System and method for determining axial magnetic disturbance in downhole orientation sensor

Abstract

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

Description

[0001] This application is a divisional application of the patent application with the application number 201980084198.7, the application date of October 24, 2019, and the invention title of "System and Method for Determining Axial Magnetic Interference in Downhole Directional Sensors". Background Art

[0002] In drilling, a measurement while drilling (MWD) directional sensor can be used to determine the orientation of the well. Directional measurements are also known as surveys in the industry. Generally, the MWD directional sensor is encapsulated in a section of the drill string near the drill bit. This 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.

[0003] During drilling, surveys are conducted regularly. Typically, the MWD sensor section and the drill string must be stationary during a survey operation. Therefore, surveys are usually conducted when adding a section of drill pipe or removing a section of drill pipe from the drill string. During the pipe-changing process, the drill string is stationary. The results of each survey are transmitted from downhole to the surface through a telemetry system. The changes in the orientation of the well and the length of the drill string at each survey are used to calculate the trajectory of the well section at or near that survey point.

[0004] The MWD directional sensor can consist of an accelerometer and a magnetometer. The magnetometer in the directional sensor can be used to measure the components of the Earth's magnetic field (i.e., the geomagnetic field) vector along the orthogonal axes (x, y, z) of the directional sensor coordinate system. The three components are (Bx, By, Bz). When the directional sensor is stationary, the accelerometer in the directional sensor can be used to measure the gravity vector The measured gravity components are (Gx, Gy, Gz).

[0005] The orientations 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 the gravity vertical and magnetic north. Since the sensor is firmly encapsulated on the drill string, once the orientation of the directional sensor is obtained, the orientation of the drill string and the well can be obtained, and vice versa.

[0006] The direction in which the axis of the drill string points downward is typically 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 calibrated and aligned with the directional sensor coordinate system. However, there may be cases 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. Additionally, the hardware axes of the magnetometer may not be calibrated and aligned with the hardware axes of the accelerometer. The measurements along the directional sensor axes are obtained through 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 referred to as the axial geomagnetic field.

[0007] The well inclination is the angle between the well and the vertical defined by the direction of the gravity vector. Thus, the well inclination can be a measure of how much the well deviates from the vertical. The accelerometer measurement results 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 onto the horizontal plane, as Figure 1 the angle shown in The horizontal plane is defined as the plane perpendicular to gravity.

[0008] The measurement results of the accelerometer and magnetometer can be used to calculate this azimuth. There are many formulas for how to calculate this azimuth. These formulas are essentially equivalent to each other. One expression for the azimuth is:

[0009]

[0010] where ATAN2[] is the arctangent function of two parameters, × is the vector cross product symbol, the subscript z represents the z-axis component of the vector, G is the magnitude of, and the horizontal component of the geomagnetic field vector is given by the following equation:

[0011]

[0012] where · is the vector dot product symbol.

[0013] In a directional survey, the measured gravity vector and magnetic vector are used in equations (1) and (2) to generate the azimuth. is the error vector in the geomagnetic field measurement. ΔB is the magnitude of. This error vector may come from two sources. The first error source may be 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 generated by a magnetic source in the drill string system near the directional sensor.

[0014] The measured magnetic field is:

[0015]

[0016] In practice, is used to replace in EQ(2). If along the vertical direction, i.e., along or opposite to then is the same as

[0017] The azimuth error originates from the sensed magnetic north vector that is away from the true magnetic north in the horizontal plane from the sensor measurement. Figure 2 A horizontal plane view of the azimuth error is shown in Figure 2 The horizontal component of the magnetic field measurement error is shown as ΔB h . This error component causes the horizontal geomagnetic vector to rotate relative to the true magnetic north vector Therefore, is determined from the measurement result and is the vector sum of h and ΔB

[0018] The azimuth error varies with the direction of the error vector. Let (ΔB N , ΔB E , ΔB V ) be along the geomagnetic north axis, geomagnetic east axis, and gravity vertical axis, respectively. Since the azimuth error caused by is given by:

[0019]

[0020] where B N is the magnitude of . This expression can be simplified to:

[0021] For |ΔB N | < B N ,

[0022] the vertical component ΔB V has no effect. It can be shown that when measuring the Earth's magnetic field, the maximum azimuth error (i.e., the largest azimuth error) due to the magnitude ΔB of the error vector is:

[0023] For ΔB < B N , It has a maximum value when:

[0024]

[0025] For most combinations of BHA and drill string, ΔB is much smaller than B N So:

[0026] For ΔB << B N ,

[0027] If ΔB << B N is true, then the maximum azimuth error occurs when the error vector is in the horizontal and east / west (HEW) direction ( and ΔB V = 0). In many cases, ΔB is indeed much smaller than B N such that EQ.8 can be used to estimate the possible maximum azimuth error. However, in these cases, the azimuth error may still be very large compared to the specifications of the directional sensor. ΔB is considered negligible only when the resulting azimuth error is less than the azimuth error specification.

