NUCLEAR MAGNETIC RESONANCE DETECTION OF GEOLOGIC STRUCTURES

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 OF GEOLOGIC STRUCTURES Field of the Invention The present invention relates to systems for obtaining quantitative and qualitative measurements of geologic structures. More specifically, the present invention provides an efficient and effective system for using nuclear magnetic resonance techniques for obtaining information relating to geologic structures. Background As is known, fluid flow properties of porous media have long been of interest in the oil industry. In an article by A. Timur, entitled "Pulsed Nuclear Magnetic Resonance Studies of Porosity, Movable Ruid, and Permeability of Sandstones," in the Journal of Petroleum Technology, June 1969, page 775, it was shown experimentally that NMR methods provide a rapid non-destructive determination of porosity, movable fluid, and permeability of rock formation. It is known that when an assembly of magnetic moments, such as those of hydrogen nuclei, are exposed to a static magnetic field they tend to align along the direction of the magnetic field, resulting in bulk magnetization. The rate at which equilibrium is established in such bulk magnetization upon provision of a static magnetic field is characterized by the parameter Tl, known as the spin-lattice relaxation time. It has been observed that the mechanism which determines the value of Tl depends on molecular dynamics. In liquids, molecular dynamics are a function of molecular size and inter-molecular interactions. Therefore, water and different types of oil have different Tl values. In the heterogeneous media, such as a porous solid which contains liquid in its pores, the dynamics of the molecules close to the solid surface are also significant and differ from the dynamics of the bulk liquid. It may thus be appreciated that the Tl parameter provides valuable information relating to well logging parameters. There exist a number of techniques for disturbing the equilibrium of an assembly of magnetic moments, such as those of hydrogen nuclei, for Tl parameter measurements. Each of these techniques provides means for measuring Tl of a rock formation within a certain volume (called the "sensitive volume") which is determined mainly by the shape of the magnetic field surrounding the magnetic structure. The signal-to-noise ratio of the measurement is limited by the ratio of the sensitive volume to the uniformity (maximum flux density minus minimum flux density) of the magnetic field within said volume, and increases in proportion to this ratio. In any given nuclear magnetic resonance instrument configuration, the apparatus will respond only to nuclei residing within the sensitive volume. In the present invention and prior art instruments described herein, the boundaries of the sensitive volume .are determined by radiation patterns of transmitting and receiving antennae as well as a combination of the detailed structure of the magnetic field with the receiver's frequency passband. The radio frequency that a given nucleus will respond to or emit when excited is proportional to the flux density of the magnetic field in which it is immersed. The 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 MHz, the instrument will be sensitive to hydrogen nuclei in regions of the magnetic field that have flux densities between 30.5 mT and 30.8 mT, providing the antenna radiation pattern allows receiving sufficient signal from that locations. If it is desired to study nuclei located with a particular region, the magnetic field structure, antenna radiation pattern 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 signal is proportional to (among other factors) the square root of the receiver passband width, it is important to minimize the variation of the magnetic field within the desired sensitive volume; smaller variations (better field uniformity) mean a better signal-to- noise ratio. Since the signal-to-noise ratio also increases with increasing frequency, the nominal magnetic field intensity within the volume is also very important. It is immaterial whether this nominal intensity is defined as the central value, average value or some other value within the range of values encompassed by the sensitive volume because only large differences in signal-to-noise ratio are significant. One technique for measuring Tl of a rock formation is exemplified by what is known as the "Schlumberger Nuclear Magnetic Logging Tool." That tool is described by R.C. Herrick, S.H. Couturie, and D.L. Best in "An Improved Nuclear Magnetic Logging System and Its Application to Formation Evaluation," SPE8361 presented at the 54th Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers 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 entitled "Earth Formation and Fluid Material Investigation by Nuclear Magnetic Relaxation Rate Determination." The Schlumberger Nuclear Magnetic Logging Tool measures the free precession of proton nuclear magnetic moments in the earth's magnetic field by applying a relatively strong DC polarizing field to the surrounding rock formation in order to align proton spins approximately perpendicularly to the earth's magnetic field. The polarizing field must be applied for a period roughly five times Tl (the spin-lattice relaxation time) for sufficient polarization (approximately two seconds). At the end of polarization, the field is turned off rapidly. Since the protons spins are unable to follow this sudden change, they are left aligned perpendicularly to the earth's magnetic field and precess about this field at the "Larmor Frequency" corresponding to the local earth's magnetic field (roughly from 1300 to 2600 Hz, depending on location). The spin precession induces in a pick-up coil a sinusoidal signal whose amplitude is proportional to the density of protons present in the formation. The signal decays with a time constant T2" (transverse relaxation time) due to non-homogeneities in the local magnetic field over the sensing