Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
  • G01V3/18—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging
  • G01V3/32—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging operating with electron or nuclear magnetic resonance
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N24/00—Investigating or analyzing materials by the use of nuclear magnetic resonance, electron paramagnetic resonance or other spin effects
  • G01N24/08—Investigating or analyzing materials by the use of nuclear magnetic resonance, electron paramagnetic resonance or other spin effects by using nuclear magnetic resonance
  • G01N24/081—Making measurements of geologic samples, e.g. measurements of moisture, pH, porosity, permeability, tortuosity or viscosity

Definitions

• 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.
• Tl 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.
• 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.
• the apparatus will respond only to nuclei residing within the sensitive volume.
• 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.
• 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.
• 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.
• 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.
• T2 transverse relaxation time
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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 500 ⁇ and interval "H” corresponding to the interval for particles of larger than 8 ⁇ , etc.
• 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.
• 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.
• 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.
• 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.
• the quantity illustrated by reference letter “A” corresponds to the porosity of pore structures greater than 2 ⁇ , 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 can be used to calculate the relative volumes of selected pore structures within the geologic medium under investigation.
• Prior art references discussed above show the NMR may be used for the determination of a rock parameter called the free-fluid index (FFI).
• FFI free-fluid index
• 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 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.
• 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:
• ⁇ ⁇ ** *r r ⁇ ⁇ RR - ⁇ ⁇ PR
• ⁇ RR represents the porosities associated with non-clay size reservoir grain structure.
• ⁇ 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.

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• 1992
  • 1992-05-15 CA CA002119785A patent/CA2119785A1/en not_active Abandoned
  • 1992-05-15 EP EP92912085A patent/EP0584224A1/en not_active Withdrawn
  • 1992-05-15 MX MX9202314A patent/MX9202314A/es unknown
  • 1992-05-15 WO PCT/US1992/004144 patent/WO1992021045A1/en not_active Application Discontinuation

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