Electricity: measuring and testing – Particle precession resonance – Using well logging device
Reexamination Certificate
2000-01-14
2002-05-21
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using well logging device
C324S300000
Reexamination Certificate
active
06392409
ABSTRACT:
FIELD OF THE INVENTION
The invention is in the field of determination of petrophysical properties, including oil saturation, of medium using data from a Nuclear Magnetic Resonance (NMR) tool.
BACKGROUND OF THE INVENTION
A variety of techniques have been utilized in determining the presence and in estimating quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, porosity, fluid content, and permeability of the rock formation surrounding the wellbore drilled for recovering hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the well bores have been drilled. More recently, wellbores have been logged while drilling of the wellbores, which is referred to as measurement-while-drilling (“MWD”) or logging-while-drilling (“LWD”). Measurements have also been made when tripping a drillstring out of a wellbore: this is called measurement-while-tripping (“MWT”).
One recently evolving technique involves utilizing Nuclear Magnetic Resonance (NMR) logging tools and methods for determining, among other things porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the fluids in the geological formations in the vicinity of the wellbore so that certain parameters such as spin density, longitudinal relaxation time (generally referred to in the art as “T
1
”), and transverse relaxation time (generally referred to as “T
2
”) of the geological formations can be estimated. From such measurements, porosity, permeability, and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
A typical NMR tool generates a static magnetic field B
0
in the vicinity of the wellbore, and an oscillating field B
1
in a direction perpendicular to B
0
. This oscillating field is usually applied in the form of short duration pulses. The purpose of the B
0
field is to polarize the magnetic moments of nuclei parallel to the static field and the purpose of the B
1
field is to rotate the magnetic moments by an angle &thgr; controlled by the width t
p
and the amplitude B
1
of the oscillating pulse. With the variation of the number of pulses, pulse duration, and pulse intervals, various pulse sequences can be designed to manipulate the magnetic moment, so that different aspects of the NMR properties can be obtained. For NMR logging, the most common sequence is the Carr-Purcell-Meiboom-Gill (“CPMG”) sequence that can be expressed as
TW−90−(t−180−t−echo)
n
After being tipped by 90°, the magnetic moment precesses around the static field at a particular frequency known as the Larmor frequency &ohgr;
0
, given by &ohgr;
0
=&ggr;B
0
, where B
0
is the field strength of the static magnetic field and &ggr; is the gyromagnetic ratio. At the same time, the magnetic moments return to the equilibrium direction (i.e., aligned with the static field) according to a decay time known as the “spin-lattice relaxation time” or T
1
. Inhomogeneities of the B
0
field result in dephasing of the magnetic moments and to remedy this, a 180° pulse is included in the sequence to refocus the magnetic moments. This gives a sequence of n echo signals.
U.S. Pat. No. 5,023,551 issued to Kleinberg discloses an NMR pulse sequence that has an NMR pulse sequence for use in the borehole environment which combines a modified inversion recovery (FIR) pulse sequence with a series of more than two, and typically hundreds, of CPMG pulses according to
[W
i
−180−TW
i
−90−(t−180−t−echo)
j
]
i
where j−1,2, . . . J and J is the number of echoes collected in a single Carr-Purcell-Meiboom-Gill (CPMG) sequence, where i=1, . . . I and I is the number of waiting times used in the pulse sequence, where W
i
are the recovery times, TW
i
are the wait times before a CPMG sequence, and where t is the spacing between the alternating 180° pulses and the echo signals. Although a conceptually valid approach for obtaining T
1
information, this method is extremely difficult to implement in wireline, MWD, LWD or MWT applications because of the long wait time that is required to acquire data with the different TWs.
Proton NMR measurement is typically performed for well logging applications since hydrogen is abundant in reservoir fluids. T2 is very short in solids, but relatively long in liquids and gases, so that the proton NMR signal from the solid rock decays quickly and only the signal from fluids in the rock pores in the region of interest is seen. This signal may arise from hydrogen in hydrocarbon or water within the pores of the formation. The local environment of the hydrogen influences the measured T2 or “spin-spin” relaxation. For example, capillary bound fluid has a shorter T2 than fluid in the center of a pore, the so-called “free fluid.” In this way, the NMR tool can be used advantageously to distinguish between producible fluid and non-producible fluid.
The NMR echo signals provide information about fluid and rock properties. Depending upon the goal of the investigation, various NMR measurement techniques can be used to obtain different petrophysical properties (e.g., partial and total porosities) or to discern multiphase fluids for hydrocarbon typing purposes. The different NMR acquisition techniques are characterized by differences in pulse timing sequences as well as repetition times between measurements. In addition, in wireline applications, multiple runs of NMR acquisition sequences with different parameters can be combined to enhance the analysis of the desired petrophysical information. However, in measurement-while-drilling applications or in measurement-while-tripping applications, it is not possible to make multiple runs, so that all the desired information must be obtained at one time while the borehole is being drilled or tripped.
The longitudinal relaxation time, T
1
, of oil phase carries important petrophysical information that is critical to hydrocarbon volumetrics, viscosity, and hydrocarbon typing analysis from NMR logs. The ratio of T
1
/T
2
is a potentially useful information revealing in-situ reservoir fluid characteristics. While T
2
can be estimated relatively easily, the estimation of T
1
is challenging particularly when reservoir fluids contain more than one fluid, e.g., oil and water, or gas and water system.
Several methods to identify and quantify hydrocarbon reservoirs have been employed during the last few years utilizing the effect of different wait times on the measured NMR signal. Depending upon the fluid properties, the wait time (TW) determines the amount of the polarization that contributes to the measured signal. For example, Akkurt et.al. disclose a Differential Spectrum Method (DSM) based upon this effect in their paper “NMR Logging of Natural Gas Reservoirs” presented at the 36
th
Annual Meeting of the Society of Professional and Well Log Analysts (SPWLA) in 1995. This approach takes advantage of the T
1
difference between hydrocarbons and water at reservoir conditions, and the short wait time (TWS) is chosen such that the fast relaxing water components are approximately fully polarized while the hydrocarbon components are not fully polarized. On the other hand, the long wait time (TWL) is typically chosen such that the hydrocarbon component is also nearly fully polarized. However, logging speed and overall signal to noise ratio (SNR) often dictates the selection of TWL to be less than optimal. Further, the TWL selected prior to acquisition may not be sufficiently long if the oil is lighter than expected. The T
1
information is critical to correct the polarization effect after the log is acquired.
Analysis of dual wait time data for T
1
estimation remains a particularly challenging task. In the prior art, a critical first step in the data analysis is t
Baker Hughes Incorporated
Madan Mossman & Sriram P.C.
Patidar Jay
Shrivastav Brij B.
LandOfFree
Determination of T1 relaxation time from multiple wait time... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Determination of T1 relaxation time from multiple wait time..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Determination of T1 relaxation time from multiple wait time... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2859996