EP1373938A1

EP1373938A1 - Method and apparatus for determining the high side of a drill string during gamma mwd operations and correlating gamma events therewith

Info

Publication number: EP1373938A1
Application number: EP02733787A
Authority: EP
Inventors: Roger P. Bartel; Michael Larronde
Original assignee: Halliburton Energy Services Inc
Current assignee: Halliburton Energy Services Inc
Priority date: 2001-04-06
Filing date: 2002-01-30
Publication date: 2004-01-02
Also published as: BR0206908A; CA2439809A1; NO20034398L; AU2006202091A1; NO20034398D0; WO2002082124A1; EP1373938A4

Abstract

A directionally sensitive MWD gamma tool (10) and method of use that facilitates geosteering operations by providing formation characteristics data and correlating such data with a referenced orientation of the tool (10). A first embodiment includes an array (14) of gamma sensors radially arranged within the tool (10) and that is circumferentially exposed to the formation. In an alternative embodiment, a single gamma sensor comprising a scintillation crystal and photomultiplier tube is arranged within a rotating shield so as to focus the direction of formation interrogation. In another aspect of the preferred embodiments, the invention provides an array of magnetic and gravitational sensors that together provide data sufficient to establish an absolute reference orientation of the directional gamma sensor positioned on the tool (10). The methods for implementing the apparatus of the invention include transmitting individual channel counts for gathered gamma sensor data independently to the surface for analysis, or correlating and identifying high side and low side portions of the data prior to transmission to the surface and selectively transmitting such information as has been identified for geosteering operations.

Description

METHOD AND APPARATUS FOR DETERMINING THE HIGH SIDE OF A

DRILL STRING DURING GAMMA MWD OPERATIONS AND

CORRELATING GAMMA EVENTS THEREWITH

INVENTORS

ROGERP. BARTEL MICHAEL LARRONDE

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The present invention relates generally to methods and devices for investigating formation characteristics within a borehole during the drilling process. The present invention relates more specifically to methods and devices that facilitate directional drilling (especially geosteering operations) by producing a circumferential set of formation data for the borehole, correlated to a desired reference. The reference in a first embodiment is the gravitational high side of the borehole while alternative embodiments may use a magnetic reference or a combination of the two. The present invention finds primary application in natural gamma radiation based MWD operations but may be adapted to provide an orientational reference for any borehole measurement that can be directionally rendered.

2. DESCRIPTION OF THE RELATED ART

Geosteering has gained increased popularity in the oil field industry especially in horizontal directional drilling. Geosteering is the process of directing the drill bit (using various mechanical devices and systems) along geologic structures and formations by relying upon real time measurements of formation characteristics made while drilling. These measurements are often electric resistivity measurements but may also be radiation based measurements, ultrasonic measurements, or any of a number of other types of measurements capable of being made while drilling.

Horizontal drilling operations, as an example, benefit greatly from knowing what formations are being approached or exited relative to the "high side" of the bent drill sub. Geosteering may typically involve directing the bit off axis in a direction toward or away from the high side as various formations are penetrated. Following a formation in a horizontal drilling operation will depend upon an accurate detection of the formation boundary and the proper reference to the high side of the bent drill sub. Natural radiation gamma sensors (often called "gamma" sensors) are well known to provide data regarding formation characteristics sufficient to identify a formation for the purposes of geosteering or the like. A problem frequently encountered, however, with standard measurement while drilling (MWD) gamma sensors is that the orientation of the gamma sensor relative the high side of the drill string is often random. The orientation of such sensors can only be determined after the bottom hole assembly is made up and torqued. If the fixed banks of gamma detectors do not align with the bent sub in a known orientation, a condition that is usually the case, the reading provided by the gamma detectors will not be very useful for the purposes of geosteering. The driller must periodically rotate the drill string specifically for the purpose of putting the detector banks in the proper orientation. This process is time consuming and cumbersome, especially with small drill collars in which 300° of rotation at the surface may correspond to only 50° of rotation at the bit over an interval long enough to be of significance for LWD measurements.

Various attempts have been made in the past to incorporate directionally sensitive components into both wire line logging methods and MWD methods. Examples of these efforts include the following:

U.S. Patent No. 5,539,225 issued to Loom is, et al. on July 23, 1996, entitled Accelerator-Based Methods and Apparatus for Measurement- While-Drilling, describes the use of a neutron accelerator in conjunction with a variety of radiation detectors spaced along the drill string immediately above the drill bit. Various combinations of the radiation source and the detectors provide measurements of porosity, density, and lithology, as well as the detection of gas. Various outputs from the logging tool are incidentally correlated to the angular or azimuthal orientation of a measurement device in the borehole. Such measurements are made using orthogonally arranged magnetometers.

U.S. Patent No. 5,017,778, issued to Wraight on May 21, 1991, entitled Methods and Apparatus for Evaluating Formation Characteristics While Drilling a Borehole Through Earth Formations, also describes an MWD method and apparatus for rotating a directionally-responsive radiation sensor having an outwardly directed response axis. Various computational methods are defined and described for providing output signals indicative of variations in the cross-sectional configuration of the borehole and for providing indications of the surrounding formation characteristics. While not responsive to a specific rotational reference, the apparatus is directional in its interrogation.