[0028] Most sections of the drill string and many sections and components of the BHA are made of iron, steel, and other ferromagnetic metals. The unknown magnetization in these metals generates a magnetic field near the directional sensor section. This magnetic field is superimposed on 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 disrupted by this additional field. The additional field generated by the magnetization of ferromagnetic metals near the directional sensor interferes with the measurement of the Earth's magnetic field. To correctly determine the orientation of the directional sensor, this interference needs to be reduced to a negligible level and / or corrected for this interference.

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

[0030] Due to various limitations, in some drilling operations, it is not practical to have very long non-magnetic sections above and below the sensor location. In these cases, magnetic interference may be inevitable. 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 interference due to short non-magnetic sections is known in the drilling industry as the short drill collar correction method or short drill collar algorithm. The conventional azimuth obtained from the output of a directional sensor without any magnetic interference correction is sometimes referred to as the long drill collar azimuth.

[0032] Due to the limited transverse dimensions of the drill string and BHA, most of the magnetization in the drill string and BHA is along the axial direction. The axial magnetization above the non-magnetic section can be approximated as a long magnetic dipole along the drill string axis. The magnetization in the transverse direction can also be considered as a magnetic dipole along the transverse direction. Even if the 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 at a certain 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 directional sensor is along the z-axis direction. The relevant error vector in the Earth's magnetic field measurement is dominated by the term where is the unit vector of the sensor z-axis, and ΔB z is the axial component. If the true inclination and azimuth of the directional sensor are θ and respectively, then:

[0035]

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

[0037] Due to the term, the azimuth error is given by EQ.5 as:

[0038]

[0039] When |ΔB z | << B N then:

[0040]

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

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

[0043]

[0044] where B ref is the reference value of the magnitude of the geomagnetic field at the well site during the measurement by the directional sensor, (B x , B y ) are the measured magnetic field components along the sensor (x, y) directions, and B z is the best estimate of the z-axis component of the geomagnetic field. This is the Total Field Matching Short Drill Collar (TFMSC) method. The reference value is obtained from an independent source independent of the directional sensor measurement. In TFMSC, the reference value is used as a constraint on how B z must be. That is, the B z satisfying EQ.12 is considered 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. EQ.12 has two solutions. Subsequently, some criteria can be used to select one solution rather than the other as the correct solution. For example, for a single survey point, when no other information is available, the solution resulting in a smaller interference term can be selected. For multiple survey points along the well section, if the interference term is expected to be constant along the well section, the solution producing a common interference term can be selected.

[0045] In the method described in the patent document under the names of van Dongen et al. (U.S. Patent No. 4,682,421, hereinafter referred to as "van Dongen"), reference values regarding the magnitude and inclination of the geomagnetic field are used to determine the correct value of B z . The Bz that minimizes the function E defined in van Dongen is considered the correct B z of the geomagnetic field. In van Dongen, is the geomagnetic vector measured by the directional sensor after correction, and is given by the reference source. These values are assumed to be in the same plane. E is the difference vector The size. The method of van Dongen can be labeled as the Minimum Vector Difference Short Drill Collar (MVDSC) algorithm.

[0046] In another method, B is selected z , such that using the selected B z plus the measured (G x , G y , G z, B x , B y ) the calculated B N is equal to the reference value of B N . This method can be called the Horizontal Component Matching 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 one solution as the correct solution.

[0047] TFMSC, MVDSC, and HCMSC are all reference-based correction methods. The axial geomagnetic field component B z is selected such that when it is used together with other measured field components (G x , G y , G z, B x , B y ) to calculate a property of the geomagnetic field, the result matches the reference value exactly or as closely as possible. When there are multiple solutions for B z , the sensor output B z can be used to select one as the solution. The difference between the solution of the correction method and the measured B z is the interference term ΔB z . If the section of the drill string and BHA near the directional sensor has not passed 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 directly subtracted from the output of the axial magnetic sensor to obtain the corrected B z , which can then be used to calculate the directional parameters.

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

[0049] If the transverse magnetometer measurement is error-free, then according to EQ.12, there is:

[0050]

[0051] where dB ref is the error in the reference value of the geomagnetic field magnitude, and dB z is in Bz B due to short drill collar correction when given by the solution of EQ.12 z error.