volume. Improved nuclear magnetic resonance logging tools and methods for using these tools are described generally in U.S. Patent Nos. 4,710,713; 4,717,876; 4,717,877; and 4,717,878, all of which are commonly owned by the assignee of the present invention. The method and apparatus of the present invention, described in greater detail below, 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, including the surface-to-volume ratio of the pore system, average grain size, and bulk volume of irreducible water associated with the pore-space effectively available for hydrocarbon accumulation. Summary of the Invention The method and apparatus of the present invention provides an improved system for using nuclear magnetic resonance techniques for obtaining information relating to geologic structures. In the system of the present invention, a nuclear magnetic resonance logging tool is used to impart magnetic polarization fields on a portion of a geologic formation. Nuclear magnetic resonance signals from the excited nuclei in the formation are then detected to obtain data for calculating a number of important petrophysical parameters of geologic interest. In the preferred embodiment, the present invention provides a method for determining the composition of a geologic structure, comprising the steps of: impaning 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 spin-echo signals; and determining the petrophysical characteristics of said geologic structure from said chain of spin-echo signals. Brief Description of the Drawings FIG. 1 is a a partially pictorial, partially block diagram illustration of a well logging apparatus for obtaining nuclear magnetic resonance measurements of a geologic structure. FIG. 2 is a graphical illustration of a chain of spin-echo relaxation signals as a function of amplitude versus time for a geologic structure investigated using a nuclear magnetic resonance system such as that shown in FIG. 1. FIG. 3 is a graphical illustration the use of time windows to selectively eliminate signals corresponding to particular pore sizes to allow determination of petrophysical properties of a geologic structure. Detailed Description of the Preferred Embodiment Referring to FIG. 1, a borehole 10 is shown adjacent to formations 12 and 14 having structures to be examined using the method and apparatus of the present invention. Within the borehole, there is a logging tool 16 which is suspended by a cable 18 routed over pulleys 20 and 22, with the position of the cable 18 being determined by a motor 24. The upper portion of the logging tool 16 comprises telemetry electronics 26, gamma ray sensing electronics 28 and magnetic resonance imaging (MRI) electronics 30. A MRI probe 32 is suspended at the bottom of the probe to provide excitation to the surrounding geologic formation. The excitation field has a generally cylindrical shape as represented by reference numeral 34. Improved devices which can be used for the probe 32 are described generally in U.S. Patent Nos. 4,710,713; 4,717,876; 4,717,877; and 4,717,878, which, by this reference, are incorporated herein for all purposes. The spin-spin pulse-echo measurement of the spin-echo relaxation of the sample, in a homogenous isotropic media, reflects the surface-to- volume characteristics of the pores. In typical rocks encountered in the well-logging environment, the rocks are complex mixtures of minerals which often include a variety of pore sizes. Consequently, the measured spin-echo relaxation in such an environment is a complex phenomenon, a reflection of the variations which exist in terms of pore surface-to-volume ratios and surface-to-fluid interactions. The method and apparatus of the present invention is based on the discovery that for a select time window of echo relaxation there is an associated select range of surface- to-volume response. Thus, by proper selection of spin-echo time windows it is possible to determine the relative fraction of select surf ace- to- volume components. In addition, these changes in relaxation time can also be used as a measure of a representative grain- size condition. FIG. 2 is a graphical illustration of a chain of spin-echo relaxation signals as a function of amplitude versus time for a geologic structure investigated using a nuclear magnetic resonance system such as that shown in FIG. 1. The spacing of the time intervals between the pulses in this application is typically between 1.5 and 3 milliseconds. The time intervals labelled "A-H" correspond to the signal intervals for various particle sizes, with interval "A" corresponding to the interval for particles larger than 500μ and interval "H" corresponding to the interval for particles of larger than 8μ, etc. Using the echoes in each time window to regress to time zero establishes an apparent porosity amplitude. Then, using first derivatives between the echo windows shown, one can determine the relative fraction of each grain-size component as part of the total porosity amplitude associated with the Bulk- Volume irreducible component determined from the complete relaxation echo-chain through the Free Fluid Analysis method. The calibration of the process is accomplished through multi-dimension regression analysis utilizing optimally selected and prepared laboratory samples. Such regression techniques are known to those skilled in the .art and are described in the following references: K. Fukunaga, Introduction to Statistical Pattern Recognition, Academic Press, 1972; Bhattacharyya & Johnson, Statistical Concepts and Methods, Wiley & Sons, 1977; and Devijver & Kittler, Pattern Recognition - A Statistical Approach, Prentice Hall, 1982. As a consequence of the actual tool operation, the measurement of spin-echo information is delayed for a few milli-seconds (typically < 5m sees for the tools described in the above referenced patents incorporated herein by reference). During this period of time (t de] ) no formation information is uniquely measured. This t deI time period includes the surface-to-volume