CA2119785A1 - Nuclear magnetic resonance detection of geologic structures - Google Patents

Abstract

A system for Nuclear Magnetic Resonance Logging is used to obtain data of petrophysical parameters. Logging tool (16) is placed adjacent formations (12, 14). An MRI probe (32) suspended at the bottom of tool (16) is used to measure spin-echo data. A chain of spin-echo signals is used to derive petrophysical characteristics based on the selection of spin-echo time windows.

Description

NUCLEAR MAGNETIC RESONANCE DETECTION

~ ' 3 Field of 4 the lnve~tion The present invention relates to systems for obtaining quantitative and qualitative 6 measurements of geologic structures. More specifically, the present invention provides 7 an efficient and effective system for using nuclear magnetic resonance techniques for , : ~
8 obtaining information relating to geologic structures. ~
~ :
g Background ~:
As is known, fluid flow properties of porous media have long been of interest inI l the oil industry. In an article by A. Timur, entitled "Pulsed Nuclear Magnetic Resonance 1~ Studies of Porosity, Mo~able Fluid, and Perrneability of Sandstones," in the Journal of 13 Petroleum Technology, June 1969, page 775, it was shown experimentally that NMR ~ :
14 methods provide a rapid non-destructive deterrnination of porosity, movable fluid, and 15 ~ ~permeabili~r of rock formation.
16~ 1t~1s~known that when an assembly of magnetic moments, such as those of 17 hydrogen nuclei, are exposed to a static magne~ic field they tend to align alon~ the 18 direction of the magnetic field, resulting in buLk magnetization. The rate at which 19 equillbrium ls established in such bulk magnetization upon provislon of a static magnehc field is characterized by the parameter T1, known as the spin-latdce relaxation time.

