Measuring and testing – Borehole or drilling – Pressure measurement
Reexamination Certificate
2001-05-30
2002-05-28
Williams, Hezron (Department: 2856)
Measuring and testing
Borehole or drilling
Pressure measurement
C166S269000
Reexamination Certificate
active
06393906
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of oil and gas exploration. More particularly, this invention relates to the use of aqueous and oil inclusions in rocks to reconstruct paleo-pressure history to evaluate hydrocarbon potential of sedimentary basins.
BACKGROUND OF THE INVENTION
A critical component of petroleum exploration involves characterizing the risk associated with the timing of hydrocarbon migration and trap fill, reservoir pressure history, and timing of seal failure. This requires reliable estimates of paleo-pressure. Paleo-pressure is the ancient pressure of the sedimentary basin (i.e., the pressure during formation of the sedimentary basin). Paleo-pressure estimation has proven to be difficult to quantify and there is a need for an accurate and low-cost method of measuring paleopressure directly from geological samples.
The only known technique for directly measuring geological paleopressure utilizes fluid inclusions in rocks. Fluid inclusions are microscopic samples of paleo-fluids that are trapped and sealed within cavities in minerals. These inclusions preserve the pressure at which they were trapped. Several methods have been used to obtain trapping pressure from fluid inclusions in petroleum systems. For example, pressure from individual methane-carbon dioxide (CH
4
—CO
2
) gas inclusions can be obtained by using Raman spectral parameters (Seitz, J. C., Pasteris, J. D., and Chou I-M., 1996. Raman spectroscopic characterization of gas mixtures. II. Quantitative composition and pressure determination of the CO
2
—CH
4
system. American Journal of Sciences, 296, 577-600). The underlying principle of Raman spectral analysis is when monochromatic (i.e. substantially single photon energy) light, such as emitted from a laser traverses a medium (e.g. gas, liquid, or solid) the majority of the scattered light will remain at the incident photon energy. However, a small proportion of the scattered light will be at changed frequencies, above and below the incident photon energy, and this is referred to as anti-Stokes and Stokes Raman scattering, respectively. The energy increment and decrement for the anti-Stokes and Stokes scattering, respectively correspond to the vibrations of the molecules of the medium that produce the scattered photons.
By measuring the energy decrement of the scattered light relative to the incident light, Raman spectroscopy is a tool to probe molecular vibrations. As Raman spectroscopy may be carried out within the ultra-violet (UV) and visible regions of the spectrum, the incident laser beam can be focused by normal light optics, i.e. microscope objectives, to give spatial resolution in the region of 1 micrometer. It therefore provides a non-destructive means of analyzing the molecular species of very small objects, including fluid inclusions in minerals. This technique has proven to be particularly successful for the analysis of species such as carbon dioxide (CO
2
), carbon monoxide (CO), methane (CH
4
), ethane (C
2
H
6
), nitrogen (N
2
), water (H
2
O), hydrogen sulfide (H
2
S), hydrogen sulfite (H
2
SO
3
), oxygen (O
2
), and sulfates (SO
4
−2
).
Laser photon energies in the range of 50000 cm
−1
to 9000 cm
−1
may be used for excitation. The Raman scattered radiation is detected over a range of 150 cm
−1
to 4600 cm
−1
below the laser excitation energy. Preferably, the inclusion to be analyzed is illuminated through a microscope and a portion of the backscattered laser excitation and Raman emission are collected and collimated by the objective. Other angles between incident and scattered laser light may be used. Filters separate the laser and Raman components and the latter is recorded by a spectrometer.
