Abstract:
NMR data are acquired using a phase-alternation of the tipping pulse. Averaged properties are estimated over a window length. The averaged properties are inverted to undo the effects of the averaging. A matrix defined in terms of Walsh functions is used in the inversion.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to a method for improving the resolution of nuclear magnetic resonance properties of an earth formation traversed by a borehole, and more particularly, to a method for compensating for the effects of commonly used processing methods for eliminating any ringing, such as magnetoacoustic ringing, and DC offset, during a nuclear magnetic resonance measurement. 
   2. Background of the Art 
   A variety of techniques are utilized in determining the presence and estimation of quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, the resistivity, porosity and permeability of the rock formation surrounding the wellbore drilled for recovering the hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the well bores have been drilled. More recently, wellbores have been logged while drilling, which is referred to as measurement-while-drilling (MWD) or logging-while-drilling (LWD). 
   One recently evolving technique involves utilizing Nuclear Magnetic Resonance (NMR) logging tools and methods for determining, among other things, porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the liquids in the geological formations surrounding the wellbore so that certain parameters such as spin density, longitudinal relaxation time (generally referred to in the art as T 1 ) and transverse relaxation time (generally referred to as T 2 ) of the geological formations can be measured. From such measurements, porosity, permeability and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons. 
   The NMR instrument also typically includes an antenna, positioned near the magnet and shaped so that a pulse of radio frequency (RF) power conducted through the antenna induces an RF magnetic field in the earth formation. The RF magnetic field is generally orthogonal to the field applied by the magnet. This RF pulse, typically called a 90° pulse, has a duration and amplitude predetermined so that the spin axes of the hydrogen nuclei generally align themselves perpendicularly both to the orthogonal magnetic field induced by the RF pulse and to the magnetic field applied by the magnet. After the 90° pulse ends, the nuclear magnetic moments of the hydrogen nuclei gradually “relax” or return to their original alignment with the magnet&#39;s field. The amount of time taken for this relaxation, referred to as T 1 , is related to petrophysical properties of interest of the earth formation. After the 90° pulse ends, the antenna is typically electrically connected to a receiver, which detects and measures voltages induced in the antenna by precessional rotation of the spin axes of the hydrogen nuclei. The spin axes of the hydrogen nuclei gradually “dephase” because of inhomogeneities in the magnet&#39;s field and because of differences in the chemical and magnetic environment within the earth formation. Dephasing results in a rapid decrease in the magnitude of the voltages induced in the antenna. Typically, a series of 180° refocusing pulses are applied to bring the spins back into focus. Each refocusing pulse produces an echo, and from analysis of the echo train, properties of the earth formation can be estimated 
   One problem with analysis of NMR measurements is that the signal detected by the antenna includes a parasitic, spurious ringing that interferes with the measurement of spin-echoes. One approach to reduce the effects of ringing has been to design the hardware to minimize the interaction between the electromagnetic fields and the materials in the device. For example U.S. Pat. No. 5,712,566 issued to Taicher et al. discloses a device in which the permanent magnet composed of a hard, ferrite magnet material that is formed into an annular cylinder having a circular hole parallel to the longitudinal axis of the apparatus. One or more receiver coils are arranged about the exterior surface of the magnet. An RF transmitting coil is located in the magnet hole where the static magnetic field is zero. The transmitting coil windings are formed around a soft ferrite rod. Thus, magnetoacoustic coil ringing is reduced by the configuration of the transmitting coil. Magnetorestrictive ringing of the magnet is reduced because the radial dependence of the RF field strength is relatively small due to use of the longitudinal dipole antenna with the ferrite rod. Further, magnetorestrictive ringing is reduced because the receiver coil substantially removes coupling of the receiver coil with parasitic magnetic flux due to the inverse effect of magnetostriction. 
   Another commonly used approach to reduce the effect of ringing is to use a so-called phase-alternated-pulse (PAP) sequence. Such a sequence is often implemented as
 
RFA ±x −τ−n·(RFB y −τ−echo−τ)−TW   (1)
 
where RFA ±x  is an A pulse, usually 90° tipping pulse and RFB is a B pulse, usually a 180° refocusing pulse. The ± phase of RFA is applied alternately in order to identify and eliminate systematic noises, such as ringing and DC offset through subsequent processing. By subtracting the echoes in the −sequence from the pulses in the adjoining +sequence, the ringing due to the 180° is suppressed.
 
