Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2000-12-14
2003-04-01
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
Reexamination Certificate
active
06541969
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns nuclear magnetic resonance (NMR) logging and more specifically relates to a method and apparatus for NMR data acquisition and processing, which for a given signal-to-noise ratio (SNR) improve the vertical resolution of data logs acquired using NMR logging tools.
BACKGROUND OF THE INVENTION
In oil and gas exploration it is desirable to understand the structure and properties of the geological formation surrounding a borehole, in order to determine if the formation contains hydrocarbon resources (oil and/or gas), to estimate the amount and producibility of hydrocarbon contained in the formation, and to evaluate the best options for completing the well in production. A significant aid in this evaluation is the use of wireline logging and/or logging-while-drilling (LWD) measurements of the formation surrounding the borehole (referred to collectively as “logs” or “log measurements”). Typically, one or more logging tools are lowered into the borehole and the tool readings or measurement logs are recorded as the tools traverse the borehole. These measurement logs are used to infer the desired formation properties.
In recent years nuclear magnetic resonance (NMR) logging has become very important for purposes of formation evaluation and is one of the preferred methods for determining formation parameters. Improvements in the NMR logging tools, as well as advances in data analysis and interpretation allow log analysts to generate detailed reservoir description reports, including clay-bound and capillary-bound related porosity, estimates of the amounts of bound and free fluids, fluid types (i.e., oil, gas and water), permeability and other properties of interest.
NMR tools used in practical applications include, for example, the centralized MRIL® tool made by NUMAR Corporation, a Halliburton company, and the sidewall CMR tool made by Schlumberger. The MRIL® tool is described, for example, in U.S. Pat. No. 4,710,713 to Taicher et al. and in various other publications including: “Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination,” by Miller, Paltiel, Gillen, Granot and Bouton, SPE 20561, 65th Annual Technical Conference of the SPE, New Orleans, La., Sep. 23-26, 1990; “Improved Log Quality With a Dual-Frequency Pulsed NMR Tool,” by Chandler, Drack, Miller and Prammer, SPE 28365, 69th Annual Technical Conference of the SPE, New Orleans, La., Sep. 25-28, 1994. Certain details of the structure and the use of the MRIL® tool, as well as the interpretation of various measurement parameters are also discussed in U.S. Pat. Nos. 4,717,876; 4,717,877; 4,717,878; 5,212,447; 5,280,243; 5,309,098; 5,412,320; 5,517,115, 5,557,200; 5,696,448 and 5,936,405. The structure and operation of the Schlumberger CMR tool is described, for example, in U.S. Pat. Nos. 4,939,648; 5,055,787 and 5,055,788 and further in “Novel NMR Apparatus for Investigating an External Sample,” by Kleinberg, Sezginer and Griffin, J. Magn. Reson. 97, 466-485, 1992; and “An Improved NMR Tool Design for Faster Logging,” D. McKeon et al., SPWLA 40
th
Annual Logging Symposium, May-June 1999. The content of the above patents is hereby expressly incorporated by reference for all purposes, and all non-patent references are incorporated by reference for background.
NMR tools of the type discussed above generally measure the time for hydrogen nuclei present in the earth formation to realign their spin axes, and consequently their bulk magnetization, either with an externally applied magnetic field, or perpendicularly to the magnetic field, after momentary reorientation due to the application of specific radio frequency (RF) pulses. The externally applied magnetic field is typically provided by a magnet disposed in the tool. The spin axes of the hydrogen nuclei in the earth formation are, in the aggregate, caused to be aligned with the magnetic field induced in the earth formation by the magnet. The NMR tool includes an antenna positioned near the magnet and shaped so that a pulse of radio frequency (RF) power conducted through the antenna induces a magnetic field in the earth formation orthogonal to the field induced by the magnet. The RF pulse has a duration predetermined so that the spin axes of the hydrogen nuclei generally align themselves perpendicular both to the orthogonal magnetic field induced by the RF pulse and to the externally applied magnetic field. After the pulse ends, the nuclear magnetic moment of the hydrogen nuclei gradually relax, i.e., return to their alignment with the externally applied magnetic field; at the same time an antenna, which is typically the same as the one used by the initial pulse, is electrically connected to a receiver, which detects and measures voltages induced in the antenna by precessional rotation of the spin axes of the hydrogen nuclei.
An actual NMR measurement involves a plurality of pulses grouped into pulse sequences, most frequently of the type known in the art as Carr-Purcell-Meiboom-Gill (CMPG) pulsed spin echo sequences. As known in the art, each CPMG sequence consists of a 90-degree (i.e., &pgr;/2) pulse followed by a large number of 180-degree (i.e., &pgr;) pulses. The 90-degree pulse rotates the proton spins into the transverse plane and the 180-degree pulses generate a sequence of spin echoes by refocusing the transverse magnetization after each spin echo.
It should be apparent that it is important for the NMR measurements to register only signals that are generated by the formation of interest. However, non-formation signals—often referred to as “offset” or “ringing” signals—arise for a variety of reasons. For example, they may be caused by the high-sensitivity tool electronics (e.g., “offsets”), or may be due to magnetostrictive effects (e.g., “ringing”) that arise from interactions between pulsed magnetic fields and electronic or magnetic components in the tool. For example, when RF pulses are applied to the antenna, the magnet can become physically deformed by magnetostriction. After each RF pulse is turned off, the magnet tends to return to its original shape in a series of damped mechanical oscillations, known as “ringing.” Ringing induces voltages in the antenna, which can interfere with measurement of the voltages induced by the spin echoes.
A method known in the art for reducing the effect of offsets, ringing and possibly other non-formation signals is to make spin echo measurements in predetermined cycles. Typically, two pulse sequences of opposite phase are acquired to cancel electronic offsets and 180-degree ringing. The pair of pulse sequences is called a phase-alternated pair (PAP). PAP measurements are performed by making a second set of spin echo measurements starting with an original transverse alignment (90 degree) RF pulse, which is inverted in phase from the 90 degree pulse used to start the first set of spin echo measurements. Voltages induced in the antenna during the second set of spin echo measurements are inverted in polarity from the voltages induced in the first set of measurements. The signals from the second set of measurements can then be subtracted from the signals in the first set of measurements to substantially remove coherent noise, such as the ringing-induced signals. (For simplicity, in the following discussion “ringing” will be used as a catch-all term designating undesirable non-formation signals). Accordingly, in the “PAP method” successive echo-train signals are acquired from the formation that are alternately in-phase and anti-phase with respect to signals that are generated outside the formation; thus, a typical PAP simply comprises any adjacent pair of in-phase and anti-phase CPMG echo-trains. An implicit assumption in this operation is that the tool-related, non-formation signals in an echo-train can somehow be characterized, and that they change little, or even not at all, between successive echo-trains.
Mathematically, the PAP method can be illustrated as follows. Suppose that an individual spin echo train (CPMG
0
) can be characterized as a summation of
Akkurt Ridvan
Bouton John C.
Cherry Ron
Day Peter Ian
Galford James Elmer
Fetzner Tiffany A.
Halliburton Energy Service,s Inc.
Lefkowitz Edward
Pennie & Edmonds LLP
LandOfFree
Method and apparatus for improving the vertical resolution... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for improving the vertical resolution..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for improving the vertical resolution... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3062356