U.S. Patent No. 5,394,941, issued to Venditto, et al. on March 7, 1995, entitled Fracture Oriented Completion Tool System, describes a completion tool for use with a casing string within a well bore. The completion tool includes a casing valve with a radioactive source, the orientation of which may be determined by use of a specific detector structure. The purpose of the completion tool and the methods associated with its use, is to orient the tool to align ports thereon with the planes of existing downhole fractures. The concern here is orientation with respect to a radioactive flag for purposes of completion rather than orientation with respect to the formation.

U.S. Patent No. 4,034,218, issued to Turcotte on July 5, 1997, entitled Focused Detection Logging Technique, describes a wire line (as opposed to MWD) based system with a sonde that emits gamma radiation in tightly collimated beams. In addition, the system incorporates a detection collimator so as to focus the detected radiation in a zone of intersection with one of the emitted beams. Because the system is implemented only in a wireline configuration, orientation is more easily known and maintained. The result is a directional sensitivity to the information gathered about the formation in the direction of the tightly collimated gamma beams. U.S. Patent No. 5,205,167 issued to Gartner, et al. on April 27, 2993, entitled Method and Apparatus for Locating Stratification in Production Fluid in a Well, describes a wire line device for measuring the volume of gas present in a multi-phase flow within a cased borehole. The device includes a low energy gamma ray source and a sodium iodide detector. Both the source and detector are surrounded by rotatable slit collimators to provide directional sensitivity to variations in the gas volume percentage in the flow. Here again the directional response is possible only because the sensors are implemented in a wire line system.

U.S. Patent No. 4,692,617, issued to Allen, et al. on September 8, 1987, entitled Method and System for Neutron Lifetime Logging describes a borehole logging tool implemented on a wire line system that includes a pulsed source of fast neutrons and a radiation detector. The radiation detector is mounted on an articulating arm and maintains contact with the borehole wall. Such contact establishes and maintains the detector in a known directional orientation. U.S. Patent No. 4,743,755, issued to Williams on May 10, 1998, entitled

Method and Apparatus for Measuring Azimuth and Speed of Horizontal Fluid Flow by a Borehole, describes another wire line based logging system for measuring the azimuth and speed of movement using a conventional well logging sonde having a neutron source and a gamma ray detection assembly. The detection assembly includes a segmented crystal and a segmented photomultiplier tube optically coupled to the crystal. Here again the system describes a sensor structure that is directional in nature but which depends on wire line implementation for orientation with respect to the formation.

U.S. Patent No. 4,705,944 issued Coope on November 10, 1987, entitled Formation Density Logging While Drilling, describes a gamma ray density sub and method of use for MWD applications. The methods described involve the computation of counting rates obtained from at least three locations in a formation sample, the locations being in a radially symmetrical pattern about the sub. The purpose of this system and method is to measure the density of a sample within the formation independent of the location of the sub and the composition of intervening drilling fluid. Directional orientation in this case is important only to provide distinct sampling and does not serve to identify an actual azimuth within the formation. U.S. Patent No. 5,012,091, issued to Moake on April 30, 1991, entitled

Production Logging Tool for Measuring Fluid Densities, describes yet another wire line based system having a shielded sonde supporting a radiation source that generates a characteristic gamma ray emission pattern. Ports aligned with a detector limit the gamma ray pathway primarily to the fluid surrounding the tool. The goal is to provide accurate data to enable a determination of the bulk density of the fluid in the well. Thus while the detector ports provide directional response, no directional reference response is obtained.

Previous attempts to provide directional reference sensitivity to gamma detection methods incorporated in MWD systems have suffered from poor resolution and limited applicability. Efforts at azimuthal or directional sensitivity for wire line based systems, although they enjoy greater resolution and accuracy, do not translate easily into similar systems for MWD operations, especially in horizontal drilling environments. The above references, therefore, while each attempting to provide azimuthal or directional information about the formation or the drilling fluid within the borehole, could not provide adequate systems for geosteering operations in horizontal drilling environments.

It would be desirable to have a gamma based MWD tool capable of interrogating the surrounding formation with sufficient resolution to provide the drilling operator with enough information to accurately geosteer the drill bit to or through the formations of interest. It would be desirable if such a system could readily identify a referenced orientation for the tool being used to interrogate the formation and thereafter associate the information gathered about the formation with the high or low side of the bent drill sub. It would desirable if such directionally referenced identification could be acquired and retained prior to the transmission of formation data to the surface for analysis in a manner that reduces the number of transmission channels required for either telemetry transmission to the surface or other methods for communicating MWD data. SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a novel method and apparatus to facilitate directional drilling (especially in geosteering operation) by producing a referenced directional response for a gamma sensor operating in an MWD environment, irrespective of the orientation of the MWD tool. It is a further object of the present invention to provide a directionally sensitive

MWD gamma tool that provides multiple channel counts for gamma based sensors that may be identified by discriminating the orientation of the tool and correlating each channel count with a particular orientation so as to characterize a high side and low side bank of sensors or to characterize any number of arc segments of the borehole circumference as desired for orientation.

