An induction logging sonde and its use in a borehole

Abstract: A new induction logging tool has a toolstring length that is shorter than the toolstring length of the induction logging tool of the prior art. The new induction logging tool includes a new induction array (22C) which appears to be folded about an axis (24) that coincides with the transmitter (22C1) of the induction array (22) of the prior art induction logging tool. The new folded induction array (22C) including a transmitter coil (22C1), located at an end of the new folded induction array (22C), and a plurality of "cowound" receiver coils (22C2) and bucking coils (22C4), a further plurality of individual receiver coils (22C3), and at least one further bucking coil (22C5) located adjacent to and on one side of the transmitter coil of the new folded induction array (22C). In the new folded induction array (22C), each of the plurality of cowound receiver coils (22C 2,3) and bucking coils (22C 4,5) include a receiver coil (N-1) and a bucking coil (N), associated with another receiver coil (N), which are cowound together about the same bobbin, the receiver coil (N-1) and the bucking coil (N) being positioned at approximately the same distance from the transmitter coil. Each of the plurality of cowound receiver coils and bucking coils are formed by winding both the bucking coil (N) and the receiver coil (N-1) in a bifilar way on the same ceramic bobbin starting from the center of the bobbin and winding the receiver coil (N-1) and the bucking coil (N) in opposite outward directions.

Description: BACKGROUND OF THE INVENTION The present invention relates to an induction logging sonde adapted to be disposed in a wellbore. The invention provides an induction logging sonde as defined in claim 5 including a "folded" sensor array, the sensor array including a transmitter coil and a plurality of bucking coils and receiver coils, the plurality of bucking coils and receiver coils further including a plurality of cowound bucking coils and receiver coils, each of the plurality of cowound bucking coils and receiver coils being cowound together on the same bobbin, the transmitter coil being located at an end of the array, the plurality of bucking coils and receiver coils including the plurality of cowound bucking coils and receiver coils being located only on one side of the transmitter coil thereby shortening the sensor array of the present invention relative to the prior art sensor array of other prior art induction logging tools. Over the past two decades, the electronics industry has been packing more and more circuitry into smaller and smaller packages. Also, in the past decade and half, since digital well logging tools were introduced, the tendency has been to pack more circuitry into the logging tools. Examples of such circuitry packed into the logging tools include self-test, automatic calibration, more measurement channels, more sensors, etc. As a result, a conventional well logging toolstring combination of today is longer, in length, than its counterpart toolstring of fifteen years ago. The measurements made by the conventional well logging toolstring are more accurate and plentiful, and the environmental corrections are more automatic, but the increased toolstring length of the conventional well logging tool means that more rathole must be drilled, and the rig-up time (the time it takes to assemble the toolstring) is much longer. One example of a conventional well logging induction tool is disclosed in U.S. Pat. No. 5,157,605 to Chandler et al, entitled "Induction Logging Method and Apparatus Including Means for Combining In-Phase and Quadrature Components of Signals Received at Varying Frequencies and Including use of Multiple Receiver Means Associated With a Single Transmitter". An example of an early logging induction tool is disclosed in U.S. Pat. No. 3,327,203 to Attali, entitled "Induction Logging System Utilizing Plural Diameter Coils." In the Chandler patent, the induction tool includes an induction array, and the induction array includes a transmitter coil, a receiver coil disposed on both sides of the transmitter coil, and a secondary or bucking coil disposed on both sides of the transmitter coil and interposed between the transmitter coil and each receiver coil. While the measurements made by the induction logging tool of the Chandler patent are accurate and plentiful, the length of the induction logging tool string which included the induction array of the Chandler patent is approximately forty (40) feet. As a result, it may be difficult to place the induction logging toolstring of the Chandler patent in wellbores having a minimum rathole, or in wellbores having severe dog legs, or in horizontal wellbores having a short kickoff radius. As a result, there is a need for a new induction logging tool, in accordance with the present invention, which has a toolstring length (16 feet) that is shorter than the toolstring length (40 feet) of the induction logging tool of the prior art Chandler patent. As a result, the new induction logging tool of the present invention is less prone to getting stuck in wellbores having poor borehole conditions, and it will provide logs in minilnum rathole, it may be placed into wellbores having severe dog legs, and it may be placed into horizontal wellbores having a short kickoff radius. SUMMARY OF THE INVENTION Accordingly, it is a primary object of the present invention to provide a new induction logging tool as defined in claim 1 for exploring an earth formation as defined in claim 9 having a toolstring length that is shorter than the toolstring length of the induction logging tool of the prior art. It is a further object of the present invention to provide a new induction logging tool which has a toolstring length