[0052] In a well close to the HEW direction, the true geomagnetic field component B z is almost zero. dB z can be much larger than the interference term ΔB z That is to say, the azimuth error of the corrected B z can be much larger than the error of the interference term. In the section of the well exactly in the HEW direction, the true axial component of the geomagnetic field is 0. Therefore, EQ.13 cannot be used. The error of B z from TFMSC can be directly calculated by EQ.12. If the magnitude of the geomagnetic field is 50000 nT (nano Tesler), and dB ref is 50010 nT, then EQ.12 gives B z = ±1000 nT. The reference value deviates by 10 nT (much smaller than the accuracy of any global geomagnetic model), and TFMSC causes an error of 1000 nT in B z If the interference term ΔB z is less than 1000 nT, the short drill collar correction makes the azimuth error larger.

[0053] As shown in EQ.11, when the well is in the HEW direction, the axial interference causes the largest error. But at this well attitude or near it, due to the inaccurate reference value, the effectiveness of TFMSC may be the lowest.

[0054] In the case of a perfect reference value, the inaccuracy of the transverse axis measurement will also cause a very large error in the axial geomagnetic component determined by the 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 magnitude. That is to say, the error of the axial component determined by TFMSC using a perfect reference value is given by the following formula:

[0057]

[0058] where dB xy is the error of B xy When B z is almost zero and B xy is almost the total magnitude of the geomagnetic field, B zThe error can be many times larger than the error of the transverse magnetometer. For example, if TFMSC is used, the xy error of 10 nT for B z will result in an error of 1000 nT for B

[0059] in a geomagnetic field of 50000 nT. The 10 nT error in the transverse direction is converted and amplified to a 1000 nT error along the axial direction by short drill collar correction. z Generally speaking, the error of B when using TFMSC is given by the following formula:

[0060]

[0061] Since the reference value is obtained from a source independent of the operation of the orientation sensor, dB xy is uncorrelated with dB ref . Therefore:

[0062]

[0063] where the overline refers to the statistical average.

[0064] It can be seen that other reference-based short drill collar correction methods have problems with inaccurate reference values and / or transverse measurement values very similar to TFMSC. It is well known in the industry that when the need for axial interference correction is most urgent, all known reference-based short drill collar corrections tend to produce larger errors in the axial geomagnetic component than the interference terms to be eliminated. When the wellbore attitude is close to the HEW direction, short drill collar correction is usually not used.

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

[0066] If the well section is very curved, the interference term may be determined by requiring that the measured values from which the interference is to be subtracted produce a geomagnetic field of one or more constant magnitudes with the smallest differences in multiple surveys. Determining the interference term is an optimization process. This correction method can be labeled as the Constant Total Field (CTF) algorithm.

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

[0068] Even if there is no explicit requirement for a reference value of the geomagnetic field, in CTF, a constant magnitude is imposed. In fact, the "constant magnitude" is information about the properties of the geomagnetic field, which comes from a source independent of 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 the inaccuracy of the reference value and / or the transverse fluxgate magnetometer measurements discussed above. For example, the constraint of "constant magnitude" may be incorrect. During the use of CTF in multiple surveys, the geomagnetic field at a specific location may change by dozens or hundreds of nT within a very short period of time.

[0069] When the implicit reference value and the transverse magnetic measurements are accurate, if multiple surveys are carried out in the curved section of the well, methods such as CTF will be effective. Some of these surveys in this set are carried out in well postures far from the HEW direction. In these postures, the magnitude of the magnetic field is sensitive to the axial offset caused by interference. The interference term can be accurately determined by the constant magnitude requirement of the survey data in these postures. For all survey data, including those of the HEW (if any), the correct interference term is removed. However, the requirement for survey data on the curved part of the well places a limitation on the location where CTF can be applied.

[0070] Nowadays, many horizontal wells are being drilled. Horizontal wells usually start vertically near the ground. Once the required depth is reached, the well quickly turns horizontal. Sometimes, the vertical section of the 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 changes 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 carried out on the curved section of the horizontal well.

[0071] On the horizontal section of the horizontal well, the well posture changes little. Although the small changes in the well posture are important for the production potential of the well, these changes are geometrically very small. In a set of multiple surveys, regardless of the z-axis offset used to calculate the magnitude, the measured magnitude of the geomagnetic field is almost constant. Therefore, the "constant magnitude" is not helpful in determining the z-axis offset. Therefore, the axial interference 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 data set.