,.: -,: : -; :~ ~, .. ,:, 2i~ ~783 WO 92J21045 - ~ ~ PCr/US92/041 It has been obselved that the mechanism which determines the value of T1 2 depends on molecular dynamics. In liquids, molecular dynamics are a function of 3 molecular size and inter-molecular interactions. Therefore, water and different types of 4 oil have different T1 values.
sIn the heterogeneous media, such as a porous solid which contains liquid in its 6 pores, the dynamics of the molecules close to the solid surface are also significant and 7differ from the dynamics of the bulk liquid. It may thus be appreciated that the T1 ~
8parameter provides valuable infolmation relating to well logging parameters ~ i 9There exist a number of techniques for disturbing the equilibrium of an assembly 10 of magnetic moments, such as those of hydrogen nuclei, for Tl parameter measurements.
1~ Each of these techniques provides means for measuring T1 of a rock formation within a 12 certain volume ~called the "sensitive volume") which is determined mainly by the shape 13 of the magne~ic field sursounding the magnetic structure. The signal-to-noise ratio of the 14 measurement is limited by the ratio of the sensitive volume to the uniforrnity (maximum 15 flux density minus minimum flux density) of the magnetic field within said volume, and 16 increases in proportion to this ratio.
17In any given nuclear magnetic resonance instrument configuration, the apparatus 18 will respond only to nuclei residing within the sensitive volume. In the present invention 19 and prior art instruments described herein, the boundaries of the sensitive volume are 20 determined by radiation pattems of transmitting and receiving antennae as well as a 21 combination of the detailed s~ucture of the magnetic field with the receiver's frequency 22 pass~and.~ ~The radio frequency that a ~iven nucleus will respond to or emit when excited 23 is proportional to the flux density of the magnetic field in which it is immersed. The , 24 proportionality factor depends upon the nuclear species. For hydrogen nuclei, that factor is 42.5759 MHz~Tesla. If the NMR receiver's passband extends from 1.30 MHz to 1.31 26 MHz, the instrument will be sensitive to hydrogen nuclei in regions of the magnetic field wo 92/21045 3 21~ ~ 7 8 ~ Pcr/uss2/o4144 that have flux densities between 30.5 mT and 30.8 mT, providing the antenna radiation pattern allows receiving sufficient signal from that locations.
3 If it is desired tO study nuclei located with a particular region, the magnetic field 4 s~ucture, antenna radiation pattenl and receiver passband must all be adjusted to be sensitive to that and only that region. Since the signal-to-noise ratio of the resulting 6 signal is proportional to (among other factors) the square root of the receiver passband 7 width, it is important to minimize the variation of the ma~netic field within the desired 8 sensitive volume; smaller variations (better field uniformity) mean a better signal-to-9 noise ratio. Since the signal-to-noise ratio also increases with increasing frequency, the 0 nominal magnetic field intensity within the volume is also very important. lt is 11 imrnaterial whether this nominal intensity is defined as the central value, average value 1~ or some other value within the range of values encompassed by the sensitive volume 13 because only large differences in signal-to-noise ratio are significant.
14 One technique for measuring T1 of a rock forrnation is exemplified by what is known as the "Schlumberger Nuclear Magnetic Logging Tool." That tool is described by 16 R.C. Herrick, S.H. Couturie, and D.L. Best in "An Improved Nuclear Magnetic Logging 17 System and Its Application to Forrnation Evaluation," SPE~361 presented at the 54th 8 Annual Fall Technical Conference and Exhibition of the Sosiety of Petroleum Engineers 19 of AIME, held in Las Vegas, Nev., Sept. 23-26, 1979, and also by R.J.S. Brown et al. in U.S. Pat. No. 3,213,357 en~itled "Earth Forrnation and Fluid Material Inves~igation by 21 Nuclear Magnetic Relaxation Rate Determination."
22 The Schlumberger Nuclear Magnetic Logging Tool measures the froe precession 23 of proton nuclear magnetic moments in the earth's magnetic ~leld by applying a relatively 24 strong DC polarizing field to the surrounding rock forrnation in order to align proton 2s spins approximately perpendicularly to the earth's magnetic field. The polarizing field 26 must be applied for a period roughly five times Tl (the spin-lattice relaxation time) for W092/2~o47Sss ,~, ~, j PCr/US92/04l44_ sufficient polarization (approximately two seconds). At the end of polarization, the field 2 is turned off rapidly. Since the protons spins are unable to follow this sudden change, 3 they are left aligned perpendicularly to the earth's magnetic field and precess about this 4 ~leld at the "Larmor Frequency" corresponding to the local earth's magnetic field S (roughly from 1300 to 2600 Hz, depending ~n location).
6 The spin precession induces in a pick-up coil a sinusoidal signal whose amplitude 7 is proportional to the density of protons present in the formation. The signal decays with 8 a time constant 1~" (transverse relaxation time) due to non-homogeneities in the local 9 magnetic field over the sensing volume.
Improved nuclear magnetic resonance logging tools and methods for using these ll tools are described generally in U.S. Patent Nos. 4,710,713; 4,717,876; 4,717,877; and 12 4,717,878, all of which are commonly owned by the assignee of the present invention.
13 The method and apparatus of the present invention, descnbed in greater detail below, 14 uses the logging tool and techniques described in the above referenced patents to obtain previously unavailable data relating to the composition of a geologic formation,16 including the surface-to-volume ratio of the pore system, average grain size, and bulk 17 volume of i~reducible water associated with the pore-space effectively available for 18 hydrocarbon accumulation.

WC~ 92/21045 2 1 ~ 7 g ~-) PCT/US92/04144 summary of the Invention 3 The method and apparatus of the present invention provides an improved system 4 for using nuclear magnetic resonance techniques for obtaining inforrnation re}ating to 5 geologic structures. In the system of the present invention, a nuclear magnetic resonance 6 logging tool is used to impart magnetic polarization fields on a portion of a geologic 7 formation. Nuclear magnetic resonance signals from the excited nuclei in the formation 8 are then detected to obtain data for calculating a number of important petrophysical 9 parameters of geologic interest.

In the preferred embodiment. the present invention provides a method for 1 1 determining the composition of a geologic structure, comprising the steps of: imparting a 12 polarizing magnetic field to a geologic structure for a predetermined period of time;
13 measuring nuclear magnetic resonance signals representing spin-echo relaxation of a 14 population of particles in said geologlc structure; constructing a chain of spin-echo 15 signals; and deten~ining ~e petrophysical characteristics of said geologic structure from 16 said chain of spin-echo signals.

