AU754992B2

AU754992B2 - A downhole tool including an electrically steerable antenna for use with a formation deployed remote sensing unit

Info

Publication number: AU754992B2
Application number: AU18316/01A
Authority: AU
Inventors: Steven Baker; Alfred Blaickner; Dean M Homan
Original assignee: Gemalto Terminals Ltd; Schlumberger Holdings Ltd
Current assignee: Schlumberger Holdings Ltd
Priority date: 2000-03-20
Filing date: 2001-02-06
Publication date: 2002-11-28
Anticipated expiration: 2021-02-06
Also published as: ID29666A; GB2360533A; AU1831601A; NO20011380L; NO20011380D0; GB0104039D0; GB2360533B; CA2334525A1

Description

TITLE: A DOWNHOLE TOOL INCLUDING AN ELECTRICALLY STEERABLE ANTENNA FOR USE WITH A FORMATION DEPLOYED REMOTE SENSING UNIT

SPECIFICATION

BACKGROUND

i. Technical Field The present invention relates generally to the discovery and production of hydrocarbons, and more particularly, to the monitoring of downhole formation properties during drilling and production.

2. Related Art Hydrocarbon exploration and production is well known.

Drilling a well and placing the well into production is expensive, oftentimes exceeding millions of dollars for a single well. Given that many "dry holes" are drilled but produce no revenue, the producing wells must pay for the exploration and drilling costs of all wells that are drilled.

Thus, there is a strong desire to produce any given reservoir at a maximum rate to recoup investment costs. However, to produce a reservoir at too great a rate may not maximize the 20 total recovery obtainable from the reservoir.

0 Thus, to satisfy competing interests, producing wells %.000 must be monitored and controlled to maximize production over time for a reservoir. Production levels depend on reservoir S0:: formation characteristics such as pressure, porosity, permeability, temperature and physical layout of the reservoir and also the nature of the hydrocarbon (or other material) extracted from the formation. Additional characteristics of a producing formation must also be determined and monitored, such characteristics including the hydrocarbon/water interface and the hydrocarbon/gas S interface, among others. In a typical reservoir, the characteristics of many different wells in a reservoir must be determined to accurately determine these reservoir properties.

An oilfield's production may be maximized by controlling production differently for the different producing wells in the reservoir. For example, if formation pressure is dropping for one well in the reservoir more quickly than it is for other wells in the reservoir, the production rate of the one well should be reduced. Alternatively, the production rate of the other wells might be increased. The manner of controlling production rates for different wells .ooo.i S for one field is generally known.

In order to measure the properties of producing wells •and to control their production, sophisticated computerized 20 controllers are positioned at the surface of production wells for the control of uphole and downhole devices such as motor valves and hydro-mechanical safety valves. Typically, microprocessor (localized) control systems are used to control production from the zones of a well. For example, these controllers are used to actuate sliding sleeves or packers by the transmission of a command from the surface to downhole electronics microprocessor controllers) or even to electromechanical control devices placed downhole.

Moreover, many prior art systems generally require a surface platform at each well for monitoring and controlling the production at a well. The associated equipment, however, is expensive. The combined costs of the equipment and the surface platform often discourage oil field producers from installing a system to monitor and control production properly. Additionally, current technologies for reliably measuring real-time formation data do not exist. Often, production of a well must be interrupted so that a wireline tool may be deployed into the well to measure desired formation properties. Accordingly, the formation data obtained is expensive in that it has high opportunity costs because of the cessation of production. It also suffers from the fact that the formation data is not true real-time formation data.

Some prior art systems measure the electrical ***resistivity of the ground in a known manner to estimate the characteristics of the reservoir. Because the resistivity of hydrocarbons is higher than water, the measured resistivity in various locations can be of assistance in mapping out the reservoir. For example, the resistivity of hydrocarbons to water is about 100 to 1 because the formation water often contains salt and, generally, is much more conductive.

Systems that map out reservoir parameters by measuring resistivity of the reservoir for a given location are not always reliable; however, because they depend upon the assumption that any present water has a salinity level that renders it more conductive that the hydrocarbons. In those situations where the salinity of the water is low, systems that measure resistivity are not as reliable.

Some prior art systems for measuring resistivity include placing an antenna within the ground for generating relatively high power signals that are transmitted through the formation to antennas at the earth surface. The amount of the received current serves to provide an indication of ground resistivity and therefore a suggestion of the formation characteristics in the path formed from the transmitting to the receiving antennas.

Other prior art systems include placing a sensor at the bottom of the well in which the sensor is electrically connected through cabling to equipment on the surface. For example, a pressure sensor is placed within the well at the S bottom to attempt to measure reservoir pressure. One shortfall of this approach, however, is that the sensor does not read reservoir pressure that is unaffected by drilling equipment and formations since the sensor is placed within the well itself.

Other prior art systems include hardwired sensors placed next to or within the well casing in an attempt to reduce the effect that the well equipment has on the reservoir pressure.

25 While such systems perhaps provide better pressure information than those in which the sensor is placed within the well itself, they still do not provide accurate pressure 6 information that is unaffected by the well or its equipment.

Alternatives to the above systems include sensors deployed temporarily in a wireline tool system. In some prior art systems, a wireline tool is lowered to a specified location (depth), secured, and deploys a probe into engagement with the formation to obtain samples from which formation parameters may be estimated. One problem with using such wireline tools, however, is that drilling and/or production must be stopped while the wireline tool is deployed and while samples are being taken or while tests are being performed. While such wireline tools provide valuable information, significant expense results from "tripping" the well, if during drilling, or stopping 15 production.

Thus, there exists a need in the art for a system and method of operation for measuring formation properties without great expense or extended disruption in drilling or production.

SUMMARY OF THE INVENTION To overcome the shortcomings of the prior systems and their operations, a plurality of remote sensing units constructed according to an embodiment of the present 25 invention are placed in subsurface formations of interest.

The remote sensing units include sensors (for measuring formation properties), an antenna coil for transmitting and receiving RF signals, and electronics for controlling the operation of the sensors and the antenna coils. The remote sensing units are deployed \\melbfiles\home$\Priyanka\eep\speci\18316-01 Amendments.doc 3/10/02 during drilling operations, or subsequently during wireline well logging operations. These remote sensing units measure various formation properties, including temperature, pressure, formation resistivity and other important properties.

With a remote sensing unit deployed in the formation, communications with the remote sensing unit are established to receive formation data that has been collected by the remote sensing units. Communication operations are performed both during drilling (using a MWD tool) and during wireline logging operations (using a wireline tool). Once the formation data has been collected, it may be reviewed locally and/or may be sent to a central location for analysis. The formation data that has been collected may be used in the continued drilling of the well, in completing the well, in producing the well, in determining where to drill subsequent wells and in determining how the production of a respective reservoir may be managed.

