Patent Application
20070030007
The present invention relates generally to an apparatus for logging a subterranean borehole. More specifically, this invention relates to a measurement tool for making substantially real time tool face angle measurements on a rotating drill collar. By linking such measurements to contemporaneously obtained real time measurements of certain formation properties, the azimuthal variation of the measured property may be determined. In this manner, an image of the measured property within the borehole may be developed. The present invention, therefore, relates specifically to a tool and method for obtaining and processing the real time tool face angle measurements required for borehole imaging applications.
Wireline and logging while drilling (LWD) tools measure physical properties of the formations through which a borehole traverses. Such logging techniques include, for example, natural gamma ray, spectral density, neutron density, inductive and galvanic resistivity, acoustic velocity, acoustic calliper, downhole pressure, and the like. Formations having recoverable hydrocarbons typically include certain well-known physical properties, for example, resistivity, porosity (density), and acoustic velocity values in a certain range. In some logging applications it is desirable to determine the azimuthal variation of particular formation properties (i.e., the extent to which such properties vary about the circumference of the borehole). Such information may be utilized, for example, to locate faults and dips that may occur in the various layers that make up the strata. Tools capable of producing azimuthally sensitive information on formation properties are typically identified as imaging tools.
Downhole imaging tools have been available in wireline form for some time. Such wireline tools typically create images by sending large quantities of circumferentially sensitive logging data uphole via a high-speed data link (e.g., a cable). Further, such wireline tools are typically stabilized and centralized in the borehole and include multiple (often times one hundred or more) sensors (e.g., resistivity sensors) extending outward from the tool into contact (or near contact) with the borehole wall. It will be appreciated by those of ordinary skill in the art that such wireline arrangements are not suitable for typical LWD applications. In particular, communication bandwidth with the surface would typically be insufficient during LWD operations (e.g., via known telemetry techniques) to carry large amounts of image-related data. Further, LWD tools are generally not centralized or stabilized during operation and thus require more rugged sensor arrangements.
Several attempts have been made to develop LWD tools and methods that may be used to provide images of various circumferentially sensitive sensor measurements related to borehole and/or formation properties. Many such attempts have made use of the rotation of the BHA (and therefore the LWD sensors) during drilling of the borehole. For example, Holenka et al., in U.S. Pat. No. 5,473,158, discloses a method in which sensor data (e.g., neutron count rate) is grouped by quadrant about the circumference of the borehole. Kurkoski, in U.S. Pat. No. 6,584,837, and Spross, in U.S. Pat. No. 6,619,395, disclose similar methods.
In prior art methods, conventional flux gate magnetometers are utilized to determine the tool face angle of the LWD sensor (which, as described in more detail below, is often referred to in the art as sensor azimuth) at the time a particular measurement or group of measurements are obtained by the sensor. While flux gate magnetometers (also referred to in the art as ring core magnetometers) can be used in borehole surveying applications, such magnetometers have some characteristics that are not ideally suited to imaging applications. For example, flux gate magnetometers typically have a relatively limited bandwidth (e.g., about 5 Hz). Increasing the bandwidth requires increased power to increase the excitation frequency at which magnetic material is saturated and unsaturated. In LWD applications, electrical power is often supplied by batteries, making such power a somewhat scarce resource. For this reason, increasing the bandwidth of flux gate magnetometers beyond about 5 Hz is not practical in many LWD applications. Flux gate magnetometers, therefore, are not well suited for making substantially real-time tool face angle measurements in many LWD settings. There exists a need for sensors and/or sensor arrangements that are suitable for making such real time tool face angle measurements.
Flux gate magnetometers are sensitive instruments requiring careful calibration and handling. Though magnetometers have been used in many LWD and MWD tools, these instruments present design challenges that add to the complexity and expense of the tools. The magnetometers are also relatively expensive, which further compounds this problem. A need exists, therefore, for a more simple, more rugged, and lower cost means for providing substantially real-time azimuthal information in LWD imaging applications.
Moreover, AC and/or DC power is often routed through a drill collar (e.g., from a turbine or a battery pack) to an LWD sensor. The magnetic field about the electrical transmission line is known to interfere with nearby magnetometers. While AC fields may be filtered in certain applications, DC fields are particularly difficult to accommodate. There also exists a need for an arrangement suitable for routing electrical power past magnetic field sensors deployed on a drill collar.
