US2767320A - Method of geochemical prospecting - Google Patents

Description

United States Patent METHOD OF GEOCHEMICAL PROSPECTING Norman D. Coggeshall, Verona, and William E. Hanson,

Pittsburgh, Pa., assignors to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware No Drawing. Application November 24, 1952, Serial No. 322,362

Claims. (Cl. 25.0-43.5)

This invention relates to a method of geochemical prospecting for hidden hydrocarbon deposits by analyzing subsurface brines or other Waters for the presence of dissolved hydrocarbon components, in particular aromatic hydrocarbons and specifically benzene.

Heretofore, one method of geochemical prospecting has been conducted by making systematic collection of soil samples over an area to be mapped. The soil samples have been tested for a variety of components indicative of the presence of hydrocarbon deposits. Methods of extracting the components from the soil samples have included the use of organic solvents or simply the application of heat to remove hydrocarbon gases trapped in the soil samples. Among the components for which these samples have been tested are the lighter paraffinic hydrocarbons which are presumed to have migrated from deep subsurface hydrocarbon deposits. One of the difliculties encountered with this method of prospecting is that generally there is a wide Variation of the sorptive and retentive capacities of the soil samples taken over a plurality of locations. This is due primarily to the differences in chemical and physical properties of the soil itself. A pattern of such concentration values may thus not give a true indication of the proximity of hydrocarbon deposits. Another difiiculty encountered is that some :of the lower molecular weight paraflinic hydrocarbons may arise from the recent decay of organic or vegetable matter instead of from petroleum deposits.

In other known methods of geochemical prospecting, techniques have been used for detecting extremely small concentrations of crude hydrocarbons dispersed in drilling fluids. These methods have involved subjecting the fluid or cuttings samples to ultraviolet light to produce fluorescence by means of which it is possible to detect extremely small concentrations of crude oil which would ordinarily be invisible to the unaided eye. One of the problems which has been encountered in this type of prospecting for crude oil deposits is that positive indications are obtained When the drilling fluid is contaminated with refined petroleum products, such as pipe-thread grease or lubricating oils.

The present invention is directed to overcome the problems encountered in the type of geochemical prospecting which involves the sampling of soils or formation waters for the detection of parafi'inic hydrocarbons as components indicative of the presence of hydrocarbon deposits and to overcome the difliculties involved in detecting hydrocarbon deposits by analyzing formation Water samples obtained by drill-stem tests or bottom-hole samplers. This invention therefore has as an object the detection of dissolved aromatic hydrocarbons in formation waters as components related to the proximity of hydrocarbon deposits. Another object of this invention is the detection of dissolved aromatic hydrocarbons in formation waters by radiation from the electromagnetic spectrum. A further object of this invention is the detection of benzene dissolved in formation brine waters. Further objects of this invention will become apparent from the subsequent description.

2,767,320 talented Oct. 16, 1956 "ice The objects of the present invention are achieved by a method of geochemical prospecting wherein information concerning the proximity of hydrocarbon deposits or source beds is derived by collecting aqueous subterranean samples from prospect areas and wildcat wells and extracting the hydrocarbon components dissolved in these samples with a suitable extracting agent and determining the concentration of these hydrocarbons by the use of electromagnetic radiation. The presence of these dissolved hydrocarbons is an indication that these subterranean waters have been in contact with or are near a petroliferous deposit.

Aromatic hydrocarbons are universal components of crude petroleum and furthermore this class of hydrocarbons is more soluble in water or brine than any other type of hydrocarbon which occurs in crude petroleum. Benzene and a considerable number of benzene homologues have been recognized in crude petroleum. The best established are benzene, toluene, the three xylenes, ethylbenzene, napthalene and the two methyl naphthalenes. The actual percentages of these components depend on the geographical area where the petroleum is found, but they are always present. The aqueous solubility of these aromatic hydrocarbons found in petroleum decreases rapidly with an increase in molecular weight. For this reason, the lower molecular weight members of this series, such as benzene, are favored as criteria for the proximity of petroleum deposits. This is based on the observation that brines or other waters when brought into contact with petroleum will preferentially dissolve out a portion of the aromatics and particularly benzene and retain them in solution, although the waters may have been later physically separated from the crude petroleum.

