Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Land-reflection type
Reexamination Certificate
1998-11-16
2001-03-20
Oda, Christine (Department: 2862)
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Land-reflection type
C367S030000, C367S032000
Reexamination Certificate
active
06205087
ABSTRACT:
DETAILED DESCRIPTION OF THE INVENTION
1. Field of the Invention
The present invention relates to logging technology for measuring physical properties of underground formations, and, in particular, to a logging method and system using sonic waves.
2. Background
In order to explore underground resources, such as those providing oil and natural gas, use has conventionally been made of logging technology by drilling a borehole into the ground, locating a measuring device called a downhole tool or sonde inside the borehole so that is can be moved up and down, and processing a measured signal from the downhole tool with processing apparatus located at the surface and connected to the downhole tool through a logging cable. In addition, sonic logging involving determining the speed of sound propagating through the underground formation using a sonic wave generator and a receiver provided on the downhole tool is also well known. For example, reference should be made to Jay Tittaman, “Geophysical Well Logging”, Academic Press, Inc., and “Illustration Physical Exploration”, 1989, Physical Exploration Society.
In conventional sonic logging a sonic wave in the form of a pulse is output by a sonic generator and transmitted into the ground, and the sonic wave propagating through the ground is detected by a receiver and, as an analog waveform, transmitted through a logging cable to the ground surface processing apparatus which processes the analog waveform to determine the arrival time of the sonic wave at the receiver. However, because of the unreliability of analog data received after transmission along a lengthy logging cable, a proposal has recently been made to convert the analog signal into a digital signal at the downhole tool, and then to transmit this digital signal to the ground surface processing apparatus for the required processing (see, for example, A. R. Harrison, C. J. Randal, J. B. Aron, C. F. Morris, A. H. Wingnall, R. A. Dwoorak, L. L. Rulledge, and J. L. Perkins, “Acquisition and Analysis of Sonic Waveforms From a Borehole Monopole and Dipole Source for the Determination of Compression and Shear Speeds and Their Relation to Rock Mechanical Properties and Surface Seismic Data”, SPE 20557, 1990, September 23-26, New Orleans, SPE 65th Annual Technical Conference and Exhibition). However, in the above-identified literature (SPE 20557), as shown in its
FIGS. 3 and 4
, the whole of the digital signal exceeding a pre-set threshold value is transmitted to the ground surface processing apparatus for analysis thereby. This requires the transmission of an exorbitant amount of digital data to the surface, and a broad bandwidth is required for the telemetry. In addition, since even that data which is not necessarily required for sonic logging analysis is also transmitted, the efficiently of the operation is rather poor. And, since a large amount of digital data is transmitted through a lengthy logging cable, there is also a chance of errors in transmission.
Tasks to be solved by the Invention
The present invention, made in view of the points raised above, suggests a sonic logging method and system capable of obviating the drawbacks of the prior art as described above. To achieve this it proposes ways of minimising the amount of digital data to be transmitted to the ground surface processing apparatus, reducing the bandwidth necessary for the telemetry, and reducing also the possibility of data errors, thereby enhancing the system's reliability.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, therefore, there is provided a sonic logging method for determining characteristics of the formations through which a borehole passes, which method uses a downhole tool which is moveably locatable up and down inside the borehole and which is coupled to ground surface processing apparatus through a logging cable, which tool is provided with at least one sonic wave generator and at least one receiver spaced apart from each other and also with a downhole processing device operative coupled to said generator and said receiver and also to said ground surface processing apparatus through said logging cable, in which method the downhole tool is first suitably located in the borehole, and then the sonic wave generator is caused to generate, and subsequently, receive a sonic wave,
which method is characterised by including the steps of:
(1) processing a detection signal from the receiver by the downhole processing device thereby to determine an arrival time of the sonic wave at its receiver; and
(2) transmitting the arrival time thus determined to the ground surface processing apparatus through the logging cable.
In accordance with another aspect of the present invention, there is provided a sonic logging downhole tool, for use in a borehole characteristic determination method, which tool includes:
at least one sonic wave generator; and
at leas tone receiver capable of receiving the sonic wave after the wave has travelled through a borehole ground formation or casing;
which tool is characterised by including a control device for controlling the generation and reception of the sonic wave, the control device comprising:
an analog-to-digital converter for digitising a detection signal from the receiver at a predetermined sampling interval;
a first memory for storing a waveform thus digitised;
a second memory for storing a predetermined program for processing the thus-stored digitised waveform; and
a microprocessor capable of executing the program stored in the second memory, thereby in operation processing the digitised waveform stored in the first memory to determine an arrival time of a sonic wave generated from the generator and arriving at the receiver.
In accordance with a further aspect of the present invention, there is provided a sonic logging system which comprises the centralisation of ground surface processing apparatus and a sonic logging downhole tool of the invention as just defined.
In sonic logging, use is commonly made of a sonic waveform having a central frequency of 15 kHz and a wavelength of 2.5 m. On digitisation this, in the case of a 16 bit resolution, leads to 250×16 bits—4 kilobits. In conventional digital sonic logging this amount of digital data is transmitted to a ground surface processing apparatus through a logging cable by way of telemetry communication, and the processing to determine the arrival time and amplitude of the P (compression) wave is carried out by the ground surface processing apparatus. However, what is actually required in sonic logging is basically only the arrival time and amplitude of the P wave—rarely if ever is the remaining digital waveform data needed. Thus, the data which is actually necessary—the two times—comprises on digitisation a mere 2×16 bits=32 bits. So, as may be understood from this simple example, if the processing of the digitised waveform is carried out in the downhole tool itself, and only the resulting arrival time and amplitude are transmitted to the ground surface processing apparatus, the amount of data to be transmitted through the logging cable can be reduced to one hundredth or less, and as a result the transmission efficiency is significantly improved, and the occurrence of error is also significantly reduced. In addition, the logging cable has an increased idle time, so that the logging cable can be used for some other purpose. The present invention has been made principally in view of these points.
Incidentally, although in accordance with the present invention, the primary object is to determine downhole the sonic signal arrival time and/or amplitude and then transmitting the result to the surface, it is of course also possible in the present invention—and in certain cases it is actually preferred—to transmit not all by only a selected portion of the digital signal up to the ground surface processing apparatus. As explained in more detail hereinafter, such a selective transmission of digital signal has the advantage of allowing the confirmation of the data's reliabilit
Fukuhara Masafumi
Tejada Mauricio
Christian Steven L.
Jolly Anthony
Oda Christine
Schlumberger Technology Corporation
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