MXPA99010135A - Downhole nmr tool having a programmable pulse sequencer - Google Patents

Abstract

A pulse sequencer for an NMR logging tool is adapted to receive state descriptors from an uphole computer that are each indicative of states of an NMR measurement sequence during a different associated time interval. The state descriptors may indicate for example the phase or frequency of an RF carrier pulse, or time interval durations. The pulse sequencer is located within the tool and is coupled to at least one coil to perform the NMR measurement sequence in response to the received state descriptors. The tool further comprises a permanent magnet 410 (see figure 14) and a ferromagnetic material 405 located adjacent to the magnet. Coils 132, 134, 136 are arranged around the ferromagnetic material 405. A metal sleeve may surround the magnet assembly.

Description

MRI TOOL WITH PROGRAMMABLE PULSATION SEQUENCER.

CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation in part to the Serial US Patent Application No. 09 / 368,341, entitled "Method and Apparatus for Performing Magnetic Resonance Measurements", recorded on August 4, 1999 and claimed priority under 35 USC § 119 a Serial Provisional Patent Application No. 60 / 107,184, registered on November 5, 1998.

BACKGROUND This invention relates to a downhole tool that includes a programmable pulse sequencer. Nuclear magnetic resonance (NMR) measurements are typically performed to investigate properties of a sample. For example, an NMR cable line or a downhole log while drilling tool (LWD) can be used to measure properties of underground formations. In this way, the typical downhole NMR tool can, for example, provide an independent measurement of the porosity lithology of a particular formation by determining the total amount of hydrogen present in the formation fluids. Equally important, the NMR tool can also provide measurements that indicate the dynamic properties and the fluid environment, insofar as these factors may be related to important petrophysical parameters. For example, NMR measurements can provide information useful for deriving the permeability of the formation and the viscosity of the fluids contained within the pore space of the formation. It may be difficult or impossible to derive this information from other conventional recording arrangements. Thus, it is the ability of the NMR tool to perform these measurements which makes it particularly attractive compared to other types of downhole tools.

Typically NMR recording tools include a magnet that is used to polarize hydrogen nuclei (protons) in the formation of a transmission wire or antenna that receives radio frequency (RF) pulses from a pulse generator of the tool and in response irradiate RF pulses into the formation. A receiving antenna can measure the response (indicated by a received RF signal called a spin echo signal) of the hydrogen polarized to the transmitted pulses. Often, the reception and transmission antennas are combined into a single transmitter / receiver antenna. The NMR techniques employed in current NMR tools typically involve some variant of a basic two-step technique that includes delay for a polarization time and the subsequent use of an acquisition sequence. During polarization time (often referred to as "waiting time"), the protons in the formation polarize in the direction of a static magnetic field (called B0) that is established by a permanent magnet (from the NMR tool). An example of an NMR sequence is a Carr-Purcell-Meiboom-Gill sequence (CPMG) 15 which is depicted in Fig.1. By applying sequence 15, a distribution of spin relaxation times can be obtained (Times T2, for example) and the distribution can be used to determine and map the properties of a formation. A technique using CPMG sequences 15 to measure times T2 may include the following steps. In the first step, the NMR tool pulses an RF field (called field B) during an appropriate time interval to apply a 90 ° excitation pulse 14a to rotate the hydrogen nucleus spin that are initially aligned along the direction of the Bo field Although not shown in detail, each pulse is effectively a sheath, or a break, of an RF carrier signal.When the spin is rotated around Bi away from the direction of the B0 field, the spin immediately begins to precede around Bo. At the end of the pulse 14a, the spin is rotated 90 ° in the plane perpendicular to the field B0.