Typically, remote sensing units are accessed over and 20 over again to receive formation data. However, the location .oo of the remote sensing units must be determined each time they are accessed so that formation data may be gathered therefrom. Thus, to assist in locating the remote sensing units, and in communicating with the remote sensing units, a S. 25 downhole tool constructed according to the present invention includes at least one steerable antenna that is employed to locate, and to wirelessly communicate with, the remote sensing units. The downhole tool may be a drilling tool, MWD tool, or a wireline tool.

Each steerable antenna is electronically controllable so that the direction of its antenna radiation pattern maximum (ARPM) may be controlled. By controlling the direction of the ARPM in the formation, the downhole tool may locate a formation deployed remote sensing unit. Further, by directing the ARPM toward the remote sensing unit, the downhole tool may efficiently power the remote sensing unit and retrieve formation data collected by the remote sensing unit. Electronics for operating each steerable antenna and for performing these operations are also contained within the downhole tool.

The steerable antenna may be constructed in a number of different manners, each construction including different antenna element numbers and configurations. In one embodiment, the steerable antenna includes a triad of coils that produce an orthonormal set of magnetic dipoles that are symmetric about a longitudinal axis of the downhole tool.

S20 With the symmetry of this structure, the magnetic dipoles .~o formed by the triad of coils are shaded equally by the downhole tool body. By separately controlling the voltage/current and polarity applied to each of the triad of coils, a transmit ARPM may be directed into the formation.

Further, by separately controlling the amplification and polarity of signals received from the triad of coils, a receive ARPM may be produced at a desired direction into the formation. The desired ARPM direction may be adjusted both longitudinally and azimuthally. In considering the operation of the steerable antenna, the term "polarity" may also be described by the phase difference between any two coil currents in the triad of coils.

In another steerable antenna construction, which is a slight modification to the orthonormal steerable antenna construction, a triad of coils forms a set of magnetic dipoles which are substantially, but not exactly orthogonal.

Preferably, the downhole tool body also shades the magnetic dipoles formed by this steerable antenna structure equally.

Because the dipoles of the triad of coils have components in all three dimensions so that a ARPM produced thereby may be adjusted both longitudinally and azimuthally.

Another steerable antenna construction includes a plurality of anti-Helmholtz coil structures, each of which produces a radial dipole uniform azimuthally about the e downhole tool body. The magnitude (and polarity) of each anti-Helmholtz coil structure may be separately controlled so that a ARPM produced by the steerable antenna as a whole may be directed longitudinally with respect to the downhole tool.

However, with this steerable antenna structure, the ARPM will be azimuthally symmetric.

In still another embodiment of the steerable antenna, two coil structures produce magnetic dipoles that extend radially from the downhole tool, normal to the longitudinal axis of the downhole tool and displaced azimuthally by 10 approximately ninety degrees. By separately controlling the magnitude and polarity of the signal provided to/received from the two coils, a composite ARPM may be controlled azimuthally about the downhole tool body. In a variation of the construction, the coil structures produce magnetic dipoles that are displaced azimuthally at an angle other than ninety degrees.

However, the teachings of the present invention extend beyond those steerable antenna constructions described above and the other particular embodiments described herein. Any steerable antenna embodiment that allows a transmit or receive ARPM to be directed into the formation from the downhole tool is within the scope of the present invention. Further, any combination of steerable antenna constructions is also within the scope S•of the present invention.

In an operation according to an embodiment of the present invention, the steerable antenna is employed to locate a remote sensing unit that has been previously deployed in the formation and to communicate with the remote sensing unit. In locating the sensor, the ARPM receive pattern is swept longitudinally and azimuthally .into the formation until a signal produced by the remote sensing unit is detected. In one operation, the remote sensing unit produces a signal under its own power.

However, in another operation, a steerable antenna ARPM transmit pattern is first swept in the formation, transmitting a powering signal to the remote sensing unit while performing the sweep. The remote sensing unit receives this power signal, enables its transmitter, and responds with a signal. A ARPM receive pattern is then swept longitudinally and azimuthally into the formation in an attempt to receive a signal produced by the remote sensing unit. This pattern of power transmission and signal receipt is performed until the remote sensing unit is located. In an alternate method of locating the remote sensing unit, the conductive properties of the remote \\melbfiles\homeS\Priyanka\Keep\speci\18316-O1 Amendments.doc 3/10/02 11 sensing alter the resistivity seen by the steerable antenna as the ARPM is swept. Using this method, the remote sensing unit is located by directing ARPM toward a minimum resistivity seen.

Once the sensor is located in the formation, the angular frequency (rotation) of the downhole tool is determined (if any) by counter sweeping the ARPM. When the strength of the signal received by the steerable antenna is constant, the angular frequency has been determined. Then, the ARPM is counter-rotated at this frequency. A coarse search is next performed so that the maximum response of the ARPM is directed toward the remote sensing unit. Then, a fine search is performed by applying jitter to the ARPM direction to ensure that the 15 ARPM continues to be directed directly toward the remote sensing unit. Finally, with the ARPM pointed directly at S: the remote sensing unit, the remote sensing unit is interrogated for its formation data.

Therefore according to a first aspect of the present invention there is provided a downhole tool used in communicating with a remote sensing unit located in a formation adjacent a wellbore, the downhole tool comprising: a steerable antenna; and 25 a downhole communication unit coupled to the steerable antenna that controls the operation of the steerable antenna to direct an antenna radiation pattern maximum toward the remote sensing unit.

According to a further aspect of the present invention there is provided a method for using a downhole tool that includes a steerable antenna to interact with a remote sensing unit located within a formation, the method comprising: directing an antenna radiation pattern maximum produced by the steerable antenna toward the remote sensing unit; and communicating with the remote sensing unit using \\melb_files\home$\Priyanka\Keep\speci\18316-01 Amendments.doc 3/10/02 Ila the steerable antenna.

Other aspects of the present invention will become apparent with further reference to the drawings and specification that follow.