It is a further object of the present invention to provide a directionally sensitive gamma based MWD tool capable of providing multiple channel counts dependent upon the radial orientation of the sensors and further capable of providing downhole discrimination of the high side and low side of the array of sensors (or of any other array subsets) prior to transmission of formation data to the surface.

It is a further object of the present invention to provide a directionally sensitive gamma tool capable of implementation in a small diameter sonde and operable by rotation of a window about a single gamma detection sensor system or operable in an insert configuration by rotation of a directional sensor about the insert to the desired reference.

It is a further object of the present invention to provide methods in an MWD environment for characterizing the high side and low side (or any other radial segment) of a bent drill sub based upon information gathered from a directionally sensitive array of sensors or a rotating sensor structure so as to facilitate a geosteering operation.

It is a further object of the present invention to provide methods and an apparatus for use in MWD environments that include the use of magnetic and gravitational sensors for establishing an referenced orientation for radiation based sensors in order to discriminate the directional orientation of formation characteristics measured by the radiation sensors.

It is a further object of the present invention to provide an apparatus for use in measurement while drilling systems that includes a plurality of magnetometers and accelerometers for appropriate identification of a directional orientation for the drill string both in terms of inclination and radial orientation of radiation sensors associated therewith.

In fulfillment of these and other objectives, the present invention provides a directionally sensitive MWD gamma tool and method of use that facilitates a geosteering operation by providing formation characteristics data and correlating such data with a referenced orientation of the tool. A first preferred embodiment of the present invention includes an array of gamma sensors radially arranged within the tool and that is circumferentially exposed to the formation. In an alternative embodiment, a single gamma sensor comprising a single scintillation crystal and photomultiplier tube are arranged within a rotating cylindrically structured shield so as to focus the direction of formation interrogation. In a further alternative embodiment, two gamma sensors include a pair of scintillation crystals and associated photomultiplier tubes that are arranged within a rotating cylindrical shield with openings positioned diametrically opposed to each other so as to interrogate the formation in opposite directions. In another aspect of the preferred embodiments, the present invention provides a plurality of magnetic and gravitational sensors that together provide data sufficient to establish an absolute reference orientation of a directional gamma sensor positioned on the tool. A bank of magnetometers and an associated array of accelerometers provide orientation of a tool (and in particular the radiation based sensor) with respect to the magnetic field and the inclination of the tool from the vertical or horizontal. The methods for implementing the apparatus of the invention include transmitting individual channel counts for gathered gamma sensor data independently to the surface for analysis, or correlating and identifying high side and low side portions of the data prior to transmission to the surface and selectively transmitting such information as has been identified for geosteering operations.

Other objects and advantages will be discerned by those skilled in the art after a reading of the following detailed description of the preferred embodiment and a consideration of the claims that follow.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic side view of one embodiment of a downhole MWD assembly of the present invention.

Fig. 2 is a cross-sectional view taken along line 2-2' in Fig. 1 showing the radially spaced position of the radiation detectors in a first embodiment of the present invention.

Fig. 3 is a schematic side view of a second embodiment of a downhole measurement assembly of the present invention when used in conjunction with a wire line based sonde. Fig. 4 is a horizontal cross-sectional view taken along line 4-4' in Fig. 3 showing the sliding window configuration of the tool.

Fig. 5 is a horizontal cross-sectional view taken along line 5-5' in Fig. 3 showing the mechanisms for rotating the shield.

Fig. 6 is a schematic perspective view of a tool sub containing sensors that provide an azimuthal response while rotating based upon an array of magnetic and gravitational sensors in conjunction with a directional gamma sensor. DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is understood that recognition of the orientation of a drill bit with respect to a specific formation depends upon two primary elements. First is the ability to interrogate the formation surrounding the borehole in a directionally sensitive manner. That is, the data gathered by a sensor about a formation characteristic must describe the formation in a specific radial direction out from the central axis of the borehole and not simply describe an average characteristic associated with the sensor's depth within the borehole. The sensor itself must be directional in nature.

The second primary element required for recognizing the orientation of a drill bit is the establishment of a directional reference against which the orientation of the sensors can be known or easily established. The present invention therefore includes components directed to each of these essential elements. They will each be described in turn and in combination.