   PCT publication WO 98/43064 of Prammer addresses the problem of ringing caused by the excitation pulse. A dual frequency acquisition is carried out with phase alternation, the separation between the two frequencies being one fourth of the reciprocal of the delay time in acquisition between the excitation pulse and the first refocusing pulse. Averaging of the two measurements then attenuates the effect of the ringing due to the excitation and refocusing pulses. 
   A drawback to the averaging of phase alternated data sequence is the requirement to combine two pulse sequence cycles. Measurements made by an NMR logging tool in this manner are therefore subjected to degradation in the vertical resolution due to the logging speed, wait time between each pulse sequence, and the data acquisition time. In addition, the logging tool moves along the longitudinal axis of the borehole between each of the measurements. The problem with logging speed is exacerbated in multifrequency NMR measurements. A pulse sequence for an eight frequency logging operation may be denoted by
 
CPMG(f 1 , TE 1 , RFA + , n 1 )−t 1 −CPMG(f 2 , TE 2 , RFA + , n 1 )−t 2 − CPMG(f 8 , TE 8 , RFA + , n 8 )−t 8 −CPMG(f 1 , TE i , RFA − , n 1 )−t 1 −CPMG(f 2 , TE 2 , RFA − , n 2 )−t 2  . . . CPMG(f 8 , TE 8 , RFA − , n 8 )−t 8    (2)
 
where f i , TE i  and n i  are the frequency, interecho time and number of echoes for the i-th CPMG echo train. The CPMG pairs that only differ in the RFA phases are 8 sequences apart. Unless the logging speed is slowed down significantly, the two sensed volumes will be spatially separate and distinct, and the resolution of the tool is impaired.
 