000.* \\melb..fies\home$\Priyw~ka\Keep\speci\18316-01 Amendments.doc 3/10/02 BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of the present invention can be obtained when the following detailed description of the preferred embodiment is considered in conjunction with the following drawings, in which: FIG. 1A is a diagrammatic sectional side view of a drilling rig, a wellbore made in the earth by the drilling rig, a remote sensing unit that has been deployed from a tool in the wellbore into a subsurface formation, and a drill string that includes a measurement while drilling tool having a downhole communication unit that retrieves subsurface formation data collected by the remote sensing unit; FIG. lB is a diagrammatic sectional side view of a drilling rig, a wellbore made in the earth by the drilling rig, a remote sensing unit that has been deployed from a tool eoe in the wellbore into a subsurface formation, and a wireline truck and open-hole wireline tool that includes a downhole communication unit that retrieves subsurface formation data collected by the remote sensing unit; FIG. 2A is a diagrammatic side view of a portion of a downhole tool having a steerable antenna constructed according to the present invention and the relationship between an antenna radiation pattern maximum (ARPM) produced by the steerable antenna and a remote sensing unit; FIG. 2B is a diagrammatic top view of the downhole tool of FIG. 2A; 13 FIG. 2C is a diagrammatic sectional side view of a magnetic dipole produced by a coil, showing the relationship between the physical structure of the coil and maximum antenna gain vectors produced by the coil; FIG. 2D is a diagrammatic section side view of a pair of maximum antenna gain vectors produced by a pair of coils that are displaced by ninety degrees and the manner in which the maximum antenna gain vectors may be combined to produce an ARPM that is steerable within the plane of the page; FIG. 3 is a diagrammatic side view of a downhole tool having a plurality of steerable antennas constructed according to the present invention; FIG. 4A is a diagrammatic side view of a downhole tool illustrating an orthonormal magnetic dipole set produced by a steerable antenna constructed according to the present invention; FIG. 4B is a diagrammatic top view illustrating the 9* orthonormal magnetic dipole set of FIG. 4A; FIG. 4C is a diagrammatic side view illustrating a 20 physical construction of the three coil steerable antenna cto o3 array of FIGs. 4A and 4B, wherein the coils are disposed upon a downhole tool as a plurality of coils wrapped about the downhole tool body; FIG. 5 is a functional block diagram illustrating the construction of adownhole communication unit according to the present invention that couples to a steerable antenna; present invention that couples to a steerable antenna; FIGs. 6A, 6B and 6C are diagrammatic views of an alternate embodiment of a steerable antenna constructed according to the present invention; FIG. 6D is a diagrammatic side view of an anti-Helmholtz antenna construction that may be used with a steerable antenna constructed according to the present invention; FIG. 7 is a logic diagram illustrating operation according to the present invention in establishing communications with a remote sensing unit; FIG 8A is a logic diagram illustrating a first methodology for locating a remote sensing unit in a formation; FIG 8B is a logic diagram illustrating a second methodology for locating a remote sensing unit in a formation; FIG 8C is a logic diagram illustrating a third methodology for locating a remote sensing unit in a S9 formation; FIG. 9 is a diagrammatic side view of a downhole tool 20 constructed according to the present invention and the manner v99999 in which a triad of coils may be wrapped about the tool body to produce a triad of magnetic dipoles of a steerable antenna; FIG. 10 is a diagrammatic side view of a downhole tool 25 and a remote sensing unit, both constructed according to the present invention; and present invention; and 15 FIGs. 11A and 11B are logic diagrams illustrating methods according to the present invention used to determine the depth at which a remote sensing unit resides within a formation and the manner in which formation properties are determined therefrom.

DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1A is a diagrammatic sectional side view of a drilling rig 126, a wellbore 112 made in the earth 118 by the drilling rig 126, a remote sensing unit 108 that has been deployed from a tool in the wellbore 112 into a subsurface formation, and a drill string that includes a measurement while drilling (MWD) tool 102 that operates to 15 locate the remote sensing unit 108 and to retrieve formation data collected by the remote sensing unit 108.

S: The MWD tool 112 includes a steerable antenna constructed according to the present invention.

The MWD tool 102 forms a portion of a drill string that also includes drill pipe 110 and a drill bit 104. MWD tools are generally known in the art to collect formation data during drilling operations. The MWD tool 102 shown forms a portion of a drill collar that resides adjacent the drill bit 104. As is known, the drill bit 25 erodes the formation to form the wellbore 112. Drilling mud circulates down through the center of the drill string, exits the drill string through nozzles or openings in the bit, and returns up through the annulus along the sides of the drill string to remove the \\melb-files\homeS\Priyanka\Keep\speci\18316-O1 Amendments.doc 3/10/02 eroded formation pieces. Casing 114 is then cemented into a portion of the wellbore 112 and a blow-out-prevention stack 116 is then attached.

During subsequent drilling operations, the open-hole portion of the wellbore 112 is extended into one or more formations of interest, the remote sensing unit 108 is deployed into a formation of interest, measurements are made by the remote sensing unit 108 and then formation data is collected from the remote sensing unit 108.

In one embodiment, the MWD tool 102 is used to deploy the remote sensing unit 108 into the subsurface formation.

In this embodiment, the MWD tool 102 includes both a deployment unit 105 and a downhole communication unit 106.

After deployment of the remote sensing unit 108, the downhole communication unit 106 communicates with the remote sensing unit 108 and provides power to the remote sensing unit 108 during such communications using the steerable antenna array.

s The downhole communication unit 106 also includes a downhole s. 0 .•rinterface (shown in FIG. 5) that communicates with an uphole 20 interface 120 via a communication link.

oo oo The uphole interface 120 may be located in a well-site ease structure 122, may be attached to the casing 114 or may be °e o placed in another location proximate to the casing 114. The uphole interface 120, in the described embodiment, is coupled to a satellite dish 124 that enables communication between the MWD tool 102 and a remote site. In other embodiments, the MWD tool 102 communicates with a remote site via a radio interface, a telephone interface, a cellular telephone interface or a combination of these so that formation data captured by the MWD tool 102 will be available at a remote location. By facilitating communication between the MWD tool 102 and a remote site, formation data collected from a number of wells from a single reservoir may be gathered, compiled and analyzed so that future drilling and production decisions may be made. However, the formation data may also be used at the drill site to assist in drilling operations.

The remote sensing unit 108 may be constructed to be battery powered, may be constructed to be remotely powered from the downhole communication unit 106, or may be powered using a combination of these techniques. A discussion of a remote sensing unit is provided in U.S. Patent Applications Serial Numbers 09/019,466, filed on February 5, 1998, and 09/382,534, filed on August 25, 1999, both assigned to the assignee of the present application, and hereby incorporated e ee by reference herein.

000.4 Because no physical connection exists between the remote sensing unit 108 and the downhole communication unit 106, an electromagnetic Radio Frequency link is established between the downhole communication unit 106 and the remote sensing unit 108 for the purpose of communicating with the remote sensing unit 108. In some embodiments, an 25 electromagnetic link also is established to provide power to eeee the remote sensing unit 108. In a typical operation, the coupling of an electromagnetic signal having a frequency of between 1 and 10 Megahertz will most efficiently allow the downhole communication unit 106 to communicate with, and to provide power to the remote sensing unit 108.

With the remote sensing unit 108 located in a subsurface formation adjacent the wellbore 112, the downhole communication unit 106 is placed in close proximity to the remote sensing unit 108. Then, power-up and/or communication operations are begun. When the remote sensing unit 108 is RF powered, or RF power is used to augment an onboard battery, power from the downhole communication unit 106 is electromagnetically coupled to the remote sensing unit 108 and used to power up the remote sensing unit 108. More specifically, the remote sensing unit 108 receives the power, charges a capacitor that will serve as its power source and commences power-up operations. Once the remote sensing unit 108 has received a specified or sufficient amount of power, it performs self-calibration operations and then makes formation measurements. These formation measurements are recorded and then communicated back the downhole communication unit 106 via the electromagnetic coupling.

