Patent Description:
Accordingly, it should be understood that these statements are to be read in this light, and not as an admission of any kind.

Producing hydrocarbons from a wellbore drilled into a geological formation is a remarkably complex endeavor. In many cases, decisions involved in hydrocarbon exploration and production may be informed by measurements from downhole well-logging tools that are conveyed deep into the wellbore. The measurements may be used to infer properties or characteristics of the geological formation surrounding the wellbore.

One type of downhole well-logging tool uses nuclear magnetic resonance (NMR) to measure the response of nuclear spins in formation fluids to applied magnetic fields. Many NMR tools have a permanent magnet that produces a static magnetic field at a desired test location (e.g., where the fluid is located). The static magnetic field produces an equilibrium magnetization in the fluid that is aligned with a magnetization vector along the direction of the static magnetic field. A transmitter antenna produces a time-dependent radio frequency magnetic field that is perpendicular to the direction of the static field. The radio frequency magnetic field produces a torque on the magnetization vector that causes it to rotate about the axis of the applied radio frequency magnetic field. The rotation results in the magnetization vector developing a component perpendicular to the direction of the static magnetic field. This causes the magnetization vector to align with the component perpendicular to the direction of the static magnetic field, and to precess around the static field.

The document <CIT> describes a nuclear magnetic resonance (NMR) measurement method and apparatus for well logging. The apparatus includes a magnet, a transmitter antenna and at least one of a receiver section of the transmit antenna, or a separate receiver antenna, having a length along the longitudinal dimension of the apparatus, which is shorter than a length of the transmitter antenna along the longitudinal dimension. The apparatus includes circuitry for applying radio frequency (RF) current pulses, for example a Carr-Purcell-Meiboom-Gill pulse sequence, to the entire transmitter antenna and for receiving signals by the at least one of the receiver section of the transmitter antenna and the separate receiver antenna. A single or multiple antennas may overcome spin echo signal amplitude reduction caused by the loss of transverse magnetization as a result of instrument motion. In a configuration, either a single antenna with different portions, or two separate antennas which are juxtaposed end-to-end and simultaneously excited during transmit, while only a lower antenna is active during receive. In transmit mode the entire length, or the longer antenna, may be used to transmit RF pulses, whereas in receive mode only the lower part of the antenna, or a shorter antenna is connected to receiver circuitry in the instrument to detect NMR signals from the measurement region. A measurement starts at a first longitudinal position in a borehole, with a measurement made at a first frequency and thus in a measurement region during receiving RF signals induced by NMR phenomena. During the measurement time Tm the instrument moves to a position located at a longitudinal distance from the first position equal to v·Tm (v = speed of motion of the well logging instrument; Tm = measurement time). At the end of the first measurement the frequency of the transmitter is rapidly changed to a second frequency and a new measurement starts on a second measurement region, but with the instrument still substantially at the same axial position.

The documents <CIT> and <CIT> describe magnetic resonance measurement systems for well logging.

The present invention resides in to a method as defined in claim <NUM>.

These described embodiments are examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description of these embodiments, certain features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions may be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would still be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

As discussed herein, downhole logging techniques may employ multiple methods (e.g., gamma-ray spectroscopy, resistivity logging, NMR spectroscopy etc.) to characterize a downhole formation. Each of the methods are complementary and the combination of multiple methods may provide a more complete picture of the downhole formation. When all the methods collect data at a similar rate, characterization of a downhole formation is more efficient. One challenge in oilfield formation evaluation is the ability to log advanced NMR applications at rates closer to the rates of other logging methods.

Certain techniques use downhole tools having an antenna array (<CIT> ) <FIG> shows an example embodiment of an apparatus for investigating subsurface formations <NUM> penetrated by a wellbore <NUM>, which can be used in performing various embodiments of a method according to the present disclosure. A well logging instrument <NUM> may be suspended in the borehole <NUM> on an armored electrical cable <NUM>, the length of which substantially determines the relative depth of the well logging instrument <NUM>. The cable <NUM> may be extended into the wellbore <NUM> and withdrawn therefrom by suitable means at the surface such as a drum and winch mechanism <NUM>. It will be appreciated that other conveyance devices for moving the well logging instrument <NUM> such as slickline or coiled tubing may also use a winch mechanism as shown in <FIG>. Surface equipment, represented at <NUM>, can be of any type know in the art for operation well logging instruments, and may include a processor subsystem, signal communication and recording devices and a telemetry transceiver for communicating with the well logging instrument <NUM>.

The well logging instrument which makes the measurements can be any suitable NMR well logging device, for use in "wireline" conveyed well logging instrumentation as shown in <FIG>, or of a type that can be used in logging while drilling (LWD) applications to be explained below with reference to <FIG>. The well logging instrument <NUM> includes, for example, a magnet such as a permanent magnet, magnet array, electromagnet or combinations thereof for inducing a static magnetic field in the formations <NUM>, and one or more radio frequency (RF) antennas for inducing a pulsed RF magnetic field in the formations <NUM> and for receiving RF energy induced by NMR phenomena excited in the formations <NUM>.

The well logging instrument described above can also be implemented, for example, in logging-while-drilling ("LWD") equipment. As shown, for example, in <FIG>, a platform and derrick <NUM> are positioned over a wellbore <NUM> that may be formed in the Earth by rotary drilling. A drill string <NUM> may be suspended within the borehole and may include a drill bit <NUM> attached thereto and rotated by a rotary table <NUM> (energized by means not shown) which engages a kelly <NUM> at the upper end of the drill string <NUM>. The drill string <NUM> is typically suspended from a hook <NUM> attached to a traveling block (not shown). The kelly <NUM> may be connected to the hook <NUM> through a rotary swivel <NUM> which permits rotation of the drill string <NUM> relative to the hook <NUM>. In some embodiments, the drill string <NUM> and drill bit <NUM> may be rotated from the surface by a "top drive" type of drilling rig.

Drilling fluid or mud <NUM> is contained in a mud pit <NUM> adjacent to the derrick <NUM>. A pump <NUM> pumps the drilling fluid <NUM> into the drill string <NUM> via a port in the swivel <NUM> to flow downward (as indicated by the flow arrow <NUM>) through the center of the drill string <NUM>. The drilling fluid exits the drill string via ports in the drill bit <NUM> and then circulates upward in the annular space between the outside of the drill string <NUM> and the wall of the wellbore <NUM>, as indicated by the flow arrows <NUM>. The drilling fluid <NUM> thereby lubricates the bit and carries formation cuttings to the surface of the earth. At the surface, the drilling fluid is returned to the mud pit <NUM> for recirculation. If desired, a directional drilling assembly (not shown) could also be employed.

