Communications – electrical: acoustic wave systems and devices – Signal transducers – Underwater type
Reexamination Certificate
2001-11-06
2003-11-04
Moskowitz, Nelson (Department: 3663)
Communications, electrical: acoustic wave systems and devices
Signal transducers
Underwater type
C367S081000, C367S177000, C367S911000, C175S050000, C166S249000, C181S102000
Reexamination Certificate
active
06643221
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to acoustic logging tools for performing acoustic investigations of subsurface geological formations traversed by a borehole. More particularly, the invention relates to the implementation of particle damping in well logging tools to attenuate tool waves.
BACKGROUND OF THE INVENTION
In the oil and gas industry, subsurface formations are typically probed by well logging tools to determine formation characteristics which can be used to predict or assess the profitability and producibility of subsequent drilling or production operations. In many cases, acoustic logging tools are used to measure formation acoustic properties which may be used to produce images or derive related characteristics for the formations.
Acoustic waves are periodic vibrational disturbances resulting from acoustic energy which propagates through a medium, such as a formation or logging tool. Acoustic waves are typically characterized in terms of their frequency, amplitude, and speed of propagation. Acoustic properties of interest for formations may include compressional (P) wave speed, shear (S) wave speed, and borehole modes, such as tube wave. Additionally, acoustic images may be used to depict borehole wall conditions and other geological features away from the borehole. These acoustic measurements have applications in seismic correlation, petrophysics, rock mechanics and other areas.
Recordings of acoustic properties as functions of depth are known as acoustic logs. Information obtained from acoustic logs may be useful in a variety of applications, including well to well correlation, determining porosity, determining mechanical or elastic parameters of rock to give an indication of lithology, detecting over-pressured formation zones, and enabling the conversion of a seismic time trace to a depth trace based on the measured speed of sound in the formation.
An acoustic logging tool typically includes one or more acoustic sources (i.e., a transmitter) for emitting acoustical energy into subsurface formations and one or more acoustic receivers for receiving acoustic energy. The receivers are typically axially spaced apart from the transmitters to allow the acoustic energy to propagating through the surrounding formation before being received at the receivers.
Transmitters and receivers for acoustic logging tools commonly comprise acoustic transducer elements, such as piezoelectric crystals. In general, an acoustic transducer converts energy between electric and acoustic forms and can be adapted to act as a source or a receiver. Acoustic transducers are typically mounted on the body of the logging tool. It is desired that the minimum amount of energy from the transmitter be transferred to the tool body and the maximum amount of energy be radiated into the borehole and the formation.
Acoustic energy emitted from a logging tool in a borehole may travel along multiple paths to reach the receivers. The part of the acoustic energy that propagates through the formation and fluid in the well is the energy that provides useful information for characterizing the formation. The part of the acoustic energy that propagates through the tool body generally provides no useful information about the formation and often presents a difficulty in measuring acoustic information from the formation.
A common issue for all acoustic tools is the part of the acoustic energy propagating along the tool body, referred to as a “tool wave.” Tool wave is undesired because it contains substantially no information about the formation and interferes with the part of the acoustic energy propagating through the formation, referred to as the “formation wave.” For many wireline tools, unwanted tool wave is reduced with design features such as slotted sleeves, isolation joints and flexible tool bodies. For logging while drilling (LWD) tools, tool waves are an even more serious challenge because these waves are carried by the prominent and stiff tool body, which is essentially a drill collar.
Various forms of acoustical energy propagating in the borehole can be used for probing different properties of the surrounding formation. For example, a monopole logging tool (wireline or logging while drilling type) uses single or multiple monopole acoustic source(s) as well as receivers which oscillate and detect uniformly in all azimuthal directions in the plane perpendicular to the tool axis.
It is well understood based on theory of wave propagation that a monopole tool can excite and detect P-waves and Stoneley waves in substantially all formations, regardless of formation acoustic speed. In addition, a monopole tool is capable of generating and detecting S-waves in so called “fast” formations where the formation shear speed is faster than the sound speed in the borehole fluid—drilling mud. However, part of the energy emitted by the monopole source couples to the tool body and generates tool waves. This tool wave propagates at a speed of about 5000 m/sec for low frequencies in a steel mandrel and typically arrives at the receivers before almost all the desirable signals from the surrounding formation. As a result, this tool wave arrival interferes with the desired formation wave signals, especially the formation P-wave.
For wireline monopole tools, the tool waves are usually delayed and suppressed by techniques such as slotted receiver housings. In logging while drilling monopole tools, which require thicker and stronger tool bodies, suppressing tool waves has proven to be a more difficult issue. One logging while drilling monopole tool operated under the trademark ISONIC by Schlumberger Technology Corporation of Sugar Land, Tex., achieves tool wave attenuation over a selected frequency band with a specially designed periodic array of grooves machined on the collar section between the transmitter and receivers, as described in U.S. Pat. No. 5,852,587 to Kostek et al.
As another example, a wireline dipole tool generates and receives flexural mode waves in a borehole. The term dipole refers to the azimuthal profile cos&thgr; for the transmitter, receivers and the acoustic field associated with the flexural mode. The flexural mode propagation speed asymptotes to the formation shear speed at the low frequencies, and to the mud-formation interface wave speed at high frequencies. Thus S-wave speed of the formation can be derived from the measured flexural mode as discussed in “Acoustic multipole sources in fluid-filled boreholes” by Kurkjian and Chang in
Geophysics,
51, 148-163 (1986).
To avoid or minimize tool wave effects on the measured borehole flexural mode, wireline dipole tools commonly use acoustically slow (i.e., mechanically flexible) housings for receivers. These tools may also include a form of acoustic isolator or attenuator between the source and receivers to reduce the transmission of tool waves.
Applying the wireline dipole shear technique to LWD tools is difficult. First, LWD tools cannot be made very flexible or acoustically slow, as done for wireline tools, because the tool body of an LWD tool is, in most cases, essentially a drill collar. This provides an easy propagation path for acoustic energy between the acoustic source and the receivers. The tool wave interferes with the borehole flexural wave and makes the measurement much more complicated and difficult, as discussed in “Mandrel effects on the dipole flexural mode in a borehole” by Hsu and Sinha in
Journal of the Acoustical Society of America,
104(4), 2025-2039 (1998) and “Acoustics of fluid-filled boreholes with pipe: Guided propagation and radiation” by Rao and Vandiver in
Journal of the Acoustical Society of America,
105, 3057-3066 (1999).
Additionally, in some cases acoustical energy reflected from formation or tool discontinuities above and below the acoustic transmitter and receivers, and acoustical energy coupled from the surrounding formation back to the tool may interfere with measurement quality or affect tool durability.
Several approaches for reducing tool waves have been proposed. For example, U.S. Pat. No. 5,510
Aron Jeffrey B.
Campanac Pierre
D'Angelo Ralph M.
Esmersoy Cengiz
Grigor Darryl
Jeffery Brigitte L.
Moskowitz Nelson
Ryberg John
Schlumberger Technology Corporation
Segura Victor H.
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