Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2001-02-07
2002-09-17
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
C324S309000
Reexamination Certificate
active
06452389
ABSTRACT:
FIELD OF THE INVENTION
The invention is related to the field of electromagnetic well logging instruments and methods. More specifically, the invention is related to the use of NMR pulse sequences for improving the efficiency of nuclear magnetic resonance (“NMR”) well logging data acquisition.
BACKGROUND OF THE INVENTION
Electromagnetic well logging instruments include circuits connected to antennas which induce alternating electromagnetic fields in earth formations surrounding a wellbore, and include circuits which measure various electromagnetic phenomena which occur as a result of interaction of the alternating electromagnetic fields with the earth formations. Such electromagnetic phenomena relate to petrophysical properties of interest of the earth formations. One type of electromagnetic well logging instrument that suffers deleterious effects of eddy currents in electrically conductive elements of the logging instrument is the nuclear magnetic resonance (“NMR”) instrument.
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 precessionally rotate 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.
A property of NMR measurements made in porous media such as earth formations is that there is typically a significant difference between the longitudinal relaxation time T
1
distribution and the transverse relaxation time T
2
distribution of fluids filling the pore spaces of the porous medium. For example, light hydrocarbons and natural gas, as commonly are present in the pore spaces of some earth formations, may have T
1
relaxation times as long as several seconds, while the T
2
relaxation times may be only about 1/100 that amount. This aspect of NMR well logging is due primarily to the effect of diffusion occurring within static magnetic field amplitude gradients. These amplitude gradients can arise from the inhomogeneous applied static magnetic field or from the earth formations itself. 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 fluid filling the pore spaces.
In order to perform precise NMR measurements on any medium, including earth formations, the nuclei of the material should be polarized by the static magnetic field for about 5 times the longest T
1
relaxation time of any individual component within the material. This is generally not the case for well logging NMR measurements, since some formation components, as previously explained, may have T
1
relaxation times as long as several seconds (requiring a polarization time of as long as about 30 seconds). This is such a long polarization time as to make impracticable having enough polarization time at commercially acceptable logging speeds. As the instrument moves along the wellbore, the earth formations that are subject to the static magnetic field induced by the instrument are constantly changing. See for example, “An Experimental Investigation of Methane in Rock Materials,” C. Straley, SPWLA Logging Symposium Transactions, paper AA (1997).
As a result of logging speed considerations, a polarization time of 8 to 10 seconds has become more common for many NMR well logging procedures, including those used for natural gas detection. See for example, “Selection of Optimal Acquisition Parameters for MRIL Logs,” R. Akkurt et al, The Log Analyst, Vol. 36, No. 6, pp. 43-52 (1996).
Typical NMR well logging measurement procedures include transmission of a series of RF energy pulses in a Carr-Purcell-Meiboom-Gill (“CPMG”) pulse sequence. For well logging instruments known in the art, the CPMG pulse sequences are about 0.5 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”.
In the typical NMR well logging procedure only about 30 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 60 percent of the time is used for polarizing 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. As a result of the small fractional amount of RF transmission time in the typical NMR measurement sequence, the RF power transmitting components in the well logging instrument are used inefficiently on a time basis. In well logging applications this inefficiency can be detrimental to the overall ability to obtain accurate NMR measurements. The signal-to-noise per unit time is proportional to square root of the average RF power used. Because the amount of electrical power which can reasonably be supplied to the NMR logging instrument (some of which, of course, is used to generate the RF pulses for the NMR measurements) is limited by the power carrying capability of an electrical cable which is used to move the logging instrument through the wellbore, inefficient use of the RF power on a time basis results in measurements with poor spatial resolution or unacceptable logging speeds.
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 purpo
Arana Louis
Baker Hughes Incorporated
Madan Mossman & Sriram P.C.
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