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. WO 92J21045 . . . . . . PCr/US92/041~4 Brief nescription .
of ~he Drawings 3 FIG. 1 is a a partially pictorial, partially block diagram illustration of a well 4 logging apparatus for obtaining nuclear magnetic resonance measurements of a geolo~ic 5 structure.
6 FIG. 2 is a graphical illustration of a chain of spin-echo relaxation signals as a 7 funct;on of amplitude versus time for a geologic structure investigated using a nuclear 8 magnetic resonance system such as that shown in FIG. 1.
9 FIG. 3 is a graphical illustration the use of time windows to selectively elirninate 10 signals corresponding to particular pore sizes to allow determination of petrophysical l l properties of a geologic structure.

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WO 92/21045 2 ~ 8 ~ PCr/US92/0414q Detailed Des~ription of the Preferr~d Embodiment :
3Refening to FIG. 1, a borehole 10 is shown adjacent to formations 12 and 14 4 having structures to be examined using the method and appara~us of the present 5 invenaion. Within the borehole, there is a logging tOOI 16 which is suspended by a cable 618 routed over pulleys 20 and 22, with the position of the cable 18 being determined by a 7 motor 24.
8The upper portion of the logging tool 16 comprises telemetry electronics 26, 9 ~amma ray sensing electronics 28 and magnetic resonance imaging (MRl) electronics 30.
10 A MRI probe 32 is suspended at the bottom of the probe to provide exci~ation to the 11 surrounding ~eologic formation. The excitation field has a ~enerally cylindrical shape as 12 represented by neference numeral 34. Improved devices which can be used for the probe 332 are describeld generally in U.S. Patent Nos. 4,710,713; 4,717,876; 4,717,877; and 44,717,878, which, by this reference, are incorporated herein for all purposes.
15The spin-spin pulse-echo measurement of the spin-echo relaxation of the sample, 16 in a homogenous isotropic media, reflects the surface-to-volume characteristics of the 17 pores. In typical rocks encountered in the well-logging environment, the rocks are 18 complex mixtures of minerals which often include a variety of pore sizes. Consequently, 19 the measured spin-echo relaxadon in such an environment is a complex phenomenon, a 20 reflecdon of the variations which exist in terms of pore surface-to-volume rados and 21 surface-to-fluid interactions.
~2The method and apparatus of the present invention is based on the discovery that 23 for a select time window of echo relaxation there is an associated select range of surface-24 to-volume response. Thus, by proper selection of spin-echo time windows it is possible 25 to determine the relahve fraction of select surface-to-volume components. In addition, 26 these changes in relaxation time can also be used as a measure of a representative ~rain-, ~., , -.

21~ 978a WO 92/21045 PCr/US92~04144, 8size condition.
2 FIG. 2 is a graphical illustration of a chain of spin-echo ~elalcation si~nals as a 3 function of amplitude versus time for a geologic s~ucture investigated usin~g a nuclear 4 magnetic resonance system such as that shown in FIG. 1. The spacin~ of the time 5 intervals between the pulses in this application is typically between l.S and 3 6 milliseconds. The time intervals labelled "A-H" correspond to the signal intervals for 7 various particle sizes, with interval "A" corresponding to the interval for particles larger 8 than S0011 and mterval "H" corresponding tO the interval for particles of larger than 8,u, 9 etc.
Using the echoes in each time window to regress to time zero establishes an 11 apparent porosity amplitude. Then, using first derivati~es between the echo windows 12 shown, one can detenTIine the relative fraction of each grain-siæ component as part of 13 the total porosity amplitude associated with the BuL~c-Volume irreducible component 14 determined from the complete relaxation echo-chain through the Free Fluid Analysis IS method. The calibration of the process is accornplished through multi-dimension 16 regression analysis utilizing optimally selected and prepared laboratory sasnples. Such 17 regression techniques are known to those skilled in the art and are described in the 18 following references: K. Fukunaga, Introduction to Sta~istical Pattern Recognition, 19 Academic Press, 1972; Bhattacharyya & Johnson, Statis~ical Concepts and Methods, 20 Wiley & Sons, 1977; and Devijver & Kittler, Pattern Recognition -- A Statistical ., : ~ , .
21 Approach, PrenoceHall, 19~2.
22 ~. As a consequence of the actual tool operation, the measurement of spin-echo -~
23 infonnation is delayed for a few milli-seconds (typically < Sm secs for the toQls 24 described in the above referenced patents incorporated herein by reference). During this 2s period of ame (tde~) no forrnation information is uniquely Dasu~d. This tdel hme period 26 includes the surface-to-volume response associated with a select pore-size group that is WO 92/21045 9 2 1 1~ 7 ~ ~ PCr/US92/04144 directly linked with the pore-sizes related tO clay size grains. Thus, by proper selection of ehe echo windows, a spin-echo measurement can be provided which measures the total 3 pore-space minus those associated with the pore surface-to-volume ratios related to the 4 clay-size particles.
s The pore surface-to-volume responses that are missed during this ~del period 6 include the clay mineral fraction of the rock-space, thus providing a direct link between 7 such a NMR measured porosity and the total porosity of the rock. In other words, in a 8 clay mineral free environment, with pores >2~, the NMR echo extrapolation provides a 9 measure of the total porosity but, in a shaly-sand that contains clay minerals and ehus 10 clay size pores, the NMR porosity measurement can be made to be free of the influence 11 of the non-reservoir quality micro-pores making the NMR measurement particularly 1~ useful in assessing the reservoir's capacity to support production. Furthermore, in the 13 event to~al porosity is known, it can be combined with such a determined NMR porosity 14 as to establish the ionically bound clay-mineral porosity and thus provide a link to 15 recognizing the clay-mineral types.