0 Alternately, RF power is supplied continually to the remote 00..

sensing unit 108 immediately after deployment and until the downhole communication unit 106 is no longer in proximity thereto.

25 According to the present invention, the downhole communication unit 106 includes a steerable antenna array (various embodiments shown subsequently in FIGs. 2A, 2B, 2C, 2D, 3, 4A, 4B, 4C, 6A, 6B, 6C, 6D, 9 and 10) that directs an antenna radiation pattern maximum (ARPM) into the formation toward the remote sensing unit 108. By directing the ARPM toward the remote sensing unit 108, the downhole communication unit 106 may perform a variety of functions.

First, the downhole communication unit 106 uses the steerable antenna array to locate the remote sensing unit 108 within the formation. Second, the downhole communication unit 106 uses the steerable antenna array to power the remote sensing unit 108. Third, the downhole communication unit 106 uses the steerable antenna array to retrieve formation data collected by the remote sensing unit 108. In each of these operations, the downhole communication unit 106 controls the direction of the ARPM into the formation.

FIG. 1B is a diagrammatic sectional side view of a drilling rig 126, a wellbore 112 made in the earth 118 by the drilling rig 126, a remote sensing unit 108 that has been deployed from a wireline tool 154 in the wellbore 112 into a subsurface formation, and a wireline truck 160 that operate in conjunction with the remote sensing unit 108 to retrieve Sformation data collected by the remote sensing unit 108. The wireline tool 154 includes a downhole communication unit 156 that includes a steerable antenna array constructed according to the present invention. The wireline tool may also include 25 a remote sensing unit deployment unit 157.

As is generally known, open-hole wireline operations are performed during the drilling of wells to collect information regarding formations penetrated by wellbore 112. In such wireline operations, a wireline truck 160 couples to a wireline tool 154 via an armored cable 158 that includes a communication path for conducting communication signals and power signals. Armored cable 158 serves both to physically couple the wireline tool 154 to the wireline truck 160 and to allow electronics contained within the wireline truck 160 to communicate with the downhole communication unit 156.

Measurements taken during wireline operations include formation resistivity (or conductivity) logs, natural radiation logs, electrical potential logs, density logs (gamma ray and neutron), micro-resistivity logs, electromagnetic propagation logs, diameter logs, formation tests, formation sampling and- other measurements. The formation data collected in these wireline operations may be coupled to a remote location via an antenna 162 that employs RF communications two-way radio, cellular communications, etc.) According to the present invention, the remote sensing unit 108 may be deployed from the wireline tool 154 (using the deployment unit 157). Further, after deployment by a MWD tool, formation data may be retrieved from the remote sensing unit 108 via the wireline tool 154. In such embodiments, the wireline tool 154 is constructed so that it couples 25 electromagnetically using an RF link with the remote sensing unit 108. In such case, the wireline tool 154 is lowered into the wellbore 112 until it is proximate to the remote sensing unit 154. In one embodiment, the remote sensing unit 108 has a radioactive signature that allows the wireline tool 154 to sense its general location in the wellbore 112 (a radioactive signature may also be employed with the remote sensing unit 108 of FIG. 1A) However, according to the present invention, the downhole communication unit 156 includes a steerable antenna array that- produces a directable ARPM that is able to locate the remote sensing unit within the wellbore 112. Because the remote sensing unit 108 is constructed of a metal, and because the wellbore 112 typically contains little metal, the downhole communication unit 106 may sense the presence of the remote sensing unit 108 using the steerable antenna without an active RF signal produced by the remote sensing unit. In performing this operation, the downhole communication unit 106 operates the steerable antenna to sweep the ARPM about the logging tool 154 into the wellbore 112 until the remote sensing unit 108 is located. When located, the downhole communication unit 106 uses this location information to direct the ARPM during further powering and formation data gathering operations.

In an alternate locating operation, the remote sensing unit produces an RF signal that is received by the downhole communication unit 156 during a sweep of the ARPM. After 25 the remote sensing unit 108 is located, the ARPM may then be directed toward the remote sensing unit 108 in subsequent powering and communication operations. During the 1_-t communication operations, the downhole communication unit 156 receives formation data collected by the remote sensing unit 108. The wireline tool 154 then transmits this formation data back to wireline truck 160 via armored cable 158. The formation data may be used immediately, stored for future use or may be immediately transmitted to a remote location for use.

According to another aspect of the present invention, the depth at which the remote sensing unit 108 resides within the formation may be also determined. As will be described further with reference to FIGs. 11A and 11B, the steerable antenna may be employed to calculate the depth of the remote sensing unit 108 within the formation. Based upon this depth, and the energy with which the remote sensing unit 108 was placed into the formation, formation properties, e.g., density, may be determined.

FIG. 2A is a diagrammatic side view of a portion of a downhole tool 206 having a steerable antenna 208 constructed according to the present invention and the relationship between an ARPM 214 produced by the steerable antenna 208 and a remote sensing unit 200. The remote sensing unit includes a housing, a sensor 202 that measures formation properties, an antenna coil 204, and electronics 205 that control the operation of the sensor 202 and the coil 204.

As shown, the downhole tool 206 locates within the wellbore 210 adjacent the location of the remote sensing unit 200. The downhole tool may be a MWD tool (as described with reference to FIG. 1A), a wireline tool (as described with reference to FIG. 1B), or another downhole tool. The steerable antenna 208 is formed upon/within the downhole tool 206 and is capable of directing the ARPM 214 at an angle 216 with respect to the longitudinal axis of the downhole tool 206 and at an azimuth angle with respect to the downhole tool 206 (as will be described with reference to FIG. 2B.

FIG. 2B is a diagrammatic top view of the downhole tool 206 of FIG. 2A. The ARPM 214 is shown to be directed at an azimuth angle 254 with respect to a reference angle 252 associated with the body of the downhole tool 206. The reference angle 252 remains fixed with respect to the structure of the downhole tool 206 such that the azimuth angle 254 of the ARPM may be controlled. In this fashion, the azimuth angle 254 of the ARPM 214 may be altered from 0 degrees to 360 degrees with respect to the reference angle 252.

Because the ARPM 214 may be directed at an angle 216 with respect to the longitudinal axis of the downhole tool 206 and at an azimuth angle 254 about the downhole tool 206, the ARPM 214 may be directed toward the remote sensing unit 200 located within the wellbore 210, independent of its location with respect to the downhole tool 206. Thus, the 25 ARPM 214 may be scanned in order to locate the remote sensing unit 200. In locating the remote sensing unit 200 within the wellbore 210, a signal received by the steerable antenna is measured at differing directions of the ARPM 214 during the formation scan. When the signal is received, the direction of the ARPM 214 is adjusted to maximize the received signal.

When the signal is maximized, the remote sensing unit's 200 location relative to the position of the downhole tool 206 is determined.

