CA2199091A1 - Method for determining oil content of an underground formation using wetcuttings - Google Patents

Abstract

Methods are disclosed for evaluating wet cutting from a well borehole to determine the hydrocarbon content of the earth formations penetrated by the well borehole. The wet cuttings to be measured are mixed with a polar hydrocarbon solvent which has both hydrophilic and hydrophilic properties. The solution of the solvent and cutting is filtered and then its emission (fluorescence) spectrum is measured by irradiating it with ultraviolet radiation excitation at wavelengths at which most petroleum compounds fluoresce. The hydrocarbon content of the sample wet cuttings is then determined by comparing its fluorescence emission to the emission fluorescence of known samples.

Description

MET~OD FOR DETERMINING OIL CONTENT OF AN UNDERGROUND
FORMATION USING WET CUTT INGS
(D# 91,137) Field of the Invention This invention is related to methods for determining the presence of hydrocarbons in an underground formation. More particularly, this invention concerns a new method which makes it possible to use wet cuttings in quantitative fluorescenc~
measurements. Benefits of this method are significant reduction in cycle time and improved accuracy. According to the current state of the art, samples to be evaluated must be dried before being solvated in order to obtain accurate fluorescence analysis.
Background of the Invention Fluorescence has been used for decades as a logging technique for detecting oil in drill cuttings. For much of that time the method used to determine the presence of oil in drili cuttings was a crude method in which an operator shined a broad spectrum ultraviolet light source on the cuttings in order to see fluorescence which would indicate the presence of oil. This visu21 method is highly subjective and inconsistent.
Ultra-violet fluorescence spectroscopy is used for the measurement of petroleum hydrocarbons in drill cuttings, cores, and 2l9~09~

soil samples by fluorescence examination of solvent extr2cts of these solids.
Molecular fluorescence is discussed in general in P~inciples of I~strumental Analysis, by Skoog, Douglas, Sanders College Publishing, Philadelphia (3rd ed. 1985), pp 225-240. The discussion in this reference indicates that the greatest fluorescence behavior occurs with compounds containing aromatic functional groups. The authors also disclose seve,al analytical profiles of hydrocarbons wherein fluorescence intensity is plotted over multiple excitation and emission wavelengths.
Fluorescence spectrophotometry is used for oil and gas prospecting via remote sensing or near surface sampling methods.
In U.S.G.S. Open-File Report 84-385, 34 pp (1984), in an article by M. E. Henry and T.S. Donovan entitled "Luminescense Properties and Chemical Geochemical Prospecting", there is a discussion of the technology for this use.
The use of fluorescence techniques for geochemical prospecting is discussed in an article by C. F. Hebert entitled, "Geochemical Prospecting for Oil and Gas Using Hydrocarbon Fluorescence Techniques", 3RD Southern Methodist Univ. Symp. -Unconventional Methods in Exploration for Petroleum and Natural Gas, Processing, (1984), pp. 40-58.
The emission fluorescence of crude oil samples has been studied and recorded over various r~avelengths, including ultraviolet wavelengths below 400nm. There have been "fingerprint"
studies, for example, at Bartlesville Energy Technology Center, where the emission fluorescence of vario~s types of crude oils has been recorded at different eYcitation wavelengths. Research of this type at the Department of Energy was related to earlier work by the Bureau of Mines, the object of which was to identify crude oil by emission fluorescence for the purpose of pollution control.
See Chisholm, B. R., Eldering, H. G., Giering, L.P., and Horning, A.~., r~ Total Luminescence Contour Spectra of Si~ Topped Crude Oils", BETC/RI-76/15, a paper prepared for ERDA for the Bartlesville Energy Research Center in Bartlesville, Oklahoma, November 1976; and Brownrigg, J.T., and Hornig, A.W.,"Low Temperature Total Luminescence Contour Spectra of Six Topped Crude Oils and Their Vacuum Distillate and Residuum Fractionsr', BETC/RI -78/13, a paper prepared for DOE for the Bartlesville EnergyTechnology Center, Bartlesville, Oklahoma, July 1978.
In U.S. Patent No. 4,977,319, incorporated herein by reference in its entirety, there is disclosed a method of determining the presence and concentration of hydrocarbons in a formation. The method involves the steps of solvating a sample from the formation in a known volume of solvent and measuring the emission fluorescence of the e~cited sample below about 400 nm and comparing the emission fluorescence to previous correlations drawn 21 ~9o~