that is shorter than the toolstring length of the induction logging tool of the prior art, the new induction logging tool including a new induction array which appears to be folded about an axis relative to the induction array of the prior art induction logging tool, the new folded induction array including a transmitter coil and a plurality of receiver coils and bucking coils which further include a plurality of cowound receiver coils and bucking coils, the transmitter coil being located at an end of the new folded induction array and the plurality of receiver coils and bucking coils including the plurality of cowound receiver coils and bucking coils being located adjacent to and only on one side of the transmitter coil of the new folded induction array, the length of the new folded induction array being approximately half the length of the induction array of the prior art induction tool in view of the cowinding of the receiver coils and bucking coils. Yet another aspect of the present invention provides a new induction logging tool which has a toolstring length that is shorter than the toolstring length of the induction logging tool of the prior art, the new induction logging tool including a new induction array which appears to be folded about an axis relative to the induction array of the prior art induction logging tool, the new folded induction array including a transmitter coil and a plurality of cowound receiver coils and bucking coils, the transmitter coil being located at an end of the new folded induction array and the plurality of cowound receiver coils and bucking coils being located adjacent to and on one side of the transmitter coil of the new folded induction array, the length of the new folded induction array being approximately half the length of the induction array of the prior art induction tool, each of the plurality of cowound receiver coils and bucking coils including a receiver coil (N-1) and a bucking coil (N) associated with another receiver coil (N), the receiver coil (N-1) being positioned at the same distance from the transmitter coil as the bucking coil (N) associated with said another receiver coil (N). A still further aspect of the present invention provides a new induction logging tool which has a toolstring length that is shorter than the toolstring length of the induction logging tool of the prior art, the new induction logging tool including a new induction array which appears to be folded about an axis which coincides with the transmitter of the prior art induction array of the prior art induction logging tool, the new folded induction array including a transmitter coil and a plurality of cowound receiver coils and bucking coils, the transmitter coil being located at an end of the new folded induction array and the plurality of cowound receiver coils and bucking coils being located adjacent to and on one side of the transmitter coil of the new folded induction array, the length of the new folded induction array being approximately half the length of the induction array of the prior art induction tool, each of the plurality of cowound receiver coils and bucking coils including a receiver coil (N-1) and a bucking coil (N), associated with another receiver coil (N), which are wound together on the same ceramic bobbin, the receiver coil (N-1) being positioned at approximately the same distance from the transmitter coil as the bucking coil (N), each of the plurality of cowound receiver coils and bucking coils being formed by winding both the bucking coil (N) and the receiver coil (N-1) in a bifilar way on the same ceramic bobbin starting from the center of the bobbin and winding the receiver coil (N-1) and the bucking coil (N) in opposite outward directions. Advantageous embodiments are defined in the dependent claims. A preferred embodiment of the new induction logging tool includes a new induction array which appears to be "folded" about an axis which coincides with the transmitter of the prior art induction array of the prior art induction logging tool, the "new folded induction array" including: (1) a transmitter coil, (2) a plurality of"cowound" receiver coils and bucking coils, (3) a further plurality of individual receiver coils, and (4) at least one further bucking coil. The transmitter coil is located at an end of the new folded induction array; however, the plurality of cowound receiver coils and bucking coils, the individual receiver coils, and the one further bucking coil are all located adjacent to and only on one side of the transmitter coil of the new folded induction array. In preferred forms of the new folded induction array, each of the plurality of cowound receiver coils and bucking coils include a receiver coil (N-1) and a bucking coil (N), which is really associated with another receiver coil (N), the receiver coil (N-1) and the bucking coil (N) being cowound together on the same ceramic bobbin and therefore being positioned at approximately the same distance from the transmitter coil. For each of the cowound receiver coils and bucking coils, the receiver coil (N-1) and the bucking coil (N), which is really associated with another receiver coil (N), are both positioned at approximately the same distance from the transmitter coil because each of the cowound receiver coils and bucking coils are formed by winding both its bucking coil (N) and its receiver coil (N-1) in a bifilar way on the same ceramic bobbin starting from the center of the bobbin and winding the receiver coil (N-1) and the bucking coil (N) in opposite outward directions. However, although it is important in the preferred embodiment that each cowound receiver coil and bucking coil must be formed by winding the bucking coil (N) and the receiver coil (N-1) on the same ceramic bobbin, recalling that each