The spin continues to precede in this plane, first in unison, then gradually losing synchronization. For step 2, at a set time Tcp following the excitation pulse 14a, the NMR tool pulses to the Bi field for a longer period of time (than the excitation pulse 14a) to apply a refocusing pulse 14b to rotate the spin precedents through an angle of 180 ° with the carrier phase changed by + _ 90 °. The NMR pulse 14b causes the spin to resynchronize and radiate an associated echo-spin 16 signal (see Fig.2) that peaks at 2.Tcp after the 90 ° tilt 14a. Step two can be repeated "k" times (where "k" is called the number of echoes and can assume any value from several to as many as several thousand, as an example) in the interval of 2.tcp. For step three, after completing the echo-spin sequence, a waiting period (usually called a wait time) is required to allow the splens to return to equilibrium along the B0 field. before beginning the next CPMG 15 sequence to collect another set of echo-spin signals. The decay of the amplitudes of each set of echo-spin signals 16 can be used to derive a timing distribution T2. Although it may be desirable to vary the characteristics of the measurement sequence to optimize performance to a particular formation, unfortunately, a conventional NMR tool can be specifically designed to perform a predefined NMR measurement sequence. Thus, the conventional tool can provide limited flexibility to change the sequence, while the parameters that can be programmed in the tool can affect the overall time of the sequence without allowing the flexibility to change a portion of the particular sequence. For example, a conventional NMR tool can be programmed with the above described time Tcr. the time between the inclination pulse 14a and the first refocusing pulse 14b. However, this value also determines the time (2, TCP) between successive refocusing pulses 14b. Thus, although a time between refocusing pulses 14b different than 2. Tcp may be desired to optimize the performance of the tool, the tool may not provide the flexibility to change this time. Thus, there is a continuing need for an arrangement that addresses one or more of the problems outlined above.

SUMMARY OF THE INVENTION In one aspect of the invention, an NMR measuring apparatus includes at least one antenna and a pulse sequencer. The pulse sequencer is coupled to said at least one antenna and is adapted to receive status descriptors that are indicative of states of an NMR measurement sequence. The pulse sequencer uses the antenna (s) to perform the sequence of NMR measurements in a downhole formation in response to the state descriptors. Other aspects of the invention as well as advantages thereof will become apparent from the following description, illustrations and claims.

BRIEF DESCRIPTION OF THE ILLUSTRATIONS Figure 1 is an illustration of an NMR measurement sequence according to the prior art. Figure 2 is an illustration of spin echo signals produced in response to the NMR measurement sequence of Figure 1. Figure 2 is an illustration of spin echo signals produced in response to the sequence of NMR measurements of Figure 1. Figure 3 is a schematic diagram of a system using a programmable NMR measurement tool according to an embodiment of the invention.

Figure 4 is an illustration of an exemplary portion of an NMR measurement sequence according to an embodiment of the invention. Figure 5 is a state diagram illustrating states of an NMR measurement sequence according to an embodiment of the invention.

Figure 6 is an illustration of status descriptors according to an embodiment of the invention. Figure 7 is an illustration of a graphical user interface that can be used to program the tool of Figure 3 according to an embodiment of the invention. Figure 8 is an illustration of the package of state descriptors before transmission to the NMR measurement tool according to an embodiment of the invention. Figure 8A is a pictorial illustration of the packing of the status descriptors of Figure 6. Figure 9 is a schematic diagram of the circuitry of the tool according to an embodiment of the invention. Figure 10 is a schematic diagram of a pulse sequencer of the tool of Fig. 9 according to an embodiment of the invention. Figure 11 is an illustration of the organization of the data in a memory of the pulsation sequencer of Fig. 10 according to an embodiment of the invention. Figure 11A is an illustration of the unfolding of the state descriptors to remove curves. Figure 12 is a waveform illustrating the decay of a radio frequency (RF) signal that is used to automatically tune the resonance frequency of an antenna of the pulse sequencer according to an embodiment of the invention. Figure 13 is a spectral distribution of the signal of Fig.12, Figure 14 is a schematic diagram of a tool sensor according to an embodiment of the invention, Figure 15 is a schematic diagram of a portion of the sensor of the Figure 14 according to an embodiment of the invention, Figure 16 is a top view of the sensor of Figure 14 according to an embodiment of the invention.

Figure 17 is a graphic representation of the magnetic permeability of a ferromagnetic material of the sensor according to an embodiment of the invention. Figure 18 is a graphical representation illustrating the relationships between the frequency of transmission pulses and the static magnetic field versus the depth of the investigation.