A bottom hole assembly ("BHA") <NUM> may be mounted within the drill string <NUM>, in some cases near the drill bit <NUM>. The BHA <NUM> may include subassemblies for making measurements, processing and storing information and for communicating with the Earth's surface. Such measurements may correspond to those made using the NMR well logging instrument explained above with reference to <FIG>. The bottom hole assembly is typically located within several drill collar lengths of the drill bit <NUM>. In the illustrated BHA <NUM>, a stabilizer collar section <NUM> is shown disposed immediately above the drill bit <NUM>, followed in the upward direction by a drill collar section <NUM>, another stabilizer collar section <NUM> and another drill collar section <NUM>. This arrangement of drill collar sections and stabilizer collar sections is illustrative only, and other arrangements of components in any implementation of the BHA <NUM> may be used. The need for or desirability of the stabilizer collars will depend on drilling conditions as well as on the demands of the measurement.

In the arrangement shown in <FIG>, the components of the NMR well logging instrument may be located, for example, in the drill collar section <NUM> above the stabilizer collar <NUM>. Such components could, if desired, be located closer to or farther from the drill bit <NUM>, such as, for example, in either stabilizer collar section <NUM> or <NUM> or the drill collar section <NUM>.

The BHA <NUM> may also include a telemetry subassembly (not shown) for data and control communication with the Earth's surface. Such telemetry subassembly may be of any suitable type, e.g., a mud pulse (pressure or acoustic) telemetry system, wired drill pipe, etc., which receives output signals from LWD measuring instruments in the BHA <NUM> (including the NMR well logging instrument) and transmits encoded signals representative of such outputs to the surface where the signals are detected, decoded in a receiver subsystem <NUM>, and applied to a processor <NUM> and/or a recorder <NUM>. The processor <NUM> may comprise, for example, a suitably programmed general or special purpose processor. A surface transmitter subsystem <NUM> may also be provided for establishing downward communication with the bottom hole assembly.

The BHA <NUM> may also include conventional acquisition and processing electronics (not shown) comprising a microprocessor system (with associated memory, clock and timing circuitry, and interface circuitry) capable of timing the operation of the source and the data measuring sensors, storing data from the measuring sensors, processing the data and storing the results, and coupling any desired portion of the data to the telemetry components for transmission to the surface. The data may also be stored in the instrument and retrieved at the surface upon removal of the drill string. Power for the LWD instrumentation may be provided by battery or, as known in the art, by a turbine generator disposed in the BHA <NUM> and powered by the flow of drilling fluid. The LWD instrumentation may also include directional sensors (not shown separately) that make measurements of the geomagnetic orientation or geodetic orientation of the BHA <NUM> and the gravitational orientation of the BHA <NUM>, both rotationally and axially.

The foregoing computations may be performed on a computer system such as one shown in the processor at <NUM> in <FIG>, or in the surface unit <NUM> in <FIG>. However, any computer or computers may be used to equal effect.

The present example embodiment of NMR well logging instrument may be of a type that can be operated to obtain separate measurements from a plurality of closely spaced thin, cylindrical or other shape "shell"-like regions in the surrounding formations <NUM>. A simplified representation of some of the components of a suitable type of well logging instrument is shown in <FIG> and <FIG>. <FIG> shows a side view of the NMR well logging instrument <NUM>. The example NMR well logging instrument shown in <FIG> may be one such as sold under the trademark MR SCANNER, which is a trademark of Schlumberger Technology Corporation, Sugar Land, Tex. A magnet or magnet array (magnet) is shown at <NUM>. The magnet <NUM> may be a permanent magnet, magnet array, an electromagnet or any combination of the foregoing. An RF antenna, represented at <NUM>, may be a suitably oriented wire coil or coils. The MR SCANNER instrument may include a separate RF antenna 37C, however this separate antenna is not important for purposes of explaining apparatus and methods according to the present disclosure.

<FIG> also illustrates a general representation of the type of closely spaced cylindrical thin shells, <NUM>-<NUM>, <NUM>-<NUM>,. <NUM>-N, that can be selectively excited to produce NMR phenomena using a multi-frequency transmitter and receiver circuit. As is known in the art, for example as disclosed in <CIT> issued to Strickman, the well logging instrument is programmed select the shell region to be investigated by appropriately selecting the frequency of the RF energy in the current pulses applied to the antenna <NUM>. The magnet <NUM> is arranged such that a static magnetic field induced in the formations (<NUM> in <FIG>) has substantially equal amplitude along the longitudinal dimension of the magnet <NUM>, and in some embodiments, the amplitude of the static magnetic field changes with respect to lateral distance from the magnet <NUM>. The RF antenna <NUM> are arranged such that RF magnetic fields induced in the formations by passing pulses of RF current through the antenna <NUM> are substantially orthogonally polarized with reference to the polarization direction of the static magnetic field. The RF magnetic fields induced by the antenna <NUM> also have substantially equal amplitude along the longitudinal dimension of the antenna <NUM> and the magnet <NUM>. Similarly, the sensitivity distribution of the antenna <NUM> is such that the antenna <NUM> has substantially equal sensitivity along the longitudinal dimension of the antenna <NUM>. Longitudinal dimension as used in the present context is used to mean along a line parallel to the longitudinal axis of the well logging instrument <NUM>, shown approximately by the arrow labeled Z in <FIG>.

The well logging instrument <NUM> comprises circuitry for applying pulses of RF current through the antenna <NUM> and for detecting voltages induced in the antenna <NUM>, of the well logging instrument <NUM>. A non-limiting example embodiment of suitable circuitry is described in the Strickman '<NUM> patent referred to above. The exact configuration of the circuitry is not intended to limit the scope of the present disclosure; those skilled in the art will be able to design different forms of suitable circuitry for use in the well logging instrument <NUM>.

