Abstract:
A tool for electromagnetic logging of a formation includes a tool body configured to move in a borehole penetrating the formation; an antenna array disposed on the tool body; and an electronic unit configured to control operation of the antenna array, wherein the antenna array comprises at least one transmitter and at least one receiver, wherein at least one selected from the group consisting of the at least one transmitter and the at least one receiver comprises a printed circuit antenna.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The invention relates generally to electromagnetic tools for well logging. More particularly, the invention relates to improved antennas and electromagnetic tools having improved antennas. 
   2. Background Art 
   The measurement of dielectric constant (or electric permittivity) of formations surrounding a borehole is known to provide very useful information about the formations. The dielectric constant of the different materials of earth formations vary widely (for example, 2.2 for oil, 7.5 limestone, and 80 for water), so measurement of dielectric properties is a useful means of formation evaluation. For example, if the lithology and the water saturation of a particular formation are known, then the porosity may be determined if the dielectric constant of the formation could be obtained. Similarly, if the lithology and porosity are known, information as to the degree of water saturation can be obtained by measuring the dielectric constant of the formation. 
   A logging device that improved the art of measuring formation dielectric constant was the electromagnetic propagation tool as disclosed, for example, in the U.S. Pat. No. 3,944,910 (“the &#39;910 patent”) issued to Rau and assigned to the present assignee. This patent discloses a logging device including a transmitter and spaced apart receivers mounted in a pad that is urged against the borehole wall. Microwave electromagnetic energy is transmitted into the formations, and energy which has propagated through the formations is received at the receiving antennas. The phase shift and attenuation of the energy propagating in the formations are determined from the received signals. The dielectric constant and, if desired, the conductivity of the formations can then be derived from the phase shift and attenuation measurements. 
   The configuration of antennas is an important aspect of successful operation of electromagnetic propagation logging tools. At a relatively high frequency of operation (for example 1100 MHz.), the signal attenuates quite rapidly. Therefore, it is important to have transmitting antennas that can efficiently generate energy and inject it into the formations, and to have receiving antennas that can efficiently receive energy that has propagated through the formations. Because the accuracy of the dielectric constant and conductivity measurements depends upon accurate measurements of attenuation and phase shift of the received signals, it is essential that the antennas operate in a stable manner over time and that the antennas are in, and remain in, a substantially balanced condition. 
   In the &#39;910 patent, the antennas in the electromagnetic propagation logging device are cavity-backed slot antennas, which are filled with a dielectric material and include a probe that is an extension of the center conductor of a coaxial cable. The center conductor of the coaxial cable extends across the cavity-backed slot connects to the wall on the opposite side of the cavity-backed slot (see  FIG. 2 ). 
   The probe (or conductor) of the cavity-backed antenna, as disclosed in the &#39;910 patent, extends across the slot in a direction parallel to the longitudinal direction of the borehole. This configuration is known as a “broadside” array. U.S. Pat. No. 4,704,581 (“the &#39;581 patent”), issued to Clark and assigned to the present assignee, discloses a similar logging device, but wherein the slot (cavity-backed) antennas have probes that extend in a direction perpendicular to the longitudinal direction of the borehole. This configuration is know as an “endfire” array. The endfire array exhibits a deeper depth of investigation and is less affected by tool standoff than the broadside array. On the other hand, the broadside array exhibits a stronger signal characteristic than the endfire array and may be preferred in relatively lossy (low resistivity) logging environment. Note that most electromagnetic logging tools have two or more receiver antennas, which facilitate the measurements of difference signals between the receiver antennas. Difference measurements cancel undesirable environmental (e.g., borehole) effects and simplify data analysis. However, one of ordinary skill in the art would appreciate that these measurements may also be performed with a single receiver antenna. In this case, the characteristics of the single antenna should be calibrated so that the true signals may be extracted from the raw measurements. With a single receiver antenna, it is more accurate to refer to the setup as a “mode” rather than an “array.” However, for simplicity, this description uses “array” to generally refer to a tool configuration that includes a transmitter and one or more receivers. One of ordinary skill in the art would appreciate that embodiments of the invention are applicable to tool configurations having one or more receivers. 