The present invention addresses one or more of the above-described drawbacks in prior art apparatuses used to measure tool face angles on a rotating drill collar. Exemplary embodiments of this invention include a measurement tool having a tri-axial arrangement of magnetoresistive magnetic field sensors deployed therein. The magnetoresistive sensors are configured to make substantially real time magnetic field measurements (e.g., at 10 millisecond intervals). Embodiments of the tool further include a programmed processor configured to calculate tool face angles from the magnetic field measurements. The processor may be further configured to correlate the calculated tool face angles with contemporaneously obtained logging while drilling data for use in constructing a borehole image of a formation property.
Exemplary embodiments of the present invention may advantageously provide several technical advantages. For example, embodiments of this invention advantageously enable tool face angles to be measured in substantially real time on a rotating drill collar. As such, embodiments of this invention may be utilized in conjunction with circumferentially sensitive LWD tools to form borehole images having improved circumferential sensitivity. Embodiments of the present invention also provide a less expensive and potentially more rugged means of obtaining real-time tool face angle information. Moreover, in exemplary embodiments of this invention, the magnetic field sensors are deployed to advantageously minimize or even substantially eliminate magnetic interference due to the transmission of electrical power through the tool, thereby improving the accuracy of the calculated tool face angles.
In one aspect the present invention includes a borehole imaging tool. The tool includes a tool body configured for rotating with a drill string in a subterranean borehole and at least one magnetoresistive magnetic field sensor deployed in the tool body. The magnetoresistive sensor is disposed to measure first and second cross axial components of a magnetic field in the subterranean borehole. The tool further includes a programmed processor communicatively coupled with the at least one magnetoresistive magnetic field sensor. The programmed processor is configured to (i) calculate tool face angles in substantially real time from the cross axial components of the magnetic field, (ii) receive logging while drilling data from a logging while drilling sensor, and (iii) correlate the logging while drilling data and the tool face angles into a set of corresponding data pairs for use in constructing a borehole image of a formation property.
In another aspect, this invention includes a borehole imaging. The tool includes a tool body configured for rotating with a drill string in a subterranean borehole, at least one magnetoresistive magnetic field sensor deployed in the tool body, and at least one logging while drilling sensor deployed in the tool body. The magnetoresistive sensor is disposed to measure first and second cross axial components of a magnetic field in the subterranean borehole, while the logging while drilling sensor is disposed to make formation property measurements in the subterranean borehole. The tool further includes a programmed processor communicatively coupled with the at least one magnetoresistive magnetic field sensor and the at least one logging while drilling sensor. The programmed processor is configured to calculate tool face angles of the at least one logging while drilling sensor in substantially real time from the cross axial components of the magnetic field.
In a further aspect, this invention includes a downhole measurement tool. The measurement tool includes a tool body configured to be operatively coupled with a drill string and deployed in a subterranean borehole. The measurement tool further includes an electrical transmission path for conducting electrical power from one longitudinal end of the tool to another longitudinal end thereof. The transmission path includes an electrically conductive, non-magnetic tube, deployed in the tool body. At least one magnetic field sensor is deployed in the conductive tube.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter, which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
Before proceeding with a discussion of the present invention, it is necessary to make clear what is meant by “azimuth” as used herein. The term azimuth has been used in the downhole drilling art in two contexts, with a somewhat different meaning in each context. In a general sense, an azimuth angle is a horizontal angle from a fixed reference position. Mariners performing celestial navigation used the term, and it is this use that apparently forms the basis for the generally understood meaning of the term azimuth. In celestial navigation, a particular celestial object is selected and then a vertical circle, with the mariner at its center, is constructed such that the circle passes through the celestial object. The angular distance from a reference point (usually magnetic north) to the point at which the vertical circle intersects the horizon is the azimuth. As a matter of practice, the azimuth angle was usually measured in the clockwise direction.