bucking coil (N) is really associated with its own corresponding receiver coil (N), it is also important that the number of coil-turns for each bucking coil (N) and its corresponding receiver coil (N) must be carefully selected. That is, for each cowound receiver coil and bucking coil, two criteria must exist and be satisfied at the same time: (1) the turns ratio between the turns of the receiver coil (N) and its corresponding bucking coil (N) must be carefully selected in order to balance or null the direct mutual coupling between the transmitter and receiver array, and (2) even though the aforementioned turns ratio must be carefully selected, the receiver coil (N-1) and the bucking coil (N) associated with another receiver coil (N) must nevertheless be cowound together on the same ceramic bobbin. As a result, since the receiver coil (N-1) and the bucking coil (N) are cowound together on the same ceramic bobbin, the bucking coil (N) and the receiver coil (N-1) are both located at approximately the same distance from the transmitter coil. Consequently, the length of the new folded induction array of the new induction tool of the present invention is approximately half the length of the induction array of the prior art induction tool. Further scope of applicability of the present invention will become apparent from the detailed description presented hereinafter. It should be understood, however, that the detailed description and the specific examples, while representing a preferred embodiment of the present invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become obvious to one skilled in the art from a reading of the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS A full understanding of the present invention will be obtained from the detailed description of the preferred embodiment presented hereinbelow, and the accompanying drawings, which are given by way of illustration only and are not intended to be limitative of the present invention, and wherein: figure 1 illustrates the prior art induction logging tool disclosed in the Chandler patent which further includes an array section; figure 2 illustrates the induction logging tool of the present invention which is shorter in length than the prior art induction logging tool of figure 1; figure 3 illustrates in greater detail the induction logging tool of the present invention shown in figure 2 which further includes an array section called the new "folded array" in accordance with the present invention; figure 4 illustrates a detailed construction of the array section of the prior art induction logging tool of figure 1; figure 5 illustrates a detailed construction of the new "folded array" section of the induction logging tool of figure 3 of the present invention; figure 6 illustrates a further more detailed construction of the new folded array section of the induction logging tool of the present invention illustrated in figure 5, the new folded array section including a "plurality of cowound receiver coils and bucking coils"; and figures 7 through 12 illustrate further, more detailed, construction views of each of the plurality of cowound receiver coils and bucking coils of figure 6. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Refering to figure 1, a prior art induction logging tool 10 includes a sensor array 12, an electronics section 14, a transmitter electronics section 16, and a spontaneous potential (SP) sub 18. While the measurements made by the prior art induction logging tool 10 of figure 1 are accurate and plentiful, the length of the tool 10, including the sensor array, is approximately fourty (40) feet. As a result of this 40 foot length, it may be difficult to place the prior art induction logging tool 10 of figure 1 in wellbores having a minimum rathole, or in wellbores having severe dog legs, or in horizontal wellbores having a short kickoff radius. The sensor array 12 of the prior art induction logging tool 10 of figure 1 is discussed in U.S. Patent 5,157,605 to Chandler et al. Other patents also discuss various other features of the prior art induction logging tool 10 of figure 1, and these other patents include: (1) U.S. Patent 5,041,975 to Minerbo et al directed to borehole correction, (2) U.S. Patent 5,355,088 to Howard Jr directed to a method and apparatus for determining invasion parameters, (3) U.S. Patent 5,184,079 to Barber directed to a method and apparatus for eliminating the effects of a dip angle on collected data, and (4) U.S. Patent 5,379,216 to Head directed to a volume transform system. Referring to figure 2, an induction logging tool 22 in accordance with the present invention is illustrated. This induction logging tool 22 includes a sensor array 22c, an electronics section 22a, and a spontaneous potential (SP) electrode 22e. However, the length of the logging tool 22 of the present invention in figure 2 is approximately 16 feet. Compare the length of the induction logging tool 22 of the present invention shown in figure 2 (16 feet) with the length of the prior art induction logging tool 10 shown in figure 1 (40 feet). This reduction in the length of the induction logging tool 22, from 40 feet to 16 feet, will allow the induction logging tool 22 of figure 2 to be placed in wellbores having a minimum rathole, or in wellbores having severe dog legs, or in horizontal wellbores having a short kickoff radius. Referring to figure 3, the induction logging tool 22 of the present invention shown in figure 2 is illustrated again in greater detail in figure 3. In figure 3, the induction logging tool 22 of the present invention shown in figure 3 includes an electronics section 22a including upper standoffs 22al, a compensator section 22b, a sensor array 22c (also illustrated as the sensor array 22c in figure 2), a lower