the wellbore wall as it passes
through the well. For such a continuous-operation method to be
successful, vibrational wave source 100 must move along well 300
slowly enough that it radiates vibrational waves at a particular
portion of wellbore wall 302 for long enough to loosen blocking
particles. The length of time needed to loosen blocking particles
will depend on the particular skin effect present in the well.
[0036] Once vibrational wave source 100 is activated, it will
remove the solid particles from second portion 303 just as it did
at portion 301. The process may be repeated at any location along
the circumference of wellbore wall 302. It is not necessary,
however, to clean all of wellbore wall 302 because fluid in the
formation can migrate radially, axially and circumferentially to
the nearest cleaned portion of wellbore wall 302. Example
apparatuses may therefore achieve improved production flow rates
using shorter operation times and lower power than tools that clean
the entire circumference of wellbore wall 302. Example apparatuses
may also allow for cleaning in situations when the power available
is insufficient to adequately stimulate the wellbore wall around
its full circumference. Because its outer diameter is smaller than
the inner diameter of well 300, vibrational wave source 100 can fit
through other passages having inner diameters smaller than the
inner diameter of well 300, such as through the landing nipple
inside production tubing 401 shown in cross-section in FIG. 4. The
desired cleaning rate will determine the suitable size for
vibrational wave source 100, as persons of ordinary skill in the
art having the benefit of this disclosure will realize. Use of
longer or wider vibrational wave sources will result in greater
cleaning rates than smaller acoustic wave sources, for example,
assuming that adequate power is available.
[0037] In certain exemplary embodiments, vibrational wave source
100 will produce vibrational waves with frequencies in the range of
about 8 kHz to about 40 kHz. In certain preferred embodiments, the
vibrational waves will have frequencies ranging from about 10 kHz
to about 20 kHz. Bursts of vibrational waves with intervening
periods of inactivation are preferred so that fluid flow, such as
production-fluid flow from the formation into the well or
injection-fluid flow from the well into the formation, can flush
away the loosened particles. The activation period for the
vibrational wave source should last approximately 2,000 to
approximately 20,000 cycles to bring the motion of the solid
particles to the full resonance amplitude. Longer activation
periods are acceptable, however, and may even be desirable in wells
with severe skin-effect damage. The inactivation period between
bursts should be selected empirically to optimize cleaning relative
to the permeability of the skin effect.
[0038] Certain example apparatuses include a means for placing the
vibrational wave source in a well. A suitable placement means will
be apparent to persons of ordinary skill in the art having the
benefit of this disclosure. The placement means may be a prior-art
wireline, as shown in FIG. 4. Wireline 400 supplies the power
necessary to operate vibrational wave source 402. Wireline 400 may
also transmit the control telemetry needed to operate the
vibrational wave source 402. Example apparatuses incorporating a
wireline 400 may also comprise a driver unit located in a tool body
403 that is placed downhole with the wireline 400 and vibrational
wave source 402. Tool body 403 may contain a conventional
electrical converter that converts wireline power to power at the
preferred driver frequency. Tool body 403 also may contain control
circuitry that permits adjustment of the activation and
inactivation periods. As will be apparent to persons of ordinary
skill in the art having the benefit of this disclosure, apparatuses
utilizing a wireline will require procedures to overcome friction
along the wellbore wall surface in highly deviated or horizontal
wells.
[0039] Apparatus 1000 includes a means for positioning vibrational
wave source 100 proximate the wellbore wall to focus the
vibrational energy at only one portion of the wellbore wall at a
time. This position helps apparatus 1000 avoid dispersing
vibrational energy over the entire circumference of the wellbore
wall. A suitable positioning means will depend on the chosen
placement means, as persons of ordinary skill in the art having the
benefit of this disclosure will realize. The positioning means may
include a decentralizer. In particular, apparatuses may include a
decentralizer with at least one bowed spring member that pushes
against one side of the wellbore wall to position the vibrational
wave source proximate an opposing side of the wellbore wall. FIG. 4
illustrates an exemplary apparatus that comprises such a
"bow-spring decentralizer," labeled with numeral 404.
[0040] The means for placing the vibrational wave source in a well
also may be prior-art coiled tubing, as shown in FIGS. 5 and 6.
