US4933638A
Expired / Lapsed
US
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Borehole measurement of NMR characteristics of earth formations, and interpretations thereof
Assignee: Schlumberger Technology Corp
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Filing: 1989-06-19
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Granted: 1990-06-12
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Expires: 2009-06-19
Abstract
Abstract Borehole NMR logging apparatus and methods, and methods for the interpretation thereof. A logging tool is provided which produces a strong, static and homogeneous magnetic field B 0 in a Volume of an adjacent formation on one side of the tool to measure nuclear magnetic resonance characteristics thereof. In the preferred embodiment, the tool has an RF antenna mounted on the outside of the metal body of the tool, directing focussed oscillating magnetic fields B 1 at said Volume to polarize or tip the magnetic moments of hydrogen nuclei of fluids within rock pores. The same antenna can be used to receive signals of proton precession in the Volume of interest immediately after transmission of the RF polarizing field B 1 . Extremely rapid damping of the antenna between the transmitting and receiving modes of operation is accomplished by a Q-switch disclosed herein. The invention provides for the direct measurement of NMR decay having transverse relaxation time T 2 behavior, and further provides for the fast repetition of pulsed measurements from within a borehole. An additional magnet array may be mounted offset from the first magnet configuration to prepolarize a formation before it is measured in order to pre-align a larger number of protons than the single magnet configuration could do by itself. Additional features of the invention are disclosed which increase the Signal/Noise ratio of the measured data, and improve the quality and quantity of borehole NMR measurements, per unit of time spent. Disclosed interpretation methods determine fluid flow permeability and longitudinal relaxation time T 1 -type parameters by directly comparing the measured decay signals (such as T 2 and T 2 * type decay) to a representation which responds to both the decay time t dec and the imposed polarization period prior to such decay, t pol . The parameters of amplitude and T 1 are determined and combined with certain preferred methods to generate robust values of formation characteristics such as fluid flow permeability. Other related methods are disclosed.
Inventors
- 👤 Robert L. Kleinberg
- 👤 Douglas D. Griffin
- 👤 Masafumi Fukuhara
- 👤 Abdurranhman Sezginer
- 👤 Weng C. Chew
- 👤 William E. Kenyon
- 👤 Peter I. Day
- 👤 Max Lipsicas
Bibliographic Details
Application Number
—
Jurisdiction
US
Data Source
google_patents
Last Synchronized
2026-10-30 01:34
CPC Classifications
G; G01; G01R; G01R33/00; G01R33/20; G01R33/28; G01R33/32; G01R33/34; G01R33/341; G01N; G01N24/00; G01N24/08; G01N24/081; G01R33/36; G01R33/3671; G01R33/38; G01R33/3808; G01V; G01V3/00; G01V3/18; G01V3/32
Claims Analysis
Total Claims: 19
Claim 1
Independent Claim
150 words
A method of determining a characteristic of an earth formation traversed by a borehole comprising the steps of: 1. measuring nuclear magnetic resonance decay signals representative of transverse (T 2 ) or observed (T 2 *) magnetic relaxation of a population of particles in the formation following a plurality of different magnetic polarization periods; 2. selecting a set of values of said signals; 3. comparing the set of values directly to a representation of NMR decay which includes amplitude A and longitudinal relaxation time T 1 terms and simultaneously depends on both the decay time t dec and polarization periods t pol associated with said values; 4. generating values of amplitude and T 1 parameters which optimize the fit between the compared set of values and said representation; 5. determining the formation characteristic in response to the generated values of at least one of the terms--amplitude or T 1 .
Claim 2
Independent Claim
150 words
A method of determining a characteristic of an earth formation traversed by a borehole comprising the steps of: 1. measuring nuclear magnetic resonance decay signals representative of transverse (T 2 ) or observed (T 2 *) magnetic relaxation of a population of particles in the formation following a plurality of different magnetic polarization periods; 2. selecting a set of values of said signals; 3. comparing the set of values directly to a representation of NMR decay which includes amplitude A and longitudinal relaxation time T 1 terms and simultaneously depends on both the decay time t dec and polarization periods t pol associated with said values; 4. generating values of amplitude and T 1 parameters which optimize the fit between the compared set of values and said representation; 5. determining the formation characteristic in response to the generated values of at least one of the terms--amplitude or T 1 .
Claim 3
Dependent Claim
102 words
A method as in claim 1 wherein said third step comprises comparing the selected signal values to a representation of the form ##EQU16## wherein A(t pol , t dec ) is the signal amplitude, t pol is the duration of a polarizing magnetic field preceding a given measurement, t dec is the decay time at which a measurement is taken, i is the index of magnetic resonance species, A i is the signal amplitude of a given species, T 1i is the longitudinal relaxation time of a given species, and T 2i is the transverse relaxation time constant of a given species.