DETAILED DESCRIPTION Referring to Figure 3, an embodiment 48 of an NMR measurement system according to the invention includes a cable line tool 50 of nuclear magnetic resonance that can be programmed with a wide range of NMR measurement sequences. In particular, the tool 50 is constructed to receive data from register sequences 52 that define a particular NMR measurement sequence to be performed by the tool 50. The data 52, in turn, includes status descriptors, each of which indicates a state of the NMR measurement sequence during an associated time portion, or interval, of the sequence. Thus, because of this arrangement, the tool 50 can generate the NMR measurement sequence in response to the status descriptors, as described below. In some embodiments, the status descriptors can be generated by a computer 60 (located on the surface of the well, for example) that communicates the resulting data 52 via a cable line 109 to the tool 50, as described below. The computer 60 may also receive magnetic resonance (MR) data 55 from the tool 50 via the cable line 109. The data 52 may be loaded into the tool 50 via other techniques (e.g., via of a serial connection before the tool 50 is lowered down the hole) different from the above-described technique by way of cable line. Each state descriptor is associated with a particular time interval of the NMR measurement sequence and indicates the logic states of several signals that control the tool 50 during the time interval. For example, a particular status descriptor may indicate the state of a digital signal that establishes the frequency of a carrier signal of transmitted radio frequency (RF) pulses., and the same status descriptor may indicate the state of another digital signal indicating a phase of the signal carrier, such as only a few examples. As other examples, a particular status descriptor may indicate the voltage logic levels that are used to operate the tool -50 to generate the NMR measurement sequence, as described below. In some additions, each status descriptor may also indicate the duration of the associated time interval. Tool 50 can store status descriptors for various NMR measurements. In this way, the sequence (s) to be used (s) can be selected before the tool 50 is lowered down the hole.

Furthermore, due to the ability of the tool 50 to store state descriptors for multiple NMR measurement sequences, the tool 50 can use different downhole sequences. For example, the tool 50 may use sequences having different RF frequencies for purposes of establishing different resonance covers 406 (see Fig. 16) to investigate different regions of the array, as described below below. The tool 50 includes circuitry 53 which is electrically coupled to an NMR sensor 57 of the tool 50. As described below, the circuitry 53 receives the data 52 from the cable line 109 and interacts with the sensor 57 to perform a given sequence of NMR measurement and also communicate the MR 55 data (via the cable line 109) to the computer 60. Referring to Figure 4, as an example, an exemplary portion 70 of an NMR measurement sequence may encompass a duration formed by successive intervals of time to "t ?, t2, t3? t4 and ts. Each of these time intervals, in turn, is associated with a status descriptor. For example, during the time interval t ?? the corresponding status descriptor can indicate logic signal states to cause the transmission of an RF 72 pulse (a tilt pulse or a refocus pulse, as examples).

