Radiant energy – Geological testing or irradiation – Well testing apparatus and methods
Reexamination Certificate
1999-02-23
2001-08-14
Hannaher, Constantine (Department: 2878)
Radiant energy
Geological testing or irradiation
Well testing apparatus and methods
C250S255000, C250S256000
Reexamination Certificate
active
06274865
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the analysis of downhole fluids in a geological formation. More particularly, the present invention relates to apparatus and methods for downhole optical analysis of formation fluid contaminated by oil based mud filtrate.
BACKGROUND OF THE INVENTION
Schlumberger Technology Corporation, the assignee of this application, has provided a commercially successful borehole tool, the MDT (a trademark of Schlumberger), which extracts and analyzes a flow stream of fluid from a formation in a manner substantially as set forth in co-owned U.S. Pat. Nos. 3,859,851 and 3,780,575 to Urbanosky. The analyzer module of the MDT, the OFA (a trademark of Schlumberger) determines the identity of the fluids in the MDT flow stream. Mullins, in co-owned U.S. Pat. No. 5,266,800, teaches that by monitoring optical absorption spectrum of the fluid samples obtained over time, a real time determination can be made as to whether a formation oil is being obtained as opposed to oil based mud (OBM) filtrate. In particular, the Safinya patent discloses a borehole apparatus which includes a testing chamber, means for directing a sample of fluid into the chamber, a light source preferably emitting near infrared rays and visible light, a spectral detector, a data base means, and a processing means. Fluids drawn from the formation into the testing chamber are analyzed by directing the light at the fluids, detecting the spectrum of the transmitted and/or backscattered light, and processing the information accordingly. Prior art equipment is shown in
FIGS. 1A-1C
.
Because different fluid samples absorb energy differently, the fraction of incident light absorbed per unit of path length in the sample depends on the composition of the sample and the wavelength of the light. Thus, the amount of absorption as a function of the wavelength of the light, hereinafter referred to as the “absorption spectrum”, has been used in the past as an indicator of the composition of the sample. For example Safinya, in U.S. Pat. No. 4,994,671, teaches, among other things, that the absorption spectrum in the wavelength range of 0.3 to 2.5 microns can be used to analyze the composition of a fluid containing oil. The disclosed technique fits a plurality of data base spectra related to a plurality of oils and to water, etc., to the obtained absorption spectrum in order to determine the amounts of different oils and water that are present in the sample.
When the desired fluid is identified as flowing in the MDT, sample capture can begin and formation oil can be properly analyzed and quantified by type. Samples are used to determined important fluid properties such as the gas-oil ratio (GOR), saturation pressure, wax and asphaltene precipitation tendency, fluid densities and fluid composition. These parameters help set various production parameters and also relate to the economic value of the reserve.
Prevalent use of oil based mud (OBM) in some markets has resulted in a premium placed on discriminating between OBM filtrate and crude oil. A variety of oils are used as the base for OBM such as diesel, synthetics such as C
16
and C
18
monoalkenes, and even crude oil. Due to the variety of base fluids and their overlapping properties with crude oils, it is difficult to identify a single signature of OBM to contrast it with crude oil. Furthermore, the use of a label or taggant for the OBM filtrate is often discouraged in part because of the difficulty in labeling at a fixed concentration 5000 barrels of mud and in part because mud engineers do not want to use any additives which may have an unknown significant consequence on drilling characteristics. Mullins, in U.S. Pat. No. 5,266,800, teaches that by monitoring optical absorption spectrum of the fluid samples obtained over time, a real time determination can be made as to whether a formation oil is being obtained as opposed to OBM filtrate. As noted above, Mullins, in U.S. Pat. No. 5,266,800, discloses how the coloration of crude oils can be represented by a single parameter which varies of several orders of magnitude. The OFA was modified to include particular sensitivity towards the measurement of crude oil coloration, and thus filtrate coloration. During initial extraction of fluid from the formation, OBM filtrate is present in relatively high concentration. Over time, as extraction proceeds, the OBM filtrate fraction declines and crude oil becomes predominant in the MDT flowline. Using coloration, as described in U.S. Pat. No. 5,266,800, this transition from contaminated to uncontaminated flow of crude oil can be monitored.
U.S. Pat. Nos. 3,780,575 and 3,859,851 to Urbanosky, U.S. Pat. Nos. 4,860,581 and 4,936,139 to Zimmerman et al., U.S. Pat. No. 4,994,671 to Safinya et al., and U.S. Pat. Nos. 5,266,800 and 5,859,430 to Mullins are hereby incorporated herein by reference.
SUMMARY OF THE INVENTION
The applicants discovered that the measured optical density of a downhole formation fluid sample contaminated by OBM filtrate changes slowly over time and approaches an asymptotic value corresponding to the true optical density of formation fluid. The applicants also discovered that a calculated gas oil ratio (GOR), derived from measured optical density measurements of a downhole formation fluid sample contaminated by OBM filtrate also changes slowly over time and approaches an asymptotic value corresponding to the true GOR of formation fluid.
The applicants recognized the potential value, in borehole investigative logging, of a real time log of OBM filtrate fraction.
The applicants also discovered that it would be possible to estimate OBM filtrate fraction by measuring optical density values at one or more frequencies, curve fitting to solve for an asymptotic value, and using the asymptotic value to calculate OBM filtrate fraction.
The applicants also discovered that it would be possible, in like manner, to estimate GOR corrected for OBM filtrate fraction, and OD corrected for OBM filtrate fraction.
The applicants also discovered that it would be possible, in like manner, to predict future filtrate fraction as continued pumping flushes the region around the MDT substantially free of OBM filtrate.
The applicants recognized the need to provide appropriate tests to validate, or invalidate, asymptote analysis so as to screen out erroneous measurements caused, for example, by OBM filtrate entering the MDT tool through ineffective mudcake forms.
The applicants further recognized that such estimates would have value not only in boreholes, but also in established wells.
OBJECTS OF THE INVENTION
Therefore it is an object of the invention to provide a method and apparatus for determining oil based mud filtrate fraction in a downhole fluid sample that is contaminated by OBM filtrate.
It is another object of the invention to provide a method and apparatus for determining oil based mud filtrate fraction based on optical density (OD) for use when there is significant difference between the coloration of formation fluid and the coloration of oil based mud filtrate.
It is another object of the invention to provide a method and apparatus for determining oil based mud filtrate fraction based on gas oil ratio (GOR) for use when there is little or no difference between the coloration of formation fluid and the coloration of oil based mud filtrate.
It is another object of the invention to provide a method and apparatus for determining GOR of formation fluid corrected for OBM filtrate contamination.
It is another object of the invention to provide a method and apparatus for determining optical density (OD) of formation fluid corrected for OBM filtrate contamination.
It is another object of the invention to provide a method and apparatus for detecting the presence of particulates in the sample that would render optical density measurements invalid and sample capture premature, either because flushing is not yet complete or because ineffective mudcake forms are allowing continuous inflow of contaminating OBM filtrate.
It is another object of the invention to provide a
Mullins Oliver C.
Schroer Jon J.
Batzer William B.
Hannaher Constantine
Lee John L.
Schlumberger Technology Corporation
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