Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2001-06-15
2003-02-25
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
C324S300000
Reexamination Certificate
active
06525534
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns nuclear magnetic resonance (NMR) logging and more specifically relates to a system and methods for NMR data acquisition and processing, which improve the vertical resolution and/or logging speed at which NMR logs can be 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; 5,936,405; 6,005,389; 6,03,164; 6,051,973; 6,107,796 and 6,111,408. 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 T
2
logging is different from most other logging methods in that the measurement is not instantaneous. Each measurement cycle, including the wait time needed for polarization, can take several seconds. Furthermore, as discussed below, several cycles usually have to be stacked to achieve adequate signal-to-noise ratio (SNR).
If a cycle takes T seconds to complete, and N cycles must be stacked, the vertical resolution of a measurement is proportional to vNT, where v is the logging speed. Clearly, the longer the cycle times and faster the logging speeds, the worse the vertical resolution. Therefore, an ever-present challenge in NMR logging is to design tools that can log faster, while retaining acceptable vertical resolution. Overcoming this challenge is an extremely important task. Several innovations towards faster logging have been put into practice over the past several years.
One such innovation was the introduction of multi-frequency logging in the early 1990s. (With reference to the listing in the back of this section, see, for example, Chandler et al, 1994). The benefit of multi-frequency logging is that the tools acquire data simultaneously over several frequencies, and the additional SNR available can be used to speed up logging as well as to obtain higher quality results. The state-of-the-art in multi-frequency logging is the MRIL®-Prime tool by Numar Corporation, which currently can operate on 9 frequencies.
Another innovation was the introduction of simultaneous acquisition of partially and fully polarized echo trains with different SNR. (See, for example, Prammer et al, 1996). Proper total porosity measurements require: (1) a short interecho time T
e
to sample fast decays, (2) high SNR to reduce the uncertainty in the estimation of fast decays, (3) long sampling time (N
e
T
e
where N
e
is the number of echoes) for adequate sampling of longer decays. It is practically impossible to achieve all these objectives with a unique wait time T
w
, T
e
and N
e
combination; while maintaining acceptable logging speeds and vertical resolution. Therefore, a good solution is to optimize the acquisition by mixing partially and fully recovered data with different measurement parameters T
w
, T
e
, N
e
and desired SNR. Another closely related innovation was the concept of simultaneous-inversion, where data acquired with different measurement parameters is inverted simultaneously using forward models that properly account for the differences in fluid NMR properties, acquisition parameters and noise levels. (See, Looyestijn, 1996, and Dunn, et al. 1998).
Yet another innovation was the use of pre-polarization (See, for example, Akkurt, 1990). In this approach the cycle time for each measurement is shortened, by placing static magnets above the antenna section to realize additional polarization during tool motion. Current generation NMR tools generally contain pre-polarization sections, allowing overall faster logging.
However, in the search for faster NMR logging a problem still exists because of the need to remove coherent non-formation signals. Such removal has been done traditionally using Phase Alternated Pair Stacking (PAPS). PAPS is the most widely used method in NMR logging to remove coherent non-formation signals, typically referred to as bias (or ringing). Since bias is a frequency dependent phenomenon, the two CPMGs making up a phase alternated pair must be acquired at the same frequency. This requirement places an undesirable upper limit to the vertical resolution of NMR logs. The rationale for the use of PAPS in the prior art is described below.
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., instrumentation biases and 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 a
Akkurt Ridvan
Cherry Ronald E.
Halliburton Energy Service,s Inc.
Lefkowitz Edward
Shrivastav Brij B.
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