Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
1997-03-19
2001-06-05
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
C324S307000
Reexamination Certificate
active
06242912
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to nuclear magnetic resonance (NMR) logging and is directed more specifically to a system and method for detecting the presence and estimating the quantity of gaseous and liquid hydrocarbons in the near wellbore zore.
Petrophysical parameters of a geologic formation which are typically used to determine whether the formation will produce viable amounts of hydrocarbons include the formation porosity PHI, fluid saturation S, the volume of the formation, and its permeability K. Formation porosity is the pore volume per unit volume of formation; it is the fraction of the total volume of a sample that is occupied by pores or voids. The saturation S of a formation is the fraction of a its pore volume occupied by the fluid of interest. Thus, water saturation S
W
is the fraction of the pore volume which contains water. The water saturation of a formation can vary from 100% to a small value which cannot be displaced by oil, and is referred to as irreducible water saturation S
Wirr
. For practical purposes it can be assumed that the oil or hydrocarbon saturation of the formation S
O
is equal to S
O
=1−S
W
. Obviously, if the formation's pore space is completely filled with water, that is if S
W
=1, such a formation is of no interest for the purposes of an oil search. On the other hand, if the formation is at S
Wirr
it will produce all hydrocarbons and no water. Finally, the permeability K of a formation is a measure of the ease with which fluids can flow through the formation, i.e., its producibility.
Nuclear magnetic resonance (NMR) logging is among the most important methods which have been developed to determine these and other parameters of interest for a geologic formation and clearly has the potential to become the measurement of choice for determining formation porosity. At least in part this is due to the fact that unlike nuclear porosity logs, the NMR measurement is environmentally safe and is unaffected by variations in matrix mineralogy. The NMR logging method is based on the observation that when an assembly of magnetic moments, such as those of hydrogen nuclei, are exposed to a static magnetic field they tend to align along the direction of the magnetic field, resulting in bulk magnetization. The rate at which equilibrium is established in such bulk magnetization upon provision of a static magnetic field is characterized by the parameter T
1
, known as the spin-lattice relaxation time. Another related and frequently used NMR logging parameter is the so called spin-spin relaxation time constant T
2
(also known as transverse relaxation time) which is an expression of the relaxation due to non-homogeneities in the local magnetic field over the sensing volume of the logging tool.
Another measurement parameter used in NMR well logging is the formation diffusion D. Generally, diffusion refers to the motion of atoms in a gaseous or liquid state due to their thermal energy. The diffusion parameter D is dependent on the pore sizes of the formation and offers much promise as a separate permeability indicator. In an uniform magnetic field, diffusion has little effect on the decay rate of the measured NMR echoes. In a gradient magnetic field, however, diffusion causes atoms to move from their original positions to new ones, which moves also cause these atoms to acquire a different phase shifts compared to atoms that did not move, and will thus contribute to a faster rate of relaxation. Therefore, in a gradient magnetic field diffusion is a logging parameter which can provide independent information about the structure of the geologic formation of interest, the properties of the fluids in it, and their interaction.
It has been observed that the mechanisms which determine the values of T
1
, T
2
and D depend on the molecular dynamics of the sample being tested. In bulk volume liquids, typically found in large pores of the formation, molecular dynamics is a function of molecular size and inter-molecular interactions which are different for each fluid. Thus, water, gas and different types of oil each have different T
1
, T
2
and D values. On the other hand, molecular dynamics in a heterogeneous media, such as a porous solid which contains liquid in its pores, differs significantly from the dynamics of the bulk liquid and generally depends on the mechanism of interaction between the liquid and the pores of the solid media. It will thus be appreciated that a correct interpretation of the measurement parameters T
1
, T
2
and D can provide valuable information relating to the types of fluids involved, the structure of the formation and other well logging parameters of interest.
A major barrier to using NMR logging alone for determination of porosity and other parameters of interest in the past has been the widespread belief that a near-wellbore NMR measurement cannot detect hydrocarbon gases. Failure to recognize such gases may result in their contribution being misinterpreted as bound fluid, which mistake may in turn result in excessively high irreducible water saturations and correspondingly incorrect permeability estimates. It has recently been found, however, that the NMR properties of gas are in fact quite different from those of water and oil under typical reservoir conditions and thus can be used to quantify the gas phase in a reservoir. More specifically, the Magnetic Resonance Imaging Log (MRIL®) tools of NUMAR Corporation have registered the gas effect as distortion in the bound volume irreducible (BVI) and/or free fluid index (FFI) measurements.
In a recent paper, entitles “NMR Logging of Natural Gas Reservoirs,” paper N, presented at the 36
th
Annual SPWLA Symposium, Paris, Jun. 26-29, 1995, Akkurt, R. et al. have shown one approach of using the capabilities provided by NUMAR's MRIL® tool for detection of gas. The content of the Akkurt et al. paper is incorporated herein for all purposes. In this paper, the authors point out that NMR signals from gas protons are detectable, and derive T
1
relaxation and diffusion properties of methane-dominated natural gas mixtures at typical reservoir conditions. The magnetic field gradient of the MRIL® is used to separate and to quantify water, oil and gas saturations based solely on NMR data.
The results in the Akkurt paper are based on the NUMAR MRIL-C tool, the output of which is used to obtain T
2
spectra. T
2
spectra are extracted from the raw CPMG echo trains by breaking the total NMR signal M(t) into N components, called bins, according to the formula:
M
(
t
)
=
∑
i
=
1
N
a
i
exp
(
-
t
/
T
2
)
where ai is the porosity associated with the i-th bin. Each bin is characterized by a fixed center transverse relaxation time T
2i
. The total NMR porosity is then determined as the sum of the porosities a
i
in all bins. The T
2
spectrum model is discussed in detail, for example, in Prammer, M. G., “NMR Pore Size Distributions and Permeability at the Well Site,” paper SPE 28368, presented at the 69-th Annual Technical Conference and Exhibition, Society of Petroleum Engineers, New Orleans, Sep. 25-28, 1994, the content of which is expressly incorporated herein for all purposes.
On the basis of the T
2
spectra, two specific methods for as measurements are proposed in the Akkurt paper and will be considered briefly next to provide relevant background information. The first method is entitled “differential pectrum method” (DSM). The DSM is based on the observation that often light oil and natural gas exhibit distinctly separated T
2
measurements in the presence of a magnetic field gradient, even though they may have overlapping T
1
measurement values. Also, it has been observed that brine and water have distinctly different T
1
measurements, even though their D
0
values may overlap. The DSM makes use of these observations and is illustrated in
FIG. 1
in a specific example for a sandstone reservoir containing brine, light oil and gas. According to the Akkurt et al. paper, two separate logging passes are made with different
Coates George R.
Mardon Duncan
Miller Melvin N.
Prammer Manfred G.
Arana Louis
Numar Corporation
Pennie & Edmonds LLP
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