Electrical cables can pass through the bore or in the wall of the
coiled tubing to supply the electrical power necessary to operate
the vibrational wave source. The electrical cables may also
transmit the control telemetry needed to operate the vibrational
wave source. Apparatuses incorporating coiled tubing may also
include a driver unit located in a tool body that is placed
downhole with the coiled tubing and vibrational wave source. The
contents of this tool body may parallel the contents of tool body
403. With coiled tubing, however, the tool body may contain
additional controls for adjusting the spatial location of
vibrational wave source 100 relative to its circumferential
position in the well. Because coiled tubing is generally used in
horizontal wells, the bow-spring decentralizer shown in FIG. 4 may
be most useful with wireline apparatuses. Coiled tubing
apparatuses, however, may also include a bow-spring or powered
decentralizer, as persons of ordinary skill in the art having the
benefit of this disclosure will realize. Example apparatuses
including coiled tubing may be better for the continuous-operation
method described earlier, as the motion of vibrational wave source
100 can be more smooth with coiled tubing than with wireline. Other
alternative means for placing the vibrational wave source in a well
may be used. For example, the means for placing the vibrational
wave source in a well also may be a prior-art well tractor. The
well tractor may be used in conjunction with another means for
placing the vibrational wave source in a well, such as a wireline,
or even independently. The best choice for a means for placing the
vibrational wave source in the well will depend, in part, on the
configuration of the well.
[0041] In certain example apparatuses 1000, vibrational wave source
100 may be not only positioned proximate the wellbore wall but also
oriented to direct the at least one direction of greatest
vibrational energy transfer toward the wellbore wall. Certain
example apparatuses therefore include a means for orienting the
vibrational wave source in the well. In certain example apparatuses
1000, this orienting means will comprise a rotator-resolver that
rotates the vibrational wave source to the desired circumferential
orientation. To control the circumferential orientation of
vibrational wave source 100, the tool body may include conventional
equipment sufficient to detect the orientation of vibrational wave
source 100 and to instruct the rotator-resolver to readjust that
orientation. The tool body may contain controls to adjust the
spacing between vibrational wave source 100 and the wellbore wall.
A suitable design for the rotator-resolver will be apparent to
persons of ordinary skill in the art having the benefit of this
disclosure. Further, other means for orienting the vibrational wave
source may be apparent to those of ordinary skill in the art having
the benefit of this disclosure. These orienting means may be more
useful when vibrational wave source 100 stops at a particular
location to radiate waves, rather than moving continuously the
orienting means. The orienting means, however, may be successfully
used in the continuous-operation method if vibrational wave source
100 moves slowly enough.
[0042] In certain example apparatuses including a rotator-resolver,
the decentralizer may include a member with at least two
articulated joints connecting the vibrational wave source to the
rotator-resolver and at least one retractable arm. The retractable
arm may position the vibrational wave source proximate the wellbore
wall. FIG. 5 illustrates an apparatus including such a
decentralizer, a rotator-resolver, and coiled tubing in a
horizontal well 508. In horizontal wells, the vibrational wave
source may be directed toward the top wellbore wall, closest to the
ground level surface, as may generally be desired. While this
apparatus may be particularly suitable for highly deviated or
horizontal wells, however, it may also be used in vertical wells.
Vibrational wave source 500 may attach to member 501 with
articulated joint 502. A second articulated joint 503 may connect
member 501 to rotator-resolver 504. Rotator-resolver 504 may also
connect to tool body 505, which may contain controls for
vibrational wave source 500 and rotator-resolver 504. Once
vibrational wave source 500 is at the desired circumferential
orientation, retractable arms 506 and 506' unfold and position
vibrational wave source 500 proximate wellbore wall 507. While FIG.
5 displays an embodiment according to the present invention with
two retractable arms 506 and 506', an embodiment need only use at
least one retractable arm.
[0043] In example apparatuses including a rotator-resolver, the
decentralizer may comprise a vibrational wave source pad and at
least one retractable arm. FIG. 6 illustrates an apparatus
including this decentralizer, a rotator-resolver and coiled tubing
in use in a horizontal well 606. This apparatus may be particularly
useful in highly deviated or horizontal wells. In FIG. 6,
vibrational wave source 600 couples to vibrational wave source pad
601 via retractable arm 602. Alternatively, vibrational wave source
600 may directly couple to vibrational wave source pad 601, and
retractable arm 602 may couple vibrational wave source pad 601 to
tool body 604. While the apparatus shown in FIG. 6 uses only one
retractable arm, an apparatus using this particular decentralizer
may incorporate more retractable arms if necessary. Vibrational
wave source pad 601 may couple to rotator-resolver 603.
