Abstract:
An improved azimuthal gamma radiation measurement assembly configured to facilitate downhole measurement of naturally occurring radiation and the correlation of measurement information with highly accurate orientation information. The azimuthal gamma radiation measurement assembly includes a resolver section that receives azimuthal gamma sensor inputs, correlates those inputs with orientation information, and logs the combined data set for further evaluation.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to downhole radiation measurement assemblies. 
         [0003]    2. Description of the Related Art 
         [0004]    Downhole radiation measurement assemblies have been used in drilling operations for some time. In downhole drilling it is useful identify sub-surface rock formations and customize drilling assemblies and drilling methods to suit a particular geological formation. This can be useful when, for example, a drilling rig has been configured to be effective for a particular type of rock formation and characteristics of the rock formation change as the wellbore extends deeper beneath the surface. It would thus be useful to identify rock formations present at various drilling depths at a wellsite. Downhole radiation measurement assemblies measure the naturally occurring low level radiation that is given off by rock formations downhole. Different types of rock can give off differing amounts of radiation or radiation having other differing characteristics and if measured accurately, the type of rock formations at different depths can be identified. Often, radiation measurement assemblies are deployed downhole and many measurements are taken at different depths in a well. The sensor measurements can then be communicated to the surface and processed to determine the particular types of rock formations present at various depths at a particular wellsite. 
         [0005]    Radiation measurement assemblies are commonly deployed with measurement while drilling tools. The purpose of measurement while drilling tools is to collect various sensor based measurements and facilitate the communication of the measurements to the surface. Measurement while drilling tools can be deployed with sensors for measuring various downhole conditions such as temperature, flow data, drillstring rotation, location information, radiation readings, or other useful downhole conditions. The sensors deployed alongside or as a part of measurement while drilling tools will often be configured to communicate data with the microcontroller or microprocessor that is a part of the measurement while drilling tool assembly deployed downhole. This communication may be made using standard protocols that transmit over bus connections between the measurement while drilling tool and the various sensors. Measurement while drilling tools can then communicate data from the sensors to the surface to remote computers or data logging equipment. Measurement while drilling tools can be deployed by wireline or inline with the drillstring and can include remote power supplies or receive power over cabling run downhole. It is common to deploy a radiation probe that is connected to a measurement while drilling tool downhole to perform radiation measurements at various depths. The measurement while drilling tool can be configured to receive gamma probe data, which for example may be in the form of a pulse train, and then process and communicate the data to remote computers on the surface. Measurement while drilling tools can also just run off of battery power and remotely log data that will later be retrieved when the tool returns to the surface. Some tools will take an additional approach where some data is sent to the surface in real-time and other data is logged to tool memory for later retrieval when the tool returns to the surface. 
         [0006]    A particular subset of downhole radiation measurement assemblies includes azimuthal gamma radiation measurement assemblies. Azimuthal gamma assemblies can include one or more azimuthal gamma probes that measure radiation downhole. Azimuthal gamma assemblies can include a microcontroller, memory, and one or more azimuthal gamma probes, among other components. Probe measurement data can be logged and/or sent to the surface as measurements are taken. The measurement of gamma radiation facilitates the determination of downhole rock formations and also the verification of well placement in directional drilling applications. The precise identification of well placement can be particularly important and is often facilitated by azimuthal gamma measurement. 
         [0007]    To this regard, omni-directional or bulk azimuthal gamma measurements have seen use downhole. Though omni-directional or bulk gamma measurements are sufficient for many applications, in highly stratified formations or while drilling along bed boundaries, omni-directional information is generally not sufficient. To provide the operator with improved information regarding a formation, a highly accurate directional gamma measurement tool is desired. 
         [0008]    In this field, Sonde probe based and collar based directional radiation tools have seen some commercial use. In collar based tools, the gamma data interpretation and binning is complicated due to the lack of available orientation information. Current solutions do not provide adequate resolution or reliability to accurately determine the orientation of the constantly spinning tool and/or drillstring. This information is particularly desirable for azimuthal gamma based measurement assemblies. 