standoff 22d, and a nose section 22e. The nose section 22e is disclosed in prior pending application serial number 08/330,397 filed 10/27/94 corresponding to attorney docket number 20.2635, entitled "Method and Apparatus for measuring mud resistivity in a wellbore including a probe having a bottom electrode located at the bottom of a tool string and propagating a current from and to the bottom electrode in a direction parallel to a longitudinal axis of the tool string". Referring to figure 4, the sensor array 12 of the prior art induction logging tool 10 of figure 1 is illustrated. This sensor array 12 is adequately described in the U.S. Patent 5,157,605 to Chandler. The sensor array 12 of figure 4 includes a transmitter coil 12a disposed in the center, a plurality of bucking coils and receiver coils disposed on one side of the transmitter coil 12a, and a further plurality of bucking coils and receiver coils disposed on the other side of the transmitter coil 12a. For example, the following bucking coils and receiver coils exist on a first side of the transmitter coil 12a: the 6 inch bucking coil 12b, the 6 inch receiver coil 12c, the 12 inch bucking coil 12d, the 12 inch receiver coil 12e, the 21 inch bucking coil 12f, the 21 inch receiver coil 12g, the 39 inch bucking coil 12h, the 39 inch receiver coil 12i, the 72 inch bucking coil 12j, and the 72 inch receiver coil 12k. In addition, the following bucking coils and receiver coils exist on a second side (opposite to the first side) of the transmitter coil 12a: the 9 inch bucking coil 12L, the 9 inch receiver coil 12m, the 15 inch bucking coil 12n, the 15 inch receiver coil 12p, the 27 inch bucking coil 12q, and the 27 inch receiver coil 12r. In figure 4, note the dotted line 24 passing through the transmitter coil 12a. This dotted line represents a "fold line". Assume that it were possible to fold (or rotate) the second side of the sensor array 12 of figure 4 about the dotted line 24 and overlay the second side of sensor array 12 (which includes the 9 inch bucking coil 12L) over the first side of the sensor array 12 (which includes the 6 inch bucking coil 12b). If this "folding operation" were possible, the result of the "folding operation" would be the sensor array 22c (hereinafter called "the folded array 22c") shown in figure 5 in accordance with the present invention. Referring to figure 5, the folded array 22c of the induction logging tool 22 of figures 2 and 3 of the present invention is illustrated. In figure 5, the folded array 22c includes a transmitter coil 22c1 disposed on one side of the array, and (1) a 6 inch bucking coil 22c5, (2) a plurality of cowound receiver coils and bucking coils 22c2, (3) a further plurality of individual receiver coils 22c3, and (4) at least one further bucking coil 22c4, all of which are disposed only on one side of the transmitter coil 22c1. The transmitter coil 22c1 is located at an end 26 of the new folded array 22c, and the plurality of cowound receiver coils and bucking coils 22c2, the 6 inch bucking coil 22c5, the individual receiver coils 22c3, and the one further bucking coil 22c4 are located adjacent to and on only one side of the transmitter coil 22c1 of the new folded array 22c. Each of the plurality of cowound receiver coils and bucking coils 22c2 in figure 5 include a receiver coil (N-1) and a bucking coil (N) which is really associated with another receiver coil (N). Furthermore, each of the plurality of cowound receiver coils and bucking coils 22c2 in figure 5 are formed by winding both the receiver coil (N-1) and its bucking coil (N) in a bifilar way on the same ceramic bobbin starting from the center of the bobbin and winding the receiver coil (N-1) and the bucking coil (N) in opposite outward directions. In addition, although it is important that each cowound receiver coil and bucking coil 22c2 be formed by winding the receiver coil (N-1) and the bucking coil (N) on the same ceramic bobbin, recalling that each bucking coil (N) is associated with its own corresponding receiver coil (N), it is also important that the number of coil-turns for each bucking coil (N) and each receiver coil (N) be carefully selected. That is, for each cowound receiver coil and bucking coil 22c2, two criteria must exist and be satisfied at the same time: (1) The turns ratio between the turns of a particular receiver coil (N) and its corresponding bucking coil (N) must be carefully selected in order to balance or null the direct mutual coupling between the transmitter and receiver array, and (2) When the aforementioned turns ratio is carefully selected, the receiver coil (N-1) and the bucking coil (N), associated with another receiver coil (N), must nevertheless be cowound on the same ceramic bobbin. When the receiver coil (N-1) and the bucking coil (N) are cowound on the same ceramic bobbin, the bucking coil (N) and the receiver coil (N-1) are both located at approximately the same distance from the transmitter coil. As a result, the functional operation of the new folded array 22c of the present invention of figure 5 is the same manner as functional operation of the prior art sensor array 12 of figure 4, the functional operation being discussed in U.S. Patent 5,157,605 to Chandler et al; however, the new folded array 22c of the present invention of figure 5 (part of the induction logging tool 22 of figures 2 and 3) is shorter in length than the prior art sensor array 12 of figures 1 and 4 thereby allowing the new induction logging tool 22 of figures 2 and 3 to be placed in wellbores having a minimum rathole, or in wellbores having severe dog legs, or in horizontal wellbores having a short kickoff radius. Referring to figure 6, a further more detailed construction of the new folded array 22c of figures 2, 3, and 5 of the new induction