Rotator-resolver 603 may join to tool body 604, which may contain a
driver for vibrational wave source 600 and controls for
rotator-resolver 604. Once the vibrational wave source is at the
desired circumferential orientation and is proximate the portion of
the wall to be cleaned, retractable arm 602 unfolds and positions
vibrational wave source 600 proximate wellbore wall 605.
[0044] Certain example apparatuses may further include at least one
standoff contactor that maintains a standoff distance between the
vibrational wave source and the wellbore wall. FIGS. 4, 5 and 6
illustrate apparatuses that include at least one standoff contactor
550. The standoff distance is labeled by the letter "d." Each
standoff contactor 550 may have at least one contact point that
touches the wellbore wall. As persons of ordinary skill in the art
having the benefit of this disclosure will realize, an appropriate
size for the standoff distance will depend on the extent of the
skin effect at the portion of the wellbore wall and the speed of
sound downhole, as discussed later in this disclosure. In certain
exemplary embodiments, the standoff distance will be in the range
of about 1.5 inches to about 0.125 inch. In certain preferred
embodiments, the standoff distance will be in the range of about
0.25 inch to about 0.20 inch.
[0045] In certain exemplary embodiments, the standoff distance may
be varied to optimize reduction of the skin effect. The
effectiveness of cleaning can be greatly enhanced if the apparatus
is positioned at a standoff distance from the wellbore wall such
that the vibrational wave source creates a standing wave pattern
between the vibrational wave source and the wellbore wall. FIG. 7
shows two example standing wave patterns of the lowest harmonic
mode. Pattern A illustrates an acoustic standing wave traveling
through fluid in a well. The dots represent fluid molecules and
suspended particles forming the standing wave pattern. Pattern B
represents the propagation of the suspended particle motion.
Vibrational wave source 700 emits an incident vibrational wave 701,
with a wavelength .lamda.. Incident vibrational wave 701 reflects
at wellbore wall 702 and returns to vibrational wave source 700 as
reflected vibrational wave 703. If the standoff distance d between
vibrational wave source 700 and wellbore wall 702 of the wellbore
is equal to an integer multiple of .lamda./2, the incident
vibrational wave 701 and reflected vibrational wave 703 will
produce a stationary pattern of constructive interference, or a
standing wave pattern.
[0046] To create this standing wave pattern, standoff contactor 550
should be sized to keep the vibrational wave source at a distance
equal to an integer multiple of .lamda./2 away from the wellbore
wall. While the distance between vibrational wave source 700 and
wall 702 may be any integer multiple of .lamda./2 to produce a
standing wave pattern, in practice, lower-order harmonics produce
better cleaning results: the acoustic aspect ratio of vibrational
waves in lower-order harmonics tends to result in deeper
penetration into the formation or intervening structure or
material. Moreover, vibrational waves in lower-order harmonics
undergo fewer cycles per unit time, thereby decreasing the acoustic
attenuation per unit distance of the vibrational waves. Once a
standing wave pattern is generated, the intensity of the
vibrational waves is enhanced at the wellbore wall: the intensity
of the reflected vibrational wave 701 adds to the intensity of the
incident vibrational wave 702, resulting in a pressure amplitude
that is greater than the pressure caused by the incident wave along
a thin pressure zone near the wellbore wall. This pressure zone can
be seen in Pattern A. The pressure zone may be further amplified by
using a vibrational wave source that can focus generated waves on
the wellbore wall. Some example apparatuses may therefore include a
vibrational wave source that can focus its waves on the wall of the
wellbore, such as a vibrational wave source that includes focused
transducers.
[0047] The standoff distance can be chosen at the surface based on
the anticipated skin effect properties and estimated speed at which
the vibrational waves will travel in the wellbore wall. However,
the standoff distance required to establish a standing wave pattern
may vary with variations in the speed of sound downhole. As persons
of ordinary skill in the art are aware, the speed of sound in
downhole fluids will vary with the temperature and pressure of the
downhole fluids, often increasing with depth in a borehole.