In a well logging instrument and method according to the present disclosure, the relative longitudinal dimensions of the antenna or antennas are selected differently during the transmit phase compared to the receive phase, as will be further explained. An apparatus and method according to the present disclosure results in exciting NMR phenomena in the selected regions of the formations <NUM> and detecting signals resulting from such NMR phenomena only in longitudinal portions that are substantially unaffected by movement of the well logging instrument <NUM> along the wellbore <NUM>. The different selected regions of the formations <NUM> are shown schematically in <FIG> at <NUM>-<NUM>. <FIG> shows an oblique view of the well logging instrument <NUM> to more clearly show the lateral separation within the formations <NUM> of the selected regions <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>,.

There are two independent types of speed effects observed on NMR porosity and T2 distribution logs acquired at relatively fast logging speeds.

The first effect is caused by incomplete pre-polarization of the hydrogen nuclei by the static magnetic field induced by the magnet in the well logging instrument (e.g., as shown at <NUM> in <FIG>). Incomplete pre-polarization may result in underestimation of the total porosity; such effect is increased in formations which have substantial porosity with long longitudinal (T1) relaxation times of fluid filling the pore spaces. To measure accurate total porosities it is desirable that the susceptible nuclei being excited to induce NMR phenomena, e.g., hydrogen nuclei, be exposed to the static magnetic field for a sufficient time to obtain <NUM>% (i.e., exposure times of at least <NUM> T1) or greater pre-polarization prior to the start of the NMR measurement. To obtain this level of pre-polarization while moving the well logging instrument at relatively high speeds, non-overlapping measurements are conducted. For non-overlapping measurements the hydrogen nuclei are pre-polarized by a magnet that has sufficient length along the direction of motion to obtain full polarization. The pre-polarization magnet length depends on the desired maximum logging speed and the maximum T1 of the fluids in the formations to be polarized. The extent of pre-polarization that is actually obtained in any set of circumstances is related to the instrument speed and the maximum T1. The magnitude of the pre-polarization will be further explained below for different instrument designs, e.g., pre-polarization and antenna lengths. The term pre-polarization is used in present context to mean orientation of the nuclear spin axes of susceptible nuclei along the polarization direction of the static magnetic field induced by the magnet (<NUM> in <FIG>).

The second effect of logging speed is to compress the measured or apparent T2 distributions. Such compression of measured T2 distributions is greater for longer T2 times. Compression does not affect the apparent total NMR porosity; however, it leads to errors in calculated bound-fluid and free-fluid porosities. The compression effect results from longitudinal movement of the RF antenna during the NMR measurement away from the region of the formation where transverse magnetization was induced by imparting RF magnetic fields into the formation. The compression effect can be mitigated to some extent by using a larger RF antenna (larger being with reference to the direction of motion of the instrument during measurement); however, it remains a substantially adverse effect for practical antenna lengths and speeds of <NUM> per hour (<NUM> feet per hour). <NUM> per hour (<NUM> feet per hour) is a practical instrument speed for other types of porosity measurements, e.g., gamma-gamma density porosity and neutron porosity.

Below will be explained the average speed-dependent polarization over the length of the RF antenna. A schematic representation of a model NMR well logging instrument is shown in <FIG>. The modeled instrument is moving at constant speed (v). On the y-axis is the speed and position dependent polarization profile shown over the length of the modeled instrument. In <FIG> it is assumed that the static magnetic field produced by the magnet is substantially constant amplitude with reference to the direction of motion (i.e., the longitudinal dimension) of the magnet. The polarization profile shown is for an instrument that has moved a distance v·Tw since the end of the previous measurement. The time Tw is the wait time between the end of one NMR measurement and the start of the next NMR measurement. A portion of the length v·Tw of the antenna in the direction of motion has been pre-polarized by the pre-polarization length lpol of the magnet whereas the remaining length of the antenna, lant-w·Tw, has been re-polarized by the static magnetic field during the time Tw from the end of the previous NMR measurement. <FIG> shows a snapshot of the spin polarization in the formation at the instant before a new NMR measurement is initiated by application of a <NUM>° RF pulse (explained further below) to the antenna.

The speed and position dependent polarization profile is given by the equations,
<MAT>
where
<MAT>
and
<MAT>.

<FIG> and the above equations are valid for the general case of overlapping NMR measurements. In the region of the antenna in Eq. (2a) the instant NMR measurements overlap (i.e., two NMR measurements measure part of the same longitudinal interval) with the previous measurements whereas in the region in Eq. (2b) successive NMR measurements are non-overlapping. From Eq. (<NUM>) and the fact that T1 values in many formations are several seconds, wait times of 3T1 may be used to attain <NUM>% polarization. Such long wait times would lead to very slow logging speeds. Therefore, using known apparatus and methods, measurements over the total length of the RF antenna are used for fast NMR logging without overlap. The condition for non-overlapping measurements is that v·Tw=lant. In the following description the notation is simplified by replacing lant with la.

The average speed-dependent polarization over the length of the antenna is determined by a weighted average of the speed and position dependent polarization function over the antenna such as by the following expression: <MAT>
where in the integral on the right hand side of Eq. (<NUM>), to simplify the description, it is assumed that the antenna sensitivity function, Sa(z)=lant -<NUM>, is uniform along the longitudinal dimension of the antenna. (<NUM>) and (<NUM>) the polarization is determined by the expression: <MAT>.

The last term in Eq. (<NUM>) accounts for the overlapping part of the measurement where the hydrogen nuclei from the previous NMR measurement are repolarized by the static magnetic field during the wait time. It is desired to obtain non-overlapping measurements for relatively high instrument movement speeds for v·Tw=lant which is represented by the expression: <MAT>
where the ratio r is defined as: <MAT>.

It may be observed in Eq. (<NUM>) that the average polarization over the length of the antenna depends on the two dimensionless ratios, r and lp/la. Moreover, for a given well logging instrument configuration with fixed values for lp and la the average polarization depends only on the product, v·T1, i.e., the distance the instrument moves during the time T1. In subsurface formations there is a distribution of T1 times corresponding to different pores sizes and fluids. If a selected value of polarization, e.g., <NUM>%, is obtained for a certain value of T1 then formations having shorter values of T1 will have polarization that exceeds <NUM>%.

<FIG> shows plots of the average polarization given by Eq. (<NUM>) with respect to the ratio lp/la for values of the ratio r=<NUM> at curve <NUM>, r=<NUM> at curve <NUM> and r=<NUM> at curve <NUM> for a value of v·T1= <NUM> (<NUM> inches). It is convenient for purposes of the present description to express v in cm per second (inches per second) and T1 in seconds. For a tool moving at <NUM> per hour (<NUM> feet per hour) (<NUM> (<NUM> inches) per second) the plots in <FIG> are valid for a value of T1=<NUM> seconds.