In this traditional meaning of azimuth, the reference plane is the horizontal plane tangent to the earth's surface at the point from which the celestial observation is made. In other words, the mariner's location forms the point of contact between the horizontal azimuthal reference plane and the surface of the earth. This context can be easily extended to a downhole drilling application. A borehole azimuth in the downhole drilling context is the relative bearing direction of the borehole at any particular point in a horizontal reference frame. Just as a vertical circle was drawn through the celestial object in the traditional azimuth calculation, a vertical circle may also be drawn in the downhole drilling context with the point of interest within the borehole being the center of the circle and the tangent to the borehole at the point of interest being the radius of the circle. The angular distance from the point at which this circle intersects the horizontal reference plane and the fixed reference point (e.g., magnetic north) is referred to as the borehole azimuth. And just as in the celestial navigation context, the azimuth angle is typically measured in a clockwise direction.
It is this meaning of “azimuth” that is used to define the course of a drilling path. The borehole inclination is also used in this context to define a three-dimensional bearing direction of a point of interest within the borehole. Inclination is the angular separation between a tangent to the borehole at the point of interest and vertical. The azimuth and inclination values are typically used in drilling applications to identify bearing direction at various points along the length of the borehole. A set of discrete inclination and azimuth measurements along the length of the borehole is further commonly utilized to assemble a well survey (e.g., using the minimum curvature assumption). Such a survey describes the three-dimensional location of the borehole in a subterranean formation.
A somewhat different meaning of “azimuth” is found in some borehole imaging art. In this context, the azimuthal reference plane is not necessarily horizontal (indeed, it seldom is). When a borehole image of a particular formation property is desired at a particular point with the borehole, measurements of the property are taken are points around the circumference of the measurement tool. The azimuthal reference plane in this context is the plane centered at the measurement tool and perpendicular to the longitudinal direction of the borehole at that point. This plane, therefore, is fixed by the particular orientation of the borehole measurement tool at the time the relevant measurements are taken.
An azimuth in this borehole imaging context is the angular separation in the azimuthal reference plane from a reference point to the measurement point. The azimuth is typically measured in the clockwise direction, and the reference point is frequently the high side of the borehole or measurement tool, relative to the earth's gravitational field, though magnetic north may be used as a reference direction in some situations. Though this context is different, and the meaning of azimuth here is somewhat different, this use is consistent with the traditional meaning and use of the term azimuth. If the longitudinal direction of the borehole at the measurement point is equated to the vertical direction in the traditional context, then the determination of an azimuth in the borehole imaging context is essentially the same as the traditional azimuthal determination.
Another important label used in the borehole imaging context is the “tool face angle”. When a measurement tool is used to gather azimuthal imaging data, the point of the tool with the measuring sensor is identified as the “face” of the tool. The tool face angle, therefore, is defined as the angular separation from a reference point to the radial direction of the tool face. The assumption here is that data gathered by the measuring sensor will be indicative of properties of the formation along a line or path that extends radially outward from the tool face into the formation. The tool face angle is an azimuth angle, where the measurement line or direction is defined for the position of the tool sensors. In the remainder of this document, the terms azimuth and tool face angle will be used interchangeably, though the tool face angle identifier will be used predominantly.
Turning now to
Measurement tool 100 includes at least one magnetic field sensor 120. Measurement tool 100 may also further include one or more accelerometers gyroscopes. As described in more detail below with respect to
LWD tool 250 typically includes at least one LWD sensor 260 deployed thereon. Such LWD sensors may include substantially any downhole logging sensors, for example, including a natural gamma ray sensor, a neutron sensor, a density sensor, a resistivity sensor, a formation pressure sensor, an annular pressure sensor, an ultrasonic sensor, an audio-frequency acoustic sensor, and the like. While the embodiment shown on
It will be understood by those of ordinary skill in the art that the deployment illustrated on
Referring now to
As described above with respect to
It will be appreciated that according to Ampere's law, there is essentially no magnetic field inside a hollow conductor due to electrical current in the conductor. Ampere's law states that the integral of the magnetic field about any closed loop path is equal to the magnetic permeability times the electric current enclosed in the loop. This may be expressed mathematically as follows: {right arrow over (B)}d{right arrow over (l)}=μ0Ienclosd Equation 1
where {right arrow over (B)} represents the magnetic field, μ0 represents the magnetic permeability, and Ienclosd represents the electrical current closed in the loop. The cylindrical symmetry of tube 150 requires that the magnetic field {right arrow over (B)} is essentially constant about any circle whose center is coaxial with the tube 150. The magnetic field may therefore be removed from the integral yielding:
{right arrow over (B)}d{right arrow over (l)}=μ0Ienclosd Equation 2
Since the electrical current enclosed in a circular path just inside the inner wall of the tube 150 is essentially zero (Ienclosd=0 due to the lack of a conducting medium), the magnetic field due to the electrical current in the tube must also be essentially zero. As such, an electric current passing through the conductive tube 150 (e.g., from power sub 200 to LWD tool 250) creates substantially no magnetic interference inside the tube 150. Therefore, the effect of magnetic interference from electrical currents in the tool may be advantageously minimized (or even substantially eliminated) via deployment of the magnetic field sensors 120 inside the conductive tube 150.