   An example of a logging device based on the teachings of the &#39;910 and &#39;581 patents is a electromagnetic propagation tool sold under the trade name of EPT™ by Schlumberger Technology Corp. (Houston, Tex.). A similar tool, called adaptable EPT™ (“ADEPT”), can provide either broadside operation or endfire operation during a given run, depending on the logging conditions. The ADEPT logging tool has two changeable pads, one containing a broadside antenna array and the other an endfire antenna array. 
   The EPT™ or ADEPT tools use cavity-backed antennas (or slotted antennas) arrays. Other related tools based on similar arrays include U.S. Pat. No. 4,698,572 (“the &#39;572 patent”) issued to Clark. The &#39;572 patent discloses electromagnetic logging tools incorporating slot antennas that have improved properties as compared with the conventional cavity-backed antennas. The slot antennas disclosed in this patent include tuning elements to improve the operation. 
   Furthermore, U.S. Pat. No. 5,434,507 (“the &#39;507 patent”) issued to Beren et al. discloses electromagnetic logging tools with two-dimensional antenna arrays. The antenna arrays may comprise conventional cavity-backed antennas or cavity-backed antennas having two conductors arranged in a crossed-dipole configuration. The two-dimensional array of antennas makes it possible to image the formations surrounding the borehole. The above described patents, i.e., the &#39;901 patent, the &#39;572 patent, the &#39;581 patent, and the &#39;507 patents, are assigned to the present assignee and are incorporated by reference in their entireties. 
   Although the cavity-backed antennas have been very reliable in wire line applications, the rough working environment of logging-while-drilling (LWD) applications may need a new design of antennas. If the antennas are placed on a pad, which can rotate at a rate up to 120 RPM in a drilling operation, the antennas would have much higher wear rates as compared with the wire line environment. As a result, the antennas may have to be replaced often. Accordingly, it is desirable to have new design of antennas that can tolerate the harsh environments of an LWD operation and/or can be more easily replaced. 
   SUMMARY OF INVENTION 
   One aspect of the invention relates to antennas for electromagnetic logging tools. An antenna in accordance with embodiments of the invention are RF antennas that may be manufactured using printed circuit technologies. 
   Another aspect of the invention relates to tools for electromagnetic logging of a formation. A tool in accordance with one embodiment of the invention includes a tool body configured to move in a borehole penetrating the formation; an antenna array disposed on the tool body; and an electronic unit configured to control operation of the antenna array, wherein the antenna array comprises at least one transmitter and at least one receiver, wherein at least one selected from the group consisting of the at least one transmitter and the at least one receiver comprises a printed circuit antenna. 
   Another aspect of the invention relates to methods for logging a well. A method in accordance with one embodiment of the invention includes moving an electromagnetic logging tool in a borehole penetrating a formation; transmitting an electromagnetic energy from at least one transmitter disposed on the electromagnetic logging tool into the formation; and receiving a signal from the formation using at least one receiver disposed on the electromagnetic logging tool, wherein at least one selected from the group consisting of the at least one transmitter and the at least one receiver comprises a printed circuit antenna. 
   Another aspect of the invention relates to methods for manufacturing an electromagnetic logging tool. A method in accordance with one embodiment of the invention includes disposing at least one transmitter and at least one receiver on a tool body that is configured to move in a borehole penetrating a formation; and disposing an electronic unit in the tool body, wherein the electronic unit is configured to control operation of the at least one transmitter and the at least one receiver, and wherein at least one selected from the group consisting of the at least one transmitter and the at least one receiver comprises a printed circuit antenna. 
   Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  shows a conventional logging-while-drill system. 
       FIG. 2  shows a prior art cavity-backed antenna. 
       FIG. 3  shows an RF antenna in accordance with one embodiment of the invention. 
       FIG. 4  shows an RF antenna in accordance with another embodiment of the invention. 
       FIG. 5  shows an RF antenna having capacitors in accordance with one embodiment of the invention. 
       FIG. 6  shows an RF antenna in accordance with another embodiment of the invention, wherein the antenna includes multiple turns. 
