Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2001-03-01
2003-02-18
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
Reexamination Certificate
active
06522138
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention is related to the field of signal processing methods for oil well logging tools. More specifically, the present invention relates to signal processing methods for enhancing the resolution of nuclear magnetic resonance (NMR) measurements.
2. Background Art
Oil well logging tools include nuclear magnetic resonance (NMR) instruments. NMR instruments can be used to determine properties of earth formations, such as the fractional volume of pore space, the fractional volume of mobile fluid filling the pore space, and the porosity of earth formations. General background of NMR well logging is described in U.S. Pat. No. 6,140,817, assigned to the assignee hereof.
A typical NMR logging tool comprises a permanent magnet, which is used to align the nuclei of interest along its magnetic field, and an antenna, which is used to provide radio frequency (RF) pulses and to act as a receiver for the resulting resonance signals. The RF pulses transmitted through the antenna produce a magnetic field which realigns the nuclei in a different orientation. In a typical application, the RF pulse strength is controlled such that the nuclei are realigned onto a plane which is perpendicular to the direction of the magnetic field generated by the permanent magnet.
Such RF pulse is called a 90-degree pulse. Once in this perpendicular plane, the interactions between the static magnetic field and the nuclei cause these nuclei to precess around the static magnetic field axis with a characteristic frequency called Larmor frequency. The precessing of these nuclei produces signals that are detected by the antenna. In the absence of further perturbation, these nuclei will gradually return to their steady state, in which their spins are aligned with the static field generated by the permanent magnet. The process of this return to the steady state is referred to as the spin-lattice relaxation and is defined by a life time called T1. If the nuclei are kept in the perpendicular plane (e.g., by using a series of pulses as in CPMG sequence or a spin-lock sequence), the signals generated by these nuclei will decay exponentially by another mechanism, the spin-spin relaxation, which is defined by a different life time, T2. The T1 and T2 values reflect the chemical and physical properties of the observed nuclei. Therefore, they can provide information as to the properties and the environment of the nuclei.
The signals measured by nuclear magnetic resonance (NMR) logging tools typically arise from the selected nuclei present in the probed volume. Because hydrogen nuclei are the most abundant and easily detectable, most NMR logging tools are tuned to detect hydrogen resonance signals (form either water or hydrocarbons). These hydrogen nuclei have different dynamic properties (e.g., diffusion rate and rotation rate) that are dependent on their environments. The different dynamic properties of these nuclei manifest themselves in different nuclear spin relaxation times (i.e., spin-lattice relaxation time (T1) and spin—spin relaxation time (T2)). For example, Hydrogen nuclei in viscous oils have relatively short relaxation times whereas hydrogen nuclei in light oils possess relatively long relaxation times. Furthermore, the hydrogen nuclei in the free water typically have longer relaxation times than those in the bound water. Consequently, these differing NMR relaxation times can provide information on properties of the earth formations.
Most NMR logging tools measure the spin-spin relaxation times (T2) to derive the properties of the earth formations. The T2 relaxation is often measured from a train of spin-echoes that are generated with a series of pulses such as the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence or some variant of this. The CPMG pulse sequence is well known in the art. [See Meiboom, S., Gill, D., 1958, “Modified Spin Echo Method for Measuring Nuclear Relaxation Times,” Review of Scientific Instruments, 29, 688-91].
FIG. 2
shows a CPMG sequence, which is typically composed of a 90-degree pulse followed by a series of 180-degree pulses with a fixed delay time between them. The initial 90-degree pulse aligns the nuclear spins in the plane perpendicular to the magnetic field generated by the permanent magnet. The successive 180-degree pulses keep these spins roughly in this plane for the duration of the measurement The proportion of nuclear spins in the transverse plane decays mainly via the spin—spin relaxation (T2) pathway. Thus, one can derive the T2 relaxation time by analyzing the exponential decay of the spin-echo magnitude.
The fast on-and-off pulses used in the CPMG sequence generate acoustic waves in the antenna by an effect known as the “Lorenz force.” The antenna returns to its original shape in a series of damped mechanical oscillations in a process referred to as “magnetoacoustic ringing.” Ringing can induce large voltages in the antenna which interfere with the measurement of the voltages induced by the nuclear spins. In addition, the RF pulses can also cause magnetostriction in the permanent magnet, which is a deformation of the magnet. In the process of returning to its original shape, the magnet generates a series of damped mechanical oscillations in a process known as “magnetostrictive ringing.” In addition, the antenna/detectors often have inherent electronic offsets, which cause the baseline of the detected signals to deviate from the zero value. In order to cancel the electronic offsets and antenna ringing it is customary to combine two CPMG measurements of opposite phase. These pairwise-combined measurements (herein, measurements denote the detected signal amplitudes) are called phase-alternate-pair (PAP) echo trains and these constitute the datasets that are submitted to processing.
PAPs may be acquired successively or sequentially. In successive acquisition, each measurement consists of a complete PAP measurement, which includes two opposite-phased CPMG measurements, and each PAP measurement is independent of the preceding and the following PAP measurements. The sampling interval (with respect to wellbore length/depth) in successive acquisition is the distance traveled by the NMR logging tool during the acquisition of one complete PAP sequence. Thus, the axial resolution (herein, axial means along the axis of the wellbore) achievable by the successive acquisition method equals the antenna length plus the distance traveled by the NMR logging tool during the acquisition of one PAP sequence.
In contrast, with a sequential acquisition, every individual CPMG measurement contributes to two PAPs. In the first PAP a particular CPMG is combined with its preceding CPMG, which necessarily has opposite phase. In the second PAP it is combined with the following CPMG, which also has opposite phase. [Herein, measurements that are pairwise-combined from sequentially acquired data, like PAP, will be referred to as sequentially pairwise-combined measurements.] The sampling interval (with respect to the wellbore length/depth) in sequential acquisition is the distance traveled by the NMR logging tool during the acquisition of one CPMG sequence, rather than a PAP sequence (which contains two CPMG sequences). Thus, the sampling interval for a sequential acquisition is roughly half that of a successive acquisition. However, the axial resolution of a sequential PAP measurement is identical to that of a successive PAP because it takes two consecutive CPMG data sets to produce a PAP measurement in a sequential acquisition. Sequential PAP acquisition, as implemented on the CMR-PLUS NMR tool, is described in “An Improved NMR Tool Design for Faster Logging,” Society of Professional Well Log Analysts (SPWLA) 40
th
Annual Logging Symposium, paper CC (1999).
Although PAP acquisition provides a convenient means for removing electronic offsets and ringing, it degrades the axial resolution of the NMR measurement. This loss of resolution is particularly acute for non-overlapping CPMG measurements, as in the above-described sequen
Jeffery Brigitte L.
Lefkowitz Edward
McEnancy Kevin P.
Ryberg John J.
Schlumberger Technology Corporation
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