Magnetic field sensor 120 may include substantially any sensor suitable for obtaining tool face angles on a rotating drill collar, such as magnetometers or magneto-resistive sensors (either giant magneto-resistive (GMR) sensors or anisotropic magneto resistive (AMR) sensors may be used). In the exemplary embodiment shown, measurement tool 100 includes a tri-axial arrangement Mx, My, and Mz of GMR sensors deployed in tube 150. Such a tri-axial arrangement, in which one of the sensors has a known orientation relative to longitudinal axis 70 (in the exemplary embodiment shown on
With continued reference to
With reference now to
The magnetic field and gravity sensors referred to herein are preferably chosen from among commercially available sensor devices known in the art. Suitable accelerometer packages include, for example, Part Number 979-0273-001 commercially available from Honeywell, and Part Number JA-5H175-1 commercially available from Japan Aviation Electronics Industry, Ltd. (JAE). As described in more detail below, suitable magnetic field sensors include magnetoresistive sensors, for example, Part Number HMC-1021D, available from Honeywell.
In the exemplary embodiment shown, A/D converter 170 is electronically coupled to a microprocessor, for example, via a 16-bit bus. Substantially any suitable microprocessor may be utilized, for example, including an ADSP-2191 M microprocessor, available from Analog Devices, Inc. It will be understood that while not shown in
A suitable controller may also optionally include other controllable components, such as sensors, data storage devices, power supplies, timers, and the like. The controller may also be disposed to be in electronic communication with various sensors and/or probes for monitoring physical parameters of the borehole. For example, the controller may be disposed to communicate with LWD tool 250 shown on
Turning now to
The magnetoresistive elements are typically made from, a nickel-iron (permalloy) thin film deposited on a silicon wafer and patterned as a resistive strip. In the presence of a magnetic field, a change in the bridge 125 resistance causes a corresponding change in voltage output. The change in the bridge 125 resistance is referred to as the magnetoresistive effect and is directly related to the current flow in the bridge 125 and the magnitude and direction of the magnetic field (the magnetic field vector). Suitable magnetoresistive sensors include, for example, part number HMC-1021D, available from Honeywell (Plymouth, Minn.).
With continued reference to
In one exemplary method embodiment, measurement tool 100 (
In the exemplary method embodiment described above, a tri-axial set of magnetic field measurements may be obtained, for example, at 10 millisecond intervals. For a drill collar rotating at 200 rpm, tool face angles may be determined 30 times per revolution (i.e., at 12 degree intervals). It will be understood that the invention is expressly not limited in this regard, since magnetic field measurements may be made at substantially any suitable interval, either faster or slower than 10 milliseconds. Magnetoresistive sensors are known to be capable of achieving high frequency magnetic field measurements and are easily capable of obtaining magnetic field measurements at intervals of less than 1 millisecond or even at intervals less than 10 microsecond. It will be appreciated that in practice the advantages high frequency magnetic field measurements (e.g., better tool face resolution) may be offset by the challenge of storing and processing the large data sets generated by such high frequency measurements. Nevertheless, as state above, this invention is not limited to any particular magnetic field measurement frequency or to any particular time intervals.
It will be understood that gravitational and magnetic field measurements may be processed to determine tool face angles using substantially any known mathematical techniques. Such techniques are well established in the art, and may be utilized to calculate the tool face angles in substantially any suitable coordinate system, including, for example, earth, tool, and borehole coordinate systems. Moreover, known techniques may be utilized to transform tool face angles between coordinate systems.