         [0009]    Regarding the orientation of the tool, orientation determination is generally well-known and well-studied in the field of aerospace. It would thus be desirable to adapt and apply some of the orientation determination techniques known in aerospace to the downhole tool environment. With regard to orientation determination, Kalman filtering techniques in particular have seen use for some time in the aerospace field. Kalman filtering allows for increased measurement accuracy and precision when multiple samples are being taken over time when error boundaries of the measurements are generally known or can be calculated. Kalman filtering works by comparing multiple measurements over a given time period, comparing them, and reducing the “noise” associated with the known potential for error or deviation in each respective measurement. Linear estimation techniques have also been applied to nonlinear systems through Extended Kalman filtering techniques. The specific techniques for handling nonlinearities vary based on the application, but they can be generally summarized as local linearization techniques. Particularly in regard to orientation estimation, the rotation matrices necessary to rotate the sensor frame into the earth frame are nonlinear, and therefore nonlinear filtering techniques must be used. 
         [0010]    It would thus be desirable to implement a custom azimuthal gamma resolver assembly utilizing Unscented Kalman Filter orientation determination techniques, specifically for collar based deployment of a directional gamma tool. It would further be desirable for such a system to include magnetometers and/or accelerometers to provide data for orientation determination. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention provides an improved azimuthal gamma radiation measurement assembly to facilitate downhole measurement of naturally occurring radiation and correlation of measurement information with highly accurate orientation information. The azimuthal gamma radiation measurement assembly includes a resolver section that receives azimuthal gamma sensor inputs, correlates those inputs with orientation information, and logs the combined data set for further evaluation. The resolver section can include a microcontroller, memory, and input/output ports. Azimuthal gamma radiation measurements are received by the resolver section from gamma probes or other radiation measurement devices. Once deployed, the resolver section continuously or at an interval, calculates the orientation of the drillstring. The orientation of the drill string can be calculated using accelerometer and/or magnetometer readings. The readings can then be processed by the microcontroller of the resolver section optionally using a Kalman Filter or other similar processing techniques in determining orientation information. The orientation information is then correlated with the radiation measurement information, it may then be logged to memory and/or sent to the surface. Post processing can be run on the data to further analyze and/or prepare it for display to an end user. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Various aspects and attendant advantages of one or more exemplary embodiments and modifications thereto will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
           [0013]      FIG. 1  depicts an example block diagram of an azimuthal gamma resolver assembly. 
           [0014]      FIG. 2  depicts an example side perspective view of an azimuthal gamma resolver assembly tool. 
           [0015]      FIG. 3  depicts an enlarged view of an example azimuthal gamma probe housing of the azimuthal gamma resolver assembly tool of  FIG. 2 . 
           [0016]      FIG. 4  depicts a block diagram of an example resolver section of an azimuthal gamma resolver assembly of  FIG. 1 . 
           [0017]      FIG. 5  depicts an alternative block diagram of an example resolver section of an azimuthal gamma resolver assembly of  FIG. 1 . 
           [0018]      FIG. 6  depicts an example unscented Kalman filter flow chart illustrating an example set of steps performed by an azimuthal gamma resolver assembly. 
           [0019]      FIG. 7  depicts an example initialization function flow chart illustrating an example set of steps performed as part of the initialization of an azimuthal gamma resolver assembly. 
           [0020]      FIG. 8  depicts an example measurement equation flow chart illustrating an example set of steps performed to facilitate the taking of azimuthal gamma measurements by the azimuthal gamma resolver assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The present invention provides an improved downhole measurement assembly to facilitate azimuthal gamma measurement and orientation data correlation. The downhole measurement assembly can also be referred to as an azimuthal gamma resolver assembly. The downhole measurement assembly and/or azimuthal gamma resolver assembly can include one or more azimuthal gamma probes to sense radiation given off by downhole formations. In a preferred embodiment the azimuthal gamma probes are provided near the outer portions of a downhole measurement assembly enclosure, the enclosure serving to protect the azimuthal gamma probes from the harsh conditions often found downhole. 
         [0022]    The azimuthal gamma resolver assembly can further include an orientation assembly comprising one or more magnetometers and/or one or more accelerometers. The magnetometers and accelerometers can be configured to provide information that may be used or further processed to determine the orientation at least a portion of a section of drill string deployed downhole to which the azimuthal gamma resolver assembly is connected to or a part of. 
