Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Well logging
Reexamination Certificate
2000-06-07
2003-01-21
Lefkowitz, Edward (Department: 2862)
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Well logging
C181S102000
Reexamination Certificate
active
06510104
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods for characterizing physical properties of underground formations by transmitting sonic waves in the borehole and by processing resulting sonic waveform measurements and, in particular, to methods for selecting the acoustic frequency of transmitted sonic waves to optimally excite dipole flexural mode sonic waves in the borehole.
2. Description of the Related Art
In the development of natural hydrocarbon (e.g. oil) reservoirs, a borehole is typically drilled into the ground from a surface location. Downhole measurements of various phenomenon and properties are often made to determine various characteristics related to the underground resources or formations, or otherwise related to the drilling process. To make such measurements, various borehole sensors or detectors may be located in the drill bit, in the bottom hole (or borehole) assembly (BHA), in the drill string above the mud motor, or in any other part of the sub-surface drill string. Borehole sensors are often contained on a special tool, such as a wireline tool, which is lowered into the hole on a wireline cable. The downhole measuring tool may also contain various signal sources for generating signals in the borehole for detection by sensors in the tool after passing through the borehole and/or underground formations around the borehole.
The sensed or measured data is typically transmitted to the surface, where it can be stored, processed, or otherwise used, e.g. to monitor and control the drilling process. Data measured or sensed by the downhole tool is typically transmitted or telemetered back to receivers and processing equipment at the surface by various telemetry techniques and systems, such as by hard wired cables or wireline tools which contain electrical and/or fiber optic conductors which transmit data to the surface based on inductive coupling or other principles.
Telemetry techniques other than wireline telemetry are also sometimes used to transmit sensed data to the surface. MWD (measurements-while-drilling) and LWD (logging-while-drilling) techniques, for example, which are sometimes used for making downhole measurements, typically employ drilling fluid or mud pulse telemetry, electromagnetic telemetry, or acoustic telemetry through the drill string itself, to transmit sensed data to the surface. Acoustic borehole telemetry and related modulation schemes are described by S. P. Monroe, “Applying Digital Data-Encoding Techniques to Mud Pulse Telemetry,”
Proceedings of the
5
th
-
SPE Petroleum Computer Conference
, Denver, Jun. 25th-28th, 1990, SPE 20236, pp. 7-16.
It is possible to determine properties of underground formations using measurements of acoustic/sonic waves that have passed through the formations. Thus, one type of measurement made downhole is measurement of sonic waves generated by a sonic generator or transmitter, which sonic waves have passed through the borehole and/or underground formations. Sonic or acoustic logging tools are accordingly utilized during various phases of hydrocarbon development and exploration.
For example, a sonic logging tool may be lowered by a logging cable into an open borehole. Such a tool, sometimes referred to as a sonde, typically contains one or more sonic wave generators or sources (transmitters), and one or more sonic wave receivers (typically hydrophones), separated by a known distance on the tool. The sonic logging tool emits or “fires” sonic waves, typically in the form of pulses, in accordance with an excitation or drive voltage waveform applied to the transducer of the sonic wave source. These transmitted sonic waves pass through the formation around the borehole and are then detected at the receiver(s). The detected acoustic signals are then typically transmitted to the surface, via a wireline inside the logging cable, for example, for processing, storage, monitoring, or other purposes. In addition to open-hole measurements, a sonic logging tool may also be used to make cased-hole measurements.
Sonic waves can travel through rock formations in essentially two forms: body waves and surface waves. There are two types of body waves that travel in rock: compressional and shear. Compressional waves, or P-waves, are waves of compression and expansion and are created when the rock formation through which the sonic waves travel is sharply compressed. With compressional waves, small particle vibrations occur in the same direction the wave is traveling. Shear waves, or S-waves, are waves of shearing action as would occur when a body is struck from the side. In this case, rock particle motion is perpendicular to the direction of wave propagation.
Surface waves are found in a borehole environment as complicated borehole-guided waves which come from reflections of the source waves reverberating in the borehole. The most common form of borehole-guided surface wave is the Stoneley (St) wave. Such sonic waveforms may be detected by a receiver as a result of sonic waves generated or emitted from a monopole (omnidirectional, or symmetric) source, for example. A monopole source generates primarily an axisymmetric family of modes together with compressional and shear headwaves.
Dipole (directional sources and receivers may also be used in some applications. A dipole source excites the flexural family of borehole modes together with compressional and shear headwaves. The flexural mode waves may be referred to as flexural waves. Sonic waves will also travel through the fluid in the borehole and along the tool itself. With no interaction with the formation, these waves carry no useful information and can interfere with the waveforms of interest if they have similar propagation speeds.
A dipole transmitter may consist essentially of a moving coil loudspeaker capable of radiating pressure pulses from both sides of its “speaker-cone.” The speaker-cone is typically a piezoelectric source or disk such as a 2″ diameter titanium disk. Thus, when a current pulse (having a drive or excitation waveform) passes through the coil, the disk vibrates parallel to its axis, creating positive pressure on the borehole fluid on one side of the sonde, and a negative pressure on the other side. Thus, when dipole sources are employed, an additional shear/flexural wave propagates along the borehole and is caused by the flexing action of the borehole in response to the dipole signal from the source. The receivers may be hydrophone receivers, placed on the tool along the borehole axis a known distance from each other and from the sonic generator(s).
Various types of dipole signal sources and transmitters have been employed or proposed. These include, for example, electromagnetic transducer devices such as is used in Schlumberger's DSI tool (see U.S. Pat. No. 4,862,991 (Hoyle et al.), issued Sep. 5, 1989; U.S. Pat. No. 4,207,961 (Kitsunezaki), issued Jun. 17, 1980; U.S. Pat. No. 4,383,591 (Ogura), issued May 17, 1983); linked mass vibrators driven by magnetostrictive actuators (see, e.g., S. M. Cohick & J. L. Butler, “Rare-Earth Iron ‘Square Ring’ Dipole Transducer,”
J Acoust. Soc. Am
. 72(2) (August 1982), pp. 313-315); piezo-electric bender devices such as are used in the XMAC tool of Baker Atlas (see, e.g., U.S. Pat. No. 4,649,525 (Angona et al.), issued Mar. 10, 1987); magnetic repulsion transducers driving a plate in contact with a fluid in an acoustic wave guide system such as are used in the MPI XACT tool (see, e.g., U.S. Pat. No. 5,852,262 (Gill et al.)); and eccentric orbital masses as proposed in U.S. Pat. No. 4,709,362 (Cole) and U.S. Pat. No. 5,135,072 (Meynier), mainly for seismic uses.
The speeds at which sonic waves travel through underground rock formations are controlled by rock mechanical properties such as density and elastic dynamic constants, and other formation properties such as amount and type of fluid present in the rock, the makeup of the rock grains, and the degree of intergrain cementation. Thus, by measuring the speed of sonic wave propagation in a borehole, it is poss
Gutierrez Anthony
Jeffery Brigitte
Lefkowitz Edward
Nava Robin
Schlumberger Technology Corporation
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