Variations in the formation fluid constituents, fluids present in
the well or particles present in these fluids can cause the speed
of sound downhole to change. In particular, the speed of sound
might change as the skin effect is reduced. If the standoff
distance can be adjusted downhole, a standing wave can be
maintained despite fluctuations in the speed of sound. FIG. 8 shows
a vibrational wave source pad 801 with an actuator 802 that
controls the standoff distance d between the vibrational wave
source 803 and wellbore wall 302 to obtain the offset necessary to
create a standing wave pattern. Actuator 802 may move vibrational
wave source 803 relative to contact points on standoff contactors
804 that touch wellbore wall 302. Standoff contactors 804 also may
help position the vibrational wave source 803 once retractable arm
805 positions it proximate wellbore wall 302. When cleaning is
completed, retractable arm 805 pulls vibrational wave source into
recess 806 of tool body 811.
[0048] The apparatus may further include a means for detecting
accretions of particles between the vibrational wave source and the
wellbore wall. If this detecting means detects a threshold level of
accreted particles, the vibrational wave source may be moved away
from the wellbore wall. The vibrational wave source may then
radiate vibrational waves to break up the accretions and reposition
proximate the wellbore wall to continue the cleaning process. In
the example apparatus shown in FIG. 8, the detecting means includes
an accelerometer 807 connected to a vibrational wave source pad
801. Accelerometer 807, which may be a conventional prior-art
accelerometer, may produce an electrical signal for a signature
vibration pattern indicating that particles have accreted between
vibrational wave source 803 and wellbore wall 302. The signature
vibration pattern will be determined empirically by comparing the
vibrations that vibrational wave source 803 experiences when no
particles have accreted to vibrations that vibrational wave source
803 experiences when particles have accreted. Other suitable means
of detecting accretions of particles will be apparent to persons of
ordinary skill in the art having the benefit of this
disclosure.
[0049] The apparatus may also comprise a means for monitoring
energy transfer from the vibrational wave source to the wellbore
wall. If the amount of energy transferred is insufficient or
excessive, the vibrational wave source can be repositioned, or the
cleaning process repeated, to optimally reduce the skin effect. The
vibrational waves radiated by vibrational wave source 803 may also
be altered to optimally reduce the skin effect at that wellbore
wall 302 if necessary. In an example apparatus shown in FIG. 8, the
monitoring means includes a hydrophone 810 suitable for use in a
downhole environment inside tool body 811. Hydrophone 810, which
may be a conventional prior-art hydrophone, may translate
vibrational waves traveling through fluid present near wellbore
wall 302 into an electrical signal proportional to the amount of
energy transferred by the vibrational waves. The vibrational waves
may contact hydrophone 810 through port 812. A processing unit 813
then measures the electrical signal to monitor the energy
transferred from vibrational wave source 803 to wellbore wall 302.
Processing unit 813 may be in tool body 811, elsewhere downhole, or
even uphole.
[0050] In an alternate example shown in FIG. 9, the monitoring
means may include an accelerometer 901 connected to vibrational
wave source pad 801 and a processing unit 902. Accelerometer 901
may also be a conventional prior-art accelerometer. Accelerometer
901 should be acoustically isolated with an insulator 903 from
vibrational wave source pad 801, however, and produce an electrical
signal proportional to the vibrations experienced by wellbore wall
302 in response to the vibrational waves radiated by vibrational
wave source 803. In certain example apparatuses, accelerometer 901
will directly contact wellbore wall 302. Processing unit 902 then
measures the electrical signal produced by accelerometer 901 to
monitor the energy received by wellbore wall 302 from vibrational
wave source 901. Accelerometer 901 may comprise a spring-loaded
membrane having a wearface that contacts wall 302, at least one
piezoelectric element attached to this spring-loaded membrane, and
a backing mass attached to the at least one piezoelectric element.
Accelerometer 901 may further comprise a housing to protect the at
least one piezoelectric element. Other suitably designed
accelerometers may be used, however, as will be apparent to persons
of ordinary skill in the art having the benefit of this disclosure.