When the downhole tool 206 is a MWD tool, the downhole tool 206 rotates at a substantially uniform angular frequency about its longitudinal axis 212. In this case, the angular frequency of the downhole tool may be determined by locating the remote sensing unit 200 and adjusting the direction of the ARPM 214 so that it remains substantially fixed upon the remote sensing unit 200. With this condition satisfied, then, the angular frequency of the downhole tool 206 corresponds to the angular frequency at which the ARPM 214 must be counter-rotated. (rate of change of azimuth angle 254)

:.E

to track the location of the remote sensing unit 200.

In providing power to the remote sensing unit 200, if such operation is required, the ARPM 214 is directed (in a power transmission mode) toward the remote sensing unit 200.

By directing the ARPM 214 toward the remote sensing unit 200, a most efficient coupling between the steerable antenna and an antenna coil 204 of the remote sensing unit 200 is achieved and a most efficient power transfer is accomplished.

25 During formation data acquisition operations, the ARPM 214 is also directed toward the remote sensing unit 200.

Directing the ARPM 214 toward the remote sensing unit 200 during these'formation data acquisition operations maximizes the quality of received transmissions from the remote sensing unit 200 and allows a lower transmission power level to be employed by the remote sensing unit 200. Thus, a smaller S transmitter with less transmission power may be employed in the remote sensing unit 200.

FIG. 2C is a diagrammatic sectional side view of a magnetic dipole produced by a coil, showing the relationship between the physical structure of the coil 250A, 250B and maximum antenna gain vectors (252A, 252B) produced by the coil. According to principles that are known, the magnetic dipole produced by the coil 250A and 250B has an antenna gain that may be expressed as a function of the angle 254 (0) This relationship is: Gain(O)a sin 2 Equation 1 The maximum of this antenna gain is therefore one when •the angle 254 is equal to ninety (90) degrees, corresponding to maximum antenna gain vector 252A, or when the angle 254 is equal to two-hundred seventy (270) 20 degrees, corresponding to maximum antenna gain vector 252B.

Thus, if using the antenna structure of FIG. 2C to communicate with a remote sensing unit, it is desirable for the maximum antenna gain vector 252A or 252B to be directed toward the remote sensing unit 200. However, as is evident form FIG. 2C, the direction of the maximum antenna gain vector 252A or 252B produced by the coil 250A and 250B is fixed with respect to the coil.

I L6 FIG. 2D is a diagrammatic section side view of a pair of maximum antenna gain vectors produced by a pair of coils that are displaced by ninety degrees and the manner in which the maximum antenna gain vectors may be combined to produce an ARPM that is steerable within the plane of the page. Maximum antenna gain vector 262 is produced by coil 264A/264B and maximum antenna gain vector 266 is produced by coil 268A/268B. These maximum antenna gain vectors 262 and 266 combine to form ARPM 270 when the coils 264 and 268 are excited in phase with one another.

However, exciting the coils 264 and 268 with differing phases will produce differing directions of the ARPM. ARPM 272 is obtained by exciting coil 264 with a positive phase and by exciting coil 268 with negative phase. ARPM 274 is obtained by exciting coil 264 with a negative phase and by exciting coil 268 with negative phase. ARPM 276 is obtained by exciting coil 264 with a negative phase and by exciting coil 268 with positive phase. Of course, ARPM 270 is complementary with ARPM 274 and ARPM 272 is complementary with ARPM 276. In a similar manner, during signal receipt, a oo the direction of the ARPM may be altered by selectively amplifying (and setting the phase) of signals produced by the ""coils 264 and 268.

As is evident, the discrete positions of the ARPM are 25 caused by exciting the coils 264 and 268 at equal amplitudes.

eo0 By exciting the coils at differing amplitudes, an ARPM at any rotational position may be produced (0 to 360 degrees).

7- FIG. 3 is a diagrammatic side view of a downhole tool having four steerable antennas according to the present invention. The downhole tool 302 includes a first steerable antenna 304 that directs a first ARPM 306 at a first angle 308 with respect to the longitudinal axis 318 of the downhole tool 302. The downhole tool 302 includes a second steerable antenna 310 that directs a second ARPM 312 at a second angle 314 with respect to the longitudinal axis 318 of the downhole tool 302. The downhole tool 302 includes a third steerable antenna 316 that directs a third ARPM 318 at a third angle 320 with respect to the longitudinal axis 318 of the downhole tool 302. Finally, the downhole tool 302 includes a fourth steerable antenna 322 that directs a fourth ARPM 324 at a fourth angle 326 with respect to the longitudinal axis 318 of the downhole tool 302. The ARPMs 306, 312, 318 and 324 may also be directed at an azimuth angle, in the manner that was described with reference to FIG. 2B.

Having multiple (two or more, which may also be easily accomplished according to the present invention) steerable antennas improves the performance of the downhole tool 302 in Slocating, delivering power to and communicating with the remote sensing unit 200. Further, by directing the ARPMs 306 and 312 (at least 2 ARMSs or 1 ARPM at 2 at least two different depths in the borehole) toward the remote sensing 25 unit to determine the angles 314 and 316 (and knowing the distance between the steerable antennas 304 and 306), the depth at which the remotes sensing unit resides within the formation 210 may be determined. Based upon this information, formation properties, including formation density, for example, may be determined.

FIG. 4A is a diagrammatic side view of a downhole tool illustrating an orthonormal magnetic dipole set produced by a steerable antenna constructed according to the present invention. Magnetic dipoles 408A, 408B and 408C produced by the steerable antenna are symmetrically disposed with respect to a longitudinal axis 318 of the downhole tool 206.

Further, each of these magnetic dipoles 408A, 408B and 408C is oriented at an approximate angle of 54.7 degrees with respect to the longitudinal axis 318 of the downhole tool 206. Examples of physical structures that produce this orthonormal magnetic dipole set are shown in FIGs. 4C and 9.

FIG. 4B is a diagrammatic top view illustrating the orthonormal magnetic dipole set of FIG. 4A. Coils 408A, 408B and 408C are disposed symmetrically about the longitudinal axis 318 of the downhole tool 206. Further, the magnetic dipoles 408A, 408B and 408C is oriented normally with respect to each other (in three dimensions). The top view of FIG. 4B 4: illustrates that the azimuth angle separation between adjacent coils is 120 degrees.

The magnetic dipoles 408A, 408B and 408C therefore form a orthonormal set. Further, with the construction 25 illustrated, each of the magnetic dipoles 408A, 408B and 408C is shaded equally with respect to the body of the downhole tool 206. With this equality in orientation, the coupled electronics (as illustrated in FIG. 5) may treat the coils uniformly. Further, with this orthonormal set, an ARPM may be produced (a composite of the magnetic dipoles 408A, 408B and 408C) at any direction longitudinally and azimuthally with respect to the downhole tool 206.

