Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2000-05-10
2002-06-25
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
C324S300000
Reexamination Certificate
active
06411087
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to nuclear magnetic resonance and more particularly to a nuclear magnetic resonance apparatus and method using high static magnetic fields trapped in high temperature superconducting materials.
2. Description of the Related Art
A variety of techniques have 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, 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”).
When logging is done after the wellbores have been drilled, a sensor assembly is conveyed downhole on a wireline that includes electrically conducting cables for carrying electrical power downhole and for transmission of signals in an uphole and a downhole direction.
In MWD applications, a drilling assembly (also referred to as the “bottom hole assembly” or the “BHA”) carrying a drill bit at its bottom end is conveyed into the wellbore or borehole. The drilling assembly is usually conveyed into the wellbore by a coiled-tubing or a drill pipe. In the case of the coiled-tubing, the drill bit is rotated by a drilling motor or “mud motor” which provides rotational force when a drilling fluid is pumped from the surface into the coiled-tubing. In the case of the drill pipe, it is rotated by a power source (usually an electric motor) at the surface, which rotates the drill pipe and thus the drill bit.
Bottom hole assemblies generally include several formation evaluation sensors for determining various parameters of the formation surrounding the BHA during the drilling of the wellbore. Such sensors are usually referred to as the MWD sensors. Such sensors traditionally have electromagnetic propagation sensors for measuring the resistivity, dielectric constant, water saturation of the formation, nuclear sensors for determining the porosity of the formation and acoustic sensors to determine the formation acoustic velocity and porosity. Other downhole sensors that have been used or proposed for use include sensors for determining the formation density and permeability. The bottom hole assemblies also include devices to determine the BHA inclination and azimuth, pressure sensors, temperature sensors, gamma ray devices, and devices that aid in orienting the drill bit in a particular direction and to change the drilling direction. Acoustic and resistivity devices have been proposed for determining bed boundaries around and in some cases in front of the drill bit. More recently, nuclear magnetic resonance (NMR) sensors have gained extreme interest as MWD sensors as such sensors can provide direct measurement for water saturation porosity and indirect measurements for permeability and other formation parameters of interest.
NMR tools generate a near uniform static magnetic field in a region of interest surrounding the wellbore. The NMR measurement is based on the fact that the nuclei of many elements possess angular momentum (“spin”) and a magnetic moment. In the absence of an external field, the nuclear spin orientations are randomly distributed with an essentially uniform orientation in space, but when a magnetic field is applied, the nuclei tend to align themselves in one of two quantum states: either parallel or anti-parallel to the applied field. There is a net excess of spins aligned parallel to the field, so that on a macroscopic level, the material in the region of interest takes on a net magnetization aligned in the same direction as the applied magnetic field. The stronger the magnetic field, the greater the excess of parallel spins and the stronger the net magnetization. NMR sensors utilize permanent magnets to generate a static magnetic field in the formation surrounding the MWD tool.
For the purposes of this invention, the NMR measurements may be treated on a macroscopic scale rather than a quantum scale. The region surrounding the NMR tool can be uniformly divided into a grid of volume elements, commonly termed “voxels,” that can be referenced using a suitable coordinate system. One such convenient coordinate system is a cylindrical polar coordinate system. Each voxel contains many hundreds of thousands of nuclei, but each voxel is small in comparison with the dimensions of the sensor. Within each voxel, the static magnetic field can be represented by a vector B
0
and the magnetization by a vector M, both with classical properties. In this way, the quantum nature of the NMR phenomenon may be conveniently set aside. Hereafter, the magnetization in the voxels is loosely referred to as “spin.”
In equilibrium conditions, the quantities B
0
and M are related by the expression
M
=
N
A
γ
2
h
2
I
(
I
+
1
)
3
kT
B
0
(
1
)
where N
A
is Avogadro's number, &ggr; is the gyromagnetic ratio of the nucleus, h is Planck's constant, I is the nuclear spin, k is Boltzmann's constant, and T is the absolute temperature. Associated with the magnetic field strength |B
0
| is a characteristic frequency, called the Larmor frequency, given by
&ohgr;
0
=&ggr;|B
0
| (2)
The equilibrium condition can be disturbed by applying a pulse of an oscillating magnetic field, represented by B
1
; this is called a radio frequency or RF pulse. Spins that have Larmor frequency at or near the frequency of the applied oscillating magnetic field experience a torque, as described by the Larmor equation
ⅆ
M
ⅆ
t
=
M
×
γ
B
0
(
3
)
where x denotes the vector cross product. This expression describes a resonant condition: spins with a Larmor frequency that matches the applied field frequency are tipped away from the static field direction by an angle (in radians) given by the equation
&thgr;=&ggr;|
B
1
|t
p
/2 (4)
where t
p
is the duration of the pulse.
Those spins “on resonance,” i.e., having a Larmor frequency that exactly matches the applied oscillating field will precess around the static field at the Larmor frequency. At the same time, the spins return to the equilibrium direction, i.e., aligned with the static field, according to a characteristic decay time constant known as the “spin-lattice relaxation time” or “T1.”
For hydrogen nuclei, &ggr;/2&pgr;=42.58 MHz/T, so that a static field of 0.0235 Tesla, would produce a precession frequency of 1 MHz. U.S. Pat. No. 4,933,638 discloses a wireline NMR logging tool that operates at a frequency of 1 MHz, which is typical of prior art tools. The decay constant T
1
is controlled by the molecular environment and is typically ten to one thousand ms. in rocks.
At the end of a ninety degree tipping pulse, all the spins on resonance are pointed in a common direction perpendicular to the static field, and they all precess at the Larmor frequency. The precessing spins are detected by a voltage induced in a receiving coil. This may be the same coil as used to produce the B
1
field or another suitably oriented coil. According to the principle of reciprocity the component of nuclear magnetization that is precessing in a plane perpendicular to the field that would be produced by current flowing in the receiving coil induces a voltage in the receiver coil that can be amplified and measured. The voltage appearing on the receiver coil is the summation of all signals from the precessing spin system in the region of interest. The decay of the precessing pulses gives useful information about the fluid content in the formation surrounding the borehole. In particular, the dominant contribution to the signal arises from the precession
Chu Wei-Kan
Fan Nong-qiang
Lefkowitz Edward
Madan Mossman & Sriram P.C.
Shrivastav Brij B.
University of Houston
LandOfFree
NMR logging tool with Hi-Tc trap field magnet does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with NMR logging tool with Hi-Tc trap field magnet, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and NMR logging tool with Hi-Tc trap field magnet will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2913804