The three curves in <FIG> correspond to antenna lengths of <NUM>, <NUM> and <NUM> centimeters (<NUM>, <NUM>, and <NUM> inches). It may be observed that for all three antenna lengths a prepolarization length of approximately <NUM> (<NUM> inches) provides <NUM>% pre-polarization. Note that less pre-polarization length is used for longer antenna lengths.

Some additional results for non-overlapping measurements computed from Eq. (<NUM>) are shown in Table <NUM>.

It may be observed in Table <NUM> that to obtain <NUM>% or greater pre-polarization at <NUM> per hour (<NUM> feet per hour) well logging instrument speed where T1=<NUM> seconds would have a magnet pre-polarization length of <NUM> (<NUM> inches) or greater.

Having explained the effect of instrument motion on polarization, methods and apparatus according to the present disclosure may now be better understood. It should be emphasized that Table <NUM> shows respective values for non-overlapping measurements (as defined above). Given the amount of time for acquiring NMR measurements, and given that NMR RF pulsing reduces some or all of the static field-induced magnetization in front (front being with respect to direction of motion) of the antenna (e.g., <NUM> in <FIG>), the only way for known NMR apparatus to make non-overlapping measurements is to stop applying RF current pulses to the antenna and wait for the instrument to move one sensor length (i.e., the length of the antenna). In practice, skipping such a large axial portion of formation is not desirable, and thus a compromise is made to allow for an "overlapped" region of the formation in combination with the non-overlapped part of the measurement. The overlapped region may take a substantial amount of wait time to re-polarize the nuclear spins, which in turn slows down logging. This limitation is one of the bases for apparatus and methods according to the present disclosure.

The effect of well logging instrument speed on NMR spin-echo amplitude decay is to increase the apparent spin echo amplitude decay rate. This means that the apparent or observed T2 values are reduced compared to the true values that would be measured by a stationary tool. The T2 speed effect can be better understood by considering <FIG>, which shows two positions of the RF antenna at times separated by dt as the tool moves in the direction shown at a constant speed, v.

At time to in <FIG>, a radio frequency current pulse is passed through the first RF antenna (see, e.g., <NUM> in <FIG>) having duration and amplitude selected to reorient the nuclei by <NUM>° (the NMR excitation pulse) and thus creates a transverse magnetization in the formation adjacent to the antenna, wherein transverse is with reference to the direction of the static magnetic field induced by the magnet (<NUM> in <FIG>). At time to+dt the antenna has moved a distance v*dt away from the volume where the transverse magnetization was induced by the RF magnetic field at time to. To measure spin echo amplitude decay, the <NUM>° RF current pulse is followed by a plurality of RF current pulses passed through the antenna, wherein each such pulse has a duration and amplitude selected to reorient the nuclear magnetic spin axes of the susceptible nuclei by <NUM>° each such current pulse separated by a spin echo time TE. RF signals generated by NMR spin echoes following the <NUM>° RF pulses are detected and their amplitudes represent the spin echo amplitude decay rate. Motion of the instrument during the time span of the <NUM>° RF pulses that follow the <NUM>° excitation pulse is the cause of the T2 speed effect.

During the acquisition of the spin-echoes, which can take one second or more, the instrument moves into a region that has no transverse magnetization and therefore as subsequent spin echoes are acquired the spin echo signal is reduced by the loss of transverse magnetization. <FIG> shows the instrument positions at times to and to+dt. If the transverse magnetization created at time to corresponds to the length of the instrument shown in <FIG>, it is apparent that when the instrument is moved to a new position at time to+dt as shown, the antenna will be at least in part positioned adjacent an area of the formation for which no transverse magnetization has occurred. Such position-related loss of transverse magnetization, and therefore signal amplitude for this simple model shown in <FIG> is expressed as: <MAT>
and by integration the transverse magnetization at time t is determined by the expression: <MAT>.

Eq. (<NUM>) indicates that the speed-affected T2 related spin echo amplitude decay time is given by T2v=la/v and that during well logging the apparent or measured decay rate is expressed as: <MAT>.

The T2 related spin echo amplitude decay speed effect can be mitigated somewhat by using a longer RF antenna, however, it is still a substantial effect and represents an obstacle to accurate fast NMR well logging. Using Eq. (<NUM>) the apparent T2 relaxation time (and/or values of such times used to calculate a T2 distribution) is represented as: <MAT>.

Eq. (<NUM>) shows that the apparent relaxation time is less than the lesser of T2 and T2v. Table <NUM> shows the speed effect at <NUM> per hour (<NUM> feet per hour) instrument speed (<NUM> (<NUM> inches) per second) for different antenna lengths. It is observed in Table <NUM> that for longer values of T2 the apparent values, T2a, are substantially smaller than their actual values.

A pre-polarization magnet with a flat (defined above) magnetic field profile and a length chosen to be in the range from <NUM> to <NUM> centimeters (<NUM> to <NUM> inches), depending on the RF antenna length and other requirements, was shown above to be suitable for fast NMR logging (e.g., at speeds up to <NUM> per hour (<NUM> feet per hour)). The T2 speed effect, however, would use a different antenna design to enable fast logging. In NMR well logging instrument designs known in the art prior to the present disclosure the same RF antenna length which is used to transmit RF pulses is also used to receive the NMR signals from the sensitive volume (the measurement region in <FIG>) of the formation. There may be multiple RF antennas in conventional NMR tools (see, e.g., <NPL>), however, the total length of the antenna in known multiple antenna instruments is used both to transmit RF pulses, and also to receive NMR signals from the formation (see <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM> and <NUM>-<NUM> in <FIG>). During NMR well logging measurement when the well logging instrument is moving along a wellbore, the antenna moves away from the region where transverse magnetization was excited by the transmitted RF pulses and, as explained above, such movement causes the T2 speed effect as explained with reference to <FIG>.