FIG. 4C is a diagrammatic side view illustrating a physical construction' of the three coil steerable antenna array of FIGs. 4A and 4B, wherein the coils are disposed upon a downhole tool 450 as a plurality of coils wrapped about the downhole tool body. Coil 454A produces magnetic dipole 456A, coil 454B produces magnetic dipole 456B and coil 454C produces magnetic dipole 456C. As shown, the magnetic dipoles 456A, 456B and 456C are orthonormal and are oriented symmetrically with respect to a longitudinal axis 452 of the downhole tool 450 and that are shaded equally by the tool body of the downhole tool 450. In one embodiment, each

S

magnetic dipole 456A, 456B and 456C is displaced from the longitudinal axis 452 by 54.7 degrees. In another embodiment, a different angle is employed. In the differing embodiment, however, the magnetic dipoles 456A, 456B and 456C Sare symmetric with respect to the longitudinal axis 452.

The coils 454A, 454B and 454C may be wrapped about the S" downhole tool 450 body. Each of these coils may include multiple wraps to produce a desired dipole strength 25 considering other design elements. The materials used in the construction of the coils will be one of design choice considering the material used to form the downhole tool 212 body, the electronics used to drive the coils, the desired signal to be produced and other considerations which are generally known.

FIG. 5 is a functional block diagram illustrating the construction of adownhole communication unit 500 according to the present invention that couples to a steerable antenna 520. The downhole communication unit 500 (and at least part of the steerable antenna 520) is disposed within a downhole tool, for example the downhole tools discussed with reference to FIGs. 1A and lB. The downhole communication unit 500 includes a processor 502, a downhole interface 504, memory/storage 506 and a coil interface 508 that couples to the steerable antenna 520. The steerable antenna 520 of this embodiment includes three coils, coil 1 510, coil 2 512 and coil 3 514.

The processor 502 executes software instructions and processes formation data that is stored in the memory/storage 506. The memory/storage 506 may include random access memory, read only memory, disk storage, optical storage, or any other type of media that stores instructions and formation data. The downhole interface 504 supports communications between the downhole communication unit 500 S" and an uphole communication unit such as those described with reference to FIGs. 1A and 1B. Some of the components of FIG.

25 5, by way of example, the memory/storage 506 may be located remotely from the downhole tool in alternate embodiments of the present invention.

31 The coil interface 508 interfaces to the coils 510, 512 and 514 of the steerable antenna 520. In other embodiments of a steerable antenna, for example the two coil embodiment of FIGs. 6A, 6B and 6C, when other than three coils exist, the coil interface 508 will interface to a differing number of coils. The coil interface 508 provides signal creation, signal amplification and signal transmission functions during transmission operations and signal receipt, amplification functions, and vector summing during remote sensing unit location operations and formation data acquisition operations.

Thus, the coil interface 508 separately controls signals transmitted to, and received from, the coils 510, 512 and 514. By separately controlling these signals, the coil S 15 interface directs the ARPM into the formation. In directing the ARPM during receipt operations, the coil interface 508 "selectively amplifies and selects the phase of signals received from the coils 510, 512 and 514. Depending upon the orientation of the coils 510, 512 and 514, and whether the oooee 20 downhole tool is rotating within the well bore, this selective amplification and phase selection changes with time. Thus, the direction of the ARPM also changes with time.

During transmission operations, the coil interface 508 selectively amplifies and alters the phase of signals transmitted to the coils 510, 512 and 514 so that the ARPM is directed into the formation at a desired direction. During 31 receipt operations, the coil interface 508 performs vector summing operations by combining signals received by each of the coils 510, 512 and 514. This vector summing is also performed by altering the amplitude and phase of the received signals.

FIGs. 6A, 6B and 6C are diagrammatic views of an alternate embodiment of a steerable antenna constructed according to the present invention. Referring to FIG. 6A, a first coil of this alternate embodiment is illustrated to have two half-coils 602 and 604. These half-coils together produce a magnetic dipole 608 that extends radially from a downhole tool in which the half-coils are formed. As is generally illustrated, the half-coils 602 and 604 are formed to extend about one-half of a cylindrical portion of the 15 downhole tool and to receive current out of phase with oooo respect to each other.

Referring now to FIG. 6B, a second coil of this alternate embodiment includes two half-coils 622 and 624 that orient and receive current so as to produce a magnetic dipole coo o 20 628 that also extends radially from the downhole tool in which the half-coils are formed. However, the half-coils 622 and 624 are formed within the downhole tool to produce the magnetic dipole 628 so that dipole 628 is rotated azimuthally with respect to the magnetic dipole 608. FIG. 6C further illustrates the orientation of these magnetic dipoles 608 and 628. These magnetic dipoles 608 and 628, contained within the downhole 630 are controllable so that an ARPM may be directed axially from the downhole tool at any azimuth angle.

However, the ARPM may not be directed at other than a degree angle with respect to the longitudinal axis of the downhole tool.

FIG. 6D is a diagrammatic side view of an anti-Helmholtz antenna construction that may be used with a steerable antenina constructed according to the present invention. The anti-Helmholtz structure is formed in/on a downhole tool 650 and includes a first coil 630 that produces a first magnetic dipole 634 and a second coil 632 that produces a second magnetic dipole 636. The first magnetic dipole 634 and the second coil 632 oppose one another such that a composite magnetic field extends radially into the formation. However, the composite magnetic dipole is symmetric azimuthally about 15 the downhole tool 650. Thus, by itself, the anti-Helmholtz oooo antenna construction produces an ARPM that is steerable in the longitudinal direction.

:FIG. 7 is a logic diagram illustrating operation according to the present invention in establishing go••e: 20 communications with a remote sensing unit. By directing the ARPM produced by the steerable antenna, a search beam can oosweep axially and azimuthally to locate a signal transmitted by a remote sensing unit. If the remote sensing unit is *.*passive but conductive, the same operations may be used for searching and tracking. In lieu of a signal transmitted from the remote sensing unit, the system electromagnetically illuminates the formation and measures a large response when the antenna beam is directed toward the conductive remote sensing unit.

Through the use of axial sweeps, the remote sensing unit can be detected before the array passes proximally to the remote sensing unit. Note that using the steerable antenna, it is possible to detect, communicate with, and power the sensor before the array actually passes by the sensor and can continue until after the array has passed by the sensor. By quickly locating the remote sensing unit, the amount of time that communication with the remote sensing unit may be maintained without interrupting the drilling process (when using a MWD tool) is maximized. This reduces the power consumption and related cost for these operations.

Through azimuthal sweeps, the entire circumference around the S. 15 borehole can be searched at a rate much faster than the drill collar rotates.

The position of the remote sensing unit relative to the drill collar is not necessarily constant. Assuming the main beam of the antenna is oriented for maximum signal strength, 20 there are several ways to obtain tracking information to steer the ARPM so as to keep a lock on the remote sensing .unit as the drill collar moves. One way is to correct the beam position using external rate information: angular, axial, lateral acceleration and/or velocity. Typically radar tracking techniques are used, such as monopulse antenna tracking. However, the array must have some horizontal/longitudinal extent to determine the azimuthal/axial tracking information. As in that case, any rate information or a unique horizontal/longitudinal extent is not available, a three phase structured robust search and tracking method is introduced. The preferred embodiment therein is to rapidly apply small deviations in the beam location and compare the received power as the beam moves slightly up and down in elevation and slightly back and forth in azimuth. The measured gradient at these points gives a correction angle to adjust the beam direction for optimum signal reception.

