Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2001-06-28
2003-11-18
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
Reexamination Certificate
active
06650114
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related to methods for acquiring and processing nuclear magnetic resonance (NMR) measurements for determination of longitudinal and transverse relaxation times T
1
and T
2
. Specifically, the invention deals with methods for acquiring NMR measurements using CPMG sequences with a variable interecho spacing.
2. Description of the Related Art
Nuclear magnetic resonance is used in the oil industry, among others, and particularly in certain oil well logging tools. NMR instruments may be used for determining, among other things, the fractional volume of pore space and the fractional volume of mobile fluid filling the pore space of earth formations. Methods of using NMR measurements for determining the fractional volume of pore space and the fractional volume of mobile fluids are described, for example, in “Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination,” M. N. Miller et al., Society of Petroleum Engineers paper no. 20561, Richardson, Tex., 1990. Further description is provided in U.S. Pat. No. 5,585,720, of Edwards, issued Dec. 17, 1996 and having the same assignee as the present application, entitled “Signal Processing Method For Multiexponentially Decaying Signals And Applications To Nuclear Magnetic Resonance Well Logging Tools.” The disclosure of the Edwards patent is fully incorporated herein by reference.
Deriving accurate transverse relaxation time T
2
relaxation spectra from nuclear magnetic resonance (NMR) data from logging subterranean formations, or from cores from such formations, is critical to determining total and effective porosities, irreducible water saturations, and permeabilities of the formations. As discussed in Prammer (U.S. Pat. No. 6,005,389), the total porosity is the fractional volume of a rock that is occupied by fluids. The total porosity (measured by a density tool) includes clay bound water that typically has extremely short relaxation times, moveable water and hydrocarbons that have long relaxation times, and capillary bound water that has intermediate relaxation times. The effective porosity is defined as that portion of the pore volume containing fluids that are moveable, i.e., the total porosity minus the clay bound water. Accurate spectra are also essential to estimate T
2
cutoff values and to obtain coefficients for the film model or Spectral Bulk Volume Irreducible (SBVI) model. Effective porosities are typically summations of partial porosities; however, distortion of partial porosity distributions has been commonly observed for a variety of reasons. These reasons include poor signal-to-noise ratio (SNR), and poor resolution in the time domain of the NMR data.
U.S. Pat. No. 6,069,477 to Chen et al having the same assignee as the present application discusses the constituents of a fluid saturated rock and various porosities of interest. Referring to
FIG. 1
, the solid portion of the rock is made up of two components, the rock matrix and dry clay. The total porosity as measured by a density logging tool is the difference between the total volume and the solid portion. The total porosity includes clay-bound water, capillary bound water, movable water and hydrocarbons. The effective porosity, a quantity of interest to production engineers is the sum of the last three components and does not include the clay bound water.
The most common NMR log acquisition and core measurement method employs T
2
measurements using CPMG (Carr, Purcell, Meiboom and Gill) sequence, as taught by Meiboom and Gill in “Modified Spin-Echo Method for Measuring Nuclear Relaxation Time,” Rev. Sci. Instrum. 1958, 29, pp. 688-691. In this method, the echo data in any given echo train are collected at a fixed time interval, the interecho time (TE). Usually, a few hundred to a few thousand echoes are acquired to sample relaxation decay. However, for determination of CBW, echo sequences of as few as ten echos have been used.
Interecho time (TE), is one of the most important, controllable experimental parameters for CPMG measurements and can affect data interpretation. In logging operations using the MRIL® tool (made by Numar Corp.), TEs of 0.6 and 1.2 milliseconds (ms) are typically used to manipulate the relaxation decay data to include or exclude clay bound water (CBW) porosity.
Interpretation of NMR core or log data is often started by inverting the time-domain CPMG echo decay into a T
2
parameter domain distribution. In general, the T
2
of fluids in porous rocks depends on the pore-size distribution and the type and number of fluids saturating the pore system. Because of the heterogeneous nature of porous media, T
2
decays exhibit a multiexponential behavior. The basic equation describing the transverse relaxation of magnetization in fluid saturated porous media is
M
(
t
)
=
∫
T
2
min
T
2
max
P
(
T
2
)
ⅇ
-
t
/
T
2
ⅆ
T
2
(
1
)
where M is magnetization, and effects of diffusion in the presence of a magnetic field gradient have not been taken into consideration. Eq.(1) is based on the assumption that diffusion effects may be ignored. In a gradient magnetic field, diffusion causes atoms to move from their original positions to new ones which also causes these atoms to acquire different phase shifts compared to atoms that did not move. This contributes to a faster rate of relaxation.
The effect of field gradients is given by an equation of the form
1
T
2
=
1
T
2
bulk
+
1
T
2
surface
+
1
T
2
diffusion
(
2
)
where the first two terms on the right hand side are related to bulk relaxation and surface relaxation while the third term is related to the field gradient G by an equation of the form
T
2
diffusion
=
C
TE
2
·
G
2
·
D
(
3
)
where TE is the interecho spacing, C is a constant and D is the diffusivity of the fluid.
In CPMG measurements, the magnetization decay is recorded (sampled) at a fixed period, TE; thus, a finite number of echoes are obtained at equally spaced time intervals, t=n TE, where n is the index for the n-th echo. This may be denoted by
M
(
nTE
)
=
∫
T
2
min
T
2
max
P
(
T
2
)
ⅇ
-
n
·
TE
/
T
2
ⅆ
T
2
+
noise
(
4
)
A problem associated with conventional CPMG sequences is that the resolvability of the T
2
spectrum is not uniform. Short T
2
s are poorly resolved as only a few data points are affected by these components. Long T
2
s, on the other hand, are oversampled. In addition, due to limitations on availability of power, the number of pulses is limited: this has the undesirable effect of leading to poor resolution of short T
2
components because measurements have to be made over long time to resolve the slowly relaxing components. The actual selection of TE and number of pulses involves a tradeoff governed by the power availability and the desire for rapid acquisition to keep down rig costs.
As discussed in U.S. Pat. No. 6,069,477 to Chen et al, the contents of which are fully incorporated herein by reference, the effects of noise, sampling rate, and the ill-conditioning of inversion and regularization are to smear (broaden) the estimated T
2
distribution. In addition, because of the non-orthogonality of multi-exponential signals, CBW signals could be shifted to higher T
2
regions if the T
2
fitting region is limited or if the regularization is excessive. This distortion is not easily rectified; even adding more bins with short T
2
does not reduce the distortion of the T
2
spectra.
Chen et al teaches the use of CPMG sequences with two different values of TE (0.6 ms and 1.2 ms). A time domain correction is used to filter out the contribution of the fast relaxing T
2
components in the TE=1.2 ms echo train. High S/N echo data with sampling time TE=0.6 ms are used to obtain the CBW T
2
distribution. These data are then used to reconstruct CBW contributions to the time domain early echoes of the conventiona
Blanz Martin
Kruspe Thomas
Rottengatter Peter
Thern Holger F.
Arana Louis
Baker Hughes Incorporated
Fetzner Tiffany A.
Madan Mossman & Sriram P.C.
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