Inclusion trapping pressure from Seitz et al. method can be obtained from the peak positions of the CH
4
and CO
2
Raman bands obtained from gas inclusions. However, this application is limited to pure CH
4
, CO
2
and/or CH
4
—CO
2
-bearing inclusions, which are rare for petroleum systems. Raman spectroscopy was also used to obtain minimum trapping pressure from synthetic CH
4
-bearing aqueous fluid inclusions (Dubessy, J., Pironon, J., Lamb, W., McShane, C., Popp, C., Thiery, R., 1998. PACROFI VII, Jun. 1-4, 1998, 27, Leng, J., Sharma, A., Bodnar, R. J., Pottorf, R. J., Vityk, M. O., 1998. Quantitative Analysis of Synthetic Fluid Inclusions in the H
2
O—CH
4
System Using Raman Spectroscopy. PACROFI VII, Jun. 1-4, 1998, 41).
The method of Dubessy et al. uses microthermometry to determine the salinity of the aqueous phase, and Raman spectra of methane and water and the thermodynamic properties of the water-methane (H
2
O—CH
4
), no salt, system to model the inclusion methane content. Microthermometry is the observation of the temperatures of phase changes within fluid inclusions as they are cooled or heated on a special microscope stage. This method can provide two basic kinds of information: inclusion composition and temperature of entrapment. The temperature of ice melting tells us something about chemistry of inclusion fluid and the temperature of the inclusion bubble (gas phase) disappearance (homogenization temperature) is a minimum or true value for the temperature at which the inclusion was trapped. The inclusion composition and homogenization temperatures are used to obtain the pressure at the bubble point. Bubble point pressure is the pressure at inclusion saturation or saturation pressure. However, the Dubessy et al. method has a number of limitations.
One of these limitations is that this method can not be used to obtain the true trapping pressure. Reliable pressure measurements from fluid inclusions are entirely dependent on the specific type of fluid inclusions measured. Some fluid inclusions can be used to determine only a minimum pressure of entrapment, some may be used to determine the true pressure of entrapment. Fluid inclusions that can be used to determine the true pressure of entrapment need to be recognized in the rock based on a unique set of petrographic and microthermometric criteria. The Dubessy et al. method provides no such criteria.
Additional limitations of the Dubessy et al. method include
1
) lack of calibration with application to natural petroleum systems of interest which contain salt and generally homogenize at temperatures below 300° C., and 2) the necessity for additional calibration for the effect of salinity. Application of the method to determine the potential for hydrocarbons is not discussed.
The work of Leng et al. reports the Raman band area ratio for synthetic CH
4
- water (no salt) inclusions and discusses neither the determination of the formation pressure from the Raman data nor the use of the pressure determination for hydrocarbon exploration applications.
The bubble point pressure or saturation pressure can be obtained from individual oil inclusions by using confocal scanning laser microscopy coupled with microthermometry (Pironon, J., Canals, M., Dubessy, J., Walgenwitz, F., Laplace-Builhe, C., 1998. Volumetric Reconstruction of Individual Fluid Inclusions By Confocal Scanning Laser Microscopy. Eur. J. Mineral. 10, 1143-1150, Aplin, A. C., Macleod, G., Larter, S. R., Sorensen, H., Booth, T, 1999. Combined Use of Confocal Laser Scanning Microscopy and PVT Simulation For Estimating the Composition and Physical Properties of Petroleum in Fluid Inclusions. Mar. Petrol. Geol. 16, 97-100, Aplin, A. C., Larter, S. R., Bigge, M. A., Macleod, G., Swabrick, R. E., Grunberger, D., 2000, Confocal microscopy of fluid inclusions reveals fluid pressure histories of sediments and an unexpected origin of gas condensate. Geology, no. 11, 1047-1050). This method involves generation of three-dimensional images of an individual oil inclusion by using confocal scanning laser microscopy and calculation of the volumetric ratio of oil to gas within the inclusion. Using commercial software (Aplin et al., 1999), these data along with inclusion homogenization temperature (Th) are used to reconstruct the bubble point for the inclusion oil and to obtain
Bodnar Robert J.
Chimenti Robert J.
Kalamaras Patricia H.
Leng Jing
Pottorf Robert H.
ExxonMobil Upstream Research Company
Katz Gary P.
Politzer Jay
Williams Hezron
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