Further, other means for monitoring energy transfer from the
vibrational wave source to the wellbore wall will be apparent to
persons of ordinary skill in the art having the benefit of this
disclosure. For example, standard formation receivers may be used,
inside or outside the tool body. The vibrational wave source 100
could then be used as a transmitter between cleaning pulses.
Example apparatuses may also include conventional receivers used in
sonic logging tools. While the hydrophone described herein may be a
broad band receiver, other receivers may be tuned to specific
cleaning frequencies.
[0051] Example apparatuses include at least one vibrational wave
source. In certain examples, however, the apparatus may include a
plurality of vibrational wave sources displaced axially at the same
circumferential orientation, displaced radially at the same axial
location, or displaced in some combination of the two
configurations. The number of vibrational wave sources chosen can
depend on the power available to the apparatus as well as its
mechanical complexity. FIG. 10 displays an example apparatus 1010
that uses vibrational wave source pads and retractable arms to
position the vibrational wave sources in the borehole. Any
positioning means, however, may be used. Here, the plurality of
vibrational wave sources includes three vibrational wave source
pads, 1020, 1030 and 1040, each containing one vibrational wave
source 1020', 1030' and 1040', but any number of vibrational wave
sources may be used, as will be apparent to persons of ordinary
skill in the art having the benefit of this disclosure. Each
vibrational wave source pad 1020, 1030 and 1040 is aligned axially
with the others. In this example, gaps "g" separate vibrational
wave source pads 1020, 1030 and 1040. In certain example
apparatuses, gap g will be about 3 feet to about 5 feet. Gap g,
however, may be of any suitable size chosen for a particular
application, as a person of ordinary skill in the art having the
benefit of this disclosure will realize.
[0052] The vibrational wave sources that form the plurality
vibrational wave sources of the apparatus may be activated
continuously, or in succession, with or without intervening periods
of inactivation. For example, the vibrational wave source 1020' may
first radiate vibrational waves at portion 1021 of the wellbore
wall for a period of time. Vibrational wave source 1020' then stops
to allow fluid flow to flush away any particles from portion 1021
and from any structures or materials present at portion 1021. Once
vibrational wave source 1020' stops, vibrational wave source 1030'
will radiate vibrational waves for another period of time at
portion 1031 of the wellbore wall and then stop to allow fluid flow
to flush away any particles from portion 1031 and from any
structures or materials present at portion 1031. Once vibrational
wave source 1030' stops, vibrational wave source 1040' will radiate
vibrational waves for another period of time at portion 1041 of the
wellbore wall and then stop to allow fluid flow to flush away any
particles from portion 1041 and from any structures or materials
present at portion 1041. If necessary, vibrational wave source
1020' may radiate vibrational waves again, and the process may be
repeated with vibrational wave sources 1030' and 1040'. Moreover,
vibrational wave sources 1020', 1030', and 1040' may radiate
vibrational waves at different frequencies to optimally reduce the
skin effect.
[0053] Although this example method activates vibrational wave
sources in succession from left to right in FIG. 10, any order of
activation may be used. In alternative methods of the present
invention, the plurality of vibrational wave sources may be
activated simultaneously, either for defined periods of time or
continuously. The periods of time during which vibrational wave
sources radiate vibrational waves may vary as needed for different
skin effects and downhole environments, or to optimize use of the
available power, as a person of ordinary skill in the art having
the benefit of this disclosure will realize. A typical activation
period will be longer than approximately 2,000 cycles. The optimal
duration of the activation period depends on the rate at which the
apparatus traverses in the well. If the rate of axial traverse is
zero, and thus the apparatus is still, the activation should
typically be no longer than approximately 5 seconds at any given
location on the wellbore wall. If the apparatus axially traverses
the wellbore at a rate such that it moves to an entirely new
location in less than approximately 5 seconds, as in slow,
continuous motion, the apparatus may be continually activated. An
intervening period of inactivation may occur between multiple
periods of activation, if desired. During these intervening
periods, formation fluid flow will further flush loosened fines.
However, formation fluid flow may flush fines even during
activation.
[0054] The multiple vibrational wave sources may assume different
configurations to offer wider coverage, if desired. In an
alternative example shown in FIGS. 11A, 11B, and 11C, the apparatus
may include a plurality of vibrational wave sources on vibrational
wave source pads displaced both circumferentially and axially. Each
vibrational wave source pad 1120, 1130, and 1140 may be displaced
circumferentially by 60 degrees from the nearest vibrational wave
source pad, as shown in the cross-sectional view of FIG. 11A.