       FIG. 7  shows a cross-sectional view of an array in accordance with one embodiment of the invention. 
       FIG. 8A  shows an endfire array and  FIG. 8B  shows a broadside array, in accordance with one embodiment of the invention. 
       FIG. 9  shows a tool configuration including an endfire array and a broadside array in accordance with one embodiment of the invention. 
       FIG. 10  shows another tool configuration having both endfire and broadside antennas at the same location. 
   

   DETAILED DESCRIPTION 
   Embodiments of the invention relate to RF antennas for logging operations and tools incorporating the same. An antenna in accordance with embodiments of the invention have properties similar to those of a cavity-backed antenna. However, an antenna of the invention does not need to have a box to provide the cavity. Some antennas of the invention may be manufactured using printed circuit technologies. Such antennas can better tolerate stresses that are expected in downhole environments, especially the harsh environments encountered in logging-while-drilling operations. 
     FIG. 1  shows a logging-while-drilling (LWD) apparatus and telemetry system. As shown, a platform and derrick  10  are positioned over a borehole  11 . A drill string  12  includes a drill bit  15  at its lower end. The drill string  12  and the drill  15  are rotated by a rotating table  16 , which engages a kelly  17  at the upper end of the drill string  12 . The drill string  12  is suspended from a hook  18 . The kelly  17  is connected to the hook  18  through a rotary swivel  19 , which permits rotation of the drill string  12  relative to the hook  18 . 
   Drilling fluid or mud  26  is contained in a pit  27 . A pump  29  pumps the drilling fluid into the drill string  12  via a port in the swivel  19  to flow downward through the center of drill string  12 . The drilling fluid exits the drill string  12  via ports in the drill bit  15  and then circulates upward in the region between the outside of the drill string and the periphery of the borehole. As is well known, the drilling fluid thereby carries formation cuttings to the surface of the earth, and the drilling fluid is returned to the pit  27  for recirculation. 
   Mounted within the drill string  12 , preferably near the drill bit  15 , is a downhole sensing, processing, storing and transmitting subsystem  100 . A transmitting subsystem may include an acoustic transmitter  56 , which generates an acoustic signal in the drilling fluid. The generated acoustic mud wave travels upward and is received at the surface of the earth by transducers  31 , which convert the received acoustic signals to electronic signals. The output of the transducers  31  is coupled to the uphole receiving subsystem  90 , which demodulates the transmitted signals and relays the demodulated signals to processor  85  and recorder  45 . 
   Transmitter  56  is controlled by transmitter control and driving electronics  57  which includes analog-to-digital (A/D) circuitry that converts the signals representative of downhole conditions into digital form. The control and driving electronics  57  may also include a suitable modulator, such as a phase shift keying (PSK) modulator, which conventionally produces driving signals for application to the transmitter  56 . These driving signals can be used to apply appropriate modulation to the mud siren of transmitter  56 . It will be understood that alternative techniques can be employed for communicating logging information to the surface of the earth. 
   The downhole subsystem  100  may further include acquisition and processor electronics  58 , which include a microprocessor (with associated memory, clock circuitry, and interface circuitry) and processing circuitry. The acquisition and processor electronics  58  are coupled to the measuring apparatus  200 . The acquisition and processor electronics  58  is capable of storing data from the measuring apparatus  200 , processing the data and storing the results, and coupling any desired portion of the information it contains to the transmitter control and driving electronics  57  for transmission to the surface by transmitter  56 . A battery  53  may provide downhole power. Alternatively, a downhole generator (not shown) such as a “mud turbine” may be utilized to provide power during drilling. 
   Subsystem  100  includes a measuring apparatus  200 . In accordance with one embodiment of the invention, the measuring apparatus  200  may include an EPT™-like device having one or more antenna arrays, each comprising, for example, four antennas T 1 , T 2 , R 1 , and R 2 . The T 1 -R 1 -R 2 -T 2  configuration, shown in  FIG. 1 , can provide borehole-compensated measurements, as disclosed in U.S. Pat. No. 3,849,721 issued to Calvert. However, one of ordinary skill in the art would appreciate that embodiments of the invention are not limited to this configuration. Instead, embodiments of the invention may include one or more transmitters and one or more receivers. In accordance with embodiments of the invention, the antennas of an electromagnetic logging tool may provide similar advantages as the conventional cavity-backed antennas. However, antennas of the invention do not have physical box-like structures (a box or a slot on the tool body) or conductive wire connections to the walls of the box-like structures. 