logging tool 22 of figures 2 and 3 is illustrated. If it were possible to fold (or rotate) the second side of the induction section 12 of figure 4 about the dotted line 24 and overlay the second side of induction section 12 of figure 4 (which includes the 9 inch bucking coil 12L) over the first side of the induction section 12 of figure 4 (which includes the 6 inch bucking coil 12b), previously called "a folding operation", a plurality of the receiver coils and bucking coils disposed on one side of the array 12 of figure 4 would overlay a corresponding plurality of the receiver coils and bucking coils disposed on the other side of the array 12 of figure 4, and the new folded array section 22c of figures 5 and 6 would be the result. The induction logging tool of the prior art shown in figure 4 includes a plurality of receiver coils and bucking coils. Each of the receiver coils and bucking coils are defined as follows: 1. A 6 inch receiver coil and corresponding 6 inch bucking coil 2. A 9 inch receiver coil and corresponding 9 inch bucking coil 3. A 12 inch receiver coil and corresponding 12 inch bucking coil 4. A 15 inch receiver coil and corresponding 15 inch bucking coil 5. A 21 inch receiver coil and corresponding 21 inch bucking coil 6. A 27 inch receiver coil and corresponding 27 inch bucking coil 7. A 39 inch receiver coil and corresponding 39 inch bucking coil 8. A 57 inch receiver coil and corresponding 57 inch bucking coil 9. A 72 inch receiver coil and corresponding 72 inch bucking coil Referring to the above list of receiver coils and bucking coils, it is necessary to define certain nomenclature used in this specification: (1) If the bucking coil (N) consists of the 9 inch bucking coil, the receiver coil (N) consists of the 9 inch receiver coil, but the receiver coil (N-1) consists of the 6 inch receiver coil, the previously mentioned coil in the above list. (2) Similarly, if the bucking coil (N) consists of the 39 inch bucking coil, the receiver coil (N) consists of the 39 inch receiver coil, but the receiver coil (N-1) consists of the 27 inch receiver coil, the previously mentioned coil in the above list. In figure 6, the new folded array 22c includes the transmitter coil 22c1, the 6 inch bucking coil 22c5, the "plurality of cowound receiver coils and bucking coils" 22c2, a temperature sensor 22c6, the individual receiver coils 22c3, and the one further bucking coil 22c4, all of which are disposed on one side only of the transmitter coil 22c1. The advantage of the new folded array 22c relative to the prior art sensor array 12 is as follows: the new folded array 22c of figures 5 and 6 is shorter in length than the sensor array 12 of figure 4. This shortened length of the folded array 22c allows the new induction logging tool 22 of figures 2 and 3 to be placed in wellbores which have a minimum rathole, or in wellbores having severe dog legs, or in horizontal wellbores having a short kickoff radius. In figures 5 and 6, the plurality of cowound receiver coils and bucking coils 22c2 of the new folded array 22c include the following: (1) a 6 inch receiver/9 inch bucking coil 2a - in figures 5 and 6, the 6 inch receiver coil from the prior art sensor array 12 of figure 4 is wound on the same bobbin as the 9 inch bucking coil from the sensor array 12 of figure 4, (2) a 9 inch receiver/12 inch bucking coil 2b - in figures 5 and 6, the 9 inch receiver coil from the prior art sensor array 12 of figure 4 is wound on the same bobbin as the 12 inch bucking coil from the sensor array 12 of figure 4, (3) a 12 inch receiver/15 inch bucking coil 2c - in figures 5 and 6, the 12 inch receiver coil from the prior art sensor array 12 of figure 4 is wound on the same bobbin as the 15 inch bucking coil from the sensor array 12 of figure 4, (4) a 15 inch receiver/21 inch bucking coil 2d - in figures 5 and 6, the 15 inch receiver coil from the prior art sensor array 12 of figure 4 is wound on the same bobbin as the 21 inch bucking coil from the sensor array 12 of figure 4, (5) a 21 inch receiver/27 inch bucking coil 2e - in figures 5 and 6, the 21 inch receiver coil from the prior art sensor array 12 of figure 4 is wound on the same bobbin as the 27 inch bucking coil from the sensor array 12 of figure 4, and (6) a 27 inch receiver/39 inch bucking coil 2f- in figures 5 and 6, the 27 inch receiver coil from the prior art sensor array 12 of figure 4 is wound on the same bobbin as the 39 inch bucking coil from the sensor array 12 of figure 4. From the aforementioned description of the plurality of cowound receiver coils and bucking coils 22c2, it is evident that each of the plurality of cowound receiver coils and bucking coils 22c2 in figures 5 and 6 include a receiver coil (N-1) and a bucking coil (N). However, the bucking coil (N) is associated with another receiver coil (N); it is not associated with the receiver coil (N-1). In addition, the plurality of cowound receiver coils and bucking coils 22c2 are all located on one side of the transmitter 22c1, the transmitter 22c1 itself being located at the end 26 of the array. The bucking coil (N) and the receiver coil (N-1) of each cowound receiver coil and bucking coil 22c2 is wound on the same ceramic bobbin. As a result, the bucking coil (N) and the receiver coil (N-1) are positioned at approximately the same distance from the transmitter coil 22c1. Furthermore, the turns ratio for a particular receiver coil and bucking coil [that is, the number of turns of a particular receiver coil (N) divided by the number of turns of its corresponding bucking coil (N)] must be carefully selected in order to balance or null the direct mutual coupling between the transmitter and receiver array. Referring to figures 7 through 12, a detailed construction of each of the plurality of cowound