between known hydrocarbon contents of samples and the related emission fluorescence.
U.S. 4,990,773, incorporated by reference her~in in its entirety, concerns a method of determining the producibility of any S hydrocarbons present in a formation.
At present, in the art of evaluating hydrocarbons from underground formations it is accepted as necessary, to obtain accurate fluorescence data, to first dry drill cuttings before solvating in state of the art aliphatic solvents such as hexane, heptane, or pentane. It is known that emission fluorescence readings will vary with the degree of sample dryness. ~ater on the cuttings surface forms an impenetrable barrier to the aliphatic solvents and the amount of eYtracted crude oil is significantly reduced. Currently, samples must be air dried for evaluation.
Drying time is dependent on the samples, environment, and equipment available to air dry samples without driving off hydrocarbons. Different formations require different drying times.
Formation samples such as shale and sandstone dry in as little as 30 minutes. Clay cuttings require more time. Hot dry field conditions accelerate the drying process and damp humid conditions delay sample drying. Spin dryers and fans reduce the required drying time by removing e~Ycess water and increasing air flow, however, using these two devices, the minimum drying time is still about 15 minutes.

2~99091 An additional disadvantage of having to dry cuttings is that the emission fluorescence after drying will var~ according to the environment and volatility of the hydrocarbons present.
Laboratory studies show a 50% decline in the emission fluorescence S of a condensate over a 24 hour period due to the evaporation of volatile hyd~ocarbons. This phenomena has been particularly observed in the field where overnight drying is common.
Although the quantitative fluorescence method has provided a more accurate method of evaluating samples of underground formation than was previously available in the art, there is a need in the art for a method of extracting oil from wet drill cuttings to minimize volitalization of light prior to analysis and to improve process and safety. It would significantly reduce sample processing problems and allow a technician to keep up with the drilling rate if it were possible to evaluate wet cuttings without a loss of accuracy. It would constitute a significant advance in the field if the resulting readings were found to exhibit fewer variations and to be more accurate.

Summary of the In~ention In accordance with the foregoing, the instant invention comprises an improvement in a metAod for evaluating a sample of an underground formation to determine the hydrocarbon content of the formation by:

solvating a known volume of the dried sample, quantitatively measuring with a fluorometer the em:ssion fluorescence of the solvated sample, and determining the hydrocarbon content of any hydrocarbon present in the sample by comparing the emmission fluorescence of said solvated sample to emission fluorescence of hydrocarbon samples from known sources, the composition of which has already been determined, wherein said improvement allows for the determination of hydrocarbon concentration using wet cuttings samples, in contrast to washed and dried cuttings and comprises:
adding said sample to a polar solvent having dual functionality to solvate water and hydrocarbons, mixing said solvent and said sample, lS filtering said solvent/wet sample solution and quantitatively measuring with a fluorometer the emission fluorescence of the solvated sample below about 400 ~m at an excitation wavelength at which most petroleum compounds fluoresce, and determining the hydrocarbon content of any hydrocarbon present in the sample by comparing the emission fluorescence of said solvated sample to the emission ~luorescence of hydrocarbon samples from known sources, the composition of which has already been determined, 21990ql wherein correlations are drawn between emission fluorescence of hydrocarbon samples from known sources and the emission fluorescence of the solvated sample.

S Brief Description of the Drawings Figure 1 is a bar graph comparing the accuracy of the results from the use of dry cuttings solvated in heptane and in isopropyl alcohol with wet cuttings solvated in isopropyl alcohol.
Figure 2 is a comparison of emission fluorescence patterns for sample drill cuttings solvated in heptane after being washed and air dried and an identical sample solvated wet in isopropanol.

