Communications: electrical – Wellbore telemetering or control
Reexamination Certificate
2000-06-09
2003-09-02
Edwards, Timothy (Department: 2635)
Communications: electrical
Wellbore telemetering or control
C340S854500, C367S035000
Reexamination Certificate
active
06614360
ABSTRACT:
FIELD OF THE INVENTION
This invention is directed toward apparatus and methods for obtaining acoustic measurements or “logs” of earth formations penetrated by a borehole. More particularly, the invention is directed toward obtaining the acoustic measurements while the borehole is being drilled. In addition, the downhole apparatus comprises a plurality of segmented transmitters and receivers which allows the transmitted acoustic energy to be directionally focused at an angle ranging from essentially 0 degrees to essentially 180 degrees with respect to the axis of the borehole.
BACKGROUND OF THE INVENTION
Acoustic measurements have been used in wireline borehole logging for the past four decades. The first wireline acoustic instruments or “tools” were single transmitter and receiver devices which were sed to measure the velocity of the first arrival component of an acoustic wave pulse transmitted through the penetrated formation. This component was usually the compressional or “P” wave component. The velocity measurement, or more precisely the travel time of the wave component from the transmitter to the receiver, was used to compute formation porosity in formation evaluation applications. In addition, early acoustic logs were used in the conversion of seismic data, initially measured in the time domain, into the depth domain thereby yielding cross sectional displays of geological structure used in the industry as a guide to exploration and development drilling.
During the late 1960s and early 1970s, acoustic wireline devices became more complex and also yielded additional information. In the area of formation evaluation, multiple transmitters and receivers were introduced to reduce the adverse effects of the borehole upon the formation acoustic measurements. In the late 1970s, as the transmission rates of wireline telemetry systems increased, the full wave form of the received signal, rather than just the first arrival time, was measured at a plurality of receivers spaced axially along the primary axis of the logging tool. The analog signals were digitized downhole and digitized wave forms were transmitted to the surface for processing. Processing involved the extraction of the travel times of the compressional and shear components, as well as various tube wave components. In addition, the amplitudes of the various wave train components were determined. In formation evaluation, the full wave form information was used to obtain a more accurate and precise measure of formation “acoustic” porosity. In addition, mechanical properties of the formation were determined by combining amplitudes of the various components of the measured acoustic wave form. This information was used to optimize subsequent drilling programs within the area, to aid in the design of hydraulic fracturing programs for the drilled well, and to greatly increase the accuracy and precision of the conversion of area seismic data from the time into the depth domain.
During this same time period, multiple types of logging sensors were beginning to be run in combination, and the measurements from the various types of sensors were combined to obtain formation evaluation information which exceeded the sum of information obtained from the response of each sensor. As an example, thermal neutron porosity sensors, scattered gamma ray sensors, and acoustic sensors were run in combination. Each sensor yielded an indication of formation porosity. By combining the responses of the three types of sensors, a more precise and accurate measure of porosity was obtained. In addition, information concerning the lithology of the formation was obtained which could not be obtained from the responses of any of the individual sensors.
Much effort in the design of acoustic wireline logging tools was, and today still is, directed toward the minimization of acoustic energy transmitted directly through the body of the downhole instrument. The arrival of this energy component at the receiver or receivers usually occurs before the arrival of energy whose path traverses the formation and the borehole. The travel path is more direct and therefore shorter. In addition, the body of the tool is usually metallic and exhibits a faster acoustic travel time than the formation and the borehole. Since the latter arrivals contain parametric information of interest, the former is considered to be interference or “noise”. This direct component is reduced and/or delayed by using a variety of techniques. The component is reduced by acoustically isolating transmitters and receivers from the tool body as much as possible. The arrival of this component is delayed, preferably until after the arrival of components from the formation and borehole, by increasing the effective travel path by cutting a series of alternating slots in the metallic tool body between the transmitter and receiver arrays. This portion of the tool body is commonly referred to as the isolation subsection or “isolator sub”. In addition, various mathematical techniques have been used in the processing of full wave form data to remove the direct component of the received wave form.
In addition to noise generated by the direct transmission of acoustic energy through the wireline tool body, additional acoustic noise is generated as the tool is conveyed along the borehole wall. This noise is commonly referred to as “road noise”. The adverse effects of road noise are minimized using mechanical and mathematical techniques. The prior art teaches the use of many types of roller mechanical devices whereby the wireline tool is “rolled” rather than “dragged” along the borehole wall thereby reducing the magnitude of the road noise. In addition, since road noise is essentially incoherent, various mathematical methods are used in the processing of full wave form data to greatly reduce the effects of road noise.
The previous discussions have been directed to wireline type measurements wherein the measurements are usually made after the borehole has been drilled. In some drilling operations, wireline logs are made intermittently during the drilling operation, but such logging usually requires that the drill string be removed from the borehole prior to logging. Logging after completion of the drilling operation often reveals that the target formation or formations have been missed by perhaps either drilling too shallow or too deep. In addition, unexpected zones, such as high pressure formations or salt zones, can be encountered during, and adversely affect, the drilling operation. Such encounters can be quite costly and can be fully analyzed with wireline logging only after the encounter. Intermediate logging is likewise costly in that the drilling operation must cease during logging operations. Furthermore, the time interval between the termination of drilling and wireline logging allows the drilling fluid to penetrate or “invade” the near borehole formation thereby possibly introducing error in wireline log measurements. The adverse effects of invasion poses a particularly serious problem for wireline logs with relatively shallow depths of investigation such as most nuclear logs. Possible damage to the borehole can occur during logging and costly drilling rig time and logging equipment time is wasted during stand-by periods for each operation.
Many of the problems discussed above can be overcome by measuring various formation evaluation and other parameters during the actual borehole drilling operation. This is particularly true with acoustic measurements since they not only represent a key formation evaluation measurement but also represent a key seismic tie-in measurement. The problems associated with intermittent logging are essentially eliminated. The need for wireline logging after the drilling can also be eliminated in some cases. Formation evaluation type measurements-while-drilling (MWD) logs can indicate to the driller, in real time, when anomalies such as a fault planes or formation lenses are being penetrated. This is particularly true if the MWD device has a relatively large depth of investigation
Dubinsky Vladimir
Leggett, III James V.
Patterson Douglas
Baker Hughes Incorporated
Edwards Timothy
Madan Mossman & Sriram P.C.
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