receiver coils and bucking coils 22c2 is illustrated, these illustrations depicting the manner by which each cowound receiver coil and bucking coil 22c2 are cowound together about the same bobbin and about the same bobbin pin. In figure 7, the bucking coil 32, 34 and the receiver coil 38, 40 associated with one of the plurality of cowound receiver coils and bucking coils 22c2 are cowound about a nominal center 36 of a bobbin 30. As a result, the bucking coil 32, 34 and the receiver coil 38, 40 are located at approximately the same distance "x" from the transmitter coil 22c1. Recall that each of the plurality of cowound receiver coils and bucking coils 22c2 are cowound together about the same ceramic bobbin. Each of the plurality of cowound receiver coils and bucking coils consist of (1) a bucking coil, and (2) a receiver coil. For example, the 6 inch receiver/9 inch bucking coil 2a consists of the 6 inch receiver coil (the receiver coil) and the 9 inch bucking coil (the bucking coil). In figure 7, the bucking coil of a particular cowound receiver coil and bucking coil (one of 2a-2f of 22c2) consists of a first bucking coil 32 (hereinafter called "Buck C1 32") and a second bucking coil 34 (hereinafter called "Buck C2 34"); and the receiver coil of the same particular cowound receiver coil and bucking coil (one of 2a-2f of 22c2) consists of a first receiver coil 38 (hereinafter called "Rec C1 38"), which is interleaved in part with the first bucking coil 32, and a second receiver coil 40 (hereinafter called "Rec C2 40"), which is interleaved in part with the second bucking coil 34. Note from the above description that the first receiver coil 38 is "interleaved in part" with the first bucking coil 32 and that the second receiver coil 40 is "interleaved in part" with the second bucking coil 34. The above reference to "interleaving" will be discussed in greater detail below with reference to figure 10 of the drawings. Therefore, in our example, the cowound receiver coil and bucking coil "the 6 inch receiver/9 inch bucking coil 2a" includes the 6 inch receiver coil and the 9 inch bucking coil, the 6 inch receiver coil being represented by the first receiver coil 38 of figure 7 plus the second receiver coil 40 of figure 7 (Rec C1 38 + Rec C2 40), the 9 inch bucking coil being represented by the first bucking coil 32 of figure 7 plus to the second bucking coil 34 of figure 7 (Buck C1 32 + Buck C2 34). Similarly, the cowound receiver coil and bucking coil "the 27 inch receiver/39 inch bucking coil 2f" includes the 27 inch receiver coil and the 39 inch bucking coil, the 27 inch receiver coil being represented by the first receiver coil 38 of figure 7 plus the second receiver coil 40 of figure 7 (Rec C1 38 + Rec C2 40), the 39 inch bucking coil being represented by the first bucking coil 32 of figure 7 plus the second bucking coil 34 of figure 7 (Buck C1 32 + Buck C2 34). Therefore, each cowound receiver coil and bucking coil (2a-2f) 22c2 of figure 6 consists of a particular receiver coil (N-1) and a particular bucking coil (N), the particular receiver coil (N-1) consisting of (Rec C1 38 + Rec C2 40) of figure 7, the particular bucking coil (N) consisting of (Buck C1 32 + Buck C2 34) of figure 7. In addition, in figure 7, the particular receiver coil (N-1) and the particular bucking coil (N) are both positioned about the nominal center 36, the nominal center 36 being further positioned at a distance "x" from the transmitter coil 22c1. Therefore, the particular receiver coil (N-1) and the particular bucking coil (N) are both located at approximately the same distance "x" from the transmitter coil 22c1. As a result, the receiver coil (N-1) and the bucking coil (N) of each of the plurality of cowound receiver coils and bucking coils (2a-2f of 22c2) are each positioned at approximately the same distance "x" from the transmitter coil 22c1. This concept will be better understood by reading the following detailed description. Figure 7 illustrates the first half of the bucking coil 32 (Buck C1 32) interleaved with the first half of the receiver coil 38 (Rec C1 38), and the second half of the bucking coil 34 (Buck C2 34) interleaved with the second half of the receiver coil 40 (Rec C2 40), each being wound about the nominal center 36 of the bobbin 30. However, figure 7 does not illustrate exactly how such coils 32, 34, 38, 40 are cowound together about the bobbin 30. The following discussion with reference to figures 8 through 12 will provide a description of exactly how such coils 32, 34, 38, and 40 are cowound together about the same ceramic bobbin 30. In figure 8, recalling (from figure 7) that Buck C1 32 interleaved with Rec C1 38 is disposed on one side of the nominal center 36 and is wound about the bobbin 30, and Buck C2 34 interleaved with Rec C2 40 is disposed on the other side of nominal center 36 and is wound about the bobbin 30, figure 8 illustrates the nominal center 36 on the bobbin 30. However, in figure 8, a plurality of bobbin pins 42 are transversely disposed relative to the nominal center 36, the plurality of bobbin pins 42 being held together by an epoxy 44. The receiver coil and bucking coil 32, 34, 38, 40 of figure 7 associated with each of the plurality of cowound receiver coils and bucking coils 2a through 2f of figure 6 are initially connected to one of the bobbin pins 42 of bobbin 30 before the coils are cowound about the same bobbin 30. Figure 9 better illustrates how each cowound receiver coil and bucking coil (32, 34, 38, 40) of figure 7 of the plurality of cowound receiver coils and bucking coils (2a-2f) 22c2 of figure 6 are connected to bobbin pins 42 and cowound about the same bobbin 30. In figure 9, the "Buck C1 32" portion of the bucking coil of figure 7 interleaved with the "Rec C1 38" portion of the