Even more, during the time interval t0. the state descriptor that is associated with the time interval t0 may indicate state signals that establish a phase and a frequency of the RF signal carrier to the RF pulse 72. As another example, during the time interval ti, the status descriptor which is associated with the time interval ti, may indicate a suction state signal which causes an input to an RF receiver of the tool 50 to be trimmed (to prevent false readings) during the transmission of the RF pulse 72. Similarly, other state descriptors can indicate the appropriate signal states to cause the generation of other RF pulses (such as the RF pulses 74 and 76) during the exemplary portion 70 of the measurement sequence NMR which is represented in Figure 4, As another example, for the case where the RF pulse 72 is a refocus pulse, the status descriptor that is associated with the time interval i2 may indicate a signal state which causes the transmission antenna (which is used to radiate the RF pulse 72) is isolated from the receiving circuit (of the tool 50) during the time interval t2 when a spin echo signal is received. As noted above, in addition to indicating the signal states, in some embodiments, each status descriptor indicates its own duration. Thus, for example, the status descriptor that is associated with the time interval t2 establishes the duration of the time interval t2. Referring to Figure 5, thus, each state descriptor is associated with a general state (denoted by "STATUS" in the description below) of the NMR measurement sequence. For example, one STATE may occur during the transmission of a refocus pulse and another STATE may occur during the subsequent time interval when a spin echo signal is received. In this way, referring to Figure 5 which represents an exemplary state diagram for the NMR measurement sequence, in STATUS 1 of the NMR measurement sequence, the associated status descriptor causes the assertion / disassembly of several signals in the circuit 53 to control the output of the tool 50 during STATUS 1 and possibly set parameters (such as a bearer phase and a frequency as examples) that are used in a subsequent STATUS of the NMR measurement sequence. After the time interval that is associated with STATE 1 elapses, the NMR measurement sequence moves to STATE 2, a STATUS described by another status descriptor. In this way, the status descriptor that is associated with STATUS 2 causes the assertion / disassociation of several signals in circuit 53. As shown in Figure 5, the NMR measurement sequence can curve between STATE 1 and STATE 2 N times. To achieve this, in some additions, the status descriptor that is associated with STATUS 1 indicates the beginning of the curve, and the status descriptor that is associated with STATUS 2 indicates the end of the curve. Any of the status descriptors, the one that describes STATE 1 or the one that describes STATE 2, can indicate the number of times (N, for this example) to repeat the curve. After N curves, the NMR measurement sequence moves to STATE 3, a state controlled by another state descriptor. As shown in Figure 5, another curvature (of M times) that includes STATE 1, STATE 2 and STATE 3 can be created, as another example. Thus, state descriptors can be used to control states of the NMR measurement sequence. To summarize, each state descriptor may indicate some or all of the following attributes. First, each state descriptor indicates the states of several signals that are used to establish the associated state or the future state of the NMR measurement sequence. The status descriptor may also indicate the duration of the associated NMR measurement sequence state. The status descriptor may also indicate parameters (A carrier frequency or a bearer phase, as examples) of the next NMR measurement sequence state after the current state elapses. With respect to bends, the status descriptor may indicate a beginning of a curve or end of a curve, and the status descriptor may indicate a repeated count for a curve. Figure 6 represents four exemplary status descriptors 90, 92, 94 and 96, each of which is associated with a different state (called STATE 1, STATE 2, STATE 3 and STATE 4 but are not necessarily related to the states that they are represented in Figure 5) of an NMR measurement sequence. In this way, the status descriptor 90 (associated with STATUS 1) indicates the output states (denoted by "11111110b", where the suffix "b" denotes a binary representation) for one or more signals of the tool 50. The descriptor state 90 also indicates a duration of 500 microseconds (s) for STATUS 1 and does not indicate the beginning or end of any curve. Therefore, at the end of 500 s, the NMR measurement sequence enters STATE 2. a state described by the status descriptor 92. The status descriptor 92 indicates the output states of one or more signals of the tool 50 and it also indicates a duration of 200 microseconds (s) for STATE 2. Status descriptor 92 also indicates the beginning (represented by " {" in Fig. 6) of a round curve that is repeated three times. At the end of the 200 s, the NMR measurement sequence enters STATE 3, a state associated with the status descriptor 94, and remains at STATE 3 for the indicated duration (450 s). The status descriptor 94 indicates the end of the round curve beginning with STATE 2. Thus, after a duration of 450 s, the NMR measurement sequence makes a transition back to STATE 2 to traverse the round curve again. After the round curve is repeated three times, the NMR measurement sequence makes the transition to STATE 4 that is associated with status descriptor 96 and remains at STATE4 for 100 s. Although a round curve is described in the example above, state descriptors can indicate multiple round curves, and state descriptors can indicate round curves nested or placed one inside the other.