Directional Gamma Sensors Reference is made first to Fig. 1 for a schematic side view of a first preferred embodiment of a downhole measurement assembly implementing the structures of the present invention. The arrangement shown in Fig. 1, oriented for clarity as it would appear in a horizontal borehole, incorporates an array of Geiger Mueller tubes within a tool section above the drill bit. Tool assembly (10), positioned within tool sub (12), includes Geiger Mueller tube array (14). Geiger Mueller tube array (14) (described in more detail below) receives gamma event information from the formation surrounding the borehole. Sensor channel counter (18) and its associated electronics (20) are positioned adjacent sensor tube array (14) within tool assembly (10). Individual signal lines (22) connect each of the individual Geiger Mueller tubes in array (14) with channel counter (18). In alternate implementations of the first preferred embodiment, multiple detectors may be grouped on each channel. For example, in larger diameter tools sixteen detectors might be used with two adjacent geiger mueller tubes grouped per channel. Support and spacing structure (24) maintains sensor tube array (14) in an appropriate radial orientation as described below. Since the structures of this first embodiment shown in Fig. 1 are intended to be operable in a measurement while drilling (MWD) environment, all of the structures are amenable to being positioned around a centralized drilling fluid conduit (26) within the drill string. The directional sensor described above with respect to Fig. 1 could likewise be integrated into a sonde based tool with smaller diameter Geiger Mueller tubes to form an array. While not a preferred embodiment, such arrangement would have benefits for standard sonde based tool operations. The smaller diameter environment would permit less shielding for isolation between the tubes and may necessitate a smaller number of tubes within the array. The configuration of the sensors could be modified to take advantage of the absence of the drilling fluid conduit in the sonde based tool.

In Fig. 2, a horizontal cross-sectional view taken along line 2-2' in Fig. 1, the radial orientation of Geiger Mueller tube array (14) can best be seen. Because each of the Geiger Mueller tubes have individual channels for signal processing and transmission, they can be made directionally sensitive when appropriately identified and grouped. As an example, after the bottom hole assembly is made up, a determination can be made of which tubes need to be grouped together to form the "high side" bank and the "low side" bank. This information can be relayed to the surface either as individual tube counts, in which case the signals can be reassembled to form the high side and the low side bank, or the information may be grouped together downhole and transmitted as a group to save transmission bandwidth to the surface. In any event, "north" and "south", as well as "east" and "west" banks, can be determined and information correlated as such can be transmitted to the surface. Alternately, as indicated above, any circumferential subset may be selected and referenced to achieve a desired polar resolution. Multiple detectors that are radially arrayed may be grouped or distinguished in any of a number of different arrangements to identify both symmetric and asymmetric orientations. As a further example, the array of tubes shown in Figs. 1 and 2 can be used to orient a bent sub in the configuration depicted. If oriented with the high and low sides as indicated, then channels C, D, and E (shown in Fig. 2) may be grouped and identified as the "high side" channel, either at the surface or downhole prior to transmission to the surface. Channel G, H, and A would therefore be considered as the "low side" channel. In the preferred embodiment, channels B and F, deriving from tube sensors midway between the high side and low side of the tool, would have their counts ignored in order to improve the separation and directional response for the instrumentation.

Alternatively, within certain formational environments, Channels A, B and C could be considered the "west side" group for geosteering purposes, and Channels E, F, and G could be considered the "east side" group. In this case, Channels D and H would be ignored in order to improve separation.

Various formational characteristics might dictate other combinations of the directionally sensitive tube array. The actual structures and electronics associated with such sensors are well known in the art. It is the identification, grouping, and transmission of the appropriate group signals to the surface that provide the improved method associated with the present invention.

The above two examples find specific application when a "mud motor" is employed as the driving means for the drill bit. In such a case the drill string is held stationary while the bent sub and the rotary action of the mud motor alter the trajectory of the hole. However, even when a mud motor is employed, there are instances when the entire drill string must be rotated (rotary drilling) to maintain the hole at its then current trajectory. In such a mode other mechanisms must be employed to correlate or reference the Geiger Mueller tube array. Also shown in Fig. 2 are circuit board mounted positioning devices (designated

X-Y magnetometer and accelerometer arrays) whose structures and functions are described in more detail below. As indicated, the operation of the present invention includes both the use of directional gamma sensors for deriving information about the formation surrounding the borehole and position referencing sensors capable of establishing a directional orientation for the rotating drill string. The manner in which the directional/positional sensors shown in Fig. 2 are utilized to reference the formation data is discussed in more detail below. An alternative embodiment of the present invention with primary application to a sonde based logging system, is disclosed in Figs. 3-5. Although the structures of the present invention find particular applicability in MWD operations, many of the directional sensor structures required for implementing the methods of the present invention in MWD operations translate well into sensor structures for wire line operations. The alternative embodiment described in Figs. 3-5 is an example of the manner in which a directional characteristic can be given to a sensor system that has heretofore been random in character. Fig. 3 is a schematic side view of a sonde based tool where there would typically not be adequate room for a definitive array of tubes such as those shown in Figs. 1 and 2. In this case, a single photomultiplier tube and a scintillation crystal may be employed. In addition, a dense material such as tungsten may be used to encase the scintillator crystal and thereby block radiation from all directions except where a window is cut to admit the radiation. In Fig. 3, tool assembly (40) is comprised of rotating shield (42), with windows (44), that surround gamma sensors comprised of scintillator crystal (50)/photomultiplier tube (52), and scintillator crystal (54)/photomultiplier tube (56). The directional sensitivity of this embodiment derives from the rotatable structure of shield (42) with windows (44). Shield (42) is rotated by means of a DC stepping motor (46) connected by gear arrangement (48) to a geared track on rotating shield (42). In this manner, directionally identifiable orientations of shield (42) can be associated with the signals received through the gamma sensors made up of scintillators (50) and (54), and photomultiplier tubes (52) and (56), respectively. Appropriate identification of the directional orientation of windows (44) can be made through either a definitive knowledge of the relative position of stepping motor (46) or the use of separate positioning sensors (not shown but described in more detail below) mounted in appropriate fashion on rotating shield (42).