receiver coil of figure 7 (associated with one of the cowound receiver coils and bucking coils 22c2) is shown connected to a bobbin pin 42 of bobbin 30. In figure 9, note how the Buck C1 32 bucking coil is interleaved with the Rec C1 38 receiver coil on the one side of the nominal center 36 of the bobbin 30. In figure 9, the Buck C1 32 bucking coil includes an end which is shown to be stripped of its outer insulating layer thereby exposing a wire pin 48. The wire pin 48 has an end portion 50 which is physically wrapped around a bobbin pin 42, one of the bobbin pins 42 shown in figure 8. Further reference to figure 10 will better illustrate the co-winding of each of the receiver coils and bucking coils 32, 38 and 34, 40 about the bobbin 30 when the end portion 50 of each wire pin 48 of either the bucking coil 32, 34 or the receiver coil 38, 40 is wrapped around the bobbin pin 42 in the manner illustrated in figure 9. In figure 10, the first half of the bucking coil 32 (Buck C1 32) is shown interleaved with the first half of the receiver coil 38 (Rec C1 38) on the left side of nominal center 36 and is wound around the bobbin 30 in the winding direction indicated by arrow 52 (upwardly in figure 10). However, note that the bucking coil turns 32 are initially interleaved with the receiver coil turns 38; however, when the bucking coil turns 32 wrap around bobbin 30, in the direction indicated by arrow 52, to the point in figure 10 indicated by numeral 56, the bucking coil turns 32 begin to cut across several successive receiver coil turns 38. This happens because the bucking coil 32 ends, at 56, but the receiver coil 38 continues to wrap around the bobbin 30. As a result, the bucking coil 32 cuts across several successive receiver coil turns 38. However, the second half of the bucking coil 34 (Buck C2 34) is shown interleaved with the second half of the receiver coil 40 (Rec C2 40) on the right side of nominal center 36 and is wound around the bobbin 30 in the winding direction indicated by the arrow 54 (downwardly in figure 10). However, note that the bucking coil turns 34 are initially interleaved with the receiver coil turns 40; however, when the bucking coil turns 34 wrap around bobbin 30, in the direction indicated by arrow 54, to the point in figure 10 indicated by numeral 58, the bucking coil turns 34 begin to cut across several successive receiver coil turns 40. This happens because the bucking coil 34 ends, at 58, but the receiver coil 40 continues to wrap around the bobbin 30. As a result, the bucking coil 34 cuts across several successive receiver coil turns 40. The end portions 50 of each bucking coil (N) 32, 34 and each receiver coil (N-1) 38, 40 of figure 7 of each of the plurality of cowound bucking coils and receiver coils 2a-2f of 22c2 of figure 6 are wrapped around the following bobbin pins 42: (1) the end portion 50 of the first half of the bucking coil 32 (Buck C1 32) of figure 7 is initially wrapped around the bobbin pin No. 2 of figure 10 and the bucking coil 32 is wrapped around the bobbin 30 in the direction of arrow 52 (upwardly in figure 10); (2) the end portion 50 of the first half of the receiver coil 38 (Rec C1 38) of figure 7 is initially wrapped around bobbin pin number 4 of figure 10 and the receiver coil 38 is wrapped around bobbin 30 in the direction of arrow 52 (upwardly in figure 10); (3) the end portion 50 of the second half of the bucking coil 34 (Buck C2 34) of figure 7 is initially wrapped around bobbin pin no. 2 of figure 10 and the bucking coil 34 is wrapped around bobbin 30 in the direction of arrow 54 (downwardly in figure 10), and (4) the end portion 50 of the second half of the receiver coil 40 (Rec C2 40) of figure 7 is initially wrapped around bobbin pin number 7 of figure 10 and the receiver coil 40 is wrapped around bobbin 30 in the direction of arrow 54 (downwardly in figure 10). In figure 11, the bucking coil portion 32 and 34 of figure 10, which cuts across several successive receiver coils 38 and 40 of figure 10, is illustrated in greater detail. Since the bucking coil portion 32 and 34 ends abruptly, at 56 and 58, but the receiver coils 38, 40 continue to wrap around the bobbin 30, the bucking coil portion 32 and 34 must cut across the successively wrapped receiver coils 38 and 40, the end of the bucking coil portion 32, 34 being soldered to a bobbin pin 42, at 60. In figure 12, recall that each of the plurality of cowound bucking coils and receiver coils 2a-2f 22c2 of figure 6 includes a bucking coil (N) and a receiver coil (N-1), where the bucking coil (N) further includes a first bucking coil 32 (Buck C1 32) and a second bucking coil 34 (Buck C2 34), and the receiver coil (N-1) further includes a first receiver coil 38 (Rec C1 38) and a second receiver coil 40 (Rec C1 40). However, the first bucking coil 32 is interleaved in part with the first receiver coil 38 when the coils 32, 38 are wrapped around a bobbin 30 in one direction (arrow 52), and the second bucking coil 34 is interleaved in part with the second receiver coil 40 when the coils 34, 40 are wrapped around the same bobbin 30 in a direction opposite to the one direction (arrow 54). Therefore, when the first bucking coil 32 ("Buck C1 32") and the first receiver coil 38 ("Rec C1 38") are wound in an interleaved fashion on one side of the nominal center 36 in the manner shown in figures 7 and 10, and when the second bucking coil 34 ("Buck C2 34") and the second receiver coil 40 ("Rec C2 40") are wound in an interleaved fashion on the other side of the nominal center 36 in the manner shown in figures 7 and 10, the result is shown in figure 12. The invention being thus described, it will be obvious that there an many variations which are not to be regarded as a departure from the scope of the following claims.