         [0023]    The azimuthal gamma resolver assembly can also include one or more microcontrollers. At least one of the microcontrollers can be configured to receive output data from one or more gamma probes or other type of radiation measurement probe that are a part of the azimuthal gamma resolver assembly. At least one of the microcontrollers can be configured to receive orientation information from the orientation assembly, and can be further configured to calculate relative position information of the portion of the section of drill string deployed downhole from the orientation information, and at least one microcontroller can be configured to correlate and generate correlation information linking the relative position information and the output from the azimuthal gamma probes. In an embodiment, one or more memory elements can also be configured to store downhole measurement assembly executable code, azimuthal gamma probe data, relative position information, correlation information, and any other information that it may be desirable to log. 
         [0024]    Referring to  FIG. 1 , an example block diagram of an azimuthal gamma resolver assembly  10  is shown as can be configured around a collar  12  of a downhole housing assembly (shown in  FIGS. 2-3 ). In this example embodiment a resolver board section  20 , includes one or more microprocessors  22  (can also be configured as one or more microcontrollers or one or more DSP&#39;s), one or more accelerometers  24 , one or more magnetometers  26 , one or more blocks of memory  28 , one or more azimuthal gamma signal input lines  30 , and a pressure sensor input line  32 . While the diagram shows certain components grouped together, it should be recognized that each component can be configured or not configured, and if configured, can be configured on one or more boards and positioned at various areas throughout the downhole tool. The one or more azimuthal gamma signal input lines  30  connect to one or more gamma probes  40 . Each gamma probe is configured with a receiver crystal  42 , a photo multiplier tube (“PMT”)  44 , and a high-voltage-power-supply (“HVPS”) and/or discriminator  46 . In an embodiment the entire azimuthal gamma resolver assembly  10  is preferably housed in a non-magnetic downhole housing assembly sub (as shown in  FIGS. 2-3 ). A pressure sensor  50  can also optionally be configured in the azimuthal gamma resolver assembly  10 . Other sensors can also be configured and/or included in the azimuthal gamma resolver assembly  10 . 
         [0025]    Referring to  FIGS. 2-3 , an example embodiment of an azimuthal gamma resolver assembly tool chassis  110  is shown. Machined pockets  120  can be included for housing azimuthal gamma probes  130  or other electronic boards that may be included in the azimuthal gamma resolver assembly. Hatch doors  140  enclose and seal the probes and/or electronics from the downhole environment. In a preferred embodiment, azimuthal gamma probes having one inch diameter Sodium Iodine (NaI) crystal are configured, though modifications can be made to the tool  110  to accommodate other probe sizes and/or probe types. In addition to physical mounting space, in an embodiment the chassis of the tool  110  can be configured to provide vibration isolation to the gamma probes  130  to prevent damage to the probes when operating in highly dynamic environments. 
         [0026]    In an embodiment, the azimuthal gamma resolver assembly tool can be configured with a high voltage power supply that can be housed in the tool chassis  110  or a nearby component to power the azimuthal gamma probes  130 . In an embodiment, a gamma probe discriminator (as illustrated in  FIG. 1 ) can be configured. A discriminator, when configured, takes signals from the gamma probe photo multiplier tube and outputs, for example, a negative 5 volt pulse, when a gamma signal over a given threshold is detected. Once a gamma signal is detected, the signal is sent from the gamma probe module to the azimuthal gamma resolver board assembly (as shown in  FIG. 1 ). In a preferred embodiment, the resolver board assembly is housed in a separate pocket from the gamma probe assemblies. 