Applying the averaged correction angle, a maximum strength signal sample will be transferred to the receiver unit. Although subsets of the three phase tracking algorithm can be used to communicate with the tool, any subset is less S 15 effective or less robust because either the signal strength is not maximum, or electromagnetic contact is lost or the S"process of locating the sensor takes a long time.

Referring to FIG. 7, the tracking process cannot occur until the remote sensing unit has been located within the 20 formation (step 702). The various methods of locating the sensor (step 702) are described in FIGs. 8A, 8B, and 8C.

Referring to FIG. 7, the downhole tool acquires the angular frequency of the drill collar in which the steerable antenna resides (step 704). This step may also be performed for a wireline tool that is rotating with respect to the formation.

However, in an embodiment in which it is assumed that the 3C downhole tool has no angular motion, step 704 is not performed.

In step 704 (Phase the downhole tool acquires the instantaneous angular frequency of the drill collar. Using a closed-loop approach, the derivative of the individual incoming field components of each coil of the steerable antenna is driven to zero. In other words, the coil interface rotates the antenna at exactly the same frequency, but in the opposite sense that the downhole tool rotates.

Since there is no relative rotation between the ARPM produced by the steerable antenna and the remote sensing unit, the received signal is constant. Once the angular velocities of the ARPM produced by the steerable antenna and downhole tool are matched and the derivative of the incoming field strength is driven to zero, the angular frequency has been acquired.

:Next (Phase II), a coarse search for the global maximum :of the received signal, is conducted (step 706). By "coarse search," it is meant a search for the antenna direction to assure the main beam of the antenna is directed towards the 20 sensor, but not necessarily that the optimum direction for maximum signal has been received. Since the ARPM is a ***composite of multiple magnetic dipoles, it has multiple lobes. It is possible, therefore, that Phase I may be accomplished by the individual dipoles with side lobes receiving the response from the remote sensing unit. Although the dipoles may acquire the correct angular velocity, they may show phase ambiguities. This false lock situation may be 31 checked beforehand by correlation of the individual field strengths, and solved within Phase II by the global maximum search algorithm. These are the main reasons why Phase II is desirable.

To avoid a locking state at a side lobe instead of the main lobe, a feed-forward oriented operation allows fast angle detection of a field global maximum, by adding phase offsets to the quasi-constant rotating antenna beam. During this procedure, the system deactivates angular frequency tracking in order to avoid any interference of the two individual search procedures. Instead of keeping the antenna pointed at a constant azimuthal angle in space, the ARPM is quickly rotated over 3600. During this rotation, the downhole tool samples and stores the received signal. After the rotation, the angle corresponding to the maximum signal is determined. This angle is an offset phase correction.

SApplying the offset correction to the original direction of the beam ensures that the received signal is in the main beam of the antenna. Because step 704 angular frequency 20 tracking is deactivated in that phase, the step size may not be arbitrarily small, otherwise synchronization of the angular frequency could be lost.

The feed-forward maximum search may also be applied to the elevation angle of the ARPM. The result of these maximum searches returns near-optimum azimuth and elevation angles for maximum signal.

The global maximum search of step 706 occurs rapidly compared to the rate of downhole tool rotation and could be implemented in an iterative process where first coarse corrections and then fine corrections are made. After the initial search of step 706 is performed, additional searches could be implemented based on degradation of the received signal in terms of signal-to-noise ratio and/or signal strength.

The global maximum of the signal received by the steerable antenna array is found and tracked during additional operations (step 708). In these continued operations, the main lobe of the antenna beam receives the signal from the sensor. To track changes in the optimum elevation and azimuthal angles, the ARPM (azimuth and elevation angles) is varied by small amounts (by applying jitter to the beam direction), and the received signals after each steering perturbation are compared. This inspection indicates a feedback gradient that leads to an angle correction towards the maximum. In an optimally-directed 20 case, the resulting change in the received signal due to these small angle deviations are balanced around the maximum (a symmetric field is assumed), so the derivative is zero.

The symmetric assumption is not essential for the method to work. If the position sampling is faster than the required formation data rate for the receiver, this algorithm allows the system to orient the antenna beam to ensure the signal received for formation data communication with the remote sensing unit have a maximum value. Thus, formation data is handed over as global maximum data from the remote sensor to the downhole communication unit (step 710). The operations of steps 708 and 710 are continued until the communications are completed (as determined at step 712), at which point operation completes.

FIG 8A is a logic diagram illustrating a first methodology for locating a remote sensing unit in a formation. In this embodiment, the remote sensing unit is self-powered to produce a signal that may be received by the steerable antenna. As a first step in the operation, the ARPM of the steerable antenna is set to initial azimuth and elevation angles (step 722). Then, the downhole communication unit listens for the remote sensing unit signal (step 724). If a signal is detected (step 726), operation ends. However, if no signal is detected (at step 726), the azimuth angle and elevation angle of the steerable antenna are incremented (step 728). If, after this increase, the elevation angle is not at a final elevation angle, operation .go.•i 20 returns to step 724. If the elevation angle is at a final elevation angle, operation returns to step 722, where operation is initialized and repeated until the remote sensing unit is located.

FIG 8B is a logic diagram illustrating a second methodology for locating a remote sensing unit in a formation. In the embodiment, a powering signal is required from the steerable antenna in order for the remote sensing Lt D unit to produce a RF signal. As a first step in the operation, the ARPM of the steerable antenna is set to initial azimuth and elevation angles (step 742). Then, the downhole communication unit transmits a RF pulse via the steerable antenna (step 744) and listens for the remote sensing unit signal (step 746). If a signal is detected (step 748), operation ends. However, if no signal is detected (at step 748), the azimuth angle and elevation angle of the steerable antenna is incremented (step 750). If, after this increase, the elevation angle is not at a final elevation angle, operation returns to step 744. If the elevation angle is at a final elevation angle, operation returns to step 742, where operation is initialized and repeated until the remote sensing unit is located.

FIG. 8C is a logic diagram illustrating a third methodology for locating a remote sensing unit in a formation. In the embodiment, a signal from the steerable antenna is reflected by the remote sensing unit and the reflection is received by the tool. Further, in the o e 20 embodiment, the steerable antenna includes both transmit antennas and receive antennas. As a first step in the oeoperation, the ARPM of the steerable antennas are both set to initial azimuth and elevation angles, these angles related .such that the ARPM of the transmit antenna and the ARPM of the receive antenna are directed to the same formation location (step 762). The system may use either the same antenna for transmit and receive operations or separate antennas for transmit and receive operations.

