Electricity: measuring and testing – Of geophysical surface or subsurface in situ – With radiant energy or nonconductive-type transmitter
Reexamination Certificate
2000-10-27
2003-03-11
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
With radiant energy or nonconductive-type transmitter
C340S854600, C175S040000
Reexamination Certificate
active
06531871
ABSTRACT:
FIELD OF INVENTION
The within invention is directed a method and an electromagnetic antenna extension assembly, including at least one extension tool, for extending a downhole electromagnetic antenna providing or transmitting data, such as measurement-while-drilling telemetry data, to the surface. More particularly, the electromagnetic antenna is extended by extending the length of a lower electrode of the electromagnetic antenna such that the electromagnetic signal transmitted by the antenna is propagated or launched into the surrounding casing and/or the formation closer to the surface. Preferably, the method and the assembly provide for the extension of the antenna, and particularly the lower electrode, by providing for the connection of one or more extension tools into a pipe string, such as a drill string, downhole of the antenna.
BACKGROUND OF INVENTION
For many parts of the world there are hydrocarbon producing zones that need to be drilled underbalanced or near balance making use of aerated drilling mud, or 2 phase or air drilling applications. In these instances, electromagnetic (“EM”) data transmitters or transmission units, such as EM measurement-while-drilling (“MWD”) systems, are used as they do not rely on pressure pulses through a continuous mud or fluid column to transmit telemetry data to the surface. Rather, the telemetry data is transmitted to the surface using a downhole transmitter antenna which produces EM waves into the formation that are picked up by a receiving antenna array located at the ground surface or sea floor and the wellhead. Specifically, EM MWD systems create an alternating current circuit with the drill string as one conductor and the earth or surrounding formation as the other conductor.
More particularly, in these systems, telemetry data is input from a downhole data transmission unit, including a data transmission tool such as an EM MWD tool, located adjacent or in proximity to the drilling bit or other bottom hole equipment, to an electromagnetic antenna such as an EM MWD antenna positioned downhole in the drill string. The EM antenna transmits the electromagnetic signal, which is detected at the surface.
The downhole data transmission tool or EM MWD tool, which senses and provides the telemetry data or information signals, is typically housed or contained within an outer transmission sub or member, such as an index or hang off sub. The EM MWD tool is held or supported within the index sub, such as by a hang off ring mounted or connected with the index sub. The hang off ring also acts as a lower electrode contact ring. A lower or downhole end of the index sub is connected with the drill bit or other equipment comprising the bottom hole assembly. An upper or uphole end of the index sub is typically connected with the EM antenna.
The EM MWD antenna is typically comprised of an outer antenna sub or member having an upper portion and a lower portion separated by an electrically insulative gap. Further, the EM antenna is typically comprised of an electrically conductive inner antenna member, being a wireline or cable, hung off the EM antenna and extending downhole for connection with the EM MWD tool. More particularly, an upper end of the EM antenna wireline connects with the upper portion of the EM outer antenna sub or member above the insulative gap. A lower end of the EM antenna wireline is comprised of a connector, preferably comprised of a collet overshot, for connection with the EM MWD tool. Specifically, the upper end of the EM MWD tool provides a connector compatible with the EM antenna wireline connector, preferably a contact stinger or probe. The EM antenna wireline is preferably insulated so if it contacts the inside of the outer antenna sub or member or any other portion of the pipe or drill string it will not short out the electromagnetic signal.
As a result of the above configuration of the EM antenna and the EM MWD tool, the upper portion of the outer antenna sub or member and the uphole part or portion of the drill string to the surface provide the upper electrode of the EM antenna. Further, the lower portion of the outer antenna sub or member and the downhole part or portion of the drill string to the drill bit provide the lower electrode of the EM antenna.
However, the propagation of EM waves by the EM antenna is characterized by an increase in attenuation with an increase in distance or an increase in the depth of the EM antenna. Thus, for many of the above-described zones it is difficult or impossible to drill with a conventional EM MWD system as the range required is too far for the telemetry to work in a satisfactory manner. The primary problem is the depth at which the intermediate casing must be set at prior to entering the underbalanced zone. Since the transmitted EM signal tends to be relatively greatly attenuated by approximately 20 dB in relative signal strength when the EM antenna is inside the casing, the maximum depth of the EM antenna tends to be limited to much shallower depths than it would be if positioned within an uncased borehole. The depths of some of these zones cause the EM signal to be greatly attenuated by the time it reaches the surface and thus the signal can not typically be detected by the surface receiver system.
To address the effects of attenuation, a repeater may be used to relay the EM signal uphole. However, where a repeater is used, the repeater tends to be relatively ineffective or less effective until it exits the intermediate casing as a result of the signal loss, being in the magnitude of about a 20 dB signal loss as discussed above. If the repeater can be located close to the surface it can be detected by the surface receiver system but the repeater may not be able to receive the signal from the lower repeaters or the main bottom hole transmitter as it is typically too far away. Adding repeaters solves the problem by having many repeaters at short distances but this solution affects the reliability of the system, adds a large capital and operating cost, and results in a decrease in the signal data rate. For instance, the data rate has been found to be halved for every repeater added, i.e. 3 repeaters would mean the signal data rate can be no faster than 1/8th of the fundamental data rate. All these options are unattractive.
A relatively more attractive option or alternative approach is to add an extension to the EM antenna which allows the first or lowermost EM antenna to be positioned much further up the drill string before launching the signal into the casing and/or the formation. More particularly, a further or additional section or length of drillpipe, referred to herein as the “lengthening section”, may be connected into the drill string in-between the lower portion of the outer antenna sub or member and the outer transmission member or index sub housing the EM MWD tool. In other words, the lower portion of the outer antenna sub or member is comprised of the lengthening section. Thus, the lengthening section of drillpipe forms a part of the lower electrode of the EM antenna. The resulting lengthened lower electrode moves or positions the launching point of the EM signal much further uphole. In this instance, the length of the conductive inner antenna member, being the EM antenna wireline, is also increased so that the EM antenna wireline, being a single section or length of wireline or cable, can extend substantially from the upper portion of the outer antenna sub or member, through the lower portion of the outer antenna sub or member including the lengthening section, for connection with the EM MWD tool.
This approach has various advantages and disadvantages. The key advantage is that the lower electrode of the EM antenna may be greatly lengthened or extended by this lengthening section of drillpipe which allows for a stronger signal to be generated. Also the EM antenna is moved closer to the surface making detection of the electromagnetic signal at the surface relatively easier. Typically these lengthening sections can be from 100 feet to thousands of feet or more.
The key li
Hay Richard Thomas
Sand Byron John
Halliburton Energy Service,s Inc.
Kuharchuk Terrence N.
McCully Michael D.
Shull William
Snow Walter E.
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