Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2002-06-06
2003-07-29
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
C324S306000
Reexamination Certificate
active
06600316
ABSTRACT:
FIELD OF THE INVENTION
The invention is related to the field of nuclear magnetic resonance (“NMR”) sensing methods and measuring techniques. More specifically, the invention is related to making NMR measurements during well logging or during Measurement-While-Drilling (“MWD”) within earth formations surrounding a wellbore. The invention also relates to methods for using NMR measurements to determine petrophysical properties of reservoir rocks and properties of fluids in the earth formations surrounding the wellbore.
BACKGROUND OF THE INVENTION
The description of the background of this invention, and the description of the invention itself are approached in the context of well logging because well logging is a well known application of NMR measurement techniques. It is to be explicitly understood that the invention is not limited to the field of well logging.
An apparatus described in U.S. Pat. No. 4,710,713 issued to Taicher et al is typical of NMR instruments used to measure certain petrophysical properties of earth formations from within a wellbore drilled through the earth formations. NMR well logging instruments such as the one disclosed by Taicher et al typically include a magnet for polarizing nuclei in the earth formations surrounding the wellbore along a static magnetic field, and at least one antenna for transmitting radio frequency (“RF”) energy pulses into the formations. The RF pulses reorient the spin axes of certain nuclei in the earth formations in a predetermined direction. As the spin axes precess and reorient themselves into alignment with the static magnetic field, they emit RF energy that can be detected by the antenna. The magnitude of the RF energy emitted by the precessing nuclei, and the rate at which the magnitude changes, are related to certain petrophysical properties of interest in the earth formations.
There are several principal operating parameters in NMR well logging which should be optimized for efficient operation of an NMR well logging instrument. These parameters include the logging speed (speed of motion of the instrument along the wellbore), the average and the peak power supplied to the instrument and transmitted as RF pulses, and the signal-to-noise ratio (“SNR”). Other parameters of interest include the vertical resolution of the instrument and the radial depth of investigation of the measurements made by the instrument within the formations surrounding the wellbore. The last two of these parameters are primarily determined by the antenna and magnet configurations of the NMR logging instrument. Improvements to these two parameters are the subject of numerous patents and other publications. Providing more flexibility in the instrument's peak power requirements, and limitations on the logging speed necessitated by the physics of NMR measurement have been more difficult to overcome.
Proton NMR relaxation time varies for different fluid types in earth formations. In addition, pore-size distributions dictate relaxation rate of wetting-fluid protons, due to the fast surface relaxation and the diffusional mixing of protons near the matrix-fluid interface with those in the middle of pores. Thus, in general, a distribution of NMR relaxation times is often observed for protons of fluids in earth formation. A large number of data points acquired in the same dynamic range is crucial to improve the accuracy and resolution of the relaxation time distribution, particularly because MWD and wireline data are known to be contaminated with high levels of random noise. There is a difference between the longitudinal relaxation time T
1
distribution and the apparent transverse relaxation time T
2
distribution of fluids filling the pore spaces of the porous medium. The difference is due primarily to the effect of diffusion in the presence of magnetic field gradients. For example, light hydrocarbons and natural gas may have T
1
relaxation times of the order of several seconds, while the apparent T
2
relaxation times may be only about {fraction (1/100)} that amount because of diffusion when measurements are made in strong gradient magnetic fields. These field gradients can arise from the non-uniformly applied static magnetic field or from the earth formations themselves. The latter gradients are caused by differences in magnetic susceptibility between the solid portion of the earth formation (referred to as the rock “matrix”) and the fluids filling the pore spaces: both the magnitude and direction of these gradients are difficult to predict).
In wireline NMR logging, the types of NMR measurements affects the logging speed. NMR measurements often require the nuclei of the material be polarized by the static magnetic field for more than three times the longest T
1
relaxation time of any individual component within the material. This requires very slow logging speeds and, in many circumstances, is unacceptable.
Typical NMR well logging measurements use pulsed NMR techniques in which RF energy is transmitted to the measurement sensitive volume in the form of a series of pulses. The most commonly used pulse sequence for logging application is the Carr-Purcell-Meiboom-Gill (“CPMG”) pulse sequence. For well logging applications known in the art, the CPMG pulse sequences are about 0.01 to 1 seconds in total duration, depending on the number of individual pulses and the time span (“TE”) between the individual RF pulses. Each series of CPMG pulses can be referred to as a “measurement set”.
The efficiency of NMR logging is affected by the following three aspects. Firstly, the wait time between two acquisition cycles is dictated by the formation rock and fluid properties. Thus, maximizing the number of data and experiments to be acquired within one measurement cycle is desired. Secondly, power transmission duty cycle, defined as the ratio of the RF transmitting time vs. total time, is limited by the instrumentation design and the efficiency of heat dissipation. In some existing NMR logging tools, the duty cycle is as low as 3-4%. When the measurements are limited by duty cycle, the tool can not repeat the experiment as fast as the formation wait time allows. Thus, use of pulses or pulse sequences that minimize the RF power consumption yet provides the same information is desired. Thirdly, the number of repeated measurements depends on the required signal-to-noise ratio. Formation properties, such as porosity, affect the signal strength. The conductivity of formation and/or borehole affects the RF energy transmission efficiency and, consequently, the strength of noise. Different porosity distributions may also require different SNR in order to achieve a desired accuracy of porosity estimates. Specifically, faster relaxing components require higher SNR data compared to slower relaxing components. Thus, the number of experiment repeats is desired to be higher for the portion of the signal that represents fast relaxing protons most.
The CPMG sequence is commonly used for well logging applications because it acquires a series of NMR signal amplitudes of a vital decay range, time-spaced equally, within a single polarization cycle. Although TE is desired to be as short as the instrumentation permits, the short TE is beneficial primarily for resolving fast relaxing components. For slowly relaxing components, the choice of TE must be balanced with power requirements to avoid limiting the number of echos acquired. It is desirable to choose the time series in accordance with the relaxation distribution scale at which one wants to resolve the spectrum, rather than taking the data equally-spaced in time. Although CPMG is efficient in terms of a large number of echos that can be acquired within a single polarization cycle, it is not an efficient way to use available RF energy because the data are acquired equally time-spaced while the relaxation components are logarithmically time-spaced.
For MWD, where high frequency vibrations limit experiment time, the saturation-recovery sequence for T
1
measurement (Fukushima, and Roeder, p. 169,
Experimental Pulse NMR,
Addison-Wesley
Chen Songhua
Georgi Dan
Kruspe Thomas
Arana Louis
Baker Hughes Incorporated
Madan Mossman & Sriram P.C.
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