16 FIG. 3 is a graphical illustration of the use of time windows to selectively 17 eliminate signals corresponding to progressively larger pore sizes to allow selective 18 determination of petrophysical properties of a geologic structure. ln particular, the 19 exclusion of all clay size pore systems can be accomplished throu~h a judicious choice 20 of echo data which eliminates those very fast relaxation pores associa~ed with the clay-~I size particles. Referring to FIG. 3, the quantity illustrated by reference letter "A"
22 co~esponds to the porosity of pore structures greater than 2~1, where A is determined by 23 a regression of the full "selected echoes" time. The quantity illustrated by reference letter 2~ "B" corresponds to the total porosity of the sarnple obtained by regression of a fully 25 sensitive echo chain to time zero. The ratio of the two quantities (A/B) can be used to 26 calculate the relative volumes of selected pore structures within the geologic medium 7 ~ 5 WO 92/2104~ ; PCrtUS92/04144_ : 1 0 under investigation.
Prior art references discussed above (see, for exarnple, A. Timur, Journal of 3 Pe~roleum Technology article) show the NMR may be used for the determination of a 4 rock parameter called the free-fluid index (FFI). The FFI method relies on use of relaxations which occur during a late measurement ~ime following a select tdel. This time 6 period being referred to as the long component of the relaxation phenornenon (typically 7 td~l's ~ 22 m secs). The difference between the pore space described as the long 8 component relaxation and that provided by the full NMR spectrum provides a direct 9 measure of the pore bulk-volume that is held in place by existing surface tension and other capillary forces. This parameter, the buL~c-volume of irreducible water, is directly 11 rela~ed to pore surface-to-volume of the non-clay-size rock. The NMR measurement of 1~ porosity and buLk-volume irreducible can be readily used to ~md the intrinsic 13 perrneability of the rock since these measured pararneters reflect the principle component 14 of the rock's producibility through a model such as that of the Coates' free-fluid perm model.
16 Referring to the principles discussed above, where the pore-sizes which dominate 17 the irreducible saturation are determined by using select echo times to identify the total ; ~ ~
18 porosity less the porosity of the clay-size rock and, by using a different set of echoes, it is ~
;
19 possible to determine the pores which are known to dominate production. The' difference between these two porosities provides the porosity known by those knowledgeable in 'I reservoir engineering as the bulk-volume of irreducible water. This pnnciple is ~
;
2~ illustrated in the following forrnulas~

23 ~RR ~I)T (~ y~
24 ~, ~)RR ~PR ~ -21 ~ 9 78~ ~
. . .` PCI/US92/04144 where ~RR represents the porosilies associated with non-clay size 3 reservoir grain structure.
4 (I)PR represents the porosities S associated with the ~on-clay size 6 pores which are free of surface 7 related relaxation effects and are 8 known to contribute to the productivi~ of the well.