Referring to Figure 7, in some embodiments, the program 62 (see Figure 3), when executed by the computer 60 to form a graphical user interface (GUI) 97 (in a display display of the computer 60) that allows the Visual creation and editing of the states of the NMR measurement sequence. In this way, the GUI 97 displays (columns 1-11, for example, as shown in Fig. 7), each of which is associated with a state of the NMR measurement sequence, as shown in Fig. .7, a top row of the GUI 97 is a row of title that allows the labeling of each column for ease of reference. In this way, states can be titled and re - titled by clicking on the title of a particular state with a mouse and renaming the state using the computer 's keyboard. The displayed signal states and the durations of the states that are described below can be changed or entered in a similar way. The row below the title row displays the duration of each state, and the row between the displayed state durations displays embedded codes of curves in round. For example, in column 1, the characters "8 {" Indicate the beginning of an outer round curve that is repeated 8 times. As an example, the outer curve can define eight NMR measurements. In column 5, the characters "1200 {" Indicate the beginning of a nested internal round curve that is repeated 1200 times. As an example, the internal round curve can define refocus pulses and delays to allow spin echo acquisition. , and the portion of the outer curve that is outside the internal round curve can define a tilt pulse. The remaining UIs of GUI 97 indicate logical signal states for each state of the NMR measurement sequence. For example, a signal denoted by "RF" has a level of logic one to indicate the beginning of a beat and otherwise has a logic level of zero. As another example, a signal denoted by "ACQ" indicates an acquisition phase with a logic level one and -otherwise it has a logic level zero. Some of the other signals that are represented in Figure 7 are described below in connection with the circuit 53 of the tool 50. Referring to Figure 8, the computer 60 can pack the status descriptors in the following manner to form the data. 52 which are communicated to the tool 50. The first data block that is communicated to the tool 50 may include header information, such as the number of status descriptors being communicated. Subsequent data blocks are formed from the state descriptors in the order of the corresponding states. Thus, the second data block is the status descriptor for STATUS 1, the third data block is the status descriptor for STATUS 2, etc. Fig. 8A represents an example of the packing of state descriptors 90, 92, 94 and 96 of Fig.6. As shown, the first data block indicates that the number of states is four. The following four blocks represent the status descriptors 90,92,94, 96 respectively. As shown, the status descriptor 92 indicates a round curve count of three while the other status descriptors 90.94 and 96 indicate curve counts-in round of zero. In this way, each time the state corresponding to the status descriptor 92 occurs, the corresponding round curve count is decreased by one. Also depicted in Fig. 8A are the branch conditions (called "jumps" in Fig. 8A) that indicate the next state. If the round curve count is zero, then the control makes a transition to the next succeeding state. However, if the round curve count is not zero, then the corresponding branch condition indicates the next state. Referring to Fig. 9, in some embodiments, the circuits 53 communicate with the computer 60 to perform a given NMR measurement sequence on the basis of the state descriptors. To achieve this, a downhole controller 110 is coupled to the cable line 109 to communicate with the computer 60 to receive the data 52 and provide the resulting state descriptors to a programmable pulse sequencer 111. The programmable pulse sequencer 111, in turn, executes the state descriptors to generate signals (over the signal lines 11) that controls the NMR measurement sequence. In the course of the NMR measurement sequence, the pulse sequencer 111 can perform the following actions: generate signals that operate a force amplifier 118 to generate RF transmission pulses (via a serial bus 121) with a resonance tuning circuit 112 to control the resonance frequency of a main receiving antenna 132 (represented by an inductor) controlling (via an ACQ signal) the activation of the digital receiver circuit 114, controlling the activation of the transmission circuit and generating signals to control several switches of the circuit 53 , as described below below. In addition to the pulse sequencer 111, the circuit 53 includes a frequency synthesizer 116 which is coupled to the pulse sequencer 111 to generate clock signals for the circuit 53 on the basis of the executed state descriptors. For example, the frequency synthesizer 116 may generate clock signals based on the frequency and RF phase which are indicated by an executed status descriptor. The pulse sequencer 111 can then use one of these clock signals to generate an RF transmission pulse by interacting with the force amplifier 118. A bus 117 establishes communication between the digital receiver 114, the downstream controller 110 and the pulse sequencer 111. Circuit 53 is coupled to multiple antennas 132,134 and 136 of an NMR sensor 57 described below. The main antenna 132 can be used to transmit pulses RF and receive spin echo signals. In some embodiments, the other antennas 134 and 136 are used to receive spin echo signals. The antennas 132, 134 and 136 are distributed along the sensor 57, an array that can be used to obtain high resolution T1 measurements and multiple T1 measurements using a single NMR measurement sequence. as described further in the Patent application US Serial NSS 09/341 368. entitled "Method and Apparatus for Performing Magnetic Resonance Measurements", filed August 4 1999.La generating a transmit pulse (a pulse or a pulse refocusing inclination, as examples) can occur as follows. First, the pulse sequencer 111 executes a particular status descriptor that indicates (via a signal called RF) that an RF pulse is to be generated during the next NMR measurement state. In this way, during the next NMR measurement state. the pulse sequencer 111 uses a clock signal that is provided by the frequency synthesizer to generate signals to produce a pulse RF at the output of the power amplifier 118.