[0073] In addition to the problem that inaccurate reference values and transverse fluxgate magnetometer measurements may cause the interference correction to be more incorrect than the interference, the multi-station analysis method may also be limited by the requirement that the multi-station survey must be carried out on a very curved section of the well. This condition is often not met. Summary of the Invention

[0074] The z - axis component of the magnetic field is measured at several axially - spaced positions within the sensor package for one orientation survey point. The axially - varying interference field and the constant z - component of the Earth's magnetic field are directly determined from the sensor output. No reference field values are used. The Earth component is then used for magnetic azimuth calculation.

[0075] Since the axial magnetic interference is approximated by the magnetic field generated by a magnetic monopole located at a distance from the orientation sensor. Multiple measurements of the field component along the z - axis are made at several known axial positions and are used to determine the pole strength, pole position, and the component of the Earth's 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 will become apparent from the following detailed description of the invention, which should be considered in conjunction with the accompanying Figure 1 drawings, in which like numerals refer to like parts, and in which:

[0077] Figure 1 is a diagrammatic illustration of various angles described in the present disclosure, including well azimuth The sensor z - axis is the downward - pointing well axis, and OP represents the projection of the z - axis onto the horizontal plane.

[0078] Figure 2 is a diagrammatic illustration of a horizontal - plane view of the azimuth error. In this illustration, OP is the horizontal - plane projection of the z - axis, is the true magnetic north vector, is the magnetic north vector obtained from the sensor measurements using the magnetic north vector of the horizontal magnetic field error obtained.

[0079] Figure 3 is a schematic diagram of an exemplary orientation sensor system having three axial magnetometers in accordance with the present disclosure.

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

[0081] Figure 4B is along Figure 4A a cross - sectional view of a magnetometer assembly constructed in accordance with the present disclosure taken along line 4B - 4B in

[0082] Figure 5 is a flow chart of an exemplary method for providing a survey using single - pole 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 canceling axial magnetic interference of the earth's magnetic field. The at least three axial magnetometers may be disposed at different positions around an axis of the directional sensing system. Axial interference from the nearest magnetic poles on a 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 from axial magnetic interference.

[0084] In some examples, the directional sensing system can be used to measure the geomagnetic field and the axial magnetic interference field. Generally, the directional sensing system can obtain axial magnetic field measurements at three or more separate positions 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 components of the geomagnetic field. Additionally, in some embodiments, a set of second equations can be solved simultaneously to obtain the magnetic pole strength parameters and the pole positions. The gravity vector and the transverse magnetic field can be measured additionally.

[0085] In some embodiments, the directional sensing system can 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 is to be understood that the presently disclosed and claimed inventive concepts are not limited in their application to the details of construction, experimentation, exemplary data, and / or arrangement of components set forth in the following description or illustrated in the drawings. The presently disclosed and claimed inventive concepts are capable of other embodiments or of being practiced or carried out in various ways. Further, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.

[0087] In the following detailed description of embodiments of the inventive concept, numerous specific details are set forth in order to provide a more thorough understanding of the inventive concept. However, it will be apparent to one of ordinary skill in the art that the inventive concepts of the present disclosure may be practiced without these specific details. In other instances, certain well-known features may not be described in detail to avoid unnecessarily complicating the present disclosure.

[0088] As used herein, the terms "comprises," "comprising," "includes," "including," "has," "having," or any other modification thereof will be a non-exclusive inclusion. For example, unless otherwise specified, a process, method, article, or apparatus that comprises a series of elements does not necessarily limit to only those elements but may also include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0089] Unless expressly stated to the contrary, "or" means an inclusive and non-exclusive "or". For example, the condition A or B is satisfied by one of the following: A is true (or present) and B is false (or absent); A is false (or absent) and B is true (or present).

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

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

[0092] The use of the terms "at least one" and "one or more" will be understood to include one and any number more 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 (if applicable) therebetween. The terms "at least one" and "one or more" can extend to 100 or 1000 or more, depending on the term to which they are attached; furthermore, the quantity of 100 / 1000 should not be considered restrictive, as higher limits may also produce satisfactory results.

[0093] In addition, as used herein, any reference to "one embodiment" or "embodiments" means that the particular elements, features, structures, or characteristics described in connection with the embodiment are included in at least one embodiment. For example, the phrase "in one embodiment" that appears in different places in the specification does not necessarily refer to the same embodiment.

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

[0095] Modifiers such as "about", "approximate", and "substantially" as used herein are intended to mean that the item so modified is not limited to the specified exact value, but includes some minor variations or deviations, such as those caused by measurement errors, manufacturing tolerances, stresses applied to various components, wear, and combinations thereof.