As shown schematically in <FIG>, or single or multiple antennas may overcome spin echo signal amplitude reduction caused by the loss of transverse magnetization as a result of instrument motion. In <FIG> the schematic can be interpreted either as a single antenna with different portions, or two separate antennas which are juxtaposed end-to-end and simultaneously excited during transmit, while only the lower antenna is active during receive. <FIG> is an example of separate antennas juxtaposed side by side. In transmit mode the entire length <NUM> in <FIG>, or the longer antenna as shown at 37B in <FIG>, is used to transmit RF pulses, whereas in receive mode only the lower part of the antenna, i.e., the receiver section shown at 37A of <FIG>, or the shorter antenna 37B of <FIG> is connected to receiver circuitry in the instrument, e.g., as shown at <NUM> in <FIG>, to detect NMR signals from the measurement region. Because the receiver section 37A of the antenna <NUM> in <FIG> (or a similar length separate antenna as shown at 37B in <FIG>) remains opposite and within the excited region <NUM> as the instrument moves there is no speed induced T2 decay resulting from loss of transverse magnetization.

In the embodiment shown in <FIG> the receiver section 37A of the antenna <NUM> is active in receive mode while the remaining (upper) part of the antenna is electrically shorted out during NMR signal acquisition. Several switching technologies can be used for shorting a portion of the antenna. While the switching times of reed relays and even solid state relays are, at best, on the order of <NUM>'s of microseconds, MOSFET switches feature turn-on times on the order of <NUM>'s nanoseconds. The ON resistance of the switch (Rds) can now be as low as <NUM> milliohms for a 650V rated MOSFET. This development opens up the possibility of using MOSFET switches inside the very sensitive NMR antenna circuit. It is to be clearly understood that the scope of the present disclosure is not limited to the embodiment shown in <FIG> and that in other embodiments two separate juxtaposed antennas may be used as shown at <NUM> and 37B in <FIG>, wherein the two antennas are in the same orientation or in different orientations. It should also be clearly understood that using the lower part of the antenna (e.g., receiver section 37A in <FIG>) is appropriate for the case where the direction of motion of the NMR well logging instrument is toward the surface of the wellbore (up logging), i.e., toward shallower measured depth. In the case of moving the NMR well logging instrument away from the surface (down logging) the upper part of the antenna is appropriate. In the case of logging while drilling (LWD) instruments, wherein measurements may be made as the instrument moves in either the direction of shallower or greater measured depth, the receiver section of the antenna or a separate receiver antenna may be positioned substantially in the middle along the antenna or separate transmitter antenna.

The NMR well logging instrument disclosed herein can be used with any conveyance methods, including without limitation armored electrical cable ("wireline"), on or in a well drilling assembly, (LWD), coiled tubing, downhole tractors, or slick line. For wireline NMR logging instruments, the T2 speed effect (among other factors) limits the logging speed to be much slower than some other formation evaluation tools which can be logged at <NUM> per hour (<NUM> feet per hour) or more. Using an instrument according to the present disclosure, NMR logging speeds can be increased and thus save rig time. For LWD NMR instruments, the logging speeds are considerably slower than for wireline conveyed instruments, but speed effects still exist. Specifically, the sensitive region is sometimes axi-symmetric and short. The extent of the parasitic T2 speed effect depends on the rate of penetration of the drill bit and bottom hole assembly (BHA). Given the short sensitive region of LWD NMR well logging instruments, rates of penetration in excess of <NUM> per hour (<NUM> feet per hour) may show a T2 speed effect which is unacceptably large. Similar arguments apply to logging while tripping or measurement after drilling. The antennas used in LWD NMR logging tools are typically of a circular coil design to achieve azimuthal symmetry. An example implementation of an NMR well logging instrument according to the present disclosure is that the transmitter coil and the receiver coils can be concentrically aligned as shown at <NUM> and 137B in <FIG>. Without departing from the scope of the present disclosure, the transmitter coil would be longer while the receiver coil would be shorter, spanning only a fraction of the length of transmitter coil.

Consider a logging instrument moving with a constant speed v. <FIG> shows an example of how to use the antenna design as explained above with reference to <FIG>, and an associated multi-frequency RF pulse sequence for fast NMR logging. In this example embodiment the receiver section (37A in <FIG>) of the RF antenna is one third of the total length of the RF antenna (<NUM> in <FIG>). The measurement time Tm is equal to the time used to acquire some or all of the NMR measurements, e.g., spin-echo amplitude measurements in a pulse sequence (or sequences) for estimating NMR properties of the formations. The pulse sequence entails applying a plurality of radio frequency current pulses and receiving the signals generated therein. During each Tm the well logging instrument moves a distance v*Tm which is chosen to be equal to the length of the receiver section (37A in <FIG>) of the antenna. The vertical axis in <FIG> is the time axis and each scale increment is equal to one measurement time Tm. For purposes of the present disclosure, a measurement cycle is defined as all the NMR RF current pulses and measurements made during Tm. <FIG> shows different positions of the instrument as it moves through different measurement cycles. Each measurement cycle is labelled with a different frequency (f1 through f4) each of which corresponds to a different radial depth or shell (as originally shown with <NUM>-<NUM> through <NUM>-<NUM> in <FIG>) in the formation for a multi-frequency well logging instrument having a magnet (<NUM> in <FIG>) which has a magnetic field with an amplitude variation with respect to lateral distance from the magnet, but with substantially constant amplitude over a selected prepolarization length thereof. Different frequencies are selected such that the respective measurement regions <NUM>-<NUM> are closely spaced in the radial direction as long as the radial spacing between adjacent excited and measurement regions is sufficiently large so that RF pulses emitted at one frequency do not disturb the static magnetization in regions corresponding to any of the other frequencies. Such result may be obtained by using shaped RF pulses (e.g., and without limitation to limit the effective RF bandwidth) Pulse shaping is a standard NMR technique that includes adjusting the amplitude and phase of the RF pulses at a given frequency to ensure they do not degrade magnetization in regions to be sampled by RF pulses at other frequencies.

In the present example embodiment shown in <FIG> the lower third of the RF antenna is used to receive the spin-echo signals whereas the entire antenna is used to transmit <NUM>° and <NUM>° RF pulses. During well logging, measurements are repeated using four, closely spaced measurement regions <NUM>-<NUM> and vTm=la/<NUM> where Tm is the measurement time, v represents the instrument speed, and la represents the antenna length. The positions of the instrument at the beginning and at the end of each measurement cycle (e.g., for any individual set of RF pulses and signal detection sequence) are indicated at <NUM>, 71A, 71B, 72A, 72B, 73A, 73B, 74A, 74B, and <NUM>, where the end of one measurement and the beginning of the subsequent measurement is made at the same instrument position (e.g., 71A, 71B, 72A, 72B, 73A, 73B, 74A, and 74B) by starting the subsequent measurement using a different frequency for the RF pulses than the previous measurement. Using the foregoing technique the excitation and measurement regions in the subsequent measurement cycle are substantially unaffected by RF pulse transmission from the prior measurement. In the foregoing discussion, for cases when the antenna sensitive region is different from the physical antenna length, the antenna sensitive region is considered to be the excitation region.