response associated with a select pore-size group that is directly linked with the pore-sizes related to clay size grains. Thus, by proper selection of the echo windows, a spin-echo measurement can be provided which measures the total pore-space minus those associated with the pore surface-to-volume ratios related to the clay- size particles. The pore surface-to-volume responses that are missed during this t del period include the clay mineral fraction of the rock-space, thus providing a direct link between such a NMR measured porosity and the total porosity of the rock. In other words, in a clay mineral free environment, with pores >2μ, the NMR echo extrapolation provides a measure of the total porosity but, in a shaly-sand that contains clay minerals and thus clay size pores, the NMR porosity measurement can be made to be free of the influence of the non-reservoir quality micro-pores making the NMR measurement particularly useful in assessing the reservoir's capacity to support production. Furthermore, in the event total porosity is known, it can be combined with such a determined NMR porosity as to establish the ionically bound clay-mineral porosity and thus provide a link to recognizing the clay-mineral types. FIG. 3 is a graphical illustration of the use of time windows to selectively eliminate signals corresponding to progressively larger pore sizes to allow selective determination of petrophysical properties of a geologic structure. In particular, the exclusion of all clay size pore systems can be accomplished through a judicious choice of echo data which eliminates those very fast relaxation pores associated with the clay- size particles. Referring to FIG. 3, the quantity illustrated by reference letter "A" corresponds to the porosity of pore structures greater than 2μ, where A is determined by a regression of the full "selected echoes" time. The quantity illustrated by reference letter "B" corresponds to the total porosity of the sample obtained by regression of a fully sensitive echo chain to time zero. The ratio of the two quantities (A/B) can be used to calculate the relative volumes of selected pore structures within the geologic medium under investigation. Prior art references discussed above (see, for example, A. Timur, Journal of Petroleum Technology article) show the NMR may be used for the determination of a rock parameter called the free-fluid index (FFI). The FFI method relies on use of relaxations which occur during a late measurement time following a select t del . This time period being referred to as the long component of the relaxation phenomenon (typically t del 's > 22 m sees). The difference between the pore space described as the long component relaxation and that provided by the full NMR spectrum provides a direct measure of the pore bulk-volume that is held in place by existing surface tension and other capillary forces. This parameter, the bulk-volume of irreducible water, is directly related to pore surface-to-volume of the non-clay-size rock. The NMR measurement of porosity and bulk-volume irreducible can be readily used to find the intrinsic permeability of the rock since these measured parameters reflect the principle component of the rock's producibility through a model such as that of the Coates' free-fluid perm model. Referring to the principles discussed above, where the pore-sizes which dominate the irreducible saturation are determined by using select echo times to identify the total porosity less the porosity of the clay-size rock and, by using a different set of echoes, it is 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 reservoir engineering as the bulk- volume of irreducible water. This principle is illustrated in the following formulas: φ = φ - φ Φ ^ ** *r r= Φ ^ RR - Φ ^PR where Φ RR represents the porosities associated with non-clay size reservoir grain structure. Φ PR represents the porosities associated with the non-clay size pores which are free of surface related relaxation effects and are known to contribute to the productivity of the well. A fully self-supported synergetic log interpretation method is possible with the information provided by the tools described in the above referenced patents incorporated herein by reference when combined with traditional well logging measurements. The MRIL system described in these patents provides access to porosity MRI (PHI) (chosen so as to eliminate porosity linked to clay-size particles), which when combined with conventional total porosity T(PHI) determinations gives access to the following: 1) (1 - Swb) 3 = MRI(PHI)/T(PHI) 2) Ct = (T(PHI))"- * Sw » [Cwf +(Swb/Sw)(Ccw -Cwf)] 3) Vclay = (Swb*T(PHI)/PHI(CLAY) where: C = 1/R; the virgin rock conductivity Sw; the total water fraction of pores Cwf; the conductivity of the non-clay held water Ccw; the conductivity of the clay-held water Vclay; the bulk fraction of dry-clay minerals PHI(CLAY); the associated clay-mineral porosity, believed unique to a clay mineral type which is accessible from natural gamma-ray technology. m; typically 1.8 for sandstones n; typically 2.0 for water-wet rocks Although the present invention has been described in connection with the preferred embodiment, it is not intended to be limited to the specific form set forth herein, but on the contrary, it is intended to cover such modifications, alternatives, and equivalents as can be reasonably included within the spirit and scope of the invention as l defined by the appended claims.

Claims: I Claim: 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 spin-echo signals; and determining the petrophysical characteristics of said geologic structure from said chain of 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 time 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 geologic structure 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.

Inventors: COATES GEORGES R, COATES, GEORGES, R.
Applicants: NUMAR CORP, NUMAR CORPORATION
Application year: 1992
Main classification: G01V3
IPC codes: G01R33, G01V3