1. Field of the Invention
The invention relates to an acoustic tool for creating an image of an earth borehole or well casing, and more particularly, to an imaging caliper instrument for use primarily in a measuring while drilling (MWD) environment utilizing acoustic pulses transmitted within a borehole.
2. History of the Prior Art
It has long been recognized in the oil and gas industry that the collection of downhole data during the drilling operation is of extreme value. Such information improves the efficiency of the drilling operation by providing critical data concerning downhole conditions. For example, it is desirable that a continuous record of borehole size be provided so that variations in borehole diameter as a function of depth may be recorded for analysis in connection with other measurements of formation parameters.
Acoustic well logging is also used in the geophysical and seismic arts to provide surveys of the various formations traversed by the borehole. In particular, acoustic velocity measurements provide valuable information concerning the type of rocks and the porosity thereof in the formation surrounding the borehole. The most commonly measured acoustic parameter in the field of well logging has been the velocity of compression waves. The velocity of shear waves and acoustic impedance have also been of value in determination of both the formation characteristics and the fluid environment.
A myriad of acoustic logging systems for downhole measurements are available in the prior art. One of the most critical measurement parameters of such acoustic logging systems is the acoustic velocity in the fluid through which the acoustic pulse is transmitted. A high degree of accuracy in the interpretation of pulse data is only possible with a precise knowledge of the acoustic velocity in the medium of measurement. Moreover, a high degree of resolution and/or accuracy in acoustic velocity measurements is necessary for the accurate identification of various formation strata as well as other critical borehole parameters.
Many prior art attempts to provide accurate acoustic logging instrumentation have encountered serious problems due to the downhole environment. For example, the drilling operation necessitates the flow of high pressure drilling mud which is pumped down through a central bore in the drill pipe, out through apertures in the drill bit and back to the surface through the annular space between the drill pipe and the side walls of the borehole. The mud removes drill bit cuttings and the like and can reveal much information about the formation itself. Such a fluid system, by definition, includes wide variations in drilling mud density and character both along the borehole as well as in a direction across the borehole annulus. For example, gas present in the drilling fluid has a direct bearing on acoustic velocity within the fluid and the presence of gas varies with position and pressure within the borehole.
One prior art technique of determining acoustic velocity includes sampling the drilling mud at the wellhead for purposes of measurement. However, such a measurement cannot accurately reflect the varying conditions of the mud downhole where the acoustic measurements are actually made. Downhole acoustic pulses are generated, generally, by acoustic transducers disposed within the side walls of a sub secured above an operating drill bit within the borehole. The acoustic pulses are transmitted from the sub to the sidewalls of the borehole through the drilling fluid and the reflection time thereof is monitored. The presence of gas or cuttings within the fluid as well as downhole pressures, temperature and turbulence thus has a direct bearing on the acoustic velocity and the pulse-echo amplitude or reflectivity measurements. However, the most convenient location for measuring acoustic velocity is at the wellhead in the passive fluid collection area where the dynamic turbulent downhole conditions are not present. In addition, once received from the borehole, the drilling mud is generally allowed to settle and/or is passed through an out-gassing unit prior to its collection and recirculation. This step drastically alters the acoustic velocity parameters of the drilling fluid from its downhole gaseous and turbulent condition and leads to inaccuracies in the interpretation of the downhole acoustic reflectivity measurements.
A prior art method of overcoming the problems of accurate data collection in a measuring-while-drilling (MWD) environment is the recording of acoustic borehole measurements with a wireline logging tool. Such tools are utilized with the drill string removed from the borehole and the drilling mud being in a settled state. Such a condition lends itself to a more homogeneous configuration and the presence of mud cakes and turbulence related to nonhomogeneous regions are generally eliminated. One such acoustic caliper logging device is set forth and shown in U.S. Pat. No. 3,835,953 to Summers wherein a wire line tool is provided for positioning within a borehole. A transducer unit repeatedly generates an acoustic pulse as the transducer system is rotated to scan the walls of the borehole in a full circle. A scan of between 1 and 10 revolutions per second may be provided with the tool itself being generally centered within the borehole. The reflections of acoustic energy from the borehole wall are then from a small, centralized area whereby the system can be highly definitive of the character of the wall. Such information is obviously useful in an analysis of the borehole configuration. One distinct disadvantage is, however, the necessity of pulling the drill string from the borehole for utilization of the wireline tool. This operation is both time consuming and expensive from the standpoint of the drilling operation.
In addition, prior art downhole acoustic parameter measurement techniques have obtained acoustic velocity at a downhole location but the acoustic path over which the velocity measurements are made is different from the path over which the parameter of interest is measured. For example, an acoustic caliper measurement made across a borehole annulus which relies on acoustic velocity data obtained in a direction parallel to the borehole axis will not be precise because of the nonlinearity of the flow pattern and flow densities across the borehole.