Then, the downhole communication unit transmits a RF pulse via the transmit steerable antenna (step 764) and listens for the remote sensing unit signal via the receive steerable antenna (step 746). If a signal is detected (step 768), operation ends. However, if no signal is detected (at step 768), the azimuth angle and elevation angle of the steerable antennas are incremented (step 770). If, after this increase, the elevation angle is not at a final elevation angle, operation returns to step 764. If the elevation angle is at a final elevation angle, operation returns to step 762, where operation is initialized and repeated until the remote sensing unit is located.

FIG. 9 is a diagrammatic side view of a downhole tool :constructed according to the present invention and the manner *4 "in which a triad of coils may be wrapped about the tool body to produce a triad of magnetic dipoles of a steerable antenna. As shown, a first coil 902, a second coil 904 and a •co e 20 third coil 906 are wrapped around the downhole tool 900 such oo that they are symmetrically disposed. Further, the coils •"902, 904 and 906 are disposed such that they produce an orthonormal (or at least symmetrical) set of magnetic dipoles.

FIG. 10 is a diagrammatic side view of a downhole tool and a remote sensing unit, both constructed according to the present invention. The downhole tool is shown to have an H-t angular velocity, as indicated by rotational arrow 902. A transmit steerable antenna includes coils 904 and 906 while a receive steerable antenna includes coils 908 and 910. Both the transmit and receive steerable antennas couple to S transmission/receive hub 916. The transmit steerable antenna produces a transmit ARPM 912 that is directed toward the remote sensing unit 200. Further, the receive steerable antenna produces a receive ARPM 914 that is also directed toward the remote sensing unit.

Figs. 11A and 11B are logic diagrams illustrating methods according to the present invention to determine the depth at which a remote sensing unit resides within a formation and to determine formation properties therefrom.

The operation of FIG. 11A contemplates the availability of a single steerable antenna while the operation of FIG. 11B contemplates the availability of at least two steerable S" antennas.

Referring now to FIG. 11A, operation commences wherein a downhole tool locates a remote sensing unit within the

S

20 formation. The downhole tool then directs an ARPM at the downhole tool (as part of the locating process) and determines the ARPM's angle with respect to the longitudinal axis of the downhole tool (step 1102) At step 1102, the downhole tool also determines the depth at which this determination was made.

The operation of step 1102 is repeated for additional depths of the downhole tool (from step 1104). After the operations of step 1102 have been repeated at once, the depth at which the sensor is located in the formation is determined (step 1106). Of course, additional determinations are also required, the diameter of the wellbore, the location of the downhole tool within the wellbore, whether the downhole tool is centered in the wellbore, and any additional measurements or assumptions that may be required to locate the remote sensing unit with respect to the surface of the wellbore.

With the depth in which the remote sensing unit resides in the formation determined, formation properties of the formation in which the remote sensing unit resides may be calculated (step 1108). Additional information required in this determination will include the amount of energy expended in placing the remote sensing unit in the wellbore, the energy of an explosive charge that was used to shoot the remote sensing unit into the formation. Other information that may be required is the possible degradation in the wellbore since the remote sensing unit was deployed that may have increased the diameter of the wellbore, and other relevant information.

a. Referring now to FIG. 11B, the depth of the remote sensing unit within the wellbore is determined using a downhole tool that includes two steerable antennas. Using the same technique described at step 1102 of FIG. 11A, the angle to the remote sensing unit from a first steerable antenna is measured (step 1110). Then, the angle to the remote sensing unit from a second steerable antenna is determined (step 1112). Then, based upon these angles, the diameter of the wellbore and the location of the downhole tool within the wellbore, the depth within the formation at which the remote sensing unit is located is determined (step 1114) Then, using this information, formation properties are estimated (step 1116) The invention disclosed herein is susceptible to various modifications and alternative forms. Specific embodiments therefor have been shown by way of example in the drawings and detailed description. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the invention is to cover all modifications, equivalents and alternatives falling within *066 the spirit and scope of the present invention as defined by e e 6@ 06 Sthe claims.

It is to be understood that, if any prior art publication

S

is referred to herein, such reference does not constitute e: an admission that the publication forms a part of the common general knowledge in the art, in Australia or any 666e other country.

6e o* see*For the purposes of this specification it will be clearly understood that the word "comprising" means "including but not limited to", and that the word "comprises" has a c s n a corresponding meaning.

OOOS

Claims

1. 1 34. The method of claim 32, wherein communicating with 2 the remote sensing unit includes: 3 sending a communication signal to the remote sensing 4 unit; and receiving a communication signal from the remote sensing 6 unit. 1 1 35. The method of claim 34, wherein communicating with 2 the remote sensing unit further comprises sending a signal to 3 the remote sensing unit. 1 1 36. The method of claim 32, wherein detecting the 2 presence of the remote sensing unit in the formation 3 comprises: 4 setting an initial azimuth and elevation of the antenna 5 radiation pattern maximum produced by the steerable antenna; 6 listening for a signal from the remote sensing unit; 7 incrementing the azimuth and elevation of the antenna 8 radiation pattern maximum produced by the steerable antenna o 9 while continuing to listen for a signal from the remote 10 sensing unit; and 11 detecting a signal from the remote sensing unit. 1 1 37. The method of claim 32, wherein detecting the 2 presence of the remote sensing unit in the formation 3 comprises: 4 setting an initial azimuth and elevation of the antenna radiation pattern maximum produced by the steerable antenna; 6 transmitting a signal into the formation; 7 listening for a signal from the remote sensing unit; 8 incrementing the azimuth and elevation of the antenna 9 radiation pattern maximum produced by the steerable antenna and repeating the steps of transmitting a signal and 11 listening for a signal; and 12 detecting a signal from the remote sensing unit. 1 1 38. The method of claim 32, wherein detecting the 2 presence of the remote sensing unit in the formation 3 comprises: 4 setting an initial azimuth and elevation of the antenna 5 radiation pattern maximum produced by a transmit steerable e 6 antenna; 7 setting an initial azimuth and elevation of the antenna 8 radiation pattern maximum produced by a receive steerable 9 antenna; 1 0 transmitting a signal into the formation using the 11 transmit steerable antenna; 12 listening for a signal from the remote sensing unit 13 using the receive steerable antenna; 14 adjusting the azimuth and elevation of the antenna radiation pattern maximums produced by the transmit steerable 16 antenna and the receive steerable antenna and repeating the 17 steps of transmitting a signal and listening for a signal; 18 and 55 detecting a signal from the remote sensing unit.

39. The downhole tool of any one of claims 1 to 22 and substantially as herein described with reference to the accompanying drawings. A method as claimed in any one of claims 23 to 38 and substantially as herein described with reference to the accompanying drawings. Dated this 3rd day of October 2002 SCHLUMBERGER HOLDINGS LIMITED :By their Patent Attorneys GRIFFITH HACK 15 Fellows Institute of Patent and Trade Mark Attorneys of Australia o o* oooo* *oo oo**o *o o *o \\melbfiles\homeS\Priyanka\Keep\speci\18316-01 Amendments.doc 3/10/02