Fig. 4 is a horizontal cross-sectional view taken along line 4-4' shown in Fig. 3.

In this view rotating shield (84) can be seen surrounding scintillator crystal (80) and an internal sleeve (82), which serves to support the scintillator crystal and the photomultiplier tube within the sonde. In this view a single window (86) is shown oriented for the purposes of receiving formation radiation data from the low side of the tool.

Fig. 5 is a horizontal cross-sectional view of the structure of the sonde shown in Fig. 3, detailing a first preferred arrangement of a gearing mechanism for stepwise rotation of the shield. In this embodiment interior geared track (62) engages with a plurality of planetary gears (66) and (68) driven by DC motor gear (64) centrally positioned within the tool. Engagement of track (62) causes shield (60) to rotate in a manner that redirects the window (not shown in Fig. 5) in an appropriate and identifiable orientation.

Methods associated with utilizing the embodiments shown in Figs. 3-5 likewise involve a high side determination that is made after the bottom hole assembly is made up. Once a high side orientation has been established (a specific rotation of the shield), then subsequent gamma radiation readings will always be identifiable with a particular directional orientation.

The embodiment shown in Fig. 3 actually incorporates two separate windows (44), one for each of the sensor pairs shown. In this configuration, windows (44) are diametrically opposed, albeit positioned on the same shield (42). This provides increased data collection while maintaining directional sensitivity. Alternatively, separate shields could be utilized with separate stepping motors driving the shields so as to orient independently operable windows. Tool Orientation

A further alternative preferred embodiment of the present invention involves the construction of a tool sub with both a directional gamma sensor and an array of magnetic and gravitational sensors that provide a directional response while the gamma sensor is rotating. Such a sensor system requires information about the rotational position of the tool at all times during drilling and during the acquisition of radiation data. Various orientational sensors (both magnetic and gravitational) are known in the art that can derive the relative positions of the gamma sensors with respect to a fixed reference. These orientational sensors include accelerometers and magnetometers appropriate for detecting and quantifying changes in the orientation and position of the tool sub. As is well known in the art, accelerometers are capable of real time identification of rotational displacement from a reference while drilling operations are occurring. Likewise, as is known in the art, magnetic flux sensors or magnetometers facilitate the identification of an azimuthal orientation of the tool based upon the earth's magnetic field.

Although the use of magnetometers and accelerometers is well known in the field and techniques for implementing both such types of sensors have been developed, it is generally believed that the relevancy of a directionally sensitive sensor array is only useful geologically when the tool is at higher angles off of vertical (perhaps as much as 45° or greater off of vertical). Thus, while conventional crossover to gravity effects is made at 6° off of vertical (for tool face readings), application of the methods and systems of the present invention find more significant use at greater angles of inclination. Under such conditions, a number of data elements are required to compute actual azimuthal orientations. This information includes the local dip angle as well as accelerometer data that can be correlated with data from a magnetometer array. Clearly, magnetic based directionally sensitive sensors are more practical when the borehole orientation is vertical and become less practical with greater horizontal inclination. Gravitational based sensors, on the other hand, are more practical with horizontal drilling operations and less important within vertical boreholes. An appropriate combination, therefore, would anticipate a variety of borehole orientations and will provide real time directionally sensitive response while the drill string is rotating. Reference is made to both Fig. 1 and Fig. 6 for a description of a tool sub containing such a sensor array structure that provides a directionally sensitive response while rotating based upon the grouping of referenced magnetic and gravitational sensors that measure the angular "tool face" position of the drill collar containing the gamma sensor. In this case, the gamma sensor is directional in nature (as described above) so as to discretely correlate with measurements made by the gravitational and magnetic sensors.

As gamma events occur, the position of the detector(s) would be noted, and the gamma event would be added to an appropriate tool face angular "bin". After a period of time, the "bins" would be parsed to locate the direction in which the maximum and/or minimum number of events had accumulated. This information could be relayed to the surface to guide the drilling operation. The bins could be emptied periodically to provide a new starting point.

The preferred embodiments of the present invention implement appropriate hardware and display software to present all eight channels as vector quantities on a polar coordinate graph. In a symmetrical approach this provides eight vectors 45° apart. The magnitude of each vector represents the number of counts collected when any of the gamma tubes (as an example) was positioned in that vector orientation. If the tool is stationary, then each vector will also represent the count for a specific individual tube. When rotating, however, every tube will spend some time in every 45° orientation vector. The resulting magnitude of each vector then represents the sum of all of the counts registered while any tube is in that vector. In this approach, therefore, there are channel A through H counts (which represent the counts for each individual tube) and vector 0° through 315° counts (in 45° increments) which any tube can contribute to. While rotating, and in a gamma flux which varies according to direction around the borehole, channels A-H will show statistically similar counts, while vectors 0° through 315° will depict varying magnitudes commensurate with the location of different gamma intensities. The directionally sensitive sensor embodiments described above define systems that align detectors by appropriate orientation of the drill string at the surface after the bottom hole assembly is made up. The detector offsets can then be physically measured and used to reference readings made by the gamma tool or some other such sensor. The realignment can be made either electronically by reassigning channel numbers, or mechanically by rotating a window or the detector array. While effective, these approaches may be considered to provide information on direction or orientation that is often less refined than desired.

