Multi-depth focused resistivity imaging tool for logging while drilling applications
First Claim
1. A resistivity logging tool, comprising:
- a collar;
a transmitter array that includes a plurality of transmitters, mounted on said collar;
a receiver array that includes at least a first and a second receiver spaced apart on said collar;
a plurality of discrete electrodes spaced about the circumference of the collar; and
a microprocessor coupled to said transmitters, said receivers, and said discrete electrodes, and wherein the microprocessor;
selects a transmitter to be energized, thereby inducing current in the collar;
receives a first signal from the first receiver and a second signal from the second receiver that indicates the amount of current flowing in the collar past the first and second receiver, respectively, and from said first and second signals determines the amount of current flowing into the formation between the first and second receivers; and
receives a signal from each of the plurality of discrete electrodes indicating the voltage level at each electrode, and based upon said voltage levels, determines an azimuthally sensitive resistivity reading for each discrete electrode.
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Accused Products
Abstract
A resistivity tool for use in an LWD system includes a transmitter array with multiple transmitters positioned above a pair of receivers. The transmitters are selectively energized, causing current to be induced in the collar of the tool. The axial current in the tool collar is measured by the pair of receivers to determine the amount of current that flows into the formation between the receivers. From this, the formation resistivity can be measured for each transmitter to provide multiple depths of investigation. In addition, the lower receiver also can be used to selectively monitor the bit resistivity. A plurality of discrete electrodes are positioned between the two receivers to permit imaging of the borehole by obtaining azimuthally sensitive resistivity readings based upon the voltage level at each discrete electrode and the amount of current flowing into the formation. In one embodiment, the voltage level of each discrete electrode also is summed to provide a radial resistivity reading based on the summed voltage levels and the current flowing into the formation. In a second embodiment, a ring electrode also is provided from which a voltage level is obtained in the vicinity of the receiver array for use in computing the radial resistivity reading. According to another embodiment, the discrete electrodes are formed as lateral-logs with a conductive central disc surrounded by two concentric metal rings and a focusing metal ring. In this embodiment, the concentric rings are kept at the same voltage by changing the amount of current that is emitted from the focusing metal ring. By measuring the amount of current emitted, together with the voltage at one of the concentric rings, an azimuthally sensitive focused resistivity reading can be obtained.
209 Citations
21 Claims
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1. A resistivity logging tool, comprising:
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a collar;
a transmitter array that includes a plurality of transmitters, mounted on said collar;
a receiver array that includes at least a first and a second receiver spaced apart on said collar;
a plurality of discrete electrodes spaced about the circumference of the collar; and
a microprocessor coupled to said transmitters, said receivers, and said discrete electrodes, and wherein the microprocessor;
selects a transmitter to be energized, thereby inducing current in the collar;
receives a first signal from the first receiver and a second signal from the second receiver that indicates the amount of current flowing in the collar past the first and second receiver, respectively, and from said first and second signals determines the amount of current flowing into the formation between the first and second receivers; and
receives a signal from each of the plurality of discrete electrodes indicating the voltage level at each electrode, and based upon said voltage levels, determines an azimuthally sensitive resistivity reading for each discrete electrode. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15)
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16. A logging-while-drilling tool that measures resistivity of a formation through which a borehole has been drilled, comprising:
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a drill bit;
a collar;
a transmitter array mounted on the collar that includes a plurality of toroid transmitters capable of inducing current in the collar that flows through a part of the collar and into the formation surrounding the borehole;
a receiver array that includes two receivers for measuring the current flowing into the formation between the receivers based upon the difference in axial current detected by the two receivers; and
an array of discrete electrodes positioned between the two receivers of the receiver array, said discrete electrodes being spaced equally around the perimeter of the collar to provide an image of the borehole resistivity. - View Dependent Claims (17, 18, 19, 20, 21)
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Specification