   It would be desirable to have a method of improving the resolution of NMR data that has been averaged over a depth interval. The present invention satisfies this need. 
   SUMMARY OF THE INVENTION 
   One embodiment of the invention is a method of evaluating an earth formation. Measurements indicative of a property of the formation are obtained at more than one depth. An average value over a window length of the property is determined at one or more depths. From the averaged values of the property, the property of the formation is estimated using a representation of the property by a set of basis functions over the window length. The measurements may include NMR signals that may result from a phase alternated pair of excitation pulses. The NMR signals may include spin echo signals. The property of the formation may be a porosity, bound volume irreducible, clay bound water, and/or bound volume movable. The measurements may result from excitation pulses at more than one frequency. The basis functions may be selected to match expected changes in the property and may be Walsh functions. The logging speed of a logging tool used to obtain the measurements may be selected to provide a specified resolution in the estimation of the property. Estimating the property may be done using an inversion. 
   Another embodiment of the invention is sn apparatus for evaluating an earth formation. The apparatus includes a logging tool conveyed into a borehole in the earth formation which makes measurements indicative of a property of the formation at more than one of depth. The apparatus also includes a processor which determines an average value over a window length of the property from the measurements and estimates from the averaged values the property of the formation, the estimation being based at least in part on representing the property by a set of basis functions over the window length. The logging tool may include a magnet and a radio-frequency (RF) antenna which pulses the earth formation and the measurements include nuclear magnetic resonance (NMR) signals. The property estimated by the processor may be a porosity, bound volume irreducible, clay bound water, and/or bound volume movable. The RF antenna may pulse the earth formation at more than one frequency. The processor may select the basis functions to match expected changes in the property. The basis functions may be Walsh functions. The apparatus may include a wireline, drilling tubular, coiled tubing or a slickline which convey the logging tool into the borehole. The processor may estimate the property by performing a matrix inversion. 
   Another embodiment of the invention is a computer readable medium for use with an apparatus for evaluating an earth formation. The apparatus includes a logging tool conveyed into a borehole in the earth formation, the logging tool making measurements indicative of a property of the earth formation. The medium includes instructions which enable a processor to determine an average value over a window length of the property, and, using a set of basis functions to represent the property, to estimate a value of the property. The medium may be a ROM, an EPROM, and EAROM, a flash memory and/or an Optical disk. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a detailed understanding of the present invention, references should be made to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals, wherein: 
       FIG. 1  (prior art) depicts diagrammatically an eccentric NMR logging tool in a borehole; 
       FIGS. 2 ,  2 A, and  2 B (prior art) show configurations of magnets, antenna and shield of a device suitable for use with the present invention; 
       FIG. 3  shows exemplary Walsh functions used in the present invention; and 
       FIG. 4  shows exemplary results of using the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   FIG. 1 (from U.S. Pat. No. 6,348,792 to Beard et al.) depicts an apparatus that is suitable for use with the present invention. This is for exemplary purposes only, and the invention can be practiced with a variety of downhole NMR devices. A borehole  10  has been drilled in a typical fashion into a subsurface geological formation  12  to be investigated for potential hydrocarbon producing reservoirs. An NMR logging tool  14  has been lowered into the hole  10  by means of a cable  16  and appropriate surface equipment represented diagrammatically by a reel  18  and is being raised through the formation  12  comprising a plurality of layers  12   a  through  12   g  of differing composition, to log one or more of the formation&#39;s characteristics. The NMR logging tool is provided with bowsprings  22  to maintain the tool in an eccentric position within the borehole with one side of the tool in proximity to the borehole wall. The permanent magnets used for providing the static magnetic field are indicated by  23  and the magnet configuration is that of a line dipole. Signals generated by the tool  14  are passed to the surface through the cable  16  and from the cable  16  through another line  19  to appropriate surface equipment  20  for processing, recording and/or display or for transmission to another site for processing, recording and/or display. 
     FIG. 2  schematically illustrates a magnetic configuration that is suitable for use with the present invention to operate over a gradient field. The tool is described in U.S. Pat. No. 6,348,792 to Beard et al, having the same assignee as the present application and the contents of which are fully incorporated herein by reference. It should be pointed out that the method of the present invention is independent of the specific magnet configuration and can be used with either a side-looking or a centralized tool, or even the pad device. The method of the present invention does not require a gradient magnetic field. The method of the present invention can even be used with a single frequency logging tool. The tool cross-sectional view in  FIG. 2  illustrates a main magnet iit, a second magnet  218 , and a transceiver antenna, comprising wires  219  and core material  210 . The arrows  221  and  223  depict the polarization (e.g., from the South pole to the North pole) of the main magnet  217  and the secondary magnet  218 . A noteworthy feature of the arrangement shown in  FIG. 2  is that the polarization of the magnets providing the static field is towards the side of the tool, rather than towards the front of the tool (the right side of  FIG. 2 ) as in prior art devices. 
   The second magnet  218  is positioned to augment the shape of the static magnetic field by adding a second magnetic dipole in close proximity to the RF dipole defined by the wires  219  and the soft magnetic core  210 . This moves the center of the effective static dipole closer to the RF dipole, thereby increasing the azimuthal extent of the region of examination, the desirability of which has been discussed above. The second magnet  218  also reduces the shunting effect of the high permeability magnetic core  210  on the main magnet  217 : in the absence of the second magnet, the DC field would be effectively shorted by the core  210 . Thus, the second magnet, besides acting as a shaping magnet for shaping the static field to the front of the tool (the side of the main magnet) also acts as a bucking magnet with respect to the static field in the core  210 . Those versed in the art would recognize that the bucking function and a limited shaping could be accomplished simply by having a gap in the core; however, since some kind of field shaping is required on the front side of the tool, in a preferred embodiment of the invention, the second magnet serves both for field shaping and for bucking. If the static field in the core  210  is close to zero, then the magnetostrictive ringing from the core is substantially eliminated. 
   As noted above, within the region of investigation, the static field gradient is substantially uniform and the static field strength lies within predetermined limits to give a substantially uniform Larmor frequency. Those versed in the art would recognize that the combination of field shaping and bucking could be accomplished by other magnet configurations than those shown in  FIG. 2 . For example,  FIG. 2A  shows a single magnet  227  and magnetic core  230  that produces substantially the same static field as that produced by the combination of magnets  217  and  218  in  FIG. 2 . A substantially similar field configuration results from the arrangement in  FIG. 2B  with the magnet  237  and the core  240 . What is being accomplished by the magnet arrangements in  FIGS. 2 ,  2 A and  2 B is an asymmetry in the static magnetic field in a direction orthogonal to the direction of magnetization. In an optional embodiment of the invention (not shown) the second magnet is omitted. 
   Returning to  FIG. 2 , the transceiver wires  219  and core pieces  210  should preferably be separated as far as possible towards the sides of the tool. This separation increases the transceiver antenna efficiency by increasing the effective RF dipole of the antenna and augments the shape of the RF magnetic field isolines so that they better conform to the static magnetic field isolines. This separation is not possible in the Kleinberg design. The secondary magnet is preferably made of nonconducting material to minimize eddy currents induced by the RF field, thereby increasing the RF antenna efficiency. 
   The core is preferably made of a powdered soft magnetic material, other than ferrite. It preferably has a high saturation flux density and comprises particles of powdered material small enough to be transparent to the RF magnetic field. Such a material has been described in U.S. Pat. No. 6,452,388 to Reiderman et al 
   The objective of the present invention is to “undo” the loss of resolution resulting from application of the stacking of signals. The method is based upon representing the “signal” to be recovered, i.e., a high resolution version of a formation property, in terms of Walsh functions. The functions range takes only 2 values, 1 and −1 and their domain is [0,1). Interestingly enough, the independent variable can only take discrete values, starting with 0 and with constant increment (½)^M. Once M is determined, the functions are denoted as Wal(k, t) where k has values between 0 and 2 M −1, and t is the independent variable. Numerically, Walsh functions can be defined iteratively:
 