The preferred embodiments of the present invention therefor use accelerometers and/or magnetometers to provide a continuous reference to the high side of the hole. This approach is most useful in MWD operations because it does not require alignment to the bent sub at the surface.

In most drilling operations there is a directional sensor of some type positioned on the drill string. This sensor references to a tool face offset in order to synchronize it with the bent sub. The driller uses the information from the directional sensor to position the rotary table and cause the downhole assembly to either build angle, drop angle, or drill straight ahead (drilling straight ahead is accomplished by commencing rotary drilling from the drilling rig). Since the driller can now control the direction of the drilling, information is needed to determine what direction to go. While the methods described initially above are beneficial, aligning the detectors to the bent sub is not useful while the drill string is rotating.

It is well known to use accelerometers to derive the position of a tool relative to gravity. However, most directional tools are optimized for stability and typically require a period of time (greater than 1 second) to converge on a reading. At 200 rpm a rotating tool will move through 45° in less than 45 milliseconds. Therefore, to insure accuracy of the above described process of vector binning, it is necessary to have almost immediate directional information. While accelerometers can be highly accurate in determining gravity vectors, they suffer from sensitivity to shock and vibration. Accelerometers are also influenced by the centrifugal forces of rotation. Such factors can render a measurement difficult to process or even useless in a typical downhole drilling environment.

Magnetometers on the other hand, while largely unaffected by shock, vibration, and centrifugal forces, have their own drawbacks. The presence of ferromagnetic material near the sensor can distort earth's magnetic field and alter the measurement. Alone, magnetometers can not indicate the high side of the borehole. The local magnetic dip angle must be known and entered into a bore hole high side determination. In addition, an azimuth calculation is required for such a determination. An azimuth calculation will typically require accelerometer data to resolve. To optimally achieve the above objectives, an array of magnetometers is mounted (preferably in a circumferential arrangement) with the sensitive axis of each magnetometer pointing radially outward. As an example, eight magnetometers may be mounted every 45° on a complete circle whose center is the axis of the gamma sensor. The magnetometers in the preferred embodiment are less than 0.2 inches square and consist of a simple Wheatstone bridge of four elements made of magnetostrictive material. The magnetostrictive material changes resistance with an applied magnetic field.

If the ring of magnetometers is rotated through one full revolution, each magnetometer will produce a sine wave whose phase is +/- 45° from the adjacent magnetometers. At any given instant, there will be one magnetometer that will generate a higher output voltage level than the others. (Theoretically there could be two at the same level if the magnetometers are positioned at the crossover point between the two.) While it is possible to determine which magnetometer has the highest output at any given moment, that magnetometer is not necessarily directed toward the gravity high side of the hole. What is determined is the magnetometer that is in the highest earth's flux. A relationship between the highest earth's flux and the gravity high side must also be established. Accelerometers are capable of measuring gravitational acceleration, but are easily altered by typical downhole drilling conditions. During drilling operations there are times when drilling ceases and the bottom hole assembly is stable. Accelerometers will then produce highly reliable measurements. From accelerometers, the gravity tool face angle can be calculated and the orientation of the gamma tube array (or other sensor array) versus gravity can be established. The magnetometer array may then be interrogated to determine which magnetometer has the highest reading. With this information, the relationship between the magnetometer array and the gamma tube array is easily established. Whenever a gamma count is detected, the position of the Geiger Mueller tube versus the gravity high side can be resolved by utilizing the previously determined association between the highest reading magnetometer and the gamma tube array.

While using the highest reading magnetometer, as described above, is an effective technique, there are other methods of differentiating the positions of the magnetometers in the array. These other methods become important because there are certain effects that reduce the resolution of the above described approach. These effects include magnetometer gain differences, and errors that occur in the sine wave transform. The former effect is one that most magnetometer devices experience because of manufacturing tolerances and sensitivity to temperature. If the gains of the magnetometers are different and/or change, a compensation method must be implemented. Otherwise the highest reading magnetometer may in fact not be the device positioned in the maximum magnetic flux.

The latter effect (the sine wave transform) occurs because the area in the vicinity of the sine wave peak suffers from poor resolution due to the low slope of the waveform. The highest sensitivity (change of output versus change of angular position) of a sine wave is near 0° and 180° (the zero crossing points). It is therefore necessary that some method of discriminating between these two points be implemented. Otherwise, using the example of eight magnetometers described above, two magnetometers may be simultaneously near their zero crossing positions. One solution is to adopt a convention that pairs each magnetometer to its 90° 'cousin' in the array (clockwise or counterclockwise). The decision therefore, would be to elect a magnetometer that is closest to zero and has a 90° cousin that is positive of zero. An alternative method would be to interrogate the array and select the two adjacent magnetometers which have opposite signs (either a plus to minus or a minus to plus convention), and then determine which of these two magnetometers is closer to zero.

