System and method for determining axial magnetic disturbance in a downhole directional sensor

Abstract

Systems and methods for achieving measurements of the geomagnetic field and axial magnetic disturbance field using a directional sensor at a survey point are described. Typically, the directional sensor is used to make axial magnetic field measurements at three or more separate locations along the sensor tool axis within the directional sensor. A computing system receives the axial magnetic field measurements and solves a set of simultaneous equations to obtain the axial component of the geomagnetic field.

Description

Background Art

[0001] In drilling, a measurement while drilling (MWD) directional sensor can be used to determine the orientation of the well. Directional measurements are also called surveys in the industry. Generally speaking, the MWD directional sensor is packaged in a section of the drill string near the drill bit. The sensor section is part of the bottom hole assembly (BHA), where various sensors and mechanical devices are located. The drill bit is at the bottom end of the BHA.

[0002] During drilling, surveys are performed periodically. Typically, the MWD sensor section and drill string must be stationary during the survey operation. Therefore, surveys are typically performed when a section of drill pipe is added or removed from the drill string. During the pipe change process, the drill string is stationary. The results of each survey are transmitted from downhole to the surface via a telemetry system. The orientation of the well and the change in drill string length at each survey are used to calculate the trajectory of the well section at or near the survey point.

[0003] The MWD orientation sensor can be composed of an accelerometer and a magnetometer. The magnetometer in the orientation sensor can be used to measure the earth's magnetic field (i.e., the geomagnetic field) vector The three components along the orthogonal axes (x, y, z) of the orientation sensor coordinate system are (Bx, By, Bz). When the orientation sensor is at rest, the accelerometer in the orientation sensor can be used to measure the gravity vector The measured components of gravity are (Gx, Gy, Gz).

[0004] The orientation of the two vectors in the directional sensor coordinate system can be used to uniquely determine the orientation of the sensor coordinate system relative to gravity vertical and magnetic north. Because the sensor is securely mounted on the drill string, once the orientation of the directional sensor is known, the orientation of the drill string and well can be known, and vice versa.

[0005] The direction in which the axis of the drill string points downward is usually defined as the z-axis of the directional sensor coordinate system. This is the axial direction. The x-axis and y-axis are the transverse axes. In most directional sensors, the hardware axes of the magnetometer and accelerometer are nominally aligned with the directional sensor coordinate system. However, there may be situations where, for example due to packaging limitations, the hardware axes of the component sensors are not substantially aligned with the hardware axes of the directional sensor coordinate system. In addition, the hardware axes of the magnetometer may not be nominally aligned with the hardware axes of the accelerometer. Measurements along the directional sensor axis are obtained by vector projection and coordinate transformation. The component of a vector in one direction is also a vector. Therefore, the axial component of the geomagnetic field can also be called the axial geomagnetic field.

[0006] Well inclination is the angle between the well and the vertical defined by the direction of the gravity vector. Thus, well inclination can be a measure of the degree to which the well deviates from the vertical. Accelerometer measurements can be used to calculate this inclination. The well azimuth relative to the North component of the geomagnetic field vector is defined as the angle between magnetic north and the projection of the well axis on the horizontal plane, as Figure 1 The angle shown in The horizontal plane is defined as the plane perpendicular to gravity.

[0007] The measurements from the accelerometer and magnetometer can be used to calculate the azimuth angle. There are many formulas on how to calculate the azimuth angle. These formulas are essentially equivalent to each other. One expression for the azimuth angle is:

[0008]

[0009] Where ATAN2[] is the inverse tangent function of two parameters, × is the vector cross product symbol, the subscript z represents the z-axis component of the vector, and G is The size of is the horizontal component of the geomagnetic field vector given by:

[0010]

[0011] where · is the vector dot product symbol.

[0012] In directional surveying, the measured gravity and magnetic vectors are used in equations (1) and (2) to produce the azimuth. is the error vector in the geomagnetic field measurement. ΔB is The error vector may come from two sources. The first error source may be an inaccurate measurement of the magnetic field at the sensor location. Another error source is that the magnetic field at the sensor location is not purely geomagnetic. There may be a magnetic field near the directional sensor generated by a magnetic source in the drill string system.

[0013] The measured magnetic field is:

[0014]

[0015] In practice, Used to replace EQ(2) if Along the vertical direction, that is, along or with On the contrary, and In this way, the azimuth angle is not affected by the vertical component of the error vector.

[0016] Azimuth error results from the perceived magnetic north vector from the sensor measurement being pointed away from true magnetic north in the horizontal plane. Figure 2 A horizontal view of the azimuth error is shown in . The horizontal component of the magnetic field measurement error is Figure 2 is shown as ΔB h This error component results in the horizontal geomagnetic vector Relative to true magnetic north vector Rotation therefore, is determined by the measurement results and is With ΔB h The vector sum of .

[0017] The azimuth error varies with the direction of the error vector. N ,ΔB E ,ΔB V ) are respectively Along the geomagnetic north axis, geomagnetic east axis and gravity vertical axis. The azimuth error caused It is given by the following formula:

[0018]

[0019] Among them B N yes The size of . This expression can be simplified to:

[0020] For |ΔB N | N ,

[0021] Vertical component ΔB V It does not work. It can be shown that when measuring the Earth's magnetic field, the maximum azimuth error due to the magnitude of the error vector ΔB (i.e., the maximum azimuth error) is:

[0022] For ΔB N ,

[0023] The maximum value is when:

[0024] And ΔB V =0 (EQ.7)

[0025] For most BHA and drill string combinations, ΔB is greater than B N Much smaller. Therefore:

[0026] For ΔB<<B N , ​​

[0027] If ΔB<<B N is true, then when the error vector is in the horizontal and east / west (HEW) directions ( And ΔB V =0), the maximum azimuth error occurs. In many cases, ΔB is indeed greater than B N is much smaller, making EQ.8 useful for estimating the maximum possible azimuth error. However, in these cases, the azimuth error can still be very large compared to the specification of the directional sensor. ΔB is considered negligible only if the resulting azimuth error is smaller than the error specification for the azimuth.