Io A fully self-supported syner~etic log interpretation method is possible with the -I l infolmation provided by the tools described in the above referenced patents incorporated 12herein by reference when combined with traditional well logging measurements. The -13 MRIL system described in these patents provides access to porosity MRI(PHI) (chosen so 14as to elirninate porosity linked to clay-size particles), which when combined with ..
: . ... . ...
15convendonal total porosity T(PHI) detem~inadons gives access to the following: ~ ~
16 1) (1- Swb)3 = MRI(PHIj~I`(PHIj --:
17 2) Ct = (T(PHl))m Sw~[Cwf $(Swb/Sw)(Ccw -Cwfl] : ;
18 3) Vclay = (Swb*T(PHI~/PHI(CLAY) 19 where:
C = I/R; the virgin rock conductivity 21 Sw: ~he total water fraction of pores 22 Cwf; the conductivity of the non-clay held water '3 Ccw: the conductivity of the clay-held water 24 Vciay: the buL~; fraction of dry-clay miner~s ~5 ~ ~ PHI(CLAY): theæsociatedc1ay-mineralporosity believed 26 ~ ~ ~ unique to a clay mineral type which;s accessible 27 ~ from natural gamma-ray technologv.
28 ~ ~ m; typically 1.8 for sandstones 29 ~ n: typically 2.0 f water-we~ rocks 30~Although the present invention has~been described in connecdon with the 31~ preferred èmbodiment, it is not Intended to be limited to the specific form set forth 32 herein,` but on the contraly, it is ~intended ~o cover such modif~cations, alternatives, and 33 equivalents as can be reasonably included; witllin the~spirit and scope of the invention as , ~

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defined by the appended claims.

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Claims (9)

CLAIMS:

1. A method for determining the composition of a geologic structure, comprising the steps of:
imparting a polarizing magnetic field to a geologic structure for a predetermined period of time;
measuring nuclear magnetic resonance signals representing spin-echo relaxation of a population of particles in said geologic structure;
constructing a chain of time-windowed spin-echo relaxation signals from said measured nuclear magnetic resonance signals wherein each of said time-windowed spin-echo relaxation signals is defined by an amplitude and a time period following the application of said magnetic polarizing field; and determining petrophysical characteristics of aid geologic structure from said chain of time-windowed spin-echo signals.

2. The method according to claim 1, each of said spin-echo relaxation signals being defined by an amplitude and a time period following the application of said magnetic polarizing field, said petrophysical characteristic comprising a surface-to-volume ratio being calculated by selecting the time period corresponding to those signals being substantially dominated by the surface-to-fluid relaxation cycle.

3. The method according to claim 2, further comprising the step of calculating permeability of said geologic structure using said surface-to-volume ratio of the pore system in said geologic structure.

4. The method according to claim 3, further comprising the step of calculating the average grain size of said geologic formation by measuring echo-to-echo intervals within lime windows selected to include spin-echo signals principally associated with surface-to-fluid interaction.

5. The method according to claim 4, further comprising the steps of:
a) selectively eliminating spin-echo signals corresponding to particular pore sizes, such as those associated with clay minerals, and b) calculating a quantity corresponding to the volume of the pore system of said geologic structure which is available for the accumulation of hydrocarbons.
exclusive of that volume of said pore system occupied by irreducible water volume.

6. The method according to claim 5, further comprising the step of determining properties of said geologic structure by calculating the volume of said geologicstructure corresponding to said eliminated spin-echo signals, said volume being calculated by comparing the eliminated spin-echo signals to an extrapolated total porosity calculated from a full spin-echo chain corresponding to all pore sizes.

7. The method according to claim 6, further comprising the step of calculating the bulk volume of irreducible water associated with said volume of the pore system of said geologic structure which is available for the accumulation of hydrocarbons, said bulk volume of irreducible water being determined by first and second extrapolations of said spin-echo signals, said first extrapolation comprising an echo chain having those echo signals corresponding to clay size particles eliminated therefrom and said second extrapolation comprising an echo chain comprising only those echo signals associated with diffusion relaxation echoes.

8. The method according to claim 7, further comprising the step of using said calculated bulk-volume of irreducible water to further calculate the total permeability of said geologic structure.

9. The method according to claim 8, further comprising the step of calculating the fraction of total pore space occupied by water ionically bound by clay particles, said fraction being calculated as the difference between the total pore space and the portion of total pore space available for accumulation of hydrocarbons.