     FIG. 2  shows a prior art cavity back antenna. As shown, the cavity back antenna  210  includes a metallic box  211  having one side open to form a cavity  216 . The cavity  216  is a metallic cavity, where electromagnetic standing waves can be generated and emitted through the open face on the top of the box  211 . The electromagnetic radiation is delivered to the cavity by a coaxial cable having a center conductor  212  and an outer conductor  213 . The outer conductor  213  of the coax is shorted to the wall of the box  211  at the entrance. The center conductor  212  extends to the opposite wall of the box  211  and is connected to this wall. 
   Electrically, the current in the center conductor  212  returns along the walls of the box  211  to reach the outer conductor  213  of the coax. As a result, the current paths comprise a series of current loops around the center conductor  212  and the walls of the box  211 . The center conductor  212  acts as the common conductor for all current loops. Simple right hand rule can be used to show that the magnetic field generated from these loops is shown as H in  FIG. 2 . Such magnetic field H distribution is equivalent to a horizontal magnetic dipole. In fact, horizontal magnetic dipoles are typically used to model the behavior of this type of antennas. 
   The box  211  of a conventional cavity-backed antenna is tough enough to withstand certain degree of stress. However, the center conductor  212  of the coax cable and its connection to the cavity walls are potential weak points in the mechanical design of the antenna. In a drilling operation, axial forces as well as shear forces would be continuously applied to the cavity. The coax cable and the connections may break under stress. Therefore, it is desirable to have a more reliable design of antennas. 
   Embodiments of the invention can achieve the same antenna behavior as the cavity-backed dipole antenna shown in  FIG. 2 , without the need for a box.  FIG. 3  shows an antenna in accordance with one embodiment of the invention that can be mounted on a printed circuit or manufactured as a printed circuit. As shown, a loop antenna  300  is formed on an insulating layer  301 . The antenna  300  may be a printed circuit or part of a printed circuit. The magnetic field H generated by such an antenna is similar to that generated by a prior art cavity backed antenna shown in  FIG. 2 . The material of the insulating layer  301  can be, for example, ceramic, polyimide, thermoplastic resin, thermoset resins, plastics, etc, on which the antenna loop  302  is engraved, etched, or printed. Such antenna allows for some bending under compression. Therefore, such an antenna is less prone to break under stress. 
     FIG. 4  shows another antenna in accordance with one embodiment of the invention. In this variation, two loops are formed to produce a similar magnetic field as that shown in  FIG. 3 . As shown in  FIG. 4 , a printed circuit  402  forms two loops on an insulating material  401 . In principle, the loops formed by circuit  402  shown in  FIG. 4  are reminiscent of the current paths of the cavity backed antenna of  FIG. 2 . The design in  FIG. 4  is slightly more complicated than that in  FIG. 3  and requires a multi-level printed circuit. However, the approach shown in  FIG. 4  can potentially be used to make multi-turn antennas. It is well known that, for the same circulating current, the efficiency of a loop antenna is proportional to the loop area and the number of turns. Therefore, the approach sown in  FIG. 4  can potentially be used to produce more efficient antennas. 
   The printed circuit designs as shown in  FIG. 3  and  FIG. 4  are easier to manufacture than the conventional cavity-backed antennas and can avoid the potential weak points at the junctions of the cavity backed antenna shown in  FIG. 2 . In addition, the printed circuit antennas can be readily tuned. Because loop antennas are inductors by nature, it is common to use capacitors to tune these antennas. However, one of ordinary skill in the art would appreciate that tuning elements other than capacitors may also be used without departing from the scope of the invention. The capacitors are usually placed outside the cavity in a prior art cavity antenna. With the printed circuit approach, the capacitors may be created as well-defined discontinuities in the current paths, as shown in  FIG. 5 . For example, one or more capacitors (e.g., capacitors  501 ,  502 ,  503 , and  504  in  FIG. 5 ) may be placed at multiple locations around the current loop. Similar capacitors (e.g., capacitors  505 ,  506 ,  507 ,  508 ) may be included on the other half of the antenna loop, as shown in  FIG. 5 . This distributed capacitors approach has certain advantages. For example, these capacitors may function to reduce eddy currents. As a result, such antennas would be less sensitive to the outside environment. 