Wal(0 ,t )=1;
 
Wal(1, t )=1 when (0&lt;= t&lt; ½); or −1 when (½&lt; t&lt;= 1)  (3).
 
Each function of order N−1 generates two functions of order N, one by contraction and repetition, the other by contraction and repetition with a change of sign. The first 16 Walsh functions are shown in  FIG. 3 .
 
   Walsh functions form a complete set of orthonormal functions analogous to sines and cosines. Therefore according to the well-known mathematical theory, they can be used to represent an arbitrary function (with domain [0, 1)) in the format of series expansion. Furthermore, Walsh functions have the characteristics of step functions in that they contain a lot of high frequency components. The choice of Walsh functions is considered to be appropriate to represent the signal to be recovered particularly when formation properties have discrete step changes at layer boundaries. 
   NMR tools typically acquire echo trains while the tool is moving upward in the borehole. As discussed above, PAP is done first to eliminate ringing signals. It is equivalent to a 2-level averaging. As discussed above, for multifrequency acquisition, the two signals that are averaged at a particular frequency may not be consecutive in acquisition (and depth). In addition, there may be a stacking of the signals over n levels (n=2, 4, 8, 16, 32, 64, etc) depending on signal to noise ratio (SNR) of the PAPed echo trains. Finally the stacked echo trains are commonly inverted to yield T 2  spectra from which formation properties such as porosity, bound volume irreducible, clay bound water and bound volume movable are determined.. Due to the stacking, the vertical resolution is reduced. The present invention is based on the recognition of the fact that the operations, such as inversion, that are used to derive the formation properties are linear. Hence the operation of “unstacking” of the inverted stacked echo trains is equivalent to inverting unstacked echo trains, i.e., the operations are commutative. In what follows, a methodology for recovering a high resolution version of formation porosity is discussed. The same methodology could be used for other formation properties that are commonly determined from the NMR echo trains. 
   We denote by P m (x) the estimated effective porosity with depth variable x in the range of [x 0 , x 1 ) and constant sampling depth interval. Using linear mapping, x=x 0 +t(x 1 −x 0 ), t has domain of [0,1). Therefore P m (t)is porosity defined on [0,1). It is a quantity as a result of depth level averaging over the true porosity P t (t): 
                     P   m     ⁡     (   t   )       =       1     2   ⁢   Δ   ⁢           ⁢   x       ⁢       ∫       -   Δ     ⁢           ⁢   x       Δ   ⁢           ⁢   x       ⁢         P   t     ⁡     (     t   +   τ     )       ⁢       ⅆ   τ     .                   (   4   )               
The goal is to find out what P t (t) is. It should have more high frequency components than P m (t). P t (t) can be approximated with a Walsh expansion including the first N terms:
 
                     P   t     ⁡     (   t   )       ≈       ∑   0     N   -   1       ⁢       a   k     ⁢       Wal   ⁡     (     k   ,   t     )       .                 (   5   )               
Substituting eqn. (5) into eqn. (4) yields:
 
                       P   m     ⁡     (   t   )       =       ∑   0     N   -   1       ⁢       a   k     ⁢       z   k     ⁡     (   t   )             ,           (   6   )               
where
 
   
     
       
         
           
             
               
                 
                   
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                     1 
                     
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                         x 
                       
                       
                         Δ 
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                         x 
                       
                     
                     ⁢ 
                     
                       
                         Wal 
                         ⁡ 
                         
                           ( 
                           
                             k 
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                         . 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 7 
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   Discretizing eqns. (6) and (7) yields: 
                             P   m     ⁡     (   j   )       =       ∑   0     N   -   1       ⁢       a   k     ⁢     z   k     ⁢     (   k   )           ,             j   =   0     ,   1   ,   2   ,     &amp;   .     ,     J   -   1     ,                 (   8   )                           z   k     ⁡     (   j   )       =       1       2   ⁢   I     +   1       ⁢       ∑     i   =     -   I       I     ⁢     Wal   ⁢     (     k   ,     j   +   i       )             ,             j   =   0     ,   1   ,   2   ,     &amp;   .     ,     J   -   1     ,                 (   9   )               
where J is the total number of depth levels. Note that eqn. (9) is for averaging over an odd number of depth levels. Modification for the case of an even number of depth levels would be known to those versed in the art.
 