It may be useful to explain in more detail the tool motion and measurements shown in <FIG>. The measurement starts at the extreme left of <FIG> at longitudinal position <NUM> with a measurement made at a first frequency f1 and thus in a measurement region <NUM> during receiving RF signals induced by NMR phenomena. During the measurement time Tm the instrument moves from position <NUM> to the position 71A. The position of 71A is located at a longitudinal distance from the first position <NUM> equal to v·Tm. The measurement region <NUM> annotated with f1 indicates the formation region from which the received spin-echo signals were acquired during the first measurement. At the end of the first measurement the frequency of the transmitter is rapidly changed to f2 and a new measurement starts on the second measurement region <NUM>, but with the instrument still substantially at the same axial position as 71A, now indicated by 71B. The slight axial movement between 71A and 71B is the result of a frequency switching time which is short and is negligible compared to Tm. The measurement region <NUM> annotated with f2 shows the formation region from which the received spin-echo signals were acquired during the second measurement. Similarly, at the end of the second measurement, with the instrument at a third axial position 72A the transmitter frequency is rapidly switched to a third frequency f3 and a new measurement starts on the third measurement region <NUM> with the instrument still at the third axial position 72B. It may be observed in <FIG>, that using an effective receiver antenna length of vTm=la/<NUM>, at the end of the third measurement the lower part of the antenna overlaps part of the axial region measured during the first measurement. Therefore another measurement may be made on a fourth measurement region (e.g., at <NUM>) before a new measurement made with fully pre-polarized nuclei can be made once again in the first frequency shell (shown as measurement region <NUM>). Thus at the end of the third measurement the frequency of the transmitter is rapidly changed to f4 and a new measurement starts on the fourth frequency shell, e.g., in measurement region <NUM>). At the end of the fourth measurement, the transmitter frequency may be rapidly switched back to f1 and the process repeats until an entire axial region of interest has been measured. As may be inferred from <FIG>, the fractional length of the receiver portion (37A in <FIG>) of the antenna, or separate receiver antenna (37B in <FIG>), may be a fraction of the length of the antenna, which in turn is related to the speed of motion of the well logging instrument and the number of different RF frequencies desired to excite different regions of the formation, that is: <MAT>
where x may be at least <NUM> (in embodiments where only integer values are used, however it should be noted that non-integral values can be used as well) to eliminate both the T1 and T2 speed effects, and additionally the desired number of RF frequencies may be at least x+<NUM>, that is, the minimum choice of frequencies is <NUM>. However, the relationship <NUM> is not limited only to integer values nor that the measurement region and vTm are equal. For example, the measurement region may be ⅓ of la while the distance traveled in one measurement cycle vTm is <MAT> of la. Further, a different number frequencies may be chosen according to a desired pulsing scheme, a desired change in vertical resolution, or a petrophysical consideration for measuring multiple depths into the formation. The scope of the present disclosure is not limited by such considerations.

A few observations and comments using the multi-frequency NMR measurements shown in <FIG> are instructive. Successive measurements are adjacent in the axial direction so there are no missed axial zones of the formation. There is no delay between successive measurements apart from the time for switching frequencies (which is negligible compared to typical NMR wait times between measurements) so that the duty cycle of the transmitter will be higher than that used in NMR well logging instruments known in the art. Each of the measurements shown in <FIG> may be from within a sensitive volume that is fully pre-polarized by a magnet as described above. The rapid switching of transmitter frequencies may be performed using well known solid state switches. The example shown in <FIG> is for the case where the length of the receiver section of the antenna is one third of the total antenna length, e.g., v·Tm=1a/<NUM> which uses four frequencies to obtain a properly polarized suite of axially continuous measurements. In general, if v·Tm=la/n where n=<NUM>, <NUM>,. then n+<NUM> frequencies or more are used to obtain a properly polarized suite of axially continuous measurements. The order of frequencies does not necessarily have to be monotonically increasing or decreasing. In embodiments, the measurement region does not move substantially beyond the initial excitation region for a particular transmitter antenna length <NUM>. It is possible that some combination of the distance vTm, the excitation region, and the measurement region will result in some or all of the receiver section moving beyond the initial excitation region, and thus T2 speed effects will occur. Unintended variable speeds are one cause of such cases. The present disclosure applies to any measurement method known to those skilled in the art which involves a preparation pulse or set of pulses (for example the <NUM> degree and <NUM> degrees pulses) applied to the transmitter length, and measurement events (for example the receiving of the echo signals) in the measurement region. Examples include saturation recovery sequences, inversion recovery sequences, composite pulse sequences, relaxometry sequences, and alike. A specific case is discussed next.

Consider a specific example of the antenna and RF pulse sequence shown in <FIG> for a well logging instrument moving at a speed of <NUM> (<NUM> inches) per second (<NUM> per hour) (<NUM> feet per hour). If the receiver section (37A in <FIG>) of the RF antenna has a length of <NUM> (<NUM> inches) then the antenna length may be <NUM> (<NUM> inches) and the measurement time Tm=<NUM> second. During the measurement time a pulse sequence, e.g., a Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence with <NUM> spin echoes (caused by <NUM><NUM>° RF pulses following an initial <NUM>° RF excitation pulse) is acquired using an echo spacing TE of <NUM>× <NUM>-<NUM>. In some embodiments, a set of CPMG sequences can be operated after the first main CPMG sequence having fewer spin echoes and a plurality of, e.g., <NUM> short wait time (e.g., <NUM>) measurement sequences each having a relatively small number of spin echoes (e.g., <NUM>). These sets of CPMG sequences are sometimes referred to as bursts or trainlets, and the present disclosure includes potentially choosing the measurement cycle to include these bursts and any other plurality of pulses. If a longer measurement time is desired in some embodiments the receiver section of the antenna may be a larger fraction of the total length of the antenna, e.g., one-half of the total antenna length, v·Tm=la/<NUM> which increases the measurement time to <NUM> seconds for an <NUM> (<NUM> inch) antenna and an instrument speed of <NUM> (<NUM> inches) per second. Using such longer receiver antenna (or receiver section of the transmit/receive antenna as explained above) would use three different RF frequencies as discussed above. Other embodiments may comprise a longer or shorter total antenna length. Other embodiments of antenna design and measurement sequences may be used that are within the scope of the present disclosure.

