Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2002-03-27
2003-06-24
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
C324S300000
Reexamination Certificate
active
06583621
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a method and apparatus for making pulsed nuclear magnetic resonance (NMR) measurements of earth formations while drilling a borehole. More specifically, the invention is directed to a NMR measurement-while-drilling (MWD) tool having the required mechanical strength and measurement sensitivity, and a method and apparatus for monitoring the motion of the measuring tool in order to take this motion into account when processing NMR signals from the formation surrounding the borehole.
BACKGROUND AND SUMMARY OF THE INVENTION
Various methods exist for performing downhole measurements of petrophysical parameters in geologic formations. Pulsed NMR logging is among the most important of these methods, and was developed primarily for determining parameters such as formation porosity, fluid composition, the quantity of movable fluid, permeability, and others. Importantly, NMR measurements are environmentally safe and are unaffected by variations in the matrix mineralogy.
In a typical NMR measurement, a logging tool (measurement device) is lowered into a drilled borehole to measure properties of the geologic formation near the tool. Then, the tool is extracted at a known rate while continuously taking and recording measurements. At the end of the experiment, a log is generated showing the properties of the geologic formation along the length of the borehole. This invention relates primarily to an alternative measurement, in which pulsed NMR logging can be done while the borehole is being drilled. The advantages of the latter approach in terms of saving both time and costs are apparent. Yet, very little has been done so far in terms of developing practical NMR logging-while-drilling (LWD) or measurements-while-drilling (MWD) solutions. Two of the main stumbling blocks appear to be the very stringent requirements concerning the mechanical strength of the device, as well as problems associated with inaccuracies of the received signals due to motions of the tool. The present invention addresses successfully both issues and therefore is believed to make a significant contribution over the prior art.
In order to more fully appreciate the issues discussed in detail next, a brief overview of NMR methods for measuring characteristics of formations surrounding a borehole is presented first. The interested reader is directed, for example, to the following article: Bill Kenyon et al.,
Nuclear Magnetic Resonance Imaging—Technology for the
21
st Century
, OILFIELD REV., Autumn 1995, at 19, for a more comprehensive review. The Kenyon article is incorporated herein by reference.
Basically, in the field of NMR measurements of earth formations surrounding a borehole, a downhole static magnetic field is used to align the magnetic moment of spinning hydrogen (H) protons in the formation in a first direction, the direction of the static magnetic field. In order to establish thermal equilibrium, the hydrogen protons must be exposed to the polarizing field for a multiple of the characteristic relaxation time, T
1
. Then, the magnetic component of a radio frequency (RF) electromagnetic wave pulse, which is polarized in a second direction orthogonal to the static field, is used to tip the protons to align them in a third direction that is orthogonal to both the first and the second direction. This initial RF pulse is thus called a 90° pulse. Following the 90° pulse the protons in the formation begin to precess about the axis of the first direction. As a result, the protons produce an oscillating magnetic field. If an antenna is placed into the oscillating magnetic field, the oscillating magnetic field will produce an oscillating electric current in the antenna. Because the amplitude of the induced electrical signal is proportional to the porosity of the earth formation being measured, the signal may be calibrated to measure formation porosity. However, due to dephasing and irreversible molecular processes, the induced signal decays rapidly after the RF pulse is removed. Consequently, when the antenna is used both to transmit the RF pulses and to receive the induced NMR signal as in one embodiment of this invention, this first NMR signal may not be observable because the antenna electronics are still saturated from residual effects of the 90° RF pulse. Therefore, the NMR signal must be rebuilt as a spin echo, as discussed below, so that it may be measured.
Additionally, the behavioral characteristics of the protons after removal of the RF pulse can be used to garner information about other formation properties, such as pore size distribution and permeability. Immediately after the 90° RF pulse is turned off, the protons precess in phase. However, due to inhomogeneities in the static magnetic field and irreversible molecular processes, the protons begin to dephase, which causes the induced signal to decay. Nevertheless, the dephasing due to inhomogeneities in the static magnetic field is partially reversible. Therefore, by applying a 180° RF pulse, the instantaneous phases are reversed such that the protons gradually come back into phase, thus rebuilding the induced signal. The antenna can detect this signal because the time required for rebuilding the signal is long enough to allow the antenna electronics to recover from the 180° RF pulse. After the signal peaks at the time when the protons are back in phase, the signal will then begin to decay due to dephasing in the opposite direction. Thus, another 180° RF pulse is needed to again reverse the instantaneous phases and thereby rebuild the signal. By repeating a series of 180° RF pulses, the signal is periodically rebuilt after each dephasing, although each rebuilding is to a slightly lesser peak amplitude due to the irreversible molecular processes. Eventually, the irreversible processes prevail such that no further rephasing is possible and the signal dies out completely. Each rebuilding of the signal in this manner is called a spin echo, and the time constant associated with the decay of the spin echo amplitudes is called the transverse relaxation time, T
2
.
Because experiments have shown that T
2
is proportional to the pore size of the formation, calibration and decomposition of T
2
yields a measure of the formation's pore size distribution. Moreover, when combined with the porosity measurement, T
2
yields an estimate of the formation's permeability. As noted above, the NMR signal may also be calibrated to obtain other formation characteristics, such as free fluid volume, bound fluid volume, fluid identification, and diffusion coefficients. Because the drilling mode of operation using a preferred embodiment of this invention may allow for enough time to develop only one spin echo, or at most a few spin echos, the apparatus may achieve only porosity and limited T
2
measurements while drilling. However, the other types of NMR measurements discussed above are possible in non-drilling modes of operation, such as stationary tool, sliding or wiping tool.
From the preceding discussion it should be apparent that in order to enable accurate NMR measurements it is important that the same protons be tipped and rephased for each successive spin echo. Excessive movement of the tool in the borehole during NMR measurement can destroy the accuracy of the measurement by changing the location of the measurement volume, i.e., which protons in the formation are affected by the interaction of the static and RF pulse magnetic fields. Therefore, if the motion of the tool during NMR measurement is not known, which generally is the case in a logging-while-drilling environment, the NMR measurement may not be reliable.
The present inventors know of three issued patents directed to practical NMR measurements while drilling: U.S. Pat. No. 5,705,927 issued Jan. 6, 1998, to Sezginer et al.; U.S. Pat. No. 5,557,201 issued Sep. 17, 1996, to Kleinberg et al.; and U.S. Pat. No. 5,280,243 issued Jan. 18, 1994, to Miller. Of these references, the '201 patent more specifically shows how to improve the tool's susc
Bartel Roger P.
Chen Chen-Kang David
Dudley James H.
Goodman George D.
Jones Dale A.
Lefkowitz Edward
Numar Corporation
Pennie & Edmonds LLP
Shrivastav Brij B.
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