         [0027]    Once deployed, the azimuthal gamma resolver assembly can be configured to continuously and/or on a given interval calculate drill string section orientation information using the outputs from accelerometers and magnetometers that can be included as part of the azimuthal gamma resolver assembly. Referring to  FIGS. 4-5 , block diagrams are shown that illustrate example azimuthal gamma resolver board section configurations. For example, as shown in  FIG. 4 , a resolver board section  220  can be configured to include a 3-axis “xyz” 50 g accelerometer  224  and two magnetometers, the first magnetometer  226  of this example being configured as a 2-axis “xy” magnetometer and the second magnetometer  228  being configured as a single axis “z” magnetometer. In a preferred embodiment, the various axis&#39;s are configured to be aligned, however, it should be understood that the axis&#39;s may be configured out of alignment with known or measureable offset values being used to align the data between the same axis. Further, in a preferred embodiment the following may be configured: a DSP and/or microcontroller  230 , a memory element  222 , an optional pressure sensor conditioning element  232 , and a voltage regulation section  234  that may include one voltage regulator or may optionally include multiple voltage regulators of varying output voltages. 
         [0028]    Referring to  FIG. 5 , an alternate example configuration of an azimuthal gamma resolver board section is shown. Various embodiments of one or more accelerometers and one or more magnetometers can be configured. In the example embodiment shown the following elements are configured on a resolver board section  320 : a 3-axis “xyz” accelerometer  334 , an “x” axis magnetometer  326 , a “y” axis magnetometer  336 , a “z” axis magnetometer  328 , a DSP and/or microcontroller  330 , a memory element  322 , an optional pressure sensor conditioning element  232 , and a voltage regulation section  234  that may include one voltage regulator or may optionally include multiple voltage regulators of varying output voltages. This example demonstrates just one of the many possible configurations of accelerometer and magnetometer setups as well as DSP and/or microcontroller and associated equipment setups. 
         [0029]    The one or more microprocessors, microcontrollers, and/or DSP&#39;s of the azimuthal gamma resolver board can be configured to run a resolver algorithm that provides highly accurate orientation information of the tool section housing the azimuthal gamma resolver assembly. In particular, a preferred resolver assembly can be configured to process the information provided by the accelerometers and magnetometers and any associated or calculated orientation information by using an Euler Angle Unscented Kalman Filter. 
         [0030]    In an embodiment, the resolver board takes the inputs of the configured magnetometers and accelerometers and calculates orientation information for the tool, consisting at least of relative position information with respect to a pre-set or determined position of the tool. As the tool is deployed downhole and the drillstring is being rotated, the orientation of the tool can constantly change; this information can be tracked and logged to memory or communicated to the surface. Regarding the orientation information, it can be useful to process the information using an Unscented Kalman Filter. Generally, this will provide higher integrity orientation information as the Kalman Filter is capable of reducing the inherent error of individual measurements by leveraging information from a number of samples over a given observation window. 
         [0031]    Referring to  FIG. 6 , in an embodiment, an Euler Angle Unscented Kalman Filter can assume four states of interest: psi, theta, phi, and rate of change of phi, wherein in this application these states are represented by azimuth, inclination, tool face, and rate of change of tool face. The three angles being a standard Euler Angle representation of orientation. While the state equations are linear, the observation matrix is nonlinear, and so an unscented transformation can be used to compute the observation matrix. Implementation challenges can include initialization and filter divergence near singularities. A flowchart of the overall filter structure  400  can be seen in  FIG. 6 . The Kalman Filter is first initialized  410 . Sigma points are then calculated  420  and propagated through the set of system equations  430  and measurement equations  440 . Mean and measurement covariance can then be calculated  450 . Finally, the azimuth, inclination, tool face, rate of change of tool face, and covariance are updated  460  and the cycle continuously repeats as orientation information continues to be gathered and processed by the azimuthal gamma resolver assembly. Through this process each of the states, the azimuth, inclination, tool face, rate of change of tool face are all cycled through the Euler Angle Unscented Kalman Filter, thus providing highly accurate information due to being cycled through the filter. This can be highly beneficial in that the higher accuracy data can then be utilized by drilling personnel, offsite personnel, and/or the drilling systems themselves. 