It would be an advantage, therefore, to overcome the problems of the prior art by providing detailed acoustic caliper information of a borehole in a measuring while drilling configuration. This gives the driller immediate feedback as to the quality of the borehole being drilled and can be used to infer in situ stresses, for example, as disclosed in U.S. Pat. No. 4,599,904 to John E. Fontenot, assigned to Baroid Technology, Inc., the assignee of this present application. This patent discloses that the calipering operation itself can be accomplished through the use of any conventional calipering device, e.g., mechanical, acoustic or neutron calipering device.
Another approach to providing MWD caliper measurements of the drilled borehole is disclosed in U.S. Pat. No. 4,665,511 to Paul F. Rodney et al., also assigned to Baroid Technology, Inc., the assignee of the present application. This patent discloses a downhole, MWD logging tool in which a plurality of acoustic transceivers are placed on the tool in both azimuthal and longitudinal spacings to provide not only a measurement of the acoustic energy reflected from the borehole wall, but also a measurement of the drilling fluid acoustic velocity, coupled with an inclinometer to determine one period of rotational data for purposes of analysis. The patent suggests that the tool can be used to measure the dimensions and shape of the drilled borehole.
The prior art also includes U.S. Pat. No. 5,130,950 which discloses a pulse echo apparatus for measuring borehole standoff and borehole diameter in an MWD environment, and includes deformable material such as rubber inside the acoustic sensor stack to allow the sensor stack to move or deform under pressure or due to thermal expansion or contraction.
As additional prior art, U.K. Patent Appln. No. GB 2 254 921 A discloses means for pressure equalization in an MWD acoustic borehole caliper device.
The prior art also discloses in U.S. Pat. No. 5,341,345 an MWD, ultrasonic stand-off gauge for use in measuring the instantaneous stand-off distance between the drill stem and the borehole wall during drilling.
As additional prior art, U.S. Pat. No. 5,469,736 discloses an MWD, acoustic caliper tool which derives a caliper measurement based on an analog threshold technique.
European Patent Appln. No. 0 747 732 A2, and also U.S. Pat. No. 5,644,186 disclose an MWD, acoustic caliper tool having a movable seal assembly intended to provide temperature and pressure compensation.
Another field of acoustic logging includes the wireline acoustic borehole scanning tool, commonly referred to as a borehole televiewer, which permits the wall of the borehole or casing to be scanned in a manner that produces a visual image of the borehole or casing wall. Acoustic pulses are emitted from an acoustic transducer functioning as a transmitter, which pulses travel to the periphery of the borehole wall, where they are reflected as echo pulses back to the transducer functioning as a receiver. The received acoustic echo pulses are used to generate a visual image of the borehole or casing wall.
Imaging systems, such as the borehole televiewer tool, can provide information to assist in the description of the subsurface reservoir. Such applications include: fracture identification, stratagraphic interpretation, and thin bed analysis. This ability is based on variations in lithology, rock physical features and borehole geometry, that cause changes in the measured travel time and amplitude of the acoustic reflected signal. This signal information is utilized to provide an image of the earth borehole wall. Inside casing such changes can be used to monitor the casing for internal corrosion or damage caused by the drilling operation. The resulting images can be displayed on a high resolution color monitor and/or may be plotted.
Borehole televiewer systems operating on wireline are well known. One such system is described in U.S. Pat. No. 3,369,626 to Zemanek, wherein the wall of the borehole is scanned periodically with acoustic energy. A single transducer, functioning as both a transmitter and a receiver, is rotated and periodically actuated to produce acoustic pulses which are reflected back off the wall of the borehole. The travel times of the echo pulses are utilized to create a beam sweep, once for each rotation of the transducer, to create an image of the wall of the borehole.
Over the years the prior art reflects numerous improvements to the basic wireline borehole televiewer. U.S. Pat. No. 4,736,348 describes a borehole televiewer wherein the received reflected acoustic signals are corrected for amplitude modulation resulting from oblique angles of incidence of the transmitted acoustic pulses when the tool is off center in the borehole, is in an eliptical borehole, or is tilted from the vertical axis of the borehole. The corrected reflection signals modulate an image display so that the full circumference of the borehole can be delineated.
U.S. Pat. No. 4,837,753 describes a wireline borehole televiewer tool where the transducer is rotated using a stepper motor. A computer synchronizes the number of transducer pulses with the number of stepper motor pulses thereby generating a constant number of transducer pulses per motor revolution. Reflected acoustic energy generates an electrical pulse which is amplified and summed, with the summed signal used to create an image of the borehole.
U.S. Pat. No. 4,774,573 describes a wireline borehole scanning apparatus where the downhole tool generates a signal related to magnetic north. Using digital processing techniques, the peak value of each detected acoustic pulse and the signal related to magnetic north are used for generating a video display.
U.S. Pat. Nos. 4,847,814 and 5,043,948 describe improved systems for creating an image of the borehole wall using the transit times and the amplitudes of the reflected acoustic echo pulses.