Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2000-06-29
2003-07-22
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
Reexamination Certificate
active
06597170
ABSTRACT:
FIELD OF THE INVENTION
The invention is in the field of determination of petrophysical properties of subsurface formations using data from a Nuclear Magnetic Resonance (NMR) tool. Specifically, the invention relates to the use of shaped pulses for reducing interference of signals from different regions of the subsurface in a gradient NMR tool using multiple frequency measurements.
BACKGROUND OF THE INVENTION
A variety of techniques have been utilized in determining the presence and in estimating quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, porosity, fluid content, and permeability of the rock formation surrounding the wellbore drilled for recovering hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the well bores have been drilled. More recently, wellbores have been logged while drilling of the wellbores, which is referred to as measurement-while-drilling (“MWD”) or logging-while-drilling (“LWD”). Measurements have also been made when tripping a drillstring out of a wellbore: this is called measurement-while-tripping (“MWT”).
One recently evolving technique involves utilizing Nuclear Magnetic Resonance (NMR) logging tools and methods for determining, among other things porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the fluids in the geological formations in the vicinity of the wellbore so that certain parameters such as spin density, longitudinal relaxation time (generally referred to in the art as “T
1
”), and transverse relaxation time (generally referred to as “T
2
”) of the geological formations can be estimated. From such measurements, porosity, permeability, and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
A typical NMR tool generates a static magnetic field B
0
in the vicinity of the wellbore, and an oscillating field B
1
in a direction perpendicular to B
0
. This oscillating field is usually applied in the form of short duration pulses. The purpose of the B
0
field is to polarize the magnetic moments of nuclei parallel to the static field and the purpose of the B
1
field is to rotate the magnetic moments by an angle &thgr; controlled by the width t
p
and the amplitude B
1
of the oscillating pulse. With the variation of the number of pulses, pulse duration, and pulse intervals, various pulse sequences can be designed to manipulate the magnetic moment, so that different aspects of the NMR properties can be obtained. For NMR logging, the most common sequence is the Carr-Purcell-Meiboom-Gill (“CPMG”) sequence that can be expressed as
TW−90−(t−180−t−echo)
n
(1)
where TW is a wait time, 90 is a 90 degree tipping pulse, 180 and is a 180 degree refocusing pulse.
After being tipped by 90°, the magnetic moment precesses around the static field at a particular frequency known as the Larmor frequency &ohgr;
0
, given by &ohgr;
0
=&ggr;B
0
, where B
0
is the field strength of the static magnetic field and &ggr; is the gyromagnetic ratio. At the same time, the magnetic moments return to the equilibrium direction (i.e., aligned with the static field) according to a decay time known as the “spin-lattice relaxation time” or T
1
. Inhomogeneities of the B
0
field result in dephasing of the magnetic moments and to remedy this, a 180° pulse is included in the sequence to refocus the magnetic moments. This gives a sequence of n echo signals.
U.S. Pat. No. 5,023,551 issued to Kleinberg discloses an NMR pulse sequence that has an NMR pulse sequence for use in the borehole environment which combines a modified inversion recovery (FIR) pulse sequence with a series of more than two, and typically hundreds, of CPMG pulses according to
[W
i
−180−TW
i
−90−(&tgr;−180−&tgr;−echo)
j
]
i
(2)
where 90 is a 90 degree tipping pulse, 180 is a 180 degree refocusing pulse, j=−1,2, . . . J and J is the number of echoes collected in a single Carr-Purcell-Meiboom-Gill (CPMG) sequence, where i=1, . . . I and I is the number of waiting times used in the pulse sequence, where W
i
are the recovery times, TW
i
are the wait times before a CPMG sequence, and where &tgr; is the spacing between the alternating 180° pulses and the echo signals. Although a conceptually valid approach for obtaining T
1
information, this method is extremely difficult to implement in wireline, MWD, LWD or MWT applications because of the long wait time that is required to acquire data with the different TWS.
There is an inherent inefficiency associated with the tipping and refocusing pulses in the CPMG sequence. The 90° tipping pulse used to modulate the RF signal has a duration one half of the duration of the 180° refocusing pulse, and, as would be known to those versed in the art, the shorter duration pulse has a large bandwidth than the longer duration refocusing pulse. Accordingly, only a portion of the pulses that are tipped will be refocused by the refocusing pulse.
In the typical NMR well logging procedure only about 5 to 10 percent of the total amount of time in between each NMR measurement set is used for RF power transmission of the CPMG pulse sequence. The remaining 90 to 95 percent of the time is used for repolarizing the earth formations along the static magnetic field. Further, more than half of the total amount of time within any of the CPMG sequences actually takes place between individual RF pulses, rather than during actual transmission of RF power.
Several methods are known in the art for dealing with the problem of non-transmitting time in an NMR measurement set. The first method assumes a known, fixed relationship between T
1
and T
2
, as suggested for example, in
Processing of Data from an NMR Logging Tool,
R. Freedman et al, Society of Petroleum Engineers paper no. 30560 (1995). Based on the assumption of a fixed relationship between T
1
and T
2
, the waiting (repolarization) time between individual CPMG measurement sequences is shortened and the measurement results are adjusted using the values of T
2
measured during the CPMG sequences. Disadvantages of this method are described, for example in,
Selection of Optimal Acquisition Parameters for MRIL Logs,
R. Akkurt et al, The Log Analyst, vol. 36, no. 6, pp. 43-52 (1996). These disadvantages can be summarized as follows. First, the relationship between T
1
and T
2
is not a fixed one, and in fact can vary over a wide range, making any adjustment to the purported T
1
measurement based on the T
2
measurements inaccurate at best. Second, in porous media T
1
and T
2
are distributions rather than single values. It has proven difficult to “adjust” T
1
distributions based on distributions of T
2
values.
U.S. Pat. No. 6,049,205 to Taicher et al having the same assignee as the present application and the contents of which are fully incorporated herein by reference, teaches a method for determination of T
1
and T
2
. The static magnetic field in the disclosed device has a field gradient, so that an RF pulse of a selected frequency excites nuclei in a specific portion of the formation determined by the gyromagnetic ratio. By altering the frequency of excitation, different regions of the formation may be analyzed. U.S. Pat. No. 5,936,405 to Prammer et al teaches making interleaved measurements at different frequencies to obtain, in a single logging pass, multiple data streams corresponding to different recovery times and/or diffusivity for the same spot in the formation. The resultant data streams are processed to determine mineralogy-independent water and hydrocarbon saturations and porosity estimates.
In order for these multiple frequency measurements to be made efficiently, it is important that there be no interfer
Beard David
Chen Songhua
Itskovich Gregory
Reiderman Arcady
Baker Hughes Incorporated
Fetzner Tiffany A.
Lefkowitz Edward
Madan Mossman & Sriram P.C.
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