     FIG. 5  uses “in-line” capacitors as tuning elements, in which the capacitors are integral parts of the circuit loop. It should be understood that other types of capacitors may be used, such as a conventional capacitor, a lumped capacitor, a shunt stub or other suitable tuning elements. Such elements may be disposed or printed between the printed loop and the power supply. In addition, non-capacitor based tuning elements, such as inductive tuning elements, may also be employed to tune the parasitic capacitance of the circuit. 
   As noted above, the multi-layer printed circuit antennas can provide stronger magnetic moments because the magnitudes of magnetic moments are directly proportional to the number of turns of a coil. In addition, a multi-turn antenna loop can be energized, as shown in  FIG. 6 , by connecting to an RF source  602  at a location on the antenna such as point  605 , to provide an improved impedance matching between the RF source  602  and the antenna load. Furthermore, the antenna  601  may include a capacitor  603  for tuning the load (i.e., load balancing). The capacitor  603  may be a tuning capacitor similar to those described with reference to  FIG. 5 . In addition, a capacitor  604  may be included as part of the impedance matching network to provide AC coupling. 
   The printed circuit type antennas, in accordance with embodiments of the invention, may be assembled to form an antenna array.  FIG. 7  shows a cross-sectional view of one example of an antenna array comprising two antennas  700 . In  FIG. 7 , two antennas are assembled between two insulating layers  703  and  704  to form the antenna array. The metallic cover  702  may be mounted over the base  707  to form individual cavities, which prevent direct interaction between the antennas. In some embodiments, the cover  702  may be a layer disposed on top of the insulating layer  703 , while the separation between the antennas are achieved using other structures (not shown). 
   The cover  702  has windows  701  cut in it to allow the radiation to be transmitted into the formation or to allow signals to reach the antenna (receiver). One of ordinary skill in the art would appreciate that the openings on the metal cover  702  may be configured in certain patterns to allow magnetic moments in particular directions to pass through. See e.g., U.S. Pat. No. 6,297,639 issued to Clark et al. These windows  701  may be covered with an insulating material to prevent the borehole fluid from entering the cavity. Parts of cover  706 , may be allowed to protrude outside to provide additional protection against wear. 
   Electronic components (or electronic units) such as  705  may be mounted underneath the insulating layer  704 . In some embodiments, the electronic components  705  may be disposed outside the printed circuit antenna array module. The electronic components  705  may function to control the operations of the antenna  700  (e.g., the transmitters and the receivers). Detailed designs and functions of the electronic units for controlling the operations of the transmitters and receivers are generally known in the art. See e.g., U.S. Pat. No. 3,849,721 issued to Calvert and U.S. Pat. No. 4,689,572 issued to Clark. These patents are assigned to the present assignee and are incorporated by reference in their entirety. 
     FIGS. 8A and 8B  show the top views of two antenna arrays, a cross-sectional view of which is shown in  FIG. 7 . Here, two different types of arrays are shown, namely endfire ( FIG. 8A ) and broadside ( FIG. 8B ) arrangements of the antennas. For description of broadside arrays see the &#39;910 patent. For a description of endfile arrays see the &#39;581 patent. In each of these arrays, two antennas are shown to have the same magnetic moments orientation. Such antenna arrays are commonly used as the receivers in borehole-compensated electromagnetic measurement tools as disclosed in U.S. Pat. No. 3,849,721. Alternatively, in non-borehole compensated tools, one of the antennas in the array may function as a transmitter, while the other may function as a receiver. Thus,  FIGS. 8A and 8B  may be examples of single-receiver tool designs. 