   Using matrix notations:
 
 {right arrow over (P)}   m   =[P   m (0) P   m (1) . . .  P   m ( J− 1)] T    (10),
 
 {right arrow over (A)}=[a   0   a   1    . . . a   N−1 ] T    (11),
 
                     G   ^     =     [             z   0     ⁡     (   0   )               z   1     ⁡     (   0   )           ⋯           z     N   -   1       ⁡     (   0   )                   z   0     ⁡     (   1   )               z   1     ⁡     (   1   )           ⋯           z     N   -   1       ⁡     (   1   )               ⋮       ⋮       ⋮       ⋮               z   0     ⁡     (     J   -   1     )               z   1     ⁡     (     J   -   1     )           ⋯           z     N   -   1       ⁡     (     J   -   1     )             ]       ,           (   12   )               
eqn. (8) can be written in a compact matrix format:
   {right arrow over (P)}   m   =Ĝ·{right arrow over (A)}   (13). 
The present invention determines the matrix {right arrow over (A)} by inverting eqn. (13). The matrix Ĝ has elements that are derived from the Walsh functions, which in turn are an orthonormal set of basis functions matched to the step changes expected in the earth properties. This is an ill-conditioned inversion problem. In one embodiment of the invention, a regularization method using curvature smoothing is used together with least square approach. Other regularization methods known to those versed in the art could be used. Once {right arrow over (A)} is calculated, then substitution back into eqn. (5) gives an estimate of P t (t).
 
   A formation model of effective porosity was created for 22 layers which have 232 levels of data. This is denoted as the original high resolution effective porosity model. A stacking is performed over n levels, resulting in the low resolution effective porosity, mimicking the measured effective porosity. The low resolution effective porosity was then inverted using the method described above to provide a high resolution estimate of the porosity.  FIG. 4  shows an example of the results. The abscissa is the depth level and the ordinate is the porosity in percent. The discontinuous curve  301  is the layered porosity model. The smooth curve  303  is the 21 level average of  301  with additive noise of 0.5 added. The curve  305  is the result obtained by inversion. As can be seen,  305  tracks the discontinuities in the actual porosity  301  quite well. 
   The use of the Walsh transform for analysis of downhole data has been discussed before in U.S. Pat. No. 6,253,155 to Hagiwara. The problem addressed therein is that of compensating for the effect of tool resolution, not that of undoing the effects of processing operations such as stacking. Hagiwara teaches the application of a deconvolution filter derived by inverting a tool response correlation matrix. This is different from the present invention where a deconvolution filter is not derived. 
   The method of the present invention may be used for any type of logging in which signals from different depths are stacked and the processing from signals to formation properties is linear. This includes, for example, in acoustic logging, it is desirable to keep the receiver arrays as small as possible in order to improve the resolution. However, short arrays suffer from a reduced signal to noise ratio. With the method of the present invention, measurements made with short arrays that are averaged over many depths can be inverted to give velocity estimates with an improved resolution. 
   By the use of the present invention, it is possible to increase the logging speed for NMR measurements that are subject to PAP and multilevel stacking without significant loss of resolution. 
   The processing of the measurements made by the probe in wireline applications may be done by the surface processor  20  or may be done by a downhole processor (not shown). For MWD applications, the processing may be done by a downhole processor that is part of a bottomhole assembly BHA conveyed on a tubular such as a drillstring or coiled tubing. This downhole processing reduces the amount of data that has to be telemetered. Alternatively, some or part of the data may be telemetered to the surface. In yet another alternative, the measurements may be stored on a suitable memory device downhole and processed when the drillstring is tripped out of the borehole. Part of the processing may also be done at a remote location. 
   The operation of the logging tool may be controlled by the downhole processor and/or the surface processor. Implicit in the control and processing of the data is the use of a computer program implemented on a suitable machine readable medium that enables the processor to perform the control and processing. The machine readable medium may include ROMs, EPROMs, EAROMs, Flash Memories and Optical disks. 
   While the foregoing disclosure is directed to the preferred embodiments of the invention, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope and spirit of the appended claims be embraced by the foregoing disclosure.