Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2000-04-18
2002-10-15
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
Reexamination Certificate
active
06466013
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the invention
This invention relates generally to determining geological properties of subsurface formations using Nuclear Magnetic Resonance (NMR) methods for logging wellbores, particularly for improving the signal level and reducing the power consumption by modifying the pulse sequence timing compared to prior art.
2. Background of the art
A variety of techniques are utilized in determining the presence and estimation of quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, the resistivity, porosity and permeability of the rock formation surrounding the wellbore drilled for recovering the 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, which is referred to as measurement-while-drilling (MWD) or logging-while-drilling (LWD).
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 liquids in the geological formations surrounding 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 measured. 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.
The NMR tools generate a uniform or near uniform static magnetic field in a region of interest surrounding the wellbore. NMR is based on the fact that the nuclei of many elements have angular momentum (spin) and a magnetic moment. The nuclei have a characteristic Larmor resonant frequency related to the magnitude of the magnetic field in their locality. Over time the nuclear spins align themselves along an externally applied magnetic field. This equilibrium situation can be disturbed by a pulse of an oscillating magnetic field, which tips the spins with resonant frequency within the bandwidth of the oscillating magnetic field away from the static field direction. The angle &thgr; through which the spins exactly on resonance are tipped is given by the equation:
&thgr;=&ggr;B
1
t
p
(1)
where &ggr; is the gyromagnetic ratio, B
1
is the effective field strength of the oscillating field and t
p
is the duration of the RF pulse.
After tipping, the spins precess around the static field at a particular frequency known as the Larmor frequency &ohgr;
0
, given by
&ohgr;=&ggr;B
0
(2)
where B
0
is the static field intensity. At the same time, the spins return to the equilibrium direction (i.e., aligned with the static field) according to an exponential decay time known as the spin-lattice relaxation time or T
1
. For hydrogen nuclei, &ggr;/2&pgr;=4258 Hz/Gauss, so that a static field of 235 Gauss would produce a precession frequency of 1 MHz. T
1
of fluid in pores is controlled totally by the molecular environment and is typically ten to one thousand milliseconds in rocks.
At the end of a &thgr;=90° tipping pulse, spins on resonance are pointed in a common direction perpendicular to the static field, and they precess at the Larmor frequency. However, because of inhomogeneity in the static field due to the constraints on tool shape, imperfect instrumentation, or microscopic material heterogeneities, each nuclear spin precesses at a slightly different rate. Hence, after a time long compared to the precession period, but shorter than T
1
, the spins will no longer be precessing in phase. This de-phasing occurs with a time constant that is commonly referred to as T
2
* if it is predominantly due to the static field inhomogeneity of the apparatus, and as T
2
if it is due to properties of the material.
The receiving coil is designed so that a voltage is induced by the precessing spins. Only that component of the nuclear magnetization that is precessing in the plane perpendicular to the static field is sensed by the coil. After a 180° tipping pulse (or an “inversion pulse”), the spins on resonance are aligned opposite to the static field and the “precession” consists of a slow return along the static field axis to the equilibrium direction. Hence, a signal will be generated after a 90° tipping pulse, but not after a 180° tipping pulse in a generally uniform magnetic field.
While many different methods for measuring T
1
have been developed, a single standard known as the CPMG sequence (Carr-Purcell-Meiboom-Gill) for measuring T
2
has evolved. In contrast to laboratory NMR magnets, well logging tools have inhomogeneous magnetic fields due to the constraints on placing the magnets within a tubular tool and the inherent “inside-out” geometry. Maxwell's divergence theorem dictates that there cannot be a region of high homogeneity outside the tool. Therefore in typical well bores, T
2
* <<T
2
, and the free induction decay becomes a measurement of the apparatus-induced inhomogeneities. To measure the true T
2
in such situations, it is necessary to cancel the effect of the apparatus-induced inhomogeneities. To accomplish the same, a series of pulses is applied to repeatedly refocus the spin system, cancelling the T
2
* effects and forming a series of spin echoes. The decay of echo amplitude is a true measure of the decay due to material properties. Furthermore it can be shown that the decay is in fact composed of a number of different decay components forming a T
2
spectrum. The echo decay data can be processed to reveal this spectrum which is related to rock pore size distribution and other parameters of interest to the well log analyst.
One method to create a series of spin echoes is due to Carr and Purcell. The pulse sequence starts with a delay of several T
1
to allow spins to align themselves along the static magnetic field axis. Then a 90° tipping pulse is applied to rotate the spins into the transverse plane where they precess with angular frequency determined by local magnetic field strength. The spin system loses coherence with time constant, T
2
*. After a short time t
CP
a 180° tipping pulse is applied which continues to rotate the spins, inverting their position in the transverse plane. They continue to precess, but now their phases converge until they momentarily align a further time t
CP
after the 180° pulse. The 180° pulse is re-applied after a further time t
CP
and the process repeated many times forming a series of spin echoes with spacing 2 t
CP
.
While the Carr-Purcell sequence would appear to provide a solution to eliminating apparatus induced inhomogeneities, it was found by Meiboom and Gill that if the duration of the 180° pulses in the Carr-Purcell sequence were even slightly erroneous so that focusing is incomplete, the transverse magnetization would steadily be rotated out of the transverse plane. As a result, substantial errors would enter the T
2
determination. Thus, Meiboom and Gill devised a modification to the Carr-Purcell pulse sequence such that after the spins are tipped by 90° and start to de-phase, the carrier of the 180° pulses is phase shifted by &pgr;/2 radians relative to the carrier of the 90° pulse. This phase change causes the spins to rotate about an axis perpendicular to both the static magnetic field axis and the axis of the tipping pulse. If the phase shift between tipping and refocusing pulses deviates slightly from &pgr;/2 then the rotation axis will not be perfectly orthogonal to the static and RF fields, but this has negligible effect. For an explanation, the reader is referred to a detailed account of spin-echo NMR techniques, such as “NMR: a nuts and bolt
Hawkes Robert
Lucas Alun
Slade Robert
Baker Hughes Incorporated
Lefkowitz Edward
Madan Mossman & Sriram P.C.
Vargas Dixomara
LandOfFree
Nuclear magnetic resonance measurements in well logging... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Nuclear magnetic resonance measurements in well logging..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Nuclear magnetic resonance measurements in well logging... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2988210