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

[0097] As Figure 3 shown, each of the axial magnetometers 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 a location L1 on the z-axis of the orientation sensor system 12. The axial magnetometer 16 is positioned at a location L2 on the z-axis of the orientation sensor system 12. The axial magnetometer 18 is positioned at a location L3 on the z-axis of the orientation sensor system 12. It should be noted that the orientation sensor system 12 can include additional components, including but not limited to an accelerometer 15, a transverse axis magnetometer 17, and a data acquisition system 19. In addition, the orientation sensor system 12 can also include a bus, interconnects, and / or similar components for establishing communication between the axial magnetometers 14, 16, 18, accelerometer 15, transverse axis magnetometer 17, and the data acquisition system(s) 19. The data acquisition system(s) 19 receives sensor data from the axial magnetometers 14, 16, 18, accelerometer 15, and transverse axis magnetometer 17, and provides this sensor data to one or more computer systems 20. In addition, the accelerometer 15 or the transverse axis magnetometer 17 can be located at or near one of the three axial magnetometers 14, 16, and 18. For the sake of simplicity of description, the positions and uses 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 capable of embodying and / or executing the logic of the processes described herein. The logic embodied in the form of software instructions and / or firmware may be executed on any suitable hardware. For example, the logic embodied in the form of software instructions and / or firmware may be executed on one or more dedicated systems, distributed processing computer systems, and / or similar systems. In some embodiments, the logic may be implemented in a stand-alone environment running on a single system, and / or the logic may be implemented in a network environment, such as a distributed system using multiple computers and / or processors. The (one or more) computer systems 20 may work together or independently and execute processor-executable code using one or more memories 22 (e.g., non-transitory memories). In some embodiments, the directional sensor system 12 may have a housing 23, and the (one or more) computer systems 20 may be located within the housing 23 of the directional sensor system 12.

[0099] Axial magnetic interference can be caused by magnetization in the various sections of the drill string system 10 (e.g., near the directional sensor system 12). In traditional directional sensors, the position of a single z-axis magnetometer is fixed. The interference term can be regarded as an offset error in the measurement of the geomagnetic field. However, the interference field is not constant. The z-axis component of the magnetic field generated by these magnetizations varies numerically with 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 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 near pole and the far pole, respectively.

[0102] The magnitude of the magnetic field of a point dipole decreases as a function of 1 / r 3 , where r is the distance 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 regarded as a point dipole. However, in a short drill collar BHA system, the magnetization cannot be treated as a point dipole. The axial size of the magnetic source can and often is greater than the distance between the directional sensor and the nearest pole. Thus, |z - z n | < |z n - z f|. EQ.18 is the formula for calculating the axial magnetic field generated by a long-axis dipole.

[0103] For a long dipole, EQ.18 shows that the magnitude of the interfering field is a quadratic function of the reciprocal of the distance between the pole and the observation point. z n It can be at the end of the non-magnetic section of the sensor and the point where the ferromagnetic section of the BHA / drill string begins. If the dominant long dipole is from below the sensor section, then z f right at or near the bottom of the BHA, at z n a few dozen feet below. If the long dipole is above the directional sensor, then z f may be several hundred feet higher than z n In most drilling environments, if |z - z n | is greater than 20 feet, then ΔB z (z) can be neglected. Therefore, in cases where the interference cannot be ignored, the second term on the right side of EQ.18 is much smaller than the first term. The field from the distant pole can be neglected, and thus:

[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, 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 cut of a sub, and the diameter of the sub is the diameter of the sensor part.

[0106] The axial distance between the directional sensor system 12 and the magnetized part 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, it can be considered that the sensor measurements are 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 can also be ignored.

[0107] Reference 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 fields 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] 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 the orientation sensor system with the z-axis pointing downhole.

[0111] 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.

[0112] B z , q, and z n are unknown in Eqs. (20)-(22). The orientation sensor system 12 is made of non-magnetic material. The magnetic poles of the dominant 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 ), a valid solution to Eqs. (20)-(22) can exist.

[0113] According to Eqs. (20)-(22), the equation for z n is:

[0114]

[0115] where:

[0116]

[0117] Since z3 > z2 > z1, the inequality is equivalent to ΔBRS > 0. It can be shown that if ΔBRS > 0 and ΔBRS ≠ 1, Eq. 23 has a solution. Furthermore, if ΔBRS > 1, the solution z n is greater than z3, and if ΔBRS < 1, z n is less than z1.