Claim 4
Dependent Claim
31 words
A method as in claim 2 wherein said third step comprises comparing the measured signals to a two component representation of NMR decay in the formation, having the general form: ##EQU17##
Claim 5
Dependent Claim
49 words
A method as in claim 2 wherein said third step comprises comparing the measured signals to a representation of the general form: ##EQU18## wherein the subscript S designates a first component of NMR decay, L designates a second component of NMR decay, and T inh is a fitting parameter.
Claim 6
Dependent Claim
35 words
A method as in claim 2 wherein said third step comprises comparing the measured signals to a representation of the form: ##EQU19## where T inh is a fitting parameter, and α is an exponential parameter
Claim 7
Independent Claim
95 words
A method of determining a characteristic of an earth formation traversed by a borehole comprising the steps of: 1. measuring NMR decay signals representing magnetic relaxation of a population of particles in the formation following a plurality of different magnetic polarization periods; 2. generating, in response to the measured signals, a representation of a characteristic relaxation of the population of particles, in the form of magnetization versus time; 3. integrating said representation of magnetization versus time to obtain an integrated value; and 4. determining said characteristic of the formation in response to the integrated value.
Claim 8
Independent Claim
95 words
A method of determining a characteristic of an earth formation traversed by a borehole comprising the steps of: 1. measuring NMR decay signals representing magnetic relaxation of a population of particles in the formation following a plurality of different magnetic polarization periods; 2. generating, in response to the measured signals, a representation of a characteristic relaxation of the population of particles, in the form of magnetization versus time; 3. integrating said representation of magnetization versus time to obtain an integrated value; and 4. determining said characteristic of the formation in response to the integrated value.
Claim 9
Dependent Claim
46 words
A method as in claim 6 further comprising: measuring T 2 * decay signals following a plurality of different polarizing periods; determining, from the measured signals, a T 1 build-up curve of magnetization versus polarizing period; and integrating said build-up curve to obtain said integrated value.
Claim 10
Dependent Claim
74 words
A method as in claim 6 wherein said measurement step further comprises measuring the magnetization of said population of particles by using a spin-echo sequence of measurements whereby the successive peaks of the spin-echo signals decay with time constant T 2 , and further comprising converting said integrated value to an integrated value associated with T 1 relaxation, and determining said characteristic of the formation using said integrated value associated with T 1 relaxation.
Claim 11
Dependent Claim
19 words
A method as in claim 8 wherein the measuring step further comprises measuring said magnetization at low resonance frequencies.
Claim 12
Independent Claim
38 words
A method as in claims 6, 7 or 8, further comprising: generating a value representing the porosity of said formation; determining permeability of said formation in response to said integrated value and the square of the porosity value.
Claim 13
Dependent Claim
14 words
A method as in claim 8 wherein said spin-echo sequence comprises a Carr-Purcell sequence.
Claim 14
Independent Claim
86 words
A method of determining a pore fluid characteristic of an earth formation traversed by a borehole comprising: 1. measuring, with a borehole tool, NMR decay signals representative of transverse relaxation of a population of particles in said formation exhibiting a T 2 relaxation time constant; 2. generating a first representation of said signals; 3. converting said representation into a second representation associated with the longitudinal relaxation time constant T 1 of said population of particles; 4. determining said formation characteristic in response to said second representation.
Claim 15
Independent Claim
86 words
A method of determining a pore fluid characteristic of an earth formation traversed by a borehole comprising: 1. measuring, with a borehole tool, NMR decay signals representative of transverse relaxation of a population of particles in said formation exhibiting a T 2 relaxation time constant; 2. generating a first representation of said signals; 3. converting said representation into a second representation associated with the longitudinal relaxation time constant T 1 of said population of particles; 4. determining said formation characteristic in response to said second representation.
Claim 16
Dependent Claim
18 words
A method as in claim 12 wherein said second representation is a multiplicative factor of said first representation.
Claim 17
Dependent Claim
118 words
A method as in claim 12 further comprising: measuring NMR decay signals with a borehole tool positioned stationarily at a predetermined first depth within the borehole; generating a value representative of the longitudinal relaxation time T 1 of a formation at said first depth in response to said NMR decay signals; measuring additional NMR decay signals with said tool in continuous motion at other depths within said borehole; generating additional values representative of the transverse relaxation time T 2 of formations at said other respective depths in response to said additional signals; determining a characteristic of said formations at said other respective depths in response to both said additional values and said value representative of T 1 .
Claim 18
Dependent Claim
26 words
A method as in claim 12 wherein said borehole tool measurement is made at low frequencies, and said second representation is identical to said first representation.
Claim 19
Independent Claim
56 words
The method as in claims 1, 6 or 12 wherein the formation characteristic is formation permeability, and wherein this characteristic is proportional to a product of (constant) ·Φ m ·T n where porosity Φ is raised to the power m being approximately 4, and T is one of said relaxation parameters raised to a power n.
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