A number of investigations have been carried out in an attempt to develop a simple and positive method of distinguishing .between the hydrocarbons from pipe-thread grease or other lubricantsthat may have contaminated sample waters from drill-stem tests or bottom-hole samplers and hydrocarbon from crude oil as they occur in the brines. The basis of this distinction lies in the fact that certain hydrocarbons such as benzene are more soluble in brine than any of the other constituents of crude oil. Furthermore, since benzene has a low boiling point it is virtually absent from most refined products used around a drilling rig with the exception of light fuel oils and gasoline. It is therefore possible that these light fuel oils, gasolines or even natural gas which are commonly used as fuels for powering drilling equipment, may contain light aromatics such as benzene. Precautions should therefore be taken that contaminants from these sources are not present or if they are present they must be capable of removal. If a well or drill hole is contaminated with benzene from a source other than sub-surface waters, the well can be flowed or pumped for a time suflicient to remove all contaminated brine, so that all the benzene from the fresh brine would arise from the contact of the brine with underground crude hydrocarbon deposit. However, the sources of contamination mentioned above are not likely to occur unless careless procedures are used. In actual practice on a series of tests on several wells we have found that the wells gave no evidence of contamination from such sources and we believe that such special precautions or preliminary swabbing prior to obtaining test samples are unnecessary.

By measuring the concentration of dissolved aromatic hydrocarbons, specifically that of dissolved benzene, in fluid samples obtained for instance from wildcat wells, even though considered to be dry holes, it is possible to determine whether a particular formation is completely barren or whether nearby portions may be petroliferous. Such information obtained by detecting aromatic hydrocarbons in the subsurface waters or brines from wildcat 3 wells may guide further drilling activity in the area. Benzene in varying concentrations has been found in tests on dry holes and increasing benzene concentration is interpreted as indicating the direction toward petroleum deposits.

In practicing our invention any suitable method of extracting aromatic hydrocarbons and particularly benzene as the component for analysis may be employed. Solvent extraction of the brine or other. waters with subsequent distillation to further concentrate and remove interfering materialfrom the aromatic hydrocarbons, followed by infrared or ultraviolet spectroscopic examination of the extract, permits the detection of aromatic components in the brine in concentrations of less than one part per million.

A method based on our procedure to be discussed below and using a strong ultraviolet absorption band of benzene has given positive results and there have been no indications of benzene whatever in synthetic brines given prolonged exposure to pipe-thread grease and lubricating oil.

The samples for analysis may comprise formation waters obtained by swabbing, drill-stem test fluids obtained from a drill-stem test, or'samples or underground waters obtained by an convenient means. The preferred method of analyzing the water samples for benzene content will now be described.

If the sample is a swab water, it may be extracted directly after filtering. If it is a mixture of water and mud such as from a drill-stem test, the water for testing may be the supernatant liquid after settling, or satisfactory water may be obtained from the mixture by centrifuging, filtration, or a combination of the two.

A preferred procedure for this invention is to extract a definite volume of the sample water with aromatic-free iso-octane (2,2,4-trimethylpentane). In the extraction a two-liter sample is conveniently used. The water is placed in a glass container with 25 ml. of iso-octane (2,2,4-trimethylpentane) of spectroscopic grade. This is then vigorously agitated in a mechanical shaker for 30 minutes. After the two phases separate,'the iso-octane may in some cases be recovered directly by means of a pipette. In oher cases emulsions of varying strength will be obtained. These may be broken chemically or by means of a centrifuge. The iso-octane will contain a major portion of the total amount of benzene originally present in the sample water.

In the preferred form of our invention in which benzene is to be detected, we prefer to concentrate the hem zene from the extract by distillation. Subsequently a selected fraction of the distillation is quantitatively analyzed for benzene by ultraviolet-absorption spectroscopy. Chromatographic fractionation, however, may also be em ployed for concentration of the benzene from the extract, and should a suitable colourimetric method for determination of benzene become available such method may be employed.

The iso-octane recovered from the extraction is measured and charged to a still with a still pot volume of approximately fifty ml. and with an efiiciency equivalent to about five theoretical plates. The charge is heated slowly until condensation is observed at the take-off point. The

. still is then allowed to operateat total reflux for one-half hour. At this point the sample is continuously removed until 3 mLof distillate has accrued. This 3 ml. will contain most of the benzene in the original charge.

The distillate of the above step is then examined in an 7 calculated from the observed benzene content in the disinterfere with its spectroscopic examination.

ferred procedure referred to above the refluxing time is tillate and the ratio of distillate volume to still-charge volume. The amount of benzene in the original water sample is then calculated from the benzene concentration in the charge to the still, the ratio of the volume of iso octane used for extraction to the volume of the original water sample, and a correction factor discussed below. If less than 25 ml. of iso-octane are recovered'frcm the extraction, it is assumed that the benzene is uniformly distributed between the recovered and non-recovered octane. As the extraction and distillation steps do not yield complete recovery it is necessary to evaluate a correction factor. This is done by introducing known concentrations of benzene into water. These samples are then extracted and processed as above. The results may thus be used to provide the necessary correction factor.

