Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Well logging
Reexamination Certificate
2000-06-26
2002-10-15
Lefkowitz, Edward (Department: 2862)
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Well logging
C181S105000
Reexamination Certificate
active
06466513
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates broadly to sonic sensors. More particularly, this invention relates to an acoustic sensor used in downhole tools.
2. State of the Art
Knowledge of a diameter of a borehole while it is being drilled is important to the driller because remedial action may be taken by the driller in real time, preventing the delay inherent in tripping the drill string and conducting open-hole logging activities. If, for example, the diameter of the borehole is over-gauge, such a fact may indicate that there is inappropriate mud flow, or an improper mud chemical characteristic, or that the well hydrostatic pressure is too low, or that there is some other source of wellbore instability. If, on the other hand the diameter of the borehole is below gauge or nominal size, such a fact may indicate that the bit is worn and should be replaced so as to obviate the need for later well reaming activities.
Well bore diameter instability increases the risk that the drilling string may become stuck downhole. A stuck drill string implies an expensive and time consuming fishing job to recover the string or deviation of the hole after the loss of the bottom of the drilling string. Well bore diameter variation information is important to the driller in real time so that remedial action may be taken.
Well bore diameter as a function of depth is also important information for a driller where the borehole must be kept open for an extended portion of time. Monitoring of well bore diameters when the drill string is tripped out of the borehole provides information to the driller regarding proper drilling fluid characteristics as they relate to formation properties.
Knowledge of borehole diameter also aids the driller when deviated holes are being drilled. When a borehole is out of gauge, directional drilling is difficult because the drill-string, bottom-hole assembly, and collar stabilizers do not contact the borehole walls as predicted by the driller. Real time knowledge of borehole diameter provides information on which to base directional drilling decisions. Such decisions may eliminate the need for tripping the string so as to modify the bottom-hole assembly to correct a hole curvature deviation problem.
Real time knowledge of well bore diameter is similarly important in logging while drilling (LWD) operations. Certain measurements, especially nuclear measurements of the formation, are sensitive to borehole diameter. Knowledge of the well bore diameter under certain circumstances can be critical for validating or correcting such measurements.
Because real-time knowledge of the borehole diameter is desirable, ultrasonic sensor assemblies for measuring the diameter of a well while it is being drilled are known. For example, U.S. Pat. No. 5,130,950, which is hereby incorporated by reference in its entirety, discloses such a sensor assembly. As seen in prior art
FIG. 1
hereof, the prior art sensor assembly
10
is adapted for placement in the wall
12
or stabilizer fin of a drilling collar, which is placed above the drilling bit of a downhole drilling assembly. The ultrasonic sensor assembly
10
includes a sensor stack
14
including an inner sound absorbing backing element
16
, a piezoelectric ceramic disk transducer
18
stacked outwardly adjacent the backing element
16
, and a matching layer
20
disposed outwardly adjacent the transducer
18
and operating to match the impedance between the ceramic transducer
18
and the environment outside the sensor assembly; i.e., the borehole mud. A window
24
, made of PEEK (polyetheretherketone), available from Victrex USA. Inc. of West Chester, Pa., is disposed outwardly of the matching layer
20
of the stack
14
, and includes an outwardly facing depression
26
for focusing an ultrasonic pulse into the drilling mud
28
toward the borehole wall
30
. An elastomer or epoxy
32
fills the depression to present a smooth face to the flowing mud and the borehole wall.
The backing material
16
, the transducer
18
, and the matching layer
20
are all held together by glue
22
and provided in a protective rubber jacket
34
. The glue
22
prevents the layers of the stack from moving side-to-side relative to each other within the rubber jacket, as the layers must remain in proper alignment for satisfactory performance. An elastomeric material
36
is placed between the rubber jacket
34
and a metallic cup
38
in which the sensor stack
14
is placed. The window
24
is mounted on a spring
40
in the cup
38
outwardly of the rubber jacket
34
and the elastomeric material
36
, which surround the sensor stack
14
.
The ceramic disk transducer
18
is approximately 0.80 inches in diameter and 0.07 inch in width, and is limited in size by the space required for the rubber jacket
34
, which surrounds and seals the sensor stack
14
within the cup
38
.
The sensor assembly
10
also includes electrodes
42
attached to the outer and inner surfaces of the ceramic disk transducer
18
and connector pins
44
for connecting the assembly to an electronics module (not shown) disposed within the drilling collar. The electronics module includes control and processing circuitry and stored logic for emitting ultrasonic pulses in the range of approximately 600 KHz to 700 KHz via the ceramic disk transducer
18
and for generating return (also called echo) signals representative of echoes which return to the transducer
18
after bouncing off the borehole wall. The electronic module also preferably includes a source of electrical energy and downhole memory for storing signals as a function of time. It interfaces with an MWD telemetry module for transmitting measurement information to the surface in real time while drilling.
Two sources of noise are present in the vicinity of the sensor stack of the tool. The first can be characterized as drilling noise, which is of a lower frequency band than that of the acoustic pulse-echo of the transducer. The second is pumping noise, which is characterized by a frequency band, which extends into the frequency range of the pulse-echo apparatus.
Pumping noise is mechanically filtered not only by the rubber jacket
34
surrounding the sensor stack
14
, but also by a filter ring
46
mounted radially outwardly of the transducer
18
about the rubber jacket. The backing element
20
is shock protected by rubber packing between it and the elastomeric material, which envelops the stack.
Drilling noise, which may be of extremely high amplitude, is partially mechanically filtered by the rubber jacket
34
and filter ring
46
described above and partially electronically filtered. Electronic filtering is achieved by an electronic high-pass filter placed prior to signal amplification to avoid amplifier saturation, which could mask ultrasonic signal detection during amplifier saturation and recovery time.
Even in view of the above, it has been found that the assembly of the ultrasonic sensor system of the U.S. Pat. No. 5,130,950 is not particularly effective in signal transmission and recovery. One reason for this is that the glue between each of the layers of the stack imparts undesirable attenuation to the recovered signal. Another reason is that there is insufficient signal output from the sensor disk, which is limited in size by the rubber boot. In addition, the prior art sensor system is difficult to manufacture, and requires a relatively large amount of time to manufacture as the glue and the elastomeric material must cure between manufacturing steps. Moreover, there is inconsistency between the performance of different sensor systems due to different amount of glue that is manually placed between the layers of the stack.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an ultrasonic sensor capable of determining the tool standoff from the borehole wall, and hence the well diameter section at a given depth.
It is another object of the invention to provide an ultrasonic sensor assembly that is optimized for signal recovery.
It is also object of t
Masson Jean-Pierre
Pabon Miguel F.
Jeffery Brigitte L.
Lefkowitz Edward
Ryberg John J.
Schlumberger Technology Corporation
Segura Victor H.
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