Vibrational wave source pads 1120, 1130, and 1140 may also be
displaced circumferentially by about 120 degrees from the nearest
vibrational wave source pad, as shown in the cross-sectional view
of FIG. 11B. In addition to their circumferential displacement,
vibrational wave sources 1120, 1130, and 1140 may be displaced
axially by a gap g, as shown in the longitudinal view of FIG. 11C.
The number and orientation of vibrational wave sources may vary to
best suit the particular well, to optimize power utilization, and
to utilize different frequencies to more effectively clean, as
persons of ordinary skill in the art having the benefit of this
disclosure will realize. The apparatuses shown in FIGS. 11A, 11B,
and 11C may be activated using the methods described earlier in
this disclosure; that is, the vibrational wave sources may be
activated in succession, simultaneously for defined time periods,
or simultaneously and continuously. As with the apparatus shown in
FIG. 10, other methods may be used to produce the desired cleaning
effect.
[0055] The apparatus may also include a means for determining how
much the skin effect in the downhole environment has been reduced.
This determining means may measure a speed of sound or propagation
speed for the vibrational waves in a wellbore wall that has already
been cleaned. The speed of sound after cleaning can be compared
with a measured control speed at which vibrational waves traveled
in the same wellbore wall prior to cleaning. The control speed can
be determined empirically by measuring the speed of sound for
vibrational waves at low acoustic intensities propagating in the
wellbore wall. For example, production improvement observed in a
test well in a particular reservoir could be correlated with the
change in the speed of sound. Empirical data from other cleanings
may be useful to supplement this comparison.
[0056] Any apparatus for reducing a skin effect in a downhole
environment may incorporate a means for determining how much the
skin effect has been reduced. For example, the vibrational wave
source pads 1201 and 1202 shown in FIG. 12 connect to
accelerometers 1203 and 1204. Accelerometers 1203 and 1204 contact
the wellbore wall but are acoustically isolated via insulators 1206
from the vibrational wave sources 1201' and 1202' contained in
vibrational wave source pads 1201 and 1202, respectively. To
determine how much the skin effect in the downhole environment has
been reduced, vibrational wave source 1201' first radiates
vibrational waves at the wellbore wall. Accelerometer 1203 will
detect vibrations in the wellbore wall as a result of the radiated
vibrational waves and create an electrical signal in response to
the detected vibrations. This electrical signal is transmitted to
processing unit 1205, which calculates the speed at which the
vibrational waves traveled based on the time difference between the
radiation and detection of the vibrational waves in the wellbore
wall. Because the time at which the radiation began, the time at
which the accelerometer detected the vibration, and the distance
between vibrational wave source 1201' and accelerometer 1203 are
known quantities, the speed at which the vibrational waves travel
can be easily calculated by dividing the distance by the time
difference. A suitable method of tracking the times of radiation
and subsequent detection of the vibrational waves will be apparent
to persons of ordinary skill in the art having the benefit of this
disclosure. This process of measuring the speed at which the
vibrational waves travel may be repeated as vibrational wave
sources 1201' and 1202' are fired. Alternatively, two transmitters
with two intervening accelerometers or hydrophones may be used to
determine how much cleaning has been achieved. Standard methods for
measuring borehole compensated speed or acoustic attenuation may be
used to measure the cleaning effect. Empirical data can be used to
correlate the acoustic attenuation of the vibrational waves with
the effectiveness of the cleaning.
[0057] Therefore, the present invention is well adapted to carry
out the objects and attain the ends and advantages mentioned, as
well as those that are inherent therein. While the invention has
been depicted and described, and is defined by reference to the
exemplary embodiments of the invention, such a reference does not
imply a limitation on the invention, and no such limitation is to
be inferred. The invention is capable of considerable modification,
alteration and equivalents in form and function, as will occur to
those ordinarily skilled in the pertinent arts and having the
benefit of this disclosure. The depicted and described embodiments
of the invention are exemplary only and are not exhaustive of the
invention. Consequently, the invention is intended to be limited
only by the spirit and scope of the appended claims, giving full
cognizance to equivalents in all respects.
* * * * *