NMR spin-echo well logging measurements are typically acquired as phase alternated pairs (i.e., the phases of the <NUM>° pulses in two phase alternated pair CPMG sequences are shifted by a phase angle of <NUM>° and they are then subtracted from each other) in order to cancel baseline offsets in the CPMG spin echo signals. For the measurements shown schematically in <FIG> subsequent measurements at the same frequency may be one antenna length apart and could be acquired as phase alternated pairs, or measurements at the different closely spaced frequencies could be acquired with alternating phases and combined. In some embodiments, the offsets could be accounted for in the data processing (See, Freedman, <CIT>). The method for removing the offsets is not intended to limit the scope of the present disclosure.

Certain advanced logging modes on tools, across both wireline and LWD, may log at or below <NUM> per hour (<NUM> fph (feet per hour)) on LWD, and at or below <NUM> fph on wireline. In many cases, it may be useful to perform faster logging, such as <NUM> fph or more. Accordingly, the techniques of the present disclosure are direct to improving the acquisition rate of NMR logging tools, such as those with multiple antennae (e.g., coils). The present techniques employ transmit and receive control of each of the antenna in an antenna array to solve two problems: <NUM>) prepolarization of the magnets on the NMR tool can be used to fullest effect, and <NUM>) to acquire different patterns of NMR data on each coil to build up an entire suite of data all at one depth location, thus enabling truly fast logging.

With this in mind, <FIG> illustrates a well-logging system <NUM> that employs the systems and methods of this disclosure. The well-logging system <NUM> may be used to convey a downhole tool <NUM> through a geological formation <NUM> via a wellbore <NUM>. The downhole tool <NUM> may be conveyed on a cable <NUM> via a logging winch system <NUM>. Although the logging winch system <NUM> is schematically shown in <FIG> as a mobile logging winch system carried by a truck, the logging winch system <NUM> may be substantially fixed (e.g., a long-term installation that is substantially permanent or modular). Any suitable cable <NUM> for well logging may be used. The cable <NUM> may be spooled and unspooled on a drum <NUM> and an auxiliary power source <NUM> may provide energy to the logging winch system <NUM> and/or the downhole tool <NUM>.

Moreover, although the downhole tool <NUM> is described as a wireline downhole tool, it should be appreciated that any suitable conveyance may be used. For example, the downhole tool <NUM> may instead be conveyed as a logging-while-drilling (LWD) tool as part of a bottom hole assembly (BHA) of a drill string, conveyed on a slickline or via coiled tubing, and so forth. For the purposes of this disclosure, the downhole tool <NUM> is any suitable measurement tool that obtains NMR logging measurements through depths of the wellbore <NUM>.

Many types of downhole tools <NUM> may obtain induction logging measurements in the wellbore <NUM>. These include, for example, the Rt Scanner, AIT, and Thrubit Induction tools by Schlumberger Technology Corporation, but induction logging measurements from other downhole tools by other manufacturers may also be used. The downhole tool <NUM> may provide induction logging measurements <NUM> to a data processing system <NUM> via any suitable telemetry (e.g., via electrical signals pulsed through the geological formation <NUM> or via mud pulse telemetry). The data processing system <NUM> may process the induction logging measurements <NUM> to identify a conductivity and/or resistivity and dielectric constant at various depths of the geological formation <NUM> in the wellbore <NUM>.

To this end, the data processing system <NUM> thus may be any electronic data processing system that can be used to carry out the systems and methods of this disclosure. For example, the data processing system <NUM> may include a processor <NUM>, which may execute instructions stored in memory <NUM> and/or storage <NUM>. As such, the memory <NUM> and/or the storage <NUM> of the data processing system <NUM> may be any suitable article of manufacture that can store the instructions. The memory <NUM> and/or the storage <NUM> may be ROM memory, random-access memory (RAM), flash memory, an optical storage medium, or a hard disk drive, to name a few examples. A display <NUM>, which may be any suitable electronic display, may provide a visualization, a well log, or other indication of properties in the geological formation <NUM> or the wellbore <NUM> using the induction logging measurements <NUM>.

<FIG> illustrates an example of a downhole tool <NUM> that includes an antenna (e.g., coils) array <NUM> that generally operate to generate NMR data indicative of a composition of a formation. As illustrated in <FIG>, the antenna array <NUM> includes three antennae 42a, 42b, and 42c. In some embodiments, the antenna array <NUM> may include two or more than three antennae <NUM>.

In general, each antenna <NUM> of the antenna array <NUM> transmits and receives NMR signals to and from a geological formation <NUM>. In some embodiments, each antenna <NUM> of the antenna array <NUM> are independently controlled to both transmit and receive. That is, each antenna <NUM> pulse independently and receive information (e.g., signals) independently. However, in some embodiments, each antenna <NUM> of the antenna array <NUM> are partially coupled. In accordance with one embodiment of the present disclosure, as a downhole tool <NUM> is moving along a geological formation <NUM>, each antenna <NUM> will interact with a region <NUM> of the geological formation <NUM> that is proportional (e.g., equal or approximately equal) to the length (e.g., along the direction of arrow <NUM>) of each antenna <NUM>. Put differently, each antenna <NUM> will at least partially overlap with a region <NUM> of the geological formation <NUM> and, thus, each antenna <NUM> will interact with a region <NUM> at a given time. For example, as illustrated, antenna 42a overlaps region 46a, antenna 42b overlaps region 46b, and antenna 42c overlaps region 46c. As such, antenna 42a is interacting with region 46a, antenna 42b is interacting with region 46b, and antenna 42c is interacting with region 46c. When the downhole tool <NUM> moves (e.g., along direction <NUM>) a distance equal to the length <NUM> of an antenna <NUM>, antenna 42c is interacting with region 46d, antenna 42b is interacting with region 46c, and antenna 42a is interacting with region 46b. It is presently recognized that optimization of the control of each antenna <NUM> of the antenna array <NUM> may reduce difficulties in downhole NMR acquisition such as pre-polarization the of the magnet(s) <NUM> on the NMR tool can be used to fullest effect, and fast logging by acquiring different patterns of NMR data on each coil to build up an entire suite of data all at one depth location.

