US 6,392,409 B1Granted

Determination of T1 relaxation time from multiple wait time NMR logs acquired in the same or different logging passes

AssigneeBaker Hughes Incorporated

InventorSonghua Chen

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Abstract

Nuclear magnetic resonance measurements of a porous medium using multiple wait times are used for determining the longitudinal relaxation time T1 time of oil in a hydrocarbon reservoir. The T1 values of water in the formation are represented by a distribution. In a dual wait time implementation of the method, data are acquired with a short wait time TWS chosen such that the wetting fluid phase (e.g., brine) is completely relaxed but the hydrocarbon phase is partially relaxed, giving a sequence ECHOA and with a long wait time TWL giving an echo sequence ECHOB. Using known methods, the T2 for the oil and water may be determined. The individual echo trains are summed and the T1 values determined from the summed echo trains and the determined values of T2. The summation greatly improves the SNR. An equivalent method determines the T1 values by summation of a first echo train that is the sum of the individual echo trains and the summation of a second echo train that is the difference of the individual echo trains. The various summed values may also be used as for quality control of the data.

Claims23

Dependent claims

A method of determining a parameter of interest of a volume of earth formation in a reservoir adjacent a borehole, said parameter of interest including at least one of a longitudinal relaxation time T1 and a transverse relaxation time T2 of a fluid in the reservoir, the method comprising:

(a) using a magnet assembly on a borehole tool conveyed in the borehole at at least one depth for producing a static magnetic field in said volume of the formation thereby aligning nuclear spins within said volume parallel to a direction of the static field;
(b) producing a radio frequency (RF) magnetic field in said volume of the formation with an antenna on the borehole tool, said RF magnetic field having a direction orthogonal to a direction of the static field, the RF field including a first pulse sequence TWA−90−(t−X−t−echo)j to produce a first echo train and at least one second pulse sequence TWB−90−(t−X−t−echo)j to produce at least a second echo train,
wherein 90 is a tipping pulse for tipping the nuclear spins at an angle substantially equal to ninety degrees to cause precession thereof, TWA is a first wait time, TWB is a second wait time, X is a refocusing pulse, and j=1, 2, . . . J, where J is the number of echoes collected in a single sequence of pulses;
(c) measuring with the borehole tool the first and at least one second echo train;
(d) determining at least one summed signal selected from the group of:
(i) a sum of the first echo train, defining a first summation signal, (ii) a sum of the at least one second echo train, defining a second summation signal, (iii) a sum of an average of the first and at least one second echo train, defining a third summation signal, and, (iv) a sum of a difference of the first echo train and the at least one second echo train, giving a fourth summation signal;
and
(e) determining the parameter of interest from the at least one summed signal.

5 more elements

(a) using a magnet assembly on a borehole tool conveyed in the borehole at a plurality of depths for producing a static magnetic field in said volume of the formation thereby aligning nuclear spins within said volume parallel to a direction of the static field;
(b) producing a radio frequency (RF) magnetic field in said volume of the formation with an antenna on the borehole tool, said RF magnetic field having a direction orthogonal to a direction of the static field, the RF field including a phase alternated pulse sequence TWA−90±X−(t−X−t−echo)j to produce an in-phase echo train and a quadrature echo train,
wherein 90 is a tipping pulse for tipping the nuclear spins at an angle substantially equal to ninety degrees to cause precession thereof, TWA is a wait time, X is a refocusing pulse, and j=1, 2, . . . J, where J is the number of echoes collected in a single sequence of pulses;
(c) at each of said plurality of depths, correcting at least one of said in-phase and quadrature echo trains for ringing and DC offset;
(d) at each of said plurality of depths, determining a sum of the at least one of the corrected in-phase echo train and the corrected quadrature echo train;
(e) comparing fluctuations at said plurality of depths of a normalized value of the sum of the at least one of the corrected in-phase and quadrature echo trains with a fluctuations of a magnitude of the j-th echo, where 2≦j≦4;
(f) discarding as unacceptable echo train data from those of said plurality of depths where the fluctuations of said normalized value of the sum of the at least one of the corrected in-phase and quadrature echo trains exceed fluctuations of the magnitude of said j-th echo;
and
(g) determining said parameter of interest from at least a portion of the remaining echo train data.

6 more elements

Dependencies are inferred from the claim text.

Facts

Application: 09/483,336Filed 14 Jan 2000
Granted: 21 May 2002
Term ends: Not estimatedThe source records don’t support an estimate.

Classification9 codes

G01N 24/081
By using nuclear magnetic resonance: making measurements of geologic samples, e.g. measurements of moisture, pH, porosity, permeability,…
G01R 33/50
NMR imaging systems: based on the determination of relaxation times , e.g. T1 measurement by IR sequences; T2 measurement by multiple-echo sequences
G01V 3/32
Specially adapted for well-logging: operating with electron or nuclear magnetic resonance
G01N 24/082
By using nuclear magnetic resonance: measurement of solid, liquid or gas content