         [0032]    Referring to  FIG. 7 , an example initialization sequence  600  is shown. To initialize the Kalman Filter, a deterministic frame decomposition algorithm is used when the tool is powered on. Unlike a linear Kalman Filter, an Extended Kalman Filter needs to be well initialized to prevent divergence. Following a standard 3-2-1 Euler angle rotation sequence the method takes a single reading of the accelerometer  610  and magnetometer  620 , and produces the azimuth, elevation, and roll quaternion. The method varies from others in that it is completely algebraic (no trigonometric functions). Similar decomposition methods are the Quest and TRIAD algorithms. However, this method is highly susceptible to noise and vibration due to the lack of feedback. Azimuth is particularly sensitive due to the dependence on both the accelerometer and magnetometer readings. Due to the sensitivity, it is preferred that the tool be powered up and initialized while, relatively, stationary. For example, the tool might be initialized with the drill string revolving at 60 rpms but 120 rpms may cause too much error in the initialization sequence.  FIG. 7  shows an example implementation of an initialization function. After the accelerometer  610  and magnetometer  620  data is gathered, horizontal and/or vertical tool orientation is determined  630 , axis are switched as appropriate  640 , and psi, theta, and phi are calculated  650 . Psi, theta, and phi are then transformed to the appropriate rotation sequence  660 , thus concluding the initialization function  600 . 
         [0033]    The initialization algorithm provides an initial state estimate, x_hat, and covariance matrix, P_hat. These two parameters are fed into the Kalman Filter. A set of sigma points are then generated by taking the Cholesky decomposition of the P_hat following the methods of an Unscented Kalman Filter. These sigma points are propagated through the state equations, and, subsequently, through the measurement equation. To handle singularity issues, the algorithm switches between two Euler Angle sequences. A 3-2-1 rotation sequence, psi; theta; phi respectively, has singularities at theta of ±90 degrees while a 3-1-3 rotation sequence has a singularity at theta of 0 and 180 degrees. In the vertical section, the orientation algorithm uses a 3-2-1 rotation sequence, and in the horizontal a 3-1-3 rotation sequence. By looking at the second rotation parameter, 2 or 1, the filter can be switched from one rotation sequence to the other. A transformation is used to compute the angles from 3-2-1-&gt;3-1-3 and vice versa so that the filter avoids divergence issues due to poor initialization. 
         [0034]    Referring to  FIG. 8 , a flowchart illustrating an example of the singularity avoidance and filter switching  500  is shown. The sigma points propagated through the measurement equation yield the expected observations. These observations can then be compared with the actual sensor measurements, and a posterior mean and covariance can then be calculated. The updated mean and covariance are then used to calculate Kalman gains, and the final step is to update the state estimate and covariance matrix. The state estimate and covariance matrix are returned and fed back into the filter on the subsequent iteration. 
         [0035]    The azimuthal gamma resolver assembly additionally has two preferred modes of operation. One preferred mode involves a lower bandwidth binning of azimuthal gamma probe data by quadrant. As the drill string spins, the orientation information is further processed to determine what quadrant, 0-90, 90-180, 180-270, or 270-360, that the gamma readings were taken from. Once the quadrant has been determined, the probe data is correlated to the quadrant data. The probe data can be tagged with information denoting a particular quadrant or otherwise associated with that quadrant by storing the information in a data set for that quadrant. In an embodiment, the data can also be time-stamped so that it may later be correlated with depth information collected by the tool or at the surface. By correlating gamma probe data with highly accurate Kalman filtered orientation data, downhole formation information can be determined with substantially higher accuracy. 
         [0036]    Another preferred mode involves a much higher bandwidth, more data intensive correlation and logging of information. In this mode, at a given time interval, substantially all of the Kalman filter calculation information, orientation information, and gamma probe readings are recorded. By recording more information over shorter time periods for pre-set intervals, the information gathered over longer runs can be more precisely verified. 
         [0037]    When the azimuthal gamma tool passes formations with higher magnetite densities, it can be appreciated that magnetometer readings may contain greater error. When this is detected, the Kalman Filter can more heavily weight the accelerometer information versus the magnetometer information when calculating orientation. The will reduce the integrity of the orientation information over the zone necessary and as a result this data can optionally be tagged to indicate the reduction in orientation information integrity. 
         [0038]    Though the mentioned flowcharts and configurations provide examples of the preferred form of practice and the concepts disclosed herein have been described in connection with the preferred form of practicing them and modifications thereto, those of ordinary skill in the art will understand that many other modifications can be made thereto. Accordingly, it is not intended that the scope of these concepts in any way be limited by the above description.