The distillation step in our procedure is importantin effecting the removal of material which would interfere with the spectroscopic examination of the benzene extract. In the extraction step described above the benzene is transferred from the water phase to the hydrocarbon phase and as a consequence is concentrated in a smaller volume of liquid. The distillation step serves a twofold purpose and for this reason it is believed important. first purpose is to remove any material that may interfere with the spectroscopic examination. The second purpose of the distillation step is to further concentrate the benzene in the extract to a smaller volume. It is essential, therefore, that the charge be heated slowly and that it should be refluxed until it is certain that most of the henzene can be removed free from the materials that would In our precedure in view of the fact that other methods may be practiced. Chromatographic fractionation for example affords the basis for a very elfe'ctive method of concen- V trating and separating mixtures, and this method may be used in the detection and determination of benzene and other aromatic hydrocarbons occurring in subsurface waters which have been in contact With buried hydrocarbon deposits.

The numerical values of volumes of water and solvent, of extraction time, of still volume, distillation time, theorectical plates, and volume of distillate used are not critical and may be varied. The only requirement is that once a set of conditions are established that it be adhered to forall calibration and sample runs.

One of the advantages of our invention in using formation waters as the sampling media is that they are ultraviolet spectrophotometer with radiation whose wave The remore easily handled and sampled than soil samples or soil gases. Another advantage is that there is no evidence in the literature that benzene is a product of recent vegetable or animal decomposition and further that benzene is virtually absent from contaminants arising from lubrieating oils and greases employed in drilling machinery.

Having thus described our invention in terms of certain specific embodiments, we intend to cover all changes and modifications of the example of the invention herein chosen for purposes of the disclosure, which do not constitute departure from the spirit and scope of the invention.

What we claim as our invention is:

1. In the art of geochemical prospecting for buried hydrocarbon deposits which comprises collecting aqueous subterranean samples in a prospect area and analyzing these for hydrocarbon components indicativev of the presence of hydrocarbon deposits, the irnprovement'which The Y .5 comprises extracting the aromatic components dissolved in said aqueous samples, and testing the extract for the presence 'of aromatic hydrocarbons as a measure of the relative amounts of said aromatic hydrocarbons originally present in said aqueous samples, ,and correlating these results as an indication of the proximity of buried hydrocarbon deposits.

2. In the art of geochemical prospecting for petroliferous deposits which comprises collecting samples of subsurface formation waters in a prospect area and analyzing these for hydrocarbon components indicative of the presence of petroliferous deposits, the improvement which comprises extracting the dissolved hydrocarbon components in said samples and passing electromagnetic radiation through the extract containing the dissolved hydrocarbons for the purpose of detecting the presence of aromatic hydrocarbons identified by their characteristic pattern of absorption bands, and determining their concentration in the original water sample as an indication of the nearness of petroliferous deposits.

3. The method of exploration for petroleum deposits which comprises a systematic collection of brine water samples from a prospect area, extracting the dissolved hydrocarbon content and passing ultraviolet radiation through the extract containing the dissolved hydrocarbons for the purpose of detecting the presence of aromatic hydrocarbons identified by their characteristic pattern of absorption bands, and determining their concentration in the original brine samples as an indication of the proximity of petroleum deposits.

4. In the art of geochemical prospecting for buried hydrocarbon deposits which comprises collecting aqueous subterranean samples in a prospect area and analyzing these for hydrocarbon components indicative of the presence of hydrocarbon deposits, the improvement which comprises extracting the dissolved aromatic components in said samples, and determining the concentration of benzene in the original samples as a component indicative of the presence of hydrocarbon deposits.

5. In the art of geochemical prospecting for petroliferous deposits which comprises a systematic collection of samples of subsurface formation waters in a prospect area and analyzing these for hydrocarbon components indicative of the presence of petroliferous deposits, the improvement which comprises extracting the dissolved hydrocarbon components in said samples, pass ing electromagnetic radiation through the extract containing the dissolved hydrocarbons for the purpose of detecting the presence of benzene as identified by its characteristic pattern of absorption bands as a component indicative of the proximity of petroliferous deposits.

6. The method of exploration for petroleum deposits which comprises a systematic collection of brine water samples over a prospect area, extracting the dissolved aromatic content therein and passing ultraviolet radiation through the extract containing the dissolved aromatic hydrocarbons in solution for the purpose of detecting the presence of benzene identified by its characteristic absorption bands, and determining the concentration of benzene in the original brine sample as an indication of the proximity of petroleum deposits.