During the next state, the sequencer pulse 11 executes next state descriptor makes the sequencer pulse 111 activates the appropriate suiches for coupling the output terminal of the power amplifier 118 to one of three antennas (antennas 132.134 or 136) and isolate the two remaining antennas. The execution of this descriptor also causes the sequencer pulse 111 makes the assertion of a signal to activate the suiche 144 to trim the input terminals of a preamplifier 146 of the receiver circuit, desasercione a signal that deactivates the suiche 142 to decouple the preamplifier 146 of the power terminal of the power amplifier 118; and disabling the ACQ signal to incapacitate the digital receiver 114 (which receives an output signal from the preamp 146), as a few examples of the signals that can be controlled by a particular status descriptor. To receive a spin echo signal, the appropriate status descriptor causes the ACQ signal to assert to capacitate the digital receiver 114; causing the BS signal to decay to allow reception of a signal by the preamplifier 146; and cause the assertion / desaserción of appropriate suiches for coupling the main antenna 132 to the input terminals of the preamplifier 146 while isolating the remaining antennas 134 and 136 of the other circuits 53. As shown in Fig. 9, the suiches 180, suiches 168 and suiches 166 are controlled by the signal path that is generated from the execution of the status descriptors to selectively couple the antennas 132, 136 and 134, respectively, to an output terminal of the force amplifier 118. The switches 182, 164 and 170 they are controlled by the signal path that is generated from the execution of the state descriptors to selectively divert the wires of the antennas 132, 134 and 136, respectively, to ground. Referring to Figures 10 and 11, in some embodiments, the pulse sequencer 111 includes a processor 302 (a digital signal processor (DSP) for example) that communicates with the downstream controller 110 to receive the status descriptors. For purposes of executing the state descriptors, the processor 302 removes any round or branch curve that exists between the state descriptors to create a linear stack of pipeline 309 of descriptors 312 for execution. For example, the state descriptors that describe STATE 1 and STATE 2 can form a round curve between STATE 1 and STATE 2 that is repeated N times. To remove the branches, the processor 302 creates a stack of 2N descriptors 312. Each descriptor 312 includes a field 314 that indicates the duration of the associated state of the NMR measurement sequence. For example, field 314 may indicate the number of clock periods that occur during the associated state. In some embodiments, each clock period is set approximately equal to one divided by the Lamour frequency. Each descriptor 312 also includes a field 316 indicating the states of several signals. For example, a particular portion (bit) of field 316 may indicate a logical state of a cue signal. However, groups of bits in field 316 may collectively indicate a digital signal, such as a phase or RF frequency, for example. As a more specific example, Figure 11A depicts the splitting of the status descriptors 90, 92, 94 and 96 (see Fig. 6) to form eight descriptors 372 that can be successively executed by the processor 302. In this way, the first descriptor 372 is directly derived from descriptor 90 and indicates a duration of 500 s. The next six descriptors 372 are basically three copies of the descriptor 92 (indicating a duration of 200 s) followed by the descriptor 94 (indicating a duration of 450 s). Finally, the remaining descriptor 372 is directly derived from the descriptor 96 (indicating a duration of 100 s). Referring back to Figure 10, the processor 302 stores the unfolded state descriptors in a first-in-first-out (FIFO) form in a FIFO 304 memory. In some embodiments, the FIFO 304 memory is emptied in half of the such that the processor 302 can store additional descriptors in the FIFO memory 304. An output latch 306 of the pulse sequencer 111 receives the bits from the field 316, and a counter 308 of the pulse sequencer 111 receives the bits from the field 314 In some embodiments, both the counter 308, the output of the FIFO memory 304 and the latch 306 are timed by a clock signal (called CLK) at the Larmor frequency. In some embodiments, the counter 308 is a decrement counter which signals the processor 302 when its count is zero. In response to this signal, the processor 302 causes the latch 306 and the counter 308 to load new data from the FIFO memory 304. In this manner, for each status descriptor, the output latch 306 provides signals indicative of the field 316 for the number of Larmor clock signals indicated by field 314. Some of these signals are communicated to a pulse generator 300 (via conductive lines 305) and some of the signals are communicated to conductive lines 303 that control the various circuits described above . The pulse generator 300 generates the signals for controlling the force amplifier 118. The input of the FIFO 304 and the processor 302 are timed at a higher frequency (via a higher frequency CLKP) than the Larmour frequency. This difference in frequency allows more processing time for the processor 302 to process the state descriptors and thus promote the continuous execution of the state descriptors. Referring back to Fig. 9, among other features of the circuitry 53, a resonance tuning circuit 126 can be used to tune the main antenna 132. In this manner, the circuit 126 includes capacitors 128 that can be selectively coupled (via a serially coupled suiche 130) in parallel with the main antenna 132, another capacitor 160 can be permanently coupled in parallel with the main antenna 132 to establish a frequent base resonant for the antenna 132. Due to this arrangement, the downstream controller 110 can selectively activate the suiches 128 to adjust the resonance frequency of the main antenna 132. To achieve this, in some embodiments, the resonance tuning circuit 126 includes a control circuit 120 which is coupled to the serial bus 121. In this way, the control circuit 120 serves as interface bus to allow the selective activation of the suiches 130 by the downhole controller 110.

In some embodiments, the downhole controller 110 automatically tunes the resonance frequency of the antenna 132 after each NMR measurement sequence. In this way, at the end of the sequence, the downstream controller 110 causes the pulse sequencer 111 to generate a calibration pulse 349 which is shown in FIG. 12. The downstream controller 310 opens the suiche 144 (see FIG. 9) and closes the suiche 142 to observe a voltage drop 350 along the antenna 132 after the pulse 349. The downstream controller 310 performs a Fast Fourier Transformation (FFT) of the voltage decay 350 to derive a spectral composition Decay 350, a composition that provides the resonance frequency 352 of the antenna 132, as shown in Figure 13. Then the downstream controller 110 determines a difference between the determined resonance frequency and the Larmor frequency and makes the corresponding corrections activating the appropriate suiches 128 of the resonance tuning circuit 126. In this way, in some embodiments, after each NMR measurement sequence. the downhole controller 110 repeats the calibration described above to keep the antenna 132 tuned to a frequency close to the Larmor frequency. Referring to Figures 3 and 14, the NMR sensor 57 includes a cylindrical permanent magnet 410 to establish a static magnetic field B0 to perform the NMR measurement sequence. The magnetic field of the magnet 410 is polarized through the diameter of the magnet 410. The sensor 57 also includes a ferrous material 405 (ie, a ferromagnetic material) which is located in the adjacency and partially circumscribes the permanent magnet 410 over a longitudinal axis of the magnet. 410. The antennas 134 and 136 are located near opposite ends of the ferrous material 405 and are formed of corresponding coils wound around the ferrous material 405 in such a way that the magnetic moments of the antennas 134 and 136 are parallel to the longitudinal axis of the magnet 410 Unlike antennas 134 and 136, antenna 132 is formed of a coil having a magnetic moment tangential to the longitudinal axis of permanent magnet 410. To accomplish this, the coil forming antenna 132 extends around a section 401 of the 405 iron material, as depicted in Figure 15. In this way, the 405 iron material may be formed of stacked sections 401. The iron material 405 assists both the static magnetic field that is created by the permanent magnet 410 and the generation / reception of RF signals by the antennas 132, 134 and 136. In this way, the iron material 405 becomes radially polarized, as shown in Fig. 16, to effectively radially extend the static magnetic field. Referring to Figure 7, the static magnetic field also elevates the magnetic permeability of the iron material between a saturated level and the permeability of a vacuum to aid in the reception of spin echo signals and the transmission of RF pulses. RF coil antennas of conventional tools can circumscribe the permanent magnet. However, unlike conventional antennas, the antennas 132, 134 and 136 are formed around the iron material 405. Due to this arrangement, in some embodiments, a metallic cylindrical sleeve 410 (see Fig. 16) encloses the permanent magnet 405, an impossible arrangement when the coils circumscribe the permanent magnet 405. The sleeve 410 protects and provides structural support to prevent the permanent magnet 405 from wobbling when the tool 50 is carried up the hole. The region of the formation that is investigated by the NMR measurement is determined by the condition: - Bo I < Bi where is the center frequency of the RF pulses. is the magnetic spin relation, which is (2). (4258) radians / sec / Gauss for protons; B0 is the magnitude of the static magnetic field and Bi is the magnitude of the RF field component that is perpendicular to the static field. The magnitudes of these fields are dependent on the position. The region in which the resonance condition is satisfied is formed as a thin covering. The thickness of the resonance cover is in the order of 1 mm. The distance from the recording tool to the resonance cover is controlled by the frequency of the RF pulses as described in U.S. Patent No. 3,597,681, entitled "Nuclear Well Log," issued on 3 August 1971. Figure 18 shows that the magnitude of the static field is a decreasing function of the distance from the registration tool. Therefore, by decreasing the frequency of RF pulses, the tool is investigated deeper in the training. One of the functions of the programmable pulse sequencer 111 is to tune to the synthesizer 116 to produce a particular frequency corresponding to a predetermined depth in the training. The pulse sequencer 111 can quickly change the frequency of the synthesizer 116, thereby changing the depth of the investigation. Although the invention has been presented with respect to a limited number of incorporations, those skilled in the art, having the benefit of this presentation, will be able to appreciate numerous modifications and variations thereof. It is understood that the appended claims cover all those modifications and variations that fall within the true spirit and scope of the invention.

Claims (20)

R E I V I N D I C A C I O N E S:

1. A method usable with an NMR measurement sequence, comprising: generation of status descriptors, each state descriptor indicating a state of the NMR measurement sequence during a different associated time interval; and storing the status descriptors in a registration tool NMR hole down.

2. The method of claim 1, wherein the storage comprises: storing the status descriptors before the tool is lowered down the hole.

3. The method of any preceding claim, further comprising: storing additional state descriptors for additional NMR measurement sequences.

4. The method of any preceding claim, further comprising: generating the NMR measurement sequence in response to the state descriptors.

5. The method of any preceding claim, further comprising: communicating an indication of the status descriptors from a well surface.

6. The method of any preceding claim, further comprising: using a coil to perform additional NMR measurements; between two of the measurements, press the coil with an RF explosion; monitor a coil voltage after the RF explosion; and based on the monitored voltage. tune the coil before making the next NMR measurement.

7. The method of claim 6, wherein the monitoring step comprises: determining a frequency composition of the monitored voltage.

8. The method of claims 6-7, further comprising: using the frequency composition to determine an approximate frequency difference between a resonant frequency of the coil and a Larmor frequency.

9. A downhole NMR logging tool, comprising: at least one coil; and a pulse sequencer coupled to said at least one coil and adapted to: receive the state descriptors indicative of states of an NMR measurement sequence; and using said at least one coil to perform the downhole NMR measurement sequence in an underground formation in response to the state descriptors.

10, The tool of claim 9, further comprising: a controller adapted to communicate with another computer separated from the tool to receive the status descriptors.

11. The tool of claims 9-10, wherein each state descriptor is associated with a different time interval of the NMR measurement sequence.

12. The tool of claims 9-10. Wherein the state descriptors indicate the different states independently of one another,

13. The tool of claims 9-10, further comprising: a memory for storing indications of the state descriptors; a closure coupled to the memory and adapted to receive an indication of first data of the indications of the status descriptors, the closure using the first data to generate control signals to regulate the NMR measurement sequence; a counter coupled to the memory and adapted to: receive an indication of second data of the indications of the status descriptors, the second data indicating a duration of time, and based on the indications of the second data, establish the time that the First data remain in the closure.

14, The tool of claim 13. wherein the counter is timed at a frequency close to the Larmor frequency and the indications of the status descriptors are stored in the memory at another frequency that is independent of the Larmor frequency.

15. The tool of claim 9, further comprising: a permanent magnet; a ferromagnetic material located adjacent to the permanent magnet; a circuit coupled to the coil and adapted to use said at least one coil and the permanent magnet to perform the NMR measurements.

16. The tool of claim 15, wherein the permanent magnet comprises a cylindrical magnet having a longitudinal axis and the ferromagnetic material at least partially circumscribes the permanent magnet on the longitudinal axis of the cylindrical magnet.

17. The tool of claims 15-16, wherein the permanent magnet substantially influences the magnetic permeability of the ferro-magnetic material.

18. The tool of claims 15-16, wherein the permanent magnet establishes a static magnetic field and the ferromagnetic material substantially influences the static magnetic field.

19. The tool of claim 15. further comprising a metal housing at least partially covering the permanent magnet.

20. The tool of claims 15-19, wherein the at least one coil surrounds the ferro-magnetic material.