[0118] EQ.23 is cubic in z n . It can be shown that all three unrestricted solutions of EQ.23 are real. And among the three unrestricted solutions, exactly one is outside the domain [z1, z3]. Therefore, there exists a valid solution of EQ.23, and this solution is unique. Once the unique solution of z n is found, EQs. 20 - 22 become linear equations of B z and q. Any two of the three equations EQs. 20 - 22 can be used to solve for B z and q. The second term on the right side of EQs. 20 - 22 is the interference to the three measured values z1, z2, and z3.

[0119] If the section of the drill string system 10 close to the directional sensor system 12 has not passed through or has passed through a strong magnetic source, the magnetization of this section can remain unchanged. Once the interference term is determined at a survey point, it can be subtracted from the axial measurements at multiple subsequent survey points without having to solve EQ.23 again. Therefore, these subsequent surveys only require one of the three axial magnetic field measurements at the three known positions (z1, z2, z3). For example, at a survey point, all three axial magnetic field measurements are made and EQ.23 as well as EQs. 20 - 22 are solved. In addition to the correct axial geomagnetic field, the interference term of the first axial magnetometer located at z1, becomes known. This term can be stored. For several subsequent survey points, this term can be subtracted from B1 at each survey point to obtain the correct axial component B z of the geomagnetic field at each survey point. If the system for solving EQs. 20 - 22 is on a surface computer instead of downhole, for the first survey point, the three axial field measurements are transmitted from downhole to the surface. For subsequent survey points, it is not necessary to transmit the other two axial magnetic field measurements from downhole to the surface. Since 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.

[0120] Axial field measurements can be made at each survey point. During the drilling process, surveys can be carried out regularly. For example, surveys can be made when adding a section of drill pipe to the drill string or removing a section of drill pipe from the drill string. During the process of replacing drill pipe, the drill string is stationary. The results of each survey can be transmitted from downhole to the surface through a telemetry system. The change in the orientation of the well and the length of the drill string at each survey can be used to calculate the trajectory of the well section at or near the survey point, thereby for manipulating the drill string.

[0121] 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.

[0122] When |B3 - B2| or |B2 - B1| is less than the precision of magnetometers 14, 16, and / or 18, there may be no interference and / or the monopole approximation may be incorrect. The interference determination may not be performed.

[0123] It may be desirable to have a large distance separation between z1, z2, and z3 such that the differences between B1, B2, and B3 can be greater than the precision of the orientation sensor system 12. The limiting factor may be the length of the orientation sensor system 12. The distance separation may be selected as the maximum distance allowed by the length of the orientation sensor system 12. To detect the magnetic poles equally well whether the magnetic poles are 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.

[0124] According to EQ.21 and EQ.22, and using z to represent z n , we get:

[0125]

[0126] Thus:

[0127]

[0128] where l (= z3 - z2) is the axial magnetometer spacing, d (= z2 - z) is the distance between the interfering magnetic pole and the middle axial magnetometer, dB (= B2 - B3) is the difference in the axial magnetic field caused by the interfering magnetic pole, is the axial magnetic interference of the middle axial magnetometer. If then EQ.26 becomes:

[0129] For [[ID=3l]]For positive d, |dB| must be equal to or greater than the axial magnetometer precision dB min . Therefore:

[0130]

[0131] The minimum |B int | that satisfies the inequality in EQ.28 is |B int | min , where:

[0132]

[0133] |B int | minis the accuracy of the magnetic interference measurement of an example sensing system of the present disclosure. EQ.29 shows that the spacing, called l, between magnetometers 14, 16, and / or 18 should be as large as possible. However, in general, it may be desirable to keep the entire orientation sensor system 12 as short as possible. Thus, in some examples, magnetometers 14, 16, and / or 18 may be spaced approximately 12 inches apart, for example, resulting in a minimum length of the field sensor section of the orientation sensor system 12 of 24 inches.

[0134] If then EQ.29 holds. Without using approximations:

[0135]

[0136] EQ.30 can be used to relate the interference measurement accuracy to the accuracy of the axial magnetic field.

[0137] Figure 4A and Figure 4B shows another exemplary orientation sensor system 12a according to the present disclosure, 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 (such as stainless steel).

[0138] In some examples, the single axial magnetometer 30 can be mounted on a rod 34 (such as a worm drive) driven by a stepper motor 36 (such as 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 threaded member, and the shuttle 38 also has threads. The single axial magnetometer 30 can be connected to the shuttle 38 so as to be moved when the shuttle moves along the rod 34.

