Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2002-04-02
2004-11-30
Shrivastav, Brij B. (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
C324S300000
Reexamination Certificate
active
06825658
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to nuclear magnetic resonance (NMR) borehole measurements and more particularly to fluid typing based on separation of signals from different fluids using user-adjusted measurement parameters.
BACKGROUND
The ability to differentiate between individual fluid types is one of the main concerns in the examination of the petrophysical properties of a geologic formation. For example, in the search for oil it is important to separate signals due to producible hydrocarbons from the signal contribution of brine, which is a fluid phase of little interest. Extremely valuable is also the capability to distinguish among different fluid types, in particular, among clay-bound water, capillary-bound water, movable water, gas, light oil, medium oil, and heavy oil. However, so far no approach has been advanced to reliably perform such fluid typing in all cases.
In evaluating the hydrocarbon production potential of a subsurface formation, the formation is described in terms of a set of “petrophysical properties.” Such properties may include: (1) the lithology or the rock type, e.g., amount of sand, shale, limestone, or more detailed mineralogical description, (2) the porosity or fraction of the rock that is void or pore space, (3) the fluid saturations or fractions of the pore space occupied by oil, water and gas, and others. Various methods exist for performing measurements of petrophysical properties in a geologic formation. Nuclear magnetic resonance (NMR) logging, which is the focus of this invention, is among the best methods that have been developed for a rapid determination of such properties, which include formation porosity, composition of the formation fluid, the quantity of movable fluid and permeability, among others. At least in part this is due to the fact that NMR measurements are environmentally safe. Importantly, NMR logs differ from conventional neutron, density, sonic, and resistivity logs in that NMR logs are essentially unaffected by matrix mineralogy, i.e., provide information only on formation fluids. The reason is that NMR signals from the matrix decay too quickly to be detected by the current generation NMR logging tools. However, such tools are capable of directly measuring rock porosity filled with the fluids. Even more important is the unique capability of NMR tools, such as NUMAR's MRIL® tool, to distinguish among different fluid types, in particular, clay-bound water, capillary-bound water, movable water, gas, light oil, medium oil, and heavy oil by applying different sets of user-adjusted measurement parameters.
To better appreciate how NMR logging can be used for fluid signal separation, it is first necessary to briefly examine the type of parameters that can be measured using NMR techniques. NMR logging 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 spin-spin relaxation time 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. Both relaxation times provide information about the formation porosity, the composition and quantity of the formation fluid, and others.
Another measurement parameter obtained in NMR logging is the diffusion of fluids in the formation. Generally, diffusion refers to the motion of atoms in a gaseous or liquid state due to their thermal energy. Self-diffusion is inversely related to the viscosity of the fluid, which is a parameter of considerable importance in borehole surveys. In a 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 different phase shifts compared to atoms that did not move. This effect contributes to a faster rate of relaxation in a gradient magnetic field.
NMR measurements of these and other parameters of the geologic formation can be done using, for example, the centralized MRIL® tool made by NUMAR, 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, Millen, 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. 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 and 5,696,448, all of which are commonly owned by the assignee of the present invention. The Schlumberger CMR tool is described, for example, in U.S. Pat. Nos. 5,055,787 and 5,055,788 to Kleinberg et al. and further in “Novel NMR Apparatus for Investigating an External Sample,” by Kleinberg, Sezginer and Griffin, J. Magn. Reson. 97, 466-485, 1992. The content of the above patents is hereby incorporated by reference; the content of the publications is incorporated by reference for background.
It has been observed that the mechanisms determining the measured values of T
1
, T
2
and diffusion depend on the molecular dynamics of the formation being tested and on the types of fluids present. Thus, in bulk volume liquids, which typically are found in large pores of the formation, molecular dynamics is a function of both molecular size and inter-molecular interactions, which are different for each fluid. Water, gas and different types of oil each have different T
1
, T
2
and diffusivity values. On the other hand, molecular dynamics in a heterogeneous media, such as a porous solid that 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 measured signals can provide valuable information relating to the types of fluids involved, the structure of the formation and other well-logging parameters of interest.
It should be clear that the quality of the fluid typing depends on the magnitudes of the contrasts between measurement signals from different fluid types. Generally, as the contrasts increase, the quality of the typing improves. Table 1 below shows the ranges of the characteristic parameters for brine, gas, and oil measured by an MRIL®—C tool under typical reservoir conditions (i.e., pressure (P) from 2,000 to 10,000 psi, and temperature (T) from 100 to 350° F.). Table 2 shows typical parameter values for a Gulf of Mexico sandstone reservoir. The information in the tables clearly reveals a broad distribution for T
1
, T
2
, D, and hydrogen index (HI) that is used in accordance with the present invention in fluid typing.
TABLE 1
Ranges of the characteristic parameters of water, gas,
and oil measured with an MRIL ®-C tool under typical
reservoir conditions
Free Water
Bound Water
Gas
Oil
Hydrogen Index (HI)
~1
~1
<1
<~1
Diffusion (D)
medium
very low
very high
low
Relaxation Time (T
1
)
medium
short
long
long
Relaxation Time (T
2
)
medium
short
short
long
TABLE 2
Typical values of characteristic parameters for fluids in a G
Coates George Richard
Hou Lei Bob
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