   The two examples shown in  FIGS. 8A and 8B  are for illustration only. One of ordinary skill in the art would appreciate that many variations of these arrays are possible without departing from the scope of the invention. For example, an array may include more than two antennas. For example, a typical borehole-compensated array will include four antennas (two transmitters and two receivers) in the T 1 -R 1 -R 2 -T 2  configuration, as shown in  FIG. 9 . 
     FIG. 9  further shows that two or more antenna arrays may be organized in a two-dimensional array as disclosed in U.S. Pat. No. 5,434,507 issued to Beren et al. As shown in  FIG. 9 , a second antenna array, comprising T′-R 1 ′-R 2 ′-T 2 ′, is arranged in a different configuration (endfire) from that of the first antenna array (broadside). 
   While  FIG. 9  shows that the two antenna arrays are disposed on the same pad, one of ordinary skill in the art would appreciate that they can also be disposed on separate pads without departing from the scope of the invention. Furthermore, each of the transmitter and the receiver antennas within the same array may be an endfire or broadside magnetic dipole antenna. Thus, some of the arrays may include cross dipoles, i.e., the transmitter and receivers are not of the same polarization (i.e., magnetic dipole orientation). 
   In addition, the endfire array (T 1 -R 1 -R 2 -T 2 ) and the broadside array (T′-R 1 ′-R 2 ′-T 2 ′) shown in  FIG. 9  may be constructed at the same location by using multi-layer printed circuits.  FIG. 10  shows one such embodiment. As shown in  FIG. 10 , each of the four antennas T 1 , T 2 , R 1 , and R 2  includes two circuits arranged to produce an endfire and a broadside magnetic dipoles at the same location. Each of these antennas is reminiscent of the antennas disclosed in U.S. Pat. No. 5,434,507 issued to Beren et al. (see  FIG. 2  and  FIG. 3  in the Beren et al. patent). Note that each of the endfire and broadside antennas shown in  FIG. 10  may be independently used to produce cross-dipole and/or non-cross-dipole measurements. Note that while four antennas are shown in this example, one of ordinary skill in the art would appreciate that any combination of transmitter and receiver antennas may be used without departing from the scope of the invention. For Example, a tool in accordance with one embodiment of the invention may have the following array: R 1 -R 2 -R 3 -T 1 -T 2 -R 4 -R 5 -R 6 . 
   The above description shows that embodiments of the invention may be used in a logging tool that is like an EPT™ or ADEPT. Embodiments of the invention may be used with wireline or logging-while-drilling (LWD), Measurement-while-drill (MWD), or logging-while-tripping (LWT) tool. A tool including antennas of the invention may be operated at appropriate frequencies to achieve different types measurements, i.e., induction and propagation measurements. Therefore, a tool in accordance with the invention may be used in all kind of mud (water-based or oil-based muds). In a particular embodiment, a tool of the invention can provide borehole images in a well drilled with all types of muds, i.e., an all mud imager. 
   When used on an LWD or MWD tool, an antenna array of the invention may be disposed in an articulating pad. Such articulating pads are known in the art, for example, the articulating pads on the PowerDrive™ tool available from Schlumberger Technology Corp. (Houston, Tex.). Alternatively, these arrays may be disposed on drill collars, pads extended from drill collars, or stabilizers of the LWD or MWD tools. 
   Advantages of the invention may include one or more of the following. An RF loop antenna in accordance with embodiments of the invention may be used with various downhole tools, such as an all mud imager. An antenna in accordance with embodiment of the invention may be made more reliable than the prior art cavity backed antennas because an antenna of the invention may be able to tolerate more bending stresses. An antenna in accordance with embodiment of the invention can be made to radiate more RF power into the rock formation by using multi turns instead of a single turn. An antenna in accordance with embodiment of the invention can be tuned easier and more efficiently by designing the capacitors as an integral part of the loop and taking advantage of distributed capacitor approach. An antenna of the invention may be efficiently operated at relatively high frequency (e.g., 1 GHz or higher) for dielectric constant logging. An antenna in accordance with embodiment of the invention is easier to build because it is compatible with printed circuit technology and a tool based on such antennas is cheaper to manufacture and maintain because of the modular construction. 
   While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.