[0139] The stepper motor 36 and the rod 34 can be configured to move a single axial magnetometer 30 in the direction of arrow 32. For example, the stepper motor 36 can rotate the rod 34 such that the shuttle 36 moves axially within the orientation sensor system 12a, thereby causing the single axial magnetometer 30 to move axially within the orientation sensor system 12a. In some examples, the stepper motor 36 may not generate electromagnetic waves or have an impact on electromagnetic waves, such 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 within the housing 32 in an offset relationship (i.e., offset from the center of the housing 32 within the shuttle 36). However, the single axial magnetometer 30 can be positioned anywhere within the housing 32 (e.g., coaxial, offset).

[0140] The measurement accuracy of the differential dB is the sensor resolution of the single axial magnetometer 30, not the accuracy. Thus, for the orientation sensor system 12a, dB min is the resolution of the single axial magnetometer 30. The resolution of the single axial magnetometer 30 can be less than its accuracy. If the positions of the axial magnetometers 14, 16, 18, and 30 of each of the systems 12 and 12a are the same respectively, the accuracy of the orientation sensor system 12a can be much higher than Figure 3 the accuracy of the three separate axial magnetometers 14, 16, and 18 of the illustrated orientation sensor system 12. In some embodiments, the orientation sensor system 12a can be manufactured to be shorter than the orientation sensor system 12 having three separate axial magnetometers 14, 16, and 18, but with the same interference measurement accuracy. The trade-off is that the orientation sensor system 12a may include mechanical complexity for moving the axial magnetometer 30 to three or more separate positions.

[0141] The above process can be labeled as the single-pole axial interference removal (SPAIR) technique. Figure 5 A method 100 for obtaining a single survey using the above process is shown. In step 102, the orientation sensor system 12 can measure (G x ,G y ,G z, B x ,B y)。In step 104, the orientation sensor system 12 can measure the axial magnetic fields of z1, z2, and z3 at three separate axial positions to obtain B1, B2, and B3. Steps 102 and 104 can be performed at the same time or in any chronological 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 among B1, B2, and B3, and this 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 z is designated as interference-corrected. In step 112, the microprocessor 30 can store the q and z z on the right side of EQ.20 - 22 n or the second term, for interference correction at subsequent survey points.

[0142] The entire SPAIR system can be part of a downhole system. Optionally, the field sensor can be downhole, while the equation-solving part of the computer system can be on 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 operation of the downhole part is very similar to existing downhole orientation sensors, except that three axial measurements are made instead of one and transmitted to the surface. This allows the use of existing hardware. Three existing orientation modules can be stacked axially in the orientation sensor to provide three axial magnetic field measurements. Two of the three x - y measurements can be turned off or used as redundant transverse magnetic field measurements to improve quality.

[0143] In embodiments where the axial magnetic field is measured at more than three positions along the axis of the orientation 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 interference 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 a final result. Optionally, a numerical optimization solution can also be used, in which a penalty function is constructed from the system of simultaneous equations. Hereafter, the term "solving a system of simultaneous equations" can also include solving a numerical optimization problem based on a system of simultaneous equations.

[0144] It is clear from the foregoing description that the subject matter of the invention disclosed herein is well adapted to carry out the objects herein mentioned and to obtain the advantages herein mentioned and those objects and advantages inherent in the subject matter of the invention disclosed herein. Although the presently preferred embodiments of the subject matter of the invention disclosed herein have been described for the purpose of this disclosure, it is to be understood that various changes can be made which will readily suggest themselves to those skilled in the art and which are within the scope and spirit of the subject matter of the invention as 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 located within a drill string, the method comprising: Performing axial magnetic field measurements at more than three separate locations inside the directional sensor; And Using a computing system: Receiving the axial magnetic field measurement values; And Solving multiple sets of simultaneous equations using the axial magnetic field measurement values from the more than three separate locations to obtain a set of solutions for the axial component of the geomagnetic field affected by magnetization interference within the drill string, the multiple sets of simultaneous equations solving for the following unknowns: axial geomagnetic field, interfering pole strength, and interfering pole position, wherein obtaining the set of solutions for 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 each of the multiple sets of simultaneous equations corresponds to a different grouping of measurements in groups of three of the more than three separate locations.

2. The method according to claim 1, wherein The method further includes averaging at least two of the sets of solutions for the axial component of the geomagnetic field to obtain an average solution for the axial component of the geomagnetic field.

3. The method according to claim 1, wherein The method further includes constructing a penalty function from the multiple sets of simultaneous equations and applying a numerical optimization solution to minimize the penalty function.

4. The method according to claim 1, wherein Performing axial magnetic field measurements at more than three separate locations inside the directional sensor further includes using more than three axial magnetometers positioned at the more than three separate locations inside the directional sensor, the more than three axial magnetometers being longitudinally spaced apart from each other along the longitudinal tool axis of the directional sensor.