While it is possible to use as few as one magnetometer to determine a gravity high side, such an approach will have certain shortcomings. As indicated above, the output of a rotating magnetometer appears as a sine wave whose period is the rotational speed and whose amplitude represents the position of the tool within the earth's magnetic field. While rotating, the output of the magnetometer may be constantly measured and its peak to peak amplitude determined. With this information, and using well known mathematical techniques, it is possible to establish the angular position of the magnetometer at any given instant. As with the circumferential array described above, this information itself is of little value unless reference is made to a gravity high side by way of accelerometer measurements (primarily the x and y accelerometers) at the tool's then current azimuth.

There are further difficulties that arise when using a single magnetometer. First, the sensor must rotate through at least one revolution so as to determine a peak to peak magnetometer output. From this measurement, the angular position can be derived for various output readings between the two peaks. This is in contrast to a magnetometer array that can determine position and relational status while static. Secondly, as mentioned above, the sine wave transform demonstrates variable angular conversion sensitivity throughout its range. This can have a significant effect especially when the axis of sensitivity of the magnetometer approaches that of the local dip angle. As the magnetometer moves closer to the dip angle, the peak to peak output for a complete revolution approaches zero. It becomes more difficult to resolve angularity from a reduced output voltage swing. Even though the sensors in the magnetometer array configuration also suffer from this reduced output, the decision process using that configuration, is not as tied to absolute values, but rather to proximity to zero and the sign of an adjacent sensor. Thus the magnetometer array configuration has the ability to provide higher accuracy in low output amplitude conditions.

While the present embodiment describes mounting the appropriate electronics and sensors (magnetometers and accelerometers) within the downhole measurement tool that will use the directional information (the gamma sensor in the example), it is not necessary that these components be mounted together. All or part of the measurement system can be mounted elsewhere, and/or be supplemented by information derived from a sensor in the system that has accelerometers or magnetometers associated with it.

In a typical directional drilling scenario, a highly accurate directional sensor may be employed to provide location and guidance information to the driller. A tool containing an inclinometer (an orthogonal array of three accelerometers) can provide the necessary gravity high side information. This information can then be conveyed to the downhole measurement tool (the gamma sensor in the present example) so as to periodically (when not rotating) update the relationship between the gravity high side and the magnetometer array. Since, in such an arrangement, only the magnetometer array (and supporting electronics) would be local to the gamma sensor, an initial offset between the remote sensor (which is providing the gravity high side information) would be measured and corrected for. The random nature in which two or more sensors are screwed together, to build up the desired grouping in a drill string, makes it necessary to establish the angular offsets between them if an accurate high side relationship is to be established.

As a further alternative, a ring of accelerometers alone may be used to determine the gravity high side. Unlike a ring of magnetometers, however, the accelerometers will be affected by external accelerations caused by tool motion, shock and vibration. This may lead to convergence errors in certain situations where the acceleration due to gravity is lost in the unwanted acceleration inputs. Such effects may be compensated for either by modifying the accelerometer outputs or selectively interrogating the accelerometers during periods of reduced motion and/or vibration.

The methods described above further make it possible to use only one high side determining apparatus in a complex downhole configuration. The positional information can be broadcast through the downhole sensor communication link. As in the above example, the orientation of each downhole sensor, with respect to the high side determining apparatus, would need to be determined and programmed into the tool(s) at the surface after the sensors have been assembled.

As indicated above, various sensor configurations (accelerometers and magnetometers) are possible to lend greater accuracy to defining a reference orientation. As an example, a ring of eight accelerometers (similar in geometry to the radial array of gamma sensors) could be implemented in such a manner that the accelerometer sensitive axes would not be affected by the centrifugal force generated by rotating the tool. Such an array will provide good results at high angles (the tool being near horizontal) but will be vibration and shock sensitive as the tool becomes more vertical. On the other hand, a similar array of magnetometers arranged in a radial configuration is obviously insensitive to shock and vibration. However, a periodic gravity reference is required from a remote directional sensor or an onboard accelerometer. The accelerometer information is required in order to interpret the output of the magnetometers at the then current azimuth (as described above).

While each of the above examples provide reasonable results, it has been determined that the expense and complexity of mounting such a sensor array is undesirable. This expense and complexity is further complicated by a desire to implement such sensor technologies on several different tool configurations and diameters. It therefore becomes more practical to mount both the accelerometer array and the magnetometer array on a primary circuit board within the tool. In this manner there are no specific configuration mounting structures and harnessing required that might vary from tool to tool. Optimally, an array of three accelerometers and two magnetometers is utilized. The two magnetometers mounted to the circuit board (as shown in Fig. 1) are positioned at an included angle of 90°. In this manner a magnetic tool face can be determined by computing the arc tangent of the ratio of the sines. This will provide a tool face of 0° to 360° with appropriate quadrant correction. When the tool is stable (not rotating), it is also possible to compute a gravity tool face from the accelerometers of 0° to 360°. Appropriate processing may then compute the tool face offset (at the then present azimuth) between the gravity and the magnetic measurements. This offset is then used (until the next time the tool stops rotating and is stable) to synchronize the magnetic tool face to produce a gravity tool face (high side) without the shortcomings of the vibrational and shock sensitivity of the accelerometers.