Detailed Description of the In~ention Fluorescence is a phenomena wherein certain compounds, containing molecular arrangements generally referred to as chromophores, emit fluorescent radiation when excited by incoming light of certain wavelengths. The chromophores contained in compounds such as the asphaltenic, aromatic and resin fractions of crude, fluoresce in the ultraviolet and visible portion of the electromagnetic spectrum when bo~barded with ultraviolet radiation of the proper excitation wavelength.
Single point (fixed excitation/emission wavelength) fluorescence measurements are used to determine the approximate quantity of oil in formation samples(QFTT~). QFTr~is a tradename for Quantitative Fluorescene Technique~, a method for detecting oil in formations which was developed and patented by Texaco Inc., and is availaole for licensing. QFT~ can be accomplished with a relatively small, portable fluorometer.
By using a scale of fluorescence intensity and nstrumentally measuring the fluorescence of a formation sample from cores or drill cuttings, a number proportional to the hydrocarbon content of the sample can be derived, as discussed in U.S. 4, 977,319, supra.
The primary(most intense) peak for crude oils generally occurs in the spectral region between 30Onm and 360 nm. The position of the maximum fluorescence peak is dependent on the predominant fluorescing species(aromatics) found in the crude oil mixture.
The two to four-ring aromatics and their derivatives have considerable overlap in the 320nm to 380nm spectral range. Most of the heavier poly-aromatics consisting of five to six rings emit fluorescence from 400nm and extend into the visible (410nm to 800nm.) Total scanning (multiple excitation/emission wavelengths) fluorescence (TSF) measurements are used to further characterize oils/oil extracts and determine similarities of oils from different 21 9~091 .

sources. TSF requires a relatively large, computer controlled instrument permanently installed in a laboratory environment.
In total scanning fluorescence or 3-D fluorescence, a sample is excited over a range of discrete wavelengths and the emitted radiation is recorded at various wavelengths. Total scanning fluorescence has indicated that the optimum excitation and emission wavelengths for most crude oils fall below 400 nanometers.
This is a region undetectable by the human eye. The optimum excitation wavelength for most crude oils is in the region of about 250 to 310 nanometers. The predominant portion of emitted radiation falls in the non-visible ultraviolet region of about 300 to about 400 nanometers.
The method of the present invention would be most useful at the well site. Analysis can be obtained faster, with less time involved in analyzing, less error due to minimal volitalization of light, as well as minimal hydrocarbons and sample handling. The solvent is safer(lower flash point) and gives fewer false positives~e.g. asphaltic drilling and fluid additives).
At present hydrocarbon content is determined by first solvating a known volume of washed and air dried sample from the formation in a known volume of a solvent which will solvate hydrocarbons. The solvated sample is excited by a fluorometer at a fi~ed, relatively narrow, excitation wavelength. F1 intensity is measured at a similarly fixed, relatively narrow wavelength at _g_ 2199()~1 which most petroleum compounds fluoresce. This value is proportional to the hydrocarbon content of the sample. The emission fluorescence of the solvated sample is then compared with samples having known concentrations of various hydrocarbons.
S Correlations can be drawn between the emission fluorscence of known samples having previously identified oil concentrations and the fluorescence of the samples in said solvent.
A wide variety of solvents capable of solvating hydrocarbons are currently used in the art to extract oils from dried formation samples. Preferred solvents are low molecular weight aliphatic hydrocarbons having four or more carbon atoms such as pentane, hexane, heptane and higher. Chlorinated hydrocarbons such as methylene chloride, chloroform, carbon tetrachloride, trichloroethane and others are also effective. However, strong solvents may lessen the accuracy of the method due to their ability to dissolve sample constituents other than hydrocarbcns. Aromatic solvents are generally not preferred because of their inherent fluorescence.
It has now been discovered that certain solvents can be effectively used to solvate samples such as wet drill cuttings without the need for drylng that is necessary with methods currently available in the art; and the resulting solvated wet cuttings will provide reliable fluorescence data. Generally suitable sol-~ents are o~ygenated hydrocarbons which e~hibit 2 1 ~'~09 ~

hydrophilic and hydrophobic properties. It can also be observed that suitable solvents have a flash point in the range from about O~ to about 40~C, preferably in the range of 10~ to 15~C. Examples include, but are not limited to, methanol, ethanol, 1-propanol, 2-S propanol(isopropanol), 2-methyl-2-propanol, and allyl alcohol.
Preferred solvents include methanol, ethanol, 2-propanol, and 2-methyl-2-propanol, and combinations thereof.
In particular, 2-propanol, or isopropyl alcohol, is capable of breaking through the water barrier while, at the same time, solvating the aromatic portion without extracting the asphaltenes. The present invention demonstrates that polar solvents exhibiting dual functionalities can be used to effectively extract the desired hydrocarbons for emission fluorescence measurements in the presence of water.
The advantages of using a polar solvent with the hydrophobic and hydrophilic functionalities, such as 2-propanol (isopropyl alcohol- I~A~, for QFT~M are:
1) The method provides the capability of extracting oil from wet drill cuttings, cores, and sidewall cores. ~t also significantly