For example only, magnetic tool face angles may be determined in substantially real time relative to a “magnetic high side” of the tool (using the real time magnetic field measurements) as follows:
where MTF represents the magnetic tool face angle and Mx and My represent the x and y components (also referred to as the cross axial components) of the measured magnetic field. As described above, the magnetic tool face angle may be acquired substantially continuously in real time (e.g., at 10 millisecond intervals) while the measurement tool is rotated in the borehole, for example, during drilling. The artisan of ordinary skill in the art will readily be able to transform the magnetic tool face angles determined in Equation 3 to more conventional borehole coordinates (e.g., in which the tool face angle is defined relative to the gravitational high side of the borehole), for example, via processing with the local inclination and azimuth of the tool (or borehole).
In a typical drilling operation, an MWD survey is typically taken when the drill bit is off bottom and after a new section of drill pip has been added to the drill string. Such a survey typically includes, among other things, measuring tri-axial components of the gravitational and magnetic fields and using the measurements to calculate tool (borehole) inclination and azimuth. For example, inclination and azimuth may be determined via the following known equations:
where Inc and Azi represent the inclination and azimuth of the measurement tool in the borehole, Gx, Gy, and Gz represent the tri-axial components of the measured gravitational field, and Mx, My, and Mz represent the tri-axial components of the measured magnetic field. As stated above, the inclination and azimuth may be used to transform the magnetic tool face angles into conventional borehole coordinates.
Alternatively, tool face angles may be computed directly using the cross axial components of the gravity and magnetic field measurements. In such embodiments, the magnetic field measurements may be made in substantially real time (as described above), while the gravity measurements are typically made intermittently, for example, at an MWD survey (as described above). One such direct solution is given below in Equation 5:
where φ represents the tool face angle in conventional borehole coordinates, Mx and My represent the measured cross axial components of the magnetic field (typically measured in substantially real time as described above), and where {right arrow over (B)}x and {right arrow over (B)}y are functions of the cross axial components of the gravitational and magnetic fields measured during the MWD survey (e.g., as described above).
While the invention is not limited in this regard, tool face angles measured in substantially real time may be advantageously correlated with circumferentially sensitive logging data to form borehole images. Such logging data may be acquired from substantially any suitable logging while drilling tool (e.g., LWD tool 250 shown on
In one exemplary embodiment, a continuous LWD sensor response may be averaged at some predetermined sampling interval (e.g., 10 milliseconds). The duration of each sampling interval is preferably significantly less than the period of the tool rotation in the borehole (e.g., the sampling interval may be about 10 milliseconds, as stated above, while the rotational period of the tool may be about 0.5 seconds). The sensor response may include substantially any LWD sensor response, including for example, an AC current in a LWD resistivity tool, gamma ray radiation counts at a gamma ray detector, and acoustic energy at an acoustic sensor. The invention is not limited in this regard. Meanwhile, a tool face sensor (such as magnetic field sensor 120 shown on
Azimuthally sensitive LWD measurements are typically utilized to form a two-dimensional image of the measured borehole property, the two dimensions being the tool face angle in the borehole and the well depth. To form such a two-dimensional image, LWD sensor measurements may be acquired at a plurality of well depths using substantially any suitable procedure. For example, LWD sensor data may be acquired substantially continuously as described above during at least a portion of a drilling operation. The above-described sampling intervals may be further grouped at relatively longer time intervals (e.g., in 10 second intervals) with each group indicative of a single well depth. At a drilling rate of about 60 feet per hour, a 10 second interval represents about a two-inch depth interval. To form a two-dimensional image the sensor data may be tagged with both a measured tool face angle and a well depth. It will be appreciated that this invention is not limited to any particular sampling intervals and/or time periods. Nor is this invention limited by the description of the above exemplary embodiments.
It will be appreciated that certain LWD tools make use of a plurality of LWD sensors deployed about the periphery of the tool. Such embodiments may advantageously enable azimuthally sensitive measurements to be made about the circumference of the borehole without rotation of the drill string. Moreover, when used with a rotating drill string, such embodiments may advantageously provide for redundancy as well as reduced system noise accomplished via averaging the data acquired at the various sensors.
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alternations can be made to the embodiments set forth herein without departing from the spirit and scope of the invention as defined by the appended claims.