7. In the art of geochemical prospecting for buried hydrocarbon deposits which comprises collecting aqueous subterranean samples in a prospect area and analyzing these for hydrocarbon components indicative of the presence of hydrocarbon deposits, the improvement which comprises extracting the dissolved aromatic components from said aqueous samples, passing ultraviolet radiation through the extract containing the aromatic hydrocarbons in solution for the purpose of detecting the presence of benzene identified by its characteristic absorption bands and for determining its concentration in the original samples, and correlating these results as an indication of the proximity of buried hydrocarbon deposits.

8. In the art ofgeochemieal prospecting for petroliferous deposits which comprises collecting samples of formation waters in a prospect area and analyzin these for hydrocarbon components indicative of the presence of petroliferous deposits, the improvement which comprises extracting the dissolved benzene 'in said samples with a suitable non-aromatic solvent of spectroscopic grade, concentrating the benzene contained in said, solvclltla ld passing ultraviolet radiation through the solvent containing the benzene to determine the concentration of benzene in the original samples as an indication of the nearness of petroliferous deposits.

9. In the art of geochemical prospecting for petroliferous deposits which comprises collecting samples of formation water in a prospect area and analyzing these for hydrocarbon components indicative of the presence of petroliferous deposits, the improvement which comprises extracting the dissolved benzene in said samples with 2,2,4-trimethylpentane, concentrating the benzene contained in said solvent and passing ultraviolet radiation of wave length in the region 220 to 280 millimicrons through the solvent containing the benzene to determine the concentration of benzene in the original samples as an indication of the nearness of petroliferous deposits.

10. The method of exploration for petroleum deposits which comprises collecting samples of brine waters from wells, extracting the benzene content dissolved therein, passing ultraviolet radiation of wave length in the region 220 to 280 millimicrons through the extract containing the benzene and determining the resulting absorption spectrum, the concentration of benzene as a component indicative of the presence of petroleum deposits.

11. The method of detecting subsurface petroleum deposits by analyzing subsurface brines which comprises extracting the aromatic components dissolved in the brine samples, passing ultraviolet radiation through the extract containing the aromatics, and determining the concentration of the aromatic hydrocarbon components in the samples as in indication of the proximity of petroleum deposits.

12. The method of detecting subsurface petroleum deposits by analyzing subsurface brines which comprises extracting the benzene dissolved in the brine samples, passing ultraviolet radiation through the extract containing the benzene, determining from the resulting absorption spectrum the concentration of benzene as an indication of the proximity of petroleum deposits.

13. The method of detecting subsurface petroleum deposits by analyzing subsurface brines which comprises extracting the benzene dissolved in the brine samples with 2,2,4-trimethylpentane, passing ultraviolet radiation of wave length in the region 220 to 280 millimicrons through the extract containing the benzene and determining through its absorption spectrum the concentration of benzene as an indication of the proximity of petroleum deposits.

14. The method of detecting subsurface petroleum deposits by analyzing subsurface brines which comprises extracting the benzene dissolved in the brine samples with 2,2,4-trimethylpentane, passing ultraviolet radiation of wave length in the region 220 to 280 millimicrons through the extract containing the benzene, measuring the absorption spectrum of the extract for said radiation whereby the concentration of benzene in the brine sample may be determined, and correlating the benzene concentration with the source location of the brine sample.

15. The method of exploration for petroleum deposits which comprises a systematic collection of subsurface water samples over a prospect area, extracting the benzene dissolved in the water sample with 2,2,4-trirnethylpentane, distilling the hydrocarbon extract to further concentrate benzene and to remove spectrally-interfering materials, and passing ultraviolet radiation of wave length in the region of 220 to 280 millimicrons through the extract containing the benzene, and determining through its absorption spectrum the concentration of benzene as an 2,500,213 indication of the proximity of petroleum deposits.

References Cited in the file of this patent "5 114,477

UNITED STATES PATENTS 2,403,631 Brown July 9, 1946 FOREIGN PATENTS V 7 Australia Jan. 15, 1942

Claims (1)

1. 3. THE METHOD OF EXPLORATION FOR PETROLEUM DEPOSITS WHICH COMPRISES A SYSTEMATIC COLLECTION OF BRINE WATER SAMPLES FROM A PROSPECT AREA, EXTRACTING THE DISSOLVED HYDROCARBON CONTENT AND PASSING ULTRAVIOLENT RADIATION THROUGH THE EXTRACT CONTAINING THE DISSOLVED HYDROCARBONS FOR THE PURPOSE OF DETECTING THE PRESENCE OF AROMATIC HYDROCARBONS IDENTIFIED BY THEIR CHARACTERISTIC PATTERN OF ABSORPTION BANDS, AND DETERMINING THEIR CONCENTRATION IN THE ORIGINAL BRINE SAMPLES AS AN INDICATION OF THE PROXIMITY OF PETROLEUM DEPOSITS.