Claims: A logging tool (22) adapted to be disposed in a borehole, comprising an array section (22c), said array section including a transmitter coil (22c1) and a plurality of cowound receiver coils and bucking coils (22c2) disposed on one side of said transmitter coil, each of said plurality of cowound receiver coils and bucking coils including a bobbin (30), and a receiver coil "N-1" and a bucking coil "N" cowound together on said bobbin characterized in that said bucking coil "N" is associated with another receiver coil "N" and not with said receiver coil "N-1". The logging tool of claim 1, wherein said array section (22c) further comprises a bucking coil (22c5) disposed on said one side of said transmitter coil (22c1) and an individual receiver coil (22c3) disposed on said one side of said transmitter coil; and one further bucking coil (22c4) disposed on said one side of said transmitter coil. , The logging tool of claim 1, wherein a direct mutual coupling exists between said transmitter coil (22c1) and the receivers of said plurality of cowound bucking coils and receiver coils (22c2), said bucking coil "N" of each of said plurality of cowound bucking coils and receiver coils including a number of turns, said receiver coil "N" further including a number of turns, the ratio between the number of turns of said receiver coil "N" and the number of turns of said bucking coil "N" being selected such that said direct mutual coupling between said transmitter coil and the receivers of said plurality of cowound bucking coils and receiver coils is balanced. The logging tool of claim 3, wherein said bucking coil "N" and said receiver coil "N-1" of each said plurality of cowound receiver coils and bucking coils (22c2) are located at approximately the same distance "x" from the transmitter coil (22c1). A logging sonde (22) adapted to be disposed in a wellbore, comprising an array section (22c) having an end, said array section including a transmitter coil (22c1) located at said end of said array section, a plurality of cowound bucking coils and receiver coils (22c2) located adjacent to one side of said transmitter coil within said array section, characterized in that each of said plurality of cowound bucking coils and receiver coils includes a bobin (30) and a receiver coil "N-1" and a bucking coil "N" cowound together about said bobbin, said bucking coil "N" being associated with another receiver coil "N". The logging sonde (22) of claim 5, wherein a direct mutual coupling exists between said transmitter coil (22c1) and the receivers of said plurality of cowound bucking coils and receiver coils (22c2), said bucking coil "N" of each of said plurality of cowound bucking coils and receiver coils including a number of turns, said receiver coil "N" further including a number of turns, the ratio between the number of turns of said receiver coil "N" and the number of turns of said bucking coil "N" being such that said direct mutual coupling between said transmitter coil and plurality of receiver coils and bucking coils is nulled. The logging sonde (22) of claim 6, wherein said receiver coil "N-1" is wound in a first direction about said bobbin (30), and said bucking coil "N" is wound in a second direction about said bobbin, said second direction being opposite to said first direction. The logging sonde (22) of claim 7, wherein said bucking coil "N" includes a first number of turns about said bobbin (30), said receiver coil "N-1" including a second number of turns about said bobbin, said first number of turns of said bucking coil "N" being less than the second number of turns of said receiver coil "N-1". A method for electromagnetically exploring an earth formation traversed by a deviated borehole, comprising: a) lowering an induction logging tool (22) into the borehole, the tool comprising an array section (22c), said array section including a transmitter coil (22c1) and a plurality of cowound receiver coils and bucking coils (22c2) disposed on one side of said transmitter coil, each of said plurality of cowound receiver coils and bucking coils including a bobbin (30), and a receiver coil "N-1" and a bucking coil "N" cowound together on said bobbin whereby said bucking coil "N" is associated with another receiver coil "N", and b) applying a source of electrical energy to the transmitter coil; and c) producing a plurality of receiver signals thereby producing an output signal representative of a characteristic of the formation.

Inventors: LONG TIMOTHY, ORBAN ANDRE, LONG, TIMOTHY, ORBAN, ANDRE
Applicants: SCHLUMBERGER HOLDINGS, SCHLUMBERGER LTD A NETHERLAND, SCHLUMBERGER SERVICES PETROL, SCHLUMBERGER TECHNOLOGY BV, SCHLUMBERGER HOLDINGS LIMITED, SCHLUMBERGER LIMITED (A NETHERLAND ANTILLES CORP.), SCHLUMBERGER TECHNOLOGY B.V., SERVICES PETROLIERS SCHLUMBERGER
Application year: 1996
Main classification: G01V3
IPC codes: G01V3