[0028] Most sections of the drill string and many sections and components of the BHA are made of iron, steel and other ferromagnetic metals. Unknown magnetization in these metals creates a magnetic field near the directional sensor section. This magnetic field is superimposed with the earth's magnetic field. For directional sensing, this additional field is deviation noise. The directional information is in the earth's magnetic field. The magnetic sensor system can correctly measure the magnetic field at the sensor location, but the magnetic field itself is corrupted by this additional field. The additional field generated by the magnetization of the ferromagnetic metal near the directional sensor interferes with the measurement of the earth's magnetic field. In order to correctly determine the orientation of the directional sensor, this interference needs to be reduced to a negligible level and / or corrected.

[0029] The directional sensor section of the BHA is made of non-magnetic material. The sections axially above and below the sensor section can be non-magnetic (i.e., "above" means away from the drill bit, and "below" means toward the drill bit, as if the BHA were vertical with the drill bit at the lower end). The field generated by the magnetization in the drill string and BHA decreases as the distance between the directional sensor and the magnetization source increases. If the non-magnetic section is long enough, the magnetic interference at the sensor location is negligible, and the measured magnetic field is the Earth's magnetic field.

[0030] Due to various constraints, in some drilling operations, it is not practical to have long non-magnetic sections above and below the sensor location. In these cases, magnetic interference may be unavoidable. The total magnetic field at the sensor location is the vector sum of the interference field and the earth's magnetic field. The interference can be determined and subtracted from the sensor measurement so that the earth's magnetic field can be correctly measured.

[0031] The method of correcting for the disturbances due to the short non-magnetic section is known in the drilling industry as the short collar correction method or the short collar algorithm. The conventional azimuth obtained from the directional sensor output without any magnetic disturbance correction is sometimes referred to as the long collar azimuth.

[0032] Due to the limited transverse dimension of the drill string and BHA, the magnetization in the drill string and BHA is mostly along the axial direction. The axial magnetization above the non-magnetic section can be approximated as a long magnetic dipole along the axis of the drill string. The magnetization in the transverse direction can also be regarded as a magnetic dipole along the transverse direction. Even if the magnetic pole strengths along the axial direction and along the transverse direction are similar, the effective transverse dipole moment is much smaller than the axial dipole moment. The interference field is usually determined by the axial or longitudinal magnetization. The interference field from the section below the non-magnetic section can have similar characteristics.

[0033] The directional sensor is axially located at a distance from the axial dipole. The interfering magnetic field is along the axial direction. Therefore, almost all short drill collar corrections are about correcting for magnetic interference when measuring the magnetic component along the z-axis.

[0034] The dominant term in the magnetic field generated by the magnetization near the oriented sensor is along the z-axis. The relevant error vector in the Earth's magnetic field measurement is By item Leading, among them is the unit vector of the sensor z-axis, ΔB z yes If the true inclination and azimuth of the orientation sensor are θ and but:

[0035]

[0036] in, are the unit vectors of geomagnetic north, geomagnetic east and vertical respectively.

[0037] because The azimuth error caused by the term is given by EQ.5 as:

[0038]

[0039] When |ΔB z |<<B N When:

[0040]

[0041] Therefore, at θ = 90 degrees and In the case of 270 degrees or 270 degrees, axial magnetic interference may cause the largest azimuth error in the well section close to the HEW direction. Therefore, the need for effective short drill collar correction is the strongest when the well is at or close to the HEW direction.

[0042] In the short drill collar correction process, information from a source independent of the directional sensor can be used to determine the z-axis component of the geomagnetic field. In a short drill collar algorithm, the magnitude of the corrected measured geomagnetic field is required to be equal to the value obtained from a source other than the directional sensor, that is:

[0043]

[0044] Among them, B ref It is the reference value of the magnitude of the geomagnetic field when the well site is measured by the directional sensor. (B x ,B y ) is the measured magnetic field component along the sensor (x, y) direction, B z is the best estimate of the z-component of the Earth's magnetic field. This is the Full Field Matched Short Drill Collar (TFMSC) method. The reference value is obtained from an independent source that is not related to the directional sensor measurements. In TFMSC, the reference value is used as the reference to B z That is, B satisfies EQ.12. z is considered to be the correct value of the z-axis component of the geomagnetic field. The difference between the z-axis magnetometer output and the solution is the interference term. There are two solutions to EQ.12. Some criteria can then be used to select one solution over another as the correct solution. For example, for a single survey point, when no other information is available, the solution that results in a smaller interference term can be selected. For multiple survey points along a well section, if the interference term is expected to be constant over the well section, the solution that produces a common interference term can be selected.

[0045] In the method described in patent document in the name of van Dongen et al. (U.S. Pat. No. 4,682,421, hereinafter referred to as "van Dongen"), reference values ​​for the magnitude and inclination of the Earth's magnetic field are used to determine B z The Bz that minimizes the function E defined in vanDongen is considered to be the correct B of the Earth's magnetic field. z In van Dongen, is the geomagnetic vector measured by the directional sensor after correction, Given by the reference source. These values ​​are assumed to be in the same plane. E is the difference vector The van Dongen method can be labeled as the minimum vector difference short drill collar (MVDSC) algorithm.

[0046] In another method, select B z , so that the selected B z Add the measured (G x ,G y ,G z ,B x ,By ) Calculated B N Equal to B N The method can be called the horizontal component matched short drill collar (HCMSC) algorithm. It can be shown that in MVDSC or HVMSC, there will be multiple solutions. Some criteria are used to select a solution as the correct solution.

[0047] TFMSC, MVDSC and HCMSC are all reference-based correction methods. Select the axial magnetic field component B z , so that when it is combined with other measured field components (G x ,G y ,G z ,B x ,B y ) to calculate a property of the Earth's magnetic field, the result matches the reference value exactly or as closely as possible. z When there are multiple solutions of z can be used to select one as a solution. The solution of the modified method is the same as the measured B z The difference between them is the interference term ΔB z If the drill string and BHA sections near the directional sensor do not pass through a strong magnetic source, the interference term may not change. In this case, for the next few directional surveys, the interference term can be subtracted directly from the output of the axial magnetic sensor to obtain a corrected B z , which is used to calculate the orientation parameters.

[0048] In the reference-based correction method, the reference value and the transverse-axis magnetometer measurement value B x and B y Assumed to be accurate. In reality, neither the reference nor the horizontal axis measurement can be perfect.

[0049] If the transverse-axis magnetic force measurements are error-free, then according to EQ.12, we have:

[0050]

[0051] Where, dB ref is the error of the reference value of the magnitude of the Earth's magnetic field, dB z It is in B z The solution to EQ.12 gives B produced by the short drill collar correction z of error.

[0052] In wells close to the HEW direction, the true geomagnetic field component B z Almost zero. dB z Can be compared with the interference term ΔB z That is, using the corrected B zThe azimuth error of can be much larger than the error of the interference term. In the well section exactly in the HEW direction, the true axial component of the geomagnetic field is 0. Therefore, EQ.13 cannot be used. B from TFMSC z The error can be calculated directly by EQ.12. If the magnitude of the Earth's magnetic field is 50000nT (nano Tesler) and dB ref is 50010nT, then EQ.12 gives B z = ±1000nT. The reference value deviates by 10nT (much less than the accuracy of any global geomagnetic model), and TFMSC causes B z If the interference term ΔB Z If it is less than 1000nT, the short drill collar correction will make the azimuth error larger.

[0053] As shown in EQ. 11, axial disturbances cause the largest errors when the well is in the HEW orientation, but at or near this well attitude, the effectiveness of TFMSC may be minimal due to inaccurate reference values.

[0054] In the case of perfect reference values, the inaccuracy of transverse axis measurement will also lead to very large errors in the axial geomagnetic component determined by short drill collar correction. When the well attitude is close to the HEW direction, the actual axial component of the geomagnetic field is almost zero. The magnitude of the geomagnetic field in the transverse axis plane:

[0055]

[0056] is almost equal to the total size. That is, The error of the axial component determined by TFMSC using a perfect reference value is given by:

[0057]

[0058] Where, dB xy It is B xy When B z Almost zero and B xy When the total magnitude of the Earth's magnetic field is almost z The error of B can be many times greater than that of the transverse axis magnetometer. For example, if TFMSC is used, then xy The 10nT error in the 50000nT geomagnetic field will cause B z The 10nT error in the transverse direction is converted and amplified into a 1000nT error in the axial direction through the short drill collar correction.

[0059] In general, the use of TFMSC zThe error is given by:

[0060]

[0061] Since the reference value is obtained from a source unrelated to the operation of the directional sensor, dB xy and dB ref are irrelevant. Therefore:

[0062]

[0063] The overline indicates the statistical mean.

[0064] It can be seen that other reference-based short-collar correction methods suffer from inaccurate reference values ​​and / or transverse axis measurements very similar to TFMSC. It is well known in the industry that when the need for axial disturbance correction is most urgent, all known reference-based short-collar corrections tend to produce larger errors in the axial geomagnetic component than the disturbance term to be eliminated. Short-collar corrections are generally not used when the well attitude is close to the HEW direction.

[0065] There are correction methods that explicitly do not use a reference source. These methods are a subclass of multi-station analysis methods. As mentioned earlier, the disturbance is along the z-axis. It can be a constant for multiple surveys of a BHA and drill string combination in a well section. Therefore, the disturbance can be regarded as a z-axis magnetometer offset error in the geomagnetic field measurement.

[0066] If the well section is very curved, the interference term may be determined by requiring the measurement from which the interference is subtracted to produce one or more constant magnitudes of the geomagnetic field with minimal differences over multiple surveys. Determining the interference term is an optimization process. This correction method can be labeled a constant total field (CTF) algorithm.

[0067] In CTF, the correction procedure is the calibration process. In the calibration process, the direct sensor output is processed to produce a measurement to be compared with some reference value. The sensor calibration parameters used to convert the raw sensor output to the sensor measurement are adjusted so that the sensor measurement matches the reference value. In CTF, the z-axis magnetic offset caused by the axial magnetic disturbance is the calibration parameter to be determined. The constant magnitude of the Earth's magnetic field is the reference value to be matched.

[0068] Even though no reference value of the geomagnetic field is explicitly required, in CTF a constant magnitude is imposed. In fact, the "constant magnitude" is information about the geomagnetic field properties that comes from a source unrelated to the operation of the directional sensor. Therefore, CTF is also a reference-based short drill collar correction. It is affected by the problems caused by inaccuracies in the reference values ​​and / or transverse axis magnetometer measurements discussed earlier. For example, the constraint of the "constant magnitude" may be wrong. During multiple surveys used in CTF, the geomagnetic field at a specific location may change by tens or hundreds of nT in a very short time.

[0069] When the implicit reference and transverse axis magnetic measurements are accurate, methods such as CTF can be effective if multiple surveys are conducted in the curved section of the well. Some of the surveys in this set of surveys are conducted at well attitudes away from the HEW direction. In these attitudes, the magnitude of the magnetic field is sensitive to axial offsets caused by interference. The interference term can be accurately determined by the constant magnitude requirement of the survey data in these attitudes. For all survey data, including those of the HEW (if any), the correct interference term is removed. However, requiring survey data on the curved section of the well limits the locations where CTF can be applied.

[0070] Many horizontal wells are being drilled today. A horizontal well usually starts out vertically close to the surface. Once the desired depth is reached, the well quickly turns horizontal. Sometimes the vertical section of a horizontal well is shared by multiple horizontal wells. In a typical horizontal well, the horizontal section of the well can be very long. Although each horizontal well turns from vertical to horizontal, the BHA used for the later part of the horizontal section of the well may not be the same BHA used to drill the curved section. Therefore, multiple surveys for multi-station analysis may not be performed on the curved section of the horizontal well.

[0071] Over the horizontal section of a horizontal well, the well attitude does not change much. Although small changes in well attitude are important to the production potential of the well, these changes are geometrically small. Over a set of multiple surveys, the measured magnitude of the geomagnetic field is nearly constant regardless of the z-axis offset used to calculate the magnitude. Therefore, "constant magnitude" does not help determine the z-axis offset. Therefore, axial disturbances cannot be determined and / or eliminated.

[0072] Any multi-station analysis method uses some implicit reference field properties. These properties are used as constraints on the multi-station survey dataset.

[0073] In addition to the problem that inaccurate reference and transverse axis magnetometer measurements may result in interference corrections being more erroneous than the interference, multistation analysis methods may be limited by the requirement that the multistation survey be conducted over a very curved section of the well. This condition is often not met. Summary of the invention

[0074] The z-component of the magnetic field is measured at several locations axially spaced in the sensor package for a directional survey point. The axially varying disturbance field and the constant z-component of the Earth's magnetic field are determined directly from the sensor output. No reference field value is used. The Earth component is then used in the magnetic azimuth calculation.

[0075] Since the axial magnetic disturbance is approximated by the magnetic field generated by a magnetic monopole positioned at a certain distance from the directional sensor, multiple measurements of the field component along the z-axis are made at several known axial positions and used to determine the pole strength, pole position and component of the earth field. Measurements are required at at least three positions. BRIEF DESCRIPTION OF THE DRAWINGS

[0076] These and other objects and features of the present invention will be more fully disclosed or become apparent from the following detailed description of the present invention, which should be read in conjunction with the accompanying Figure 1 In the drawings, like numerals refer to like parts, and in the drawings:

[0077] Figure 1 is a diagram of various angles described in the present disclosure, including well orientation The sensor z-axis points downward to the well axis, and OP represents the projection of the z-axis on the horizontal plane.

[0078] Figure 2 is a diagram of the horizontal plane view of the azimuth error. In this diagram, OP is the horizontal plane projection of the z-axis, is the true magnetic north vector, is the magnetic north vector from the sensor measurement using the horizontal magnetic field error The resulting magnetic north vector.

[0079] Figure 3 is a schematic diagram of an exemplary directional sensor system having three axial magnetometers according to the present disclosure.

[0080] Figure 4A is a side view of another exemplary orientation sensor system having a single axial magnetometer according to the present disclosure.

[0081] Figure 4B It is along Figure 4A 4B-4B in FIG. 4 is a cross-sectional view of a magnetometer assembly constructed according to the present disclosure.

[0082] Figure 5 is a flow chart of an exemplary method of providing a survey using monopole axial interference removal (SPAIR) in accordance with the present disclosure. DETAILED DESCRIPTION

[0083] Examples of the present disclosure are directed to a directional sensing system that includes at least three axial magnetometers for directly measuring and / or eliminating axial magnetic interference with measuring the earth's magnetic field. The at least three axial magnetometers can be arranged at different positions around the axis of the directional sensing system. The axial interference from the nearest magnetic pole on the drill string near the directional sensing system and the earth's magnetic field can be determined simultaneously. The measured earth's magnetic field can be free of axial magnetic interference.

[0084] In some examples, the directional sensing system can be used to measure the geomagnetic field and the axial magnetic disturbance field. Generally speaking, the directional sensing system can obtain axial magnetic field measurements at three or more separate locations along the axis of the directional sensing system within the directional sensing system. A set of first equations can be solved simultaneously to obtain the axial component of the geomagnetic field. In addition, in some embodiments, a set of second equations can be solved simultaneously to obtain magnetic pole strength parameters and pole positions. The gravity vector and the transverse axis magnetic field can be additionally measured.

[0085] In some embodiments, the directional sensing system may be used to measure the geomagnetic field at several survey points along a well section.

[0086] Before explaining in detail at least one embodiment of the presently disclosed and claimed inventive concepts, it should be understood that the presently disclosed and claimed inventive concepts are not limited in their application to the details of the structures, experiments, exemplary data and / or arrangements of the components set forth in the following description or illustrated in the accompanying drawings. The presently disclosed and claimed inventive concepts are capable of other embodiments or can be implemented or performed in various ways. In addition, it should be understood that the words and terms used herein are for descriptive purposes and should not be considered limiting.

[0087] In the following detailed description of the embodiments of the inventive concept, many specific details are listed to provide a more thorough understanding of the inventive concept. However, it is obvious to those skilled in the art that the inventive concept of the present disclosure can be implemented without these specific details. In other cases, certain well-known features may not be described in detail to avoid unnecessarily complicating the instant disclosure.

[0088] As used herein, the terms "comprises," "including," "comprising," "containing," "having," "having," or any other variation thereof will cover non-exclusive inclusions. For example, unless otherwise indicated, a process, method, article, or apparatus that includes a list of elements is not necessarily limited to only those elements but may also include other elements not specifically listed or inherent to such process, method, article, or apparatus.

[0089] Unless expressly stated otherwise, "or" refers to an inclusive and non-exclusive "or". For example, condition A or B is satisfied by either: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists).

[0090] As used herein, the term "and combinations thereof" refers to all permutations or combinations of the items preceding the term. For example, "A, B, C, and combinations thereof" is intended to include at least one of: A, B, C, AB, AC, BC, or ABC, and if order is important in a particular case, BA, CA, CB, CBA, BCA, ACB, BAC, or CAB. Continuing with this example, combinations containing repetitions of one or more items or terms, such as BB, AAA, AAB, BBC, AAABCCCC, CBBAAA, CABABB, etc., are expressly included. One of ordinary skill in the art will understand that, unless otherwise apparent from the context, there is generally no limit to the number of items or terms in any combination.

[0091] The use of "a" or "an" is used to describe elements and components of the embodiments herein. This is done merely for reasons of convenience and to give a general meaning to the content of the invention. This description should be understood to include one or more and the singular also includes the plural unless it is obvious that something else is meant.

[0092] The use of the terms "at least one" and "one or more" will be understood to include one and any number greater than one, including but not limited to each of 2, 3, 4, 5, 10, 15, 20, 30, 40, 50, 100, and all integers and fractions therebetween (if applicable). The terms "at least one" and "one or more" may extend to 100 or 1000 or more, depending on the terms to which they are connected; furthermore, the number of 100 / 1000 should not be considered limiting, as higher limits may also produce satisfactory results.

[0093] Furthermore, as used herein, any reference to "one embodiment" or "an embodiment" means that a particular element, feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. For example, the phrase "in one embodiment" appearing in different places in the specification does not necessarily refer to the same embodiment.

[0094] A "computing system" or "computer" as used herein may include a microprocessor, or an FPGA, or an electronic device, or a desktop PC.

[0095] As used herein, modifiers such as "approximately," "approximately," and "substantially" are intended to indicate that the modified item is not limited to the exact value specified, but includes some slight variations or deviations, such as those caused by measurement errors, manufacturing tolerances, stresses exerted on various components, wear, and combinations thereof.

[0096] Certain exemplary embodiments of the present invention will now be described with reference to the accompanying drawings. Figure 3 , Figure 4A and Figure 4B As shown, generally, such embodiments relate to a drill string system 10, and more particularly to systems and methods for directly measuring and / or eliminating axial magnetic interference with measuring the earth's magnetic field. The drill string system 10 generally includes a directional sensor system 12 having three axial magnetometers 14, 16, and 18. The directional sensor system 12 includes a z-axis.

[0097] like Figure 3 As shown, each axial magnetometer 14, 16 and 18 can be positioned on the z-axis of the orientation sensor 12. For example, the axial magnetometer 14 is positioned at position L1 on the z-axis of the orientation sensor system 12. The axial magnetometer 16 is positioned at position L2 on the z-axis of the orientation sensor system 12. The axial magnetometer 18 is positioned at position L3 on the z-axis of the orientation sensor system 12. It should be noted that the orientation sensor system 12 may include additional components, including but not limited to accelerometer 15, transverse axis magnetometer 17, data acquisition system 19. In addition, the orientation sensor system 12 may also include a bus, interconnection and / or similar components for establishing communication between the axial magnetometers 14, 16, 18, accelerometer 15, transverse axis magnetometer 17 and (one or more) data acquisition system 19. (One or more) data acquisition system 19 receives sensor data from the axial magnetometers 14, 16, 18, accelerometer 15, transverse axis magnetometer 17, and provides these sensor data to one or more computer systems 20. Additionally, the accelerometer 15 or the transverse-axis magnetometer 17 may be located at or near one of the three axial magnetometers 14, 16, and 18. To simplify the description, the location and use of the axial magnetometers 14, 16, and 18 within the orientation sensor system 12 will be described in further detail herein.

[0098] In addition, the drill string system 10 may further include one or more computer systems 20 that can embody and / or execute the logic of the processes described herein. The logic embodied in the form of software instructions and / or firmware can be executed on any suitable hardware. For example, the logic embodied in the form of software instructions and / or firmware can be executed on one or more dedicated systems, distributed processing computer systems, and / or similar systems. In some embodiments, the logic can be implemented in a stand-alone environment running on a single system, and / or the logic can be implemented in a network environment, such as a distributed system using multiple computers and / or processors. The (one or more) computer systems 20 can work together or independently, using one or more memories 22 (e.g., non-transitory memory) to execute processor executable code. In some embodiments, the directional sensor system 12 can have a housing 23, and the (one or more) computer systems 20 can be located within the housing 23 of the directional sensor system 12.

[0099] Axial magnetic interference can be caused by magnetization in various sections of the drill string system 10 (for example, near the directional sensor system 12). In conventional directional sensors, the position of a single z-axis magnetometer is fixed. The interference term can be regarded as an offset error in the geomagnetic field measurement. However, the interference field is not a constant. The z-axis component of the magnetic field generated by these magnetizations varies in value due to the distance between the magnetization and the sensing z-axis magnetometer. Most of these magnetizations are axial dipoles. In many cases, only one long dipole is dominant. This dipole may come from a section above or below the directional sensor. The magnetic field from the long axis dipole can be calculated as the total magnetic field generated by two monopoles. These two monopoles are equal in strength but opposite in sign. They are located on the axis of the well section. At the z position on the well axis, the axial magnetic field ΔB generated by the long dipole z (z) is:

[0100]

[0101] where q is proportional to the strength of the monopole, z n and z f are the axial positions of the proximal and distal poles, respectively.

[0102] The magnitude of the magnetic field of a point dipole is 1 / r 3 where r is the distance away from the dipole. If the size of the magnetization is much smaller than the distance between the observation point and the magnetization, the magnetization can be considered a point dipole. However, in a short-collar BHA system, the magnetization cannot be considered a point dipole. The axial size of the magnetic source can be, and often is, larger than the distance between the directional sensor and the nearest pole. Therefore, |zz n |<|z n -z f|. EQ.18 is the formula for calculating the axial magnetic field produced by a long-axis dipole.

[0103] For a long dipole, EQ.18 shows that the magnitude of the interference field is a quadratic function of the inverse of the distance between the pole and the observation point. n This could be at the point where the non-magnetic section of the sensor ends and the ferromagnetic section of the BHA / drill string begins. If the dominant long dipole comes from below the sensor section, then z f At or near the bottom of the BHA, n If the long dipole is above the directional sensor, then z f Maybe better than z n Hundreds of feet high. In most drilling environments, if |zz n | Greater than 20 feet, then ΔB z (z) can be neglected. Therefore, in the case where the interference cannot be neglected, the second term on the right hand side of EQ.18 is much smaller than the first term. The field from the far pole can be neglected, and therefore:

[0104]

[0105] Due to the limited diameter of the well, the directional sensor system 12 may be much longer in the axial direction than in the transverse direction. Generally speaking, for example, in a sonar-type directional sensor, the diameter of the directional module (OM) may be about 1 to 2 inches, and the length of the sensor section is about 1 to 5 feet. For a drill collar-based directional sensor, the sensor system can be set on the cutout of a short sub (sub), and the diameter of the short sub (sub) is the diameter of the sensor section.

[0106] The axial distance between the directional sensor system 12 and the magnetized portion on the drill string system 10 may also be much greater than the diameter of the drill string system 10 and / or the BHA. Therefore, the sensor measurements may be considered to be made on the axis of the drill string system 10 and / or the BHA. In some embodiments, the curvature of the well and the misalignment between the BHA and the well may also be ignored.

[0107] refer to Figure 3 , the axial magnetic fields at three positions z1, z2 and z3 within the housing 23 of the directional sensor system 12 can be measured by at least three separate axial magnetometers 14, 16 and 18 for one survey. The housing 23 can be made of a material that does not interfere with the magnetic field received by the axial magnetometers 14, 16 and 18. For example, the housing 23 can be made of a non-ferrous material, such as copper or a copper alloy.

[0108] Over the axial dimensions of each of the magnetometers 14, 16, and 18, the geomagnetic field is constant. The axial fields at each of the positions z1, z2, and z3 are the sum of the Earth field and the interfering field. The three measurements can be given by the following equations:

[0109]

[0110]

[0111]

[0112] where B z is the axial component of the geomagnetic field, (B1, B2, B3) are the axial components of the total magnetic field measured at three known positions (z1, z2, z3) along the BHA / drill string axis, and q is proportional to the pole strength. z1, z2, and z3 are arranged in ascending order in an orientation sensor system with the z-axis pointing downhole.

[0113] The proportionality of q to the pole strength depends on the units used for pole strength, z-axis coordinates, and magnetic field. The product of the proportionality and the pole strength is q, which can be regarded as the source of the interfering magnetic field. This product is an unknown and needs to be determined as a single parameter. There is no need to know what the proportionality is. q can be the pole strength in a unit system where the proportionality is 1. Therefore, q will be referred to hereinafter as the pole strength parameter, or simply the pole strength.

[0114] B z , q, and z n are unknown in EQ. (20)-(22). The orientation sensor system 12 is made of non-magnetic material. The magnetic poles that dominate the interfering field must be above or below the orientation sensor system 12. Therefore, z n must be less than z1 or greater than z3 to be a valid solution. Thus, when B1 > B2 > B3 or B1 < B2 < B3 (which can be rewritten as ), there can be a valid solution to EQ. (20)-(22).

[0115] According to EQ. (20)-(22), the equation for z n is:

[0116]

[0117] where:

[0118]

[0119] Since z3 > z2 > z1, the inequality This is equivalent to ΔBRS>0. It can be shown that if ΔBRS>0 and ΔBRS≠1, then EQ.23 has a solution. Furthermore, if ΔBRS>1, then the solution z n Greater than z3, if ΔBRS<1, then z n Less than z1.

[0120] EQ.23 in z n is cubic. It can be shown that the three unrestricted solutions of EQ.23 are all real numbers. There is only one valid solution to EQ.23 outside the domain [z1,z3]. Therefore, there is a valid solution to EQ.23, and the solution is unique. Once z is found n The only solution of EQ.20-22 is B z and q. Any two of the three equations EQ.20-22 can be used to solve B z and q. The second term on the right side of EQ.20-22 is the interference with the three measured values ​​z1, z2, and z3.

[0121] If the section of the drill string system 10 near the directional sensor system 12 does not pass through or passes through a strong magnetic source, the magnetization of the end of the section can remain unchanged. Once the interference term is determined at one survey point, it can be subtracted from the axial measurements of multiple subsequent survey points without solving EQ.23 again. Therefore, these subsequent surveys only require one of the three axial magnetic field measurements at three known locations (z1, z2, z3). For example, all three axial magnetic field measurements are made at one survey point, and EQ.23 and EQ.20-22 are solved. In addition to the correct axial geomagnetic field, the interference term at the locations z1, z2, z3 is also solved. The interference term of the first axial magnetometer at φ becomes known. This term can be stored. For the next few survey points, this term can be subtracted from B1 at each survey point to obtain the correct axial component B of the geomagnetic field at each survey point. z If the system used to solve EQ.20-22 is on a surface computer, rather than downhole, then for the first survey point, three axial field measurements are transmitted from downhole to the surface. For subsequent survey points, it is not necessary to transmit the additional two axial magnetic field measurements from downhole to the surface. Because the data rate from downhole to the surface is very limited, not having to transmit two measurements for each survey point represents a significant improvement in operating speed.

[0122] Axial field measurements can be made at each survey point. Surveys can be made periodically during the drilling process. For example, a survey can be made when a section of drill pipe is added to or removed from the drill string. During the drill pipe replacement process, the drill string is at rest. The results of each survey can be transmitted from downhole to the surface via a telemetry system. The orientation of the well and the change in drill string length at each survey can be used to calculate the trajectory of the section at or near the survey point, and thus used to steer the drill string.

[0123] When ΔBRS≤0 or ΔBRS=1, the monopole approximation may no longer be accurate. Subsequently, one of the three axial measurements can be used to generate the long drill collar azimuth. Alternatively, a reference-based short drill collar correction method can be applied.

[0124] When |B3-B2| or |B2-B1| is less than the accuracy of magnetometers 14, 16, and / or 18, interference may not be present and / or the monopole approximation may be erroneous. Interference determination may not be performed.

[0125] It may be desirable to have a large distance interval between z1, z2, and z3 so that the difference between B1, B2, and B3 can be greater than the accuracy of the orientation sensor system 12. A limiting factor may be the length of the orientation sensor system 12. The distance interval may be selected to be the maximum distance allowed by the length of the orientation sensor system 12. In order to detect the magnetic pole equally well, whether the magnetic pole is above or below the orientation sensor, the two distances z2-z1 and z3-z2 may be similar. If these two distances are substantially different in the orientation sensor system 12, the orientation sensor system 12 may be more accurate in determining the magnetic pole on the side with the shorter distance.

[0126] According to EQ.21 and EQ.22, and use z for z n ,get:

[0127]

[0128] in this way:

[0129]

[0130] Where l (= z3-z2) is the axial magnetometer spacing, d (= z2-z) is the distance between the interfering magnetic pole and the intermediate axial magnetometer, dB (= B2-B3) is the difference in axial magnetic field caused by the interfering magnetic pole, is the axial magnetic disturbance of the intermediate axial magnetometer. If Then EQ.26 becomes:

[0131] for

[0132] For positive d, |dB| must be equal to or greater than the axial magnetometer accuracy dB min .

[0133] therefore:

[0134]

[0135] The minimum |B that satisfies the inequality in EQ.28 int |Yes|B int | min ,in:

[0136]

[0137] |B int | min is the accuracy of the sensing system of the example of the present disclosure for measuring magnetic disturbances. EQ. ​​29 shows that the spacing between the magnetometers 14, 16, and / or 18, referred to as l, should be as large as possible. However, in general, it may be desirable to keep the entire directional sensor system 12 as short as possible. Thus, in some examples, the magnetometers 14, 16, and / or 18 may be spaced approximately 12 inches apart, for example, resulting in a minimum length of the field sensor segment of the directional sensor system 12 of 24 inches.

[0138] if Then EQ.29 holds. Without using approximations:

[0139]

[0140] EQ.30 can be used to relate the interference measurement accuracy to the accuracy of the axial magnetic field.

[0141] Figure 4A and Figure 4B Another exemplary orientation sensor system 12a according to the present disclosure is shown, which has a single axial magnetometer 30. The single axial magnetometer 30 can be configured to be movable in the axial direction of the orientation sensor system 12a as indicated by arrow 32, so as to be able to measure the axial magnetic field at three separate positions z1, z2, z3 of the orientation sensor system 12a. The orientation sensor system 12a can have a housing 33 formed of a non-magnetic material (e.g., stainless steel).

[0142] In some examples, a single axial magnetometer 30 can be mounted on a rod 34 (e.g., a worm drive) driven by a stepper motor 36 (e.g., a piezoelectric motor). In some examples, the single axial magnetometer 30 can be connected to a shuttle 38, which is configured to move the single axial magnetometer 30 on the rod 34. For example, the rod 34 can be a member with threads, and the shuttle 38 also has threads. The single axial magnetometer 30 can be connected to the shuttle 38 so that it is moved when the shuttle moves along the rod 34.

[0143] The stepper motor 36 and the rod 34 can be configured to move the single axial magnetometer 30 in the direction of the arrow 32. For example, the stepper motor 36 can rotate the rod 34 so that the shuttle 36 moves axially within the directional sensor system 12a, thereby causing the single axial magnetometer 30 to move axially within the directional sensor system 12a. In some examples, the stepper motor 36 may not generate or affect electromagnetic waves, so that no additional electromagnetic parameters are generated and / or provided to the single axial magnetometer 30 due to the use of the stepper motor 36 and / or the rod 34. In some examples, the single axial magnetometer 30 can be positioned in an offset relationship within the housing 32 (i.e., offset from the center of the housing 32 within the shuttle 36). However, the single axial magnetometer 30 can be positioned at any position within the housing 32 (e.g., coaxial, offset).

[0144] The accuracy of the measurement of the difference dB is the sensor resolution, not the accuracy, of the individual axial magnetometers 30. Therefore, for the directional sensor system 12a, dB min is the resolution of a single axial magnetometer 30. The resolution of a single axial magnetometer 30 may be less than its accuracy. If the positions of the axial magnetometers 14, 16, 18, and 30 of each system 12 and 12a are the same, respectively, the accuracy of the directional sensor system 12a may be less than Figure 3 The accuracy of the three separate axial magnetometers 14, 16, and 18 of the directional sensor system 12 shown is much higher. In some embodiments, the directional sensor system 12a can be made shorter than the directional sensor system 12 with three separate axial magnetometers 14, 16, and 18, but with the same interference measurement accuracy. The trade-off is that the directional sensor system 12a may include the mechanical complexity of moving the axial magnetometer 30 to three or more separate locations.

[0145] The above process may be labeled as the Monopolar Axial Interference Removal (SPAIR) technique. Figure 5 A method 100 of obtaining a single survey using the above process is shown. In step 102, the directional sensor system 12 may measure (G x , G y , G z , B x , B y). In step 104, the directional sensor system 12 can measure the axial magnetic field at three separate axial positions z1, z2 and z3 to obtain B1, B2 and B3. Steps 102 and 104 can be performed at the same time or in any time order. In step 106, the microprocessor 20 can determine ΔBRS. If ΔBRS≤0 or ΔBRS=1, then in step 108, a direct measurement value for the azimuth of the long drill collar is output in B1, B2 and B3, and the output is designated as uncorrected. If ΔBRS>0 and ΔBRS≠1, then in step 110, EQ.20-22 is solved by first solving EQ.23, and the resulting B is output. z In step 112, the microprocessor 30 may obtain the obtained B z In step 114, the microprocessor 30 may optionally store the q and z values ​​on the right side of EQ. 20-22. n or the second one, so that interference correction can be made at subsequent survey points.

[0146] The entire system of SPAIR can be part of a downhole system. Alternatively, the field sensor can be downhole, while the equation solving portion of the computer system can be at the surface, and the three axial magnetic field measurements, along with other sensor data, are transmitted to the surface for processing and storage. In such a downhole-surface system, the downhole portion operates very similarly to an existing downhole directional sensor, except that three axial measurements are made instead of one and transmitted to the surface. This allows the use of existing hardware. Three existing directional modules can be stacked axially in a directional sensor to provide three axial magnetic field measurements. Two of the three xy measurements can be turned off or used as redundant transverse axis magnetic field measurements to improve quality.

[0147] In embodiments where the axial magnetic field is measured at more than three locations along the axis of the directional sensor tool, there are more than three equations in the system of simultaneous equations that relate the axial magnetic field measurements to three unknowns: the axial geomagnetic field, the interfering magnetic pole strength, and the position. Each three-equation subset of the system of simultaneous equations can be used to determine the axial geomagnetic field, the pole strength, and the pole position. The results of some or all of the subsets can be averaged to obtain the final result. Alternatively, a digital optimization solution can also be used, in which a penalty function is constructed from the simultaneous equations. Hereinafter, the term "solving a set of simultaneous equations" may also include solving a numerical optimization problem based on a set of simultaneous equations.

[0148] It is clear from the above description that the content of the invention disclosed herein is well suited to achieve the purposes mentioned herein and to obtain the advantages mentioned herein as well as those purposes and advantages inherent in the content of the invention disclosed herein. Although the present preferred embodiments of the content of the invention disclosed herein have been described for the purpose of such disclosure, it should be understood that various changes can be made, and the changes will be easily enlightened to those skilled in the art and implemented within the scope and field of the content of the invention disclosed and claimed herein.

Claims

1. A method for measuring the geomagnetic field and the axial magnetic interference field at a survey point using a directional sensor in a drill string, the method comprising: making axial magnetic field measurements using the orientation sensor with three or more axial magnetometers positioned at three or more separate locations within the orientation sensor, the three or more axial magnetometers being longitudinally spaced apart from one another along a longitudinal tool axis of the orientation sensor; and using a computing system to receive the axial magnetic field measurements made by the three or more axial magnetometers, and using the axial magnetic field measurements from the three or more separate locations to solve a set of simultaneous equations to obtain the axial component of the geomagnetic field under interference from magnetization in the drill string, the set of simultaneous equations being solved for the following unknowns: axial geomagnetic field, interfering magnetic pole strength, and interfering pole position, wherein obtaining the axial component of the geomagnetic field includes determining the magnetic interference generated by the magnetization of the drill string at the survey point; wherein the method also includes solving the set of simultaneous equations by first solving the equation for the interfering pole position.

2. The method according to claim 1, characterized in that: The method also includes solving a set of simultaneous equations to obtain magnetic pole strength parameters.

3. The method according to claim 1, characterized in that The method further comprises the step of measuring a gravity vector and a transverse-axis magnetic field by means of the orientation sensor.

4. The method according to claim 1, characterized in that The magnetic disturbance is the z-axis magnetometer offset error.

5. The method according to claim 1, characterized in that: The survey point is a first survey point, and the method includes: performing subsequent axial magnetic field measurements using the directional sensor at a second survey point following the first survey point, wherein a first axial magnetometer among the three or more axial magnetometers is located at a first position among the three or more separate positions within the directional sensor; and subtracting a first magnetic interference term associated with the first position within the directional sensor from the axial magnetic field measurement at the second survey point to obtain the axial component of the geomagnetic field at the second survey point, wherein the first magnetic interference term includes magnetic interference generated from within the drill string.

6. A method for measuring the geomagnetic field at a survey point along a section of a well using a directional sensor, the method comprising: Using the directional sensor at the survey point, three or more axial magnetic field measurements are performed using an axial magnetometer that can be moved to three or more separate positions within the directional sensor along the longitudinal tool axis of the directional sensor; using a computer system to receive the three or more axial magnetic field measurements at the survey point, and solving a set of simultaneous equations to obtain the axial component of the geomagnetic field under interference from magnetization in the drill string including the directional sensor, and obtain interference terms at the three or more separate positions of the axial magnetometer of the directional sensor at the survey point, wherein the interference terms include magnetic fields generated from magnetic sources within the drill string; and storing the interference terms in a non-transitory memory; wherein the method also includes solving the set of simultaneous equations by first solving equations for interference pole positions.

7. The method according to claim 6, characterized in that The method further comprises the step of determining magnetic pole strength parameters using the measurements obtained at the survey points.

8. The method according to claim 6, characterized in that The method further comprises the step of measuring a gravity vector and a transverse-axis magnetic field at a plurality of survey points.

9. The method according to claim 6, characterized in that The survey point is a first survey point, and the method further includes: performing subsequent axial magnetic field measurements using the directional sensor at a second survey point following the first survey point, wherein the axial magnetometer is located at a first position of the three or more separate positions within the directional sensor; and subtracting a first interference term associated with the first position within the directional sensor from the axial magnetic field measurement at the second survey point to obtain the axial component of the geomagnetic field at the second survey point, wherein the first interference term includes magnetic interference generated from within the drill string.

10. A directional sensing system, comprising: an axial magnetic sensor for a drill string, the axial magnetic sensor having a longitudinal tool axis and having three or more axial magnetometers longitudinally spaced from one another along the longitudinal tool axis within the axial magnetic sensor, the axial magnetic sensor comprising at least one data acquisition system configured to measure an axial magnetic field using the three or more axial magnetometers at three or more different locations within the axial magnetic sensor when the directional sensing system is stationary at a single location; and a computing system for: receiving at least three axial magnetic field measurements at the single location from the three or more axial magnetometers at the three or more different locations within the axial magnetic sensor; and using the axial magnetic field measurements from the three or more different locations to solve a set of simultaneous equations to obtain the axial component of the geomagnetic field under the interference from the magnetization in the drill string, the set of simultaneous equations being solved for the following unknowns: the axial geomagnetic field, the interfering magnetic pole strength, and the interfering pole position, wherein obtaining the axial component of the geomagnetic field includes determining the magnetic interference generated by the magnetization of the drill string at the single location; wherein the computing system is also used to determine the interfering pole position before determining the axial geomagnetic field.

11. The directional sensing system according to claim 10, characterized in that: The interfering pole position is determined before determining the interfering magnetic pole strength.

12. A directional sensing system, comprising: An directional sensor having a longitudinal tool axis, the directional sensor comprising: at least one axial magnetometer, the at least one axial magnetometer being capable of being moved to three or more separate positions within the directional sensor along the longitudinal tool axis of the directional sensor; and a data acquisition system, the data acquisition system being configured to obtain axial magnetic field measurements for the three or more separate positions within the directional sensor from the at least one axial magnetometer; and a computing system, the computing system being configured to: receive axial magnetic field measurements from the directional sensor; and use the axial magnetic field measurements to obtain an axial component of the geomagnetic field under interference from magnetization in a drill string including the directional sensor, wherein obtaining the axial component of the geomagnetic field comprises determining magnetic interference generated from within the drill string at a survey position; wherein the computing system is further configured to determine an interference pole position before determining the axial component of the geomagnetic field.

13. The directional sensing system according to claim 12, characterized in that: The computing system also determines a magnetic pole strength parameter.

14. The directional sensing system according to claim 13, characterized in that: The interfering pole position is determined before the magnetic pole strength parameter is determined.

15. A directional sensing system, comprising: An orientation sensor for a drill string, the orientation sensor having a longitudinal tool axis and comprising: a plurality of accelerometers; a plurality of magnetometers, the plurality of magnetometers comprising three axial magnetometers longitudinally spaced apart from each other at three locations within the orientation sensor along the longitudinal tool axis of the orientation sensor; and a data acquisition system configured to measure a gravity vector, a transverse magnetic field, and an axial magnetic field from the three locations within the orientation sensor using the three axial magnetometers; and a computing system having one or more non-transitory computer-readable media storing a set of computer-executable instructions for running on one or more processors, the set of computer-executable instructions comprising: The executable instructions, when executed, cause the one or more processors to: receive the gravity vector, the transverse-axis magnetic field, and the axial magnetic field measurements from the orientation sensor; solve a set of simultaneous equations using the axial magnetic field measurements from the three locations to obtain the axial component of the geomagnetic field under interference from magnetization in the drill string, the set of simultaneous equations solving for the following unknowns: the axial geomagnetic field, the interfering magnetic pole strength, and the interfering pole position, wherein obtaining the axial component of the geomagnetic field includes determining the magnetic interference generated by the magnetization of the drill string at a survey point, and solving the set of simultaneous equations by first solving the equation for the interfering pole position; and determine at least one orientation parameter using the gravity vector, the transverse-axis magnetic field, and the axial component of the geomagnetic field.

16. The directional sensing system according to claim 15, characterized in that: A first axial magnetometer of the three axial magnetometers is positioned at a distance between eight and eighteen inches from an adjacent second axial magnetometer of the three axial magnetometers.

17. The directional sensing system according to claim 16, characterized in that: The three axial magnetometers are equidistantly spaced from one another within the orientation sensor at the three locations along the longitudinal tool axis of the orientation sensor.

18. The directional sensing system according to claim 17, characterized in that: The orientation sensor has a proximal end along the longitudinal tool axis, a distal end along the longitudinal tool axis, and a midpoint segment equidistantly spaced between the proximal end and the distal end along the longitudinal tool axis, and the three axial magnetometers include a first axial magnetometer positioned at the proximal end, a second axial magnetometer positioned at the midpoint segment, and a third axial magnetometer positioned at the distal end.

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