5. The method according to claim 1, wherein, Performing axial magnetic field measurements at more than three separate locations inside the directional sensor further includes using axial magnetometers that can be moved to the more than three separate locations inside the directional sensor.

6. The method according to claim 1, wherein The method further includes solving the multiple sets of simultaneous equations by first solving a set of simultaneous equations to obtain the interfering pole position.

7. The method according to claim 1, wherein, The method further includes solving a set of simultaneous equations to obtain an interfering pole strength parameter.

8. The method according to claim 1, wherein, The method further includes the step of measuring the gravity vector and the transverse magnetic field by the directional sensor.

9. The method according to claim 1, wherein The magnetic interference is a z-axis magnetometer offset error.

10. The method according to claim 1, wherein The survey point is a first survey point, and the method includes: Performing subsequent axial magnetic field measurements at a second survey point after the first survey point, the first survey point being at a first position among the more than three separate locations inside the directional sensor; and, Subtracting a first magnetic interference term associated with the first position inside the directional sensor from the axial magnetic field measurement value 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 the magnetic interference generated from within the drill string.

11. A method for measuring the geomagnetic field at a survey point along a section of a well using a directional sensor, the method comprising: Performing three or more axial magnetic field measurements at three or more separate locations inside the directional sensor and along its longitudinal tool axis at the survey point; Using a computer system: Receive three or more axial magnetic field measurements at the survey point; Determine that the monopole approximation is valid; Solve a set of simultaneous equations to obtain the axial component of the geomagnetic field affected by magnetization interference in the drill string including the orientation sensor, and obtain interference terms at three or more separate positions inside the orientation sensor at the survey point, where the interference terms include the magnetic field generated by magnetic sources within the drill string; And Store the interference terms in non-transitory memory.

12. The method according to claim 11, wherein, Performing three or more axial magnetic field measurements at three or more separate positions inside the orientation sensor at the survey point and along its longitudinal tool axis further includes: using more than three axial magnetometers positioned at the three or more separate positions inside the orientation sensor, and the more than three axial magnetometers are longitudinally spaced apart from each other along the longitudinal tool axis of the orientation sensor.

13. The method according to claim 11, wherein, Performing three or more axial magnetic field measurements at three or more separate positions inside the orientation sensor at the survey point and along its longitudinal tool axis further includes: using axial magnetometers capable of moving to the three or more separate positions inside the orientation sensor.

14. The method according to claim 11, wherein The method further includes: solving the set of simultaneous equations by first solving the interference pole position equation.

15. The method according to claim 11, wherein, The method further includes: using the measurements obtained at the survey point to determine the magnetic pole strength parameter.

16. The method according to claim 11, wherein, The method further includes measuring the gravity vector and the transverse magnetic field at a plurality of survey points.

17. The method according to claim 13, wherein, The survey point is a first survey point, and the method further includes: Performing subsequent axial magnetic field measurements at a second survey point after the first survey point, where the axial magnetometer is located at a first position among the three or more separate positions inside the orientation sensor; and, Subtracting a first interference term associated with the first position inside the orientation sensor from the axial magnetic field measurement values at the second survey point to obtain the axial component of the geomagnetic field at the second survey point, where the first interference term includes magnetic interference generated from within the drill string.

18. An orientation sensing system, comprising: A housing having a longitudinal axis; A threaded rod disposed along the longitudinal axis within the housing; A shuttle penetrating the threaded rod and capable of moving along the threaded rod; At least one axial magnetometer coupled to the shuttle; And A motor coupled to the threaded rod and configured to rotate the threaded rod to axially move the shuttle and the at least one axial magnetometer along the threaded rod.

19. The orientation sensing system according to claim 18, wherein, The orientation sensing system further includes: A data acquisition system configured to obtain axial magnetic field measurement values from the at least one axial magnetometer at three or more separate positions inside the housing; and A computing system configured to: Receive the axial magnetic field measurement values from the at least one axial magnetometer; and, Using the measured axial magnetic field values, an axial component of the geomagnetic field affected by magnetization interference within the drill string including the directional sensing system is obtained, wherein obtaining the axial component of the geomagnetic field includes determining magnetic interference generated from within the drill string at the survey point; wherein the computing system is further configured to determine an interference pole position prior to determining the axial component of the geomagnetic field.

20. The directional sensing system according to claim 19, wherein, The computing system is configured to determine a magnetic pole strength parameter.

21. The orientation sensing system according to claim 20, wherein, The determination of the interference pole position precedes the determination of the magnetic pole strength parameter.

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