Although the methods and structures of the present invention have been described in conjunction with a number of preferred embodiments, those skilled in the art will anticipate a number of alternative applications within which the capabilities of the present invention are equally beneficial. Systems other than those utilized in conjunction with geosteering operations and horizontal drilling configurations might also benefit from the directional measurement capabilities of the present invention. Any drilling environment where MWD operations would be improved by a knowledge of the orientation of the drill bit and its position within the borehole, could benefit from the methods and devices of the present invention.

In addition, the present invention has been described primarily in conjunction with gamma based sensors, although other types of formation interrogating sensors could be utilized with the same structures and methods. Depending upon the depth of investigation and the types of formations involved, alternative well logging methods could be utilized in conjunction with the basic structures and methods described. Alternative applications based upon the descriptions above will be anticipated by those skilled in the art.

Claims

We Claim:

1. An apparatus for referencing the rotational orientation of a drill string during MWD operations, and correlating measured formation characteristics therewith, the apparatus comprising: a directionally sensitive device sensor for interrogating the formation surrounding the borehole and returning a signal indicative of relevant characteristics of said formation; and a sensor array capable of establishing a reference orientation for rotational movement of said drill string within said borehole, wherein an offset between said formation characteristic directional sensor and said sensor array is defined.

2. The apparatus of Claim 1 wherein said directional sensor comprises an array of gamma sensors radially arranged within a tool sub on said drill string.

3. The apparatus of Claim 2 wherein said radial array of gamma sensors comprises eight Geiger Mueller tubes arranged circumferentially within said sub tool and spaced at approximately 45° intervals.

4. The apparatus of Claim 1 wherein said directional sensor is established within a sonde based tool and said sensor comprises a scintillation crystal and an associated photomultiplier tube.

5. The apparatus of Claim 4 wherein said scintillation crystal is enclosed within a rotating shield, said rotating shield having a window defined therein for establishing directional sensitivity to said formation characteristics.

6. The apparatus of Claim 1 wherein said sensor array comprises a plurality of accelerometers orthogonally positioned within said drill string.

7. The apparatus of Claim 1 wherein said sensor array comprises a plurality of magnetomoeters orthogonally positioned within said drill string.

8. The apparatus of Claim 1 wherein said sensor array comprises at least two magnetometers orthogonally positioned within said drill string and at least two accelerometer orthogonally positioned within said drill string.

9. The apparatus of Claim 8 wherein said sensor array is configured on a circuit board positioned off-axis in said drill string.

10. A method for determining a reference rotational orientation for a drill string during MWD operations, and correlating measured formation characteristics therewith, the method comprising the steps of: providing a formation characteristic sensor operable in an MWD environment; discriminating a directional correlation for said formation characteristic; establishing a reference orientation for rotational movement of said drill string within said borehole; correlating said referenced orientation with said direction correlated formational characteristic; and utilizing said information as a basis for modification of a drilling direction in a geosteering operation.

11. The method of Claim 10 further comprising the step of transmitting information relating to said referenced orientation and said direction correlated formational characteristic said data to the surface for processing and analysis.

12. The method of Claim 11 wherein said step of transmitting information comprises transmitting all information gathered by said formation characteristic sensor and said step of correlating said reference orientation occurs during said processing and analysis at the surface.

13. The method of Claim 11 wherein said step of transmitting information comprises transmitting a selected relevant portion of information gathered by said formation characteristic sensor and said step of correlating said reference orientation occurs within a data processor provided within said drill string.

14. The method of Claim 10 wherein said step of establishing a reference orientation for rotational movement of said drill string comprises polling a plurality of accelerometers positioned within said drill string proximate to said formation characteristic sensor.

15. The method of Claim 10 wherein said step of establishing a reference orientation for rotational movement of said drill string comprises polling a plurality of accelerometers and a plurality of magnetometers positioned within said drill string proximate to aid formation characteristic sensor.

16. The method of Claim 15 wherein said step of establishing a reference orientation comprises establishing a reference to a gravitational high side of said drill string within said borehole.

17. The method of Claim 15 wherein said step of establishing a reference orientation comprises establishing a reference to a magnetic azimuth for said drill string within said borehole.

18. The method of Claim 10 wherein said step of discriminating a directional correlation for said formation characteristic comprises the step of progressively shielding said formation characteristic sensor through a radial scan of said formation.

19. The method of Claim 10 wherein said step of discriminating a directional correlation for said formation characteristic comprises the steps of providing a plurality of radially arranged sensors and progressively interrogating said sensors through a radial scan of said formation.

20. The method of Claim 19 wherein said step of discriminating a directional correlation further comprises ignoring sensor data associated with sensors positioned at radial segment boundaries in order to provide greater discrimination of data between said radial segments.