<FIG> is a flow diagram <NUM> for generating NMR data, in accordance with aspects of the present disclosure. The steps illustrated in the flow diagram <NUM> are performed via signals sent from the data processing system <NUM> or any suitable processing device. In general, the process uses one or more Carr-Purcell-Meiboom-Gill sequences and burst sequences to acquire a suite of NMR data. The phrase "CPMG" involves acquiring NMR echoes which give rise to T2-based information at a given wait time (polarization). The phrase "bursts" is used to refer to additional CPMG echo trains which are relatively shorter and repeated multiple times to build up signal to noise ratio for intermediate and shorter time constants, which also builds up the T1-based information in the data. In some embodiments, neither the bursts, nor the main CPMG echo trains are pure CPMGs. In some embodiments, the bursts or main CPMG echo trains comprise additional echo spacings, such as a diffusion editing method by two long echo spacings at the front of a CPMG, in order to encode diffusion into the NMR suite of data.

The flow diagram <NUM> includes generating a first pulse at a first position with a plurality of antennae (process block <NUM>). In some embodiments, the first pulse may include a single CPMG sequence. After the first pulse is generated, a second pulse is generated with a first subset of the plurality of antennae (process block <NUM>). The second pulse includes a CPMG and two "bursts", as discussed herein. It should be appreciated that this may be useful in generating T2 data. After the second pulse is generated, a third pulse is generated with a second subset of the plurality of antennae <NUM> (process block <NUM>). For example, the second subset of the plurality of antennae <NUM> may be fewer than the first subset, or a subset of the first subset. In some embodiments, the third pulse includes a CPMG and five bursts. It should be appreciated that this may be useful in generating T1/T2 data. Furthermore, it should be appreciated that NMR data is gathered at each step, and the data at each step is combined to produce a suite of NMR data (process block <NUM>).

To further help illustrate the steps in the flow diagram <NUM> of <FIG>, <FIG> is a block diagram <NUM> that illustrates an example of controlling the downhole tool <NUM> having the antenna array <NUM> shown in <FIG>, in accordance with an embodiment of the present disclosure.

Generally, the block diagram <NUM> illustrates time points <NUM> that represent a position (e.g., along axis <NUM>) of antennae <NUM> over time (e.g., axis <NUM>) as the antennae move the direction of the arrow <NUM>, which is the direction of logging. The six time points 70a, 70b, 70c, 70d, 70e, and 70f each illustrate a respective position of the antennae <NUM> over a length of time. In some embodiments, the time difference <NUM> between each time point <NUM> are equal, while in other embodiments, the time difference <NUM> may vary by a mathematical relationship, such as an exponential relationship.

As a non-limiting example, each antenna <NUM> may be between <NUM> and <NUM> (e.g., <NUM>, <NUM>, <NUM>, <NUM><NUM>) centimeters per second (<NUM> and <NUM><NUM>, <NUM>, <NUM>, <NUM>, <NUM>) inches in length <NUM> and the logging speed (e.g., rate at which the downhole tool <NUM> is moving in the direction of the arrow <NUM>) may be between <NUM> and <NUM> (e.g., <NUM>, <NUM>, <NUM>, <NUM><NUM>) centimeters per second (<NUM> and <NUM> (e.g., <NUM>, <NUM>, <NUM>, <NUM>, <NUM>)) inches per second). In one example, when each antenna is <NUM> (<NUM> inches) in length, and the downhole tool <NUM> is moving at <NUM> (<NUM> inches) per second, the time difference <NUM> between each time point <NUM> is <NUM> second. The antennae <NUM> at time point 70a are at a first position. The antennae <NUM> at time point 70b have moved a distance 74a from the first position to a second position that is equal to one length <NUM> of the antenna <NUM>. The antennae <NUM> at time point 70c have moved a distance 74b from the first position to the third position that is equal to two lengths <NUM> of an antenna. In a similar manner, the distance 74c is be equal to three lengths <NUM>.

At each time point <NUM>, a different pattern of NMR data is acquired as described herein. An antenna <NUM> that is collecting data is indicated by a filled in box and produces a suit of data every <NUM> (<NUM> inches). For example, the antenna 42c of time point 70b is not transmitting and receiving data (e.g., OFF). At time point 70a, the antennae <NUM> acquires NMR data in accordance with process block <NUM>. At time point 70b, the antennae <NUM> acquires NMR data in accordance with process block <NUM>. As such, the first subset may be antennae 42a and 42b. At time point 70c, the antennae <NUM> acquires NMR data in accordance with process block <NUM>. As such, illustrated the second subset may be antenna 42a. The data acquired at each time point 70a, 70b, and 70c is combined to generate a suite of NMR data. This process is then be repeated at time points 70d, 70e, and 70f to generate a second suite of NMR data.

Claim 1:
A method, comprising:
generating NMR data by transmitting and receiving pulses into the geological formation (<NUM>) with a plurality of antennae (42a-c) on a downhole tool (<NUM>), wherein each antenna (42a-c) has a length L and is segmented into a transmitter section and a receiver section, wherein the transmitter section is active during a transmit mode and the receiver section is active during a receive mode and wherein the transmitter section is electrically shorted during the receive mode, and wherein the downhole tool (<NUM>) is moved at a rate substantially equal to L per second and wherein at time points, subsets of the plurality of antennae (42a-c) are active;
wherein generating NMR data by transmitting and receiving pulses into the geological formation comprises:
generating a first pulse sequence at a first position along a geological formation with the plurality of antennae, wherein the first pulse sequence comprises a Carr-Purcell-Meiboom-Gill (CPMG) sequence;
generating a second pulse sequence at a second position with a first subset of antennae of the plurality of antennae, wherein the second pulse sequence comprises CPMG and a first burst sequence, wherein multiple shorter suites of pulses are repeated in succession;
generating at least a third pulse sequence at at least a third position with at least a second subset of antennae of the plurality of antennae, wherein the at least third pulse sequence comprises a CPMG sequence and at least a second burst sequence; and
generating a suite of multidimensional T1 and T2 NMR data based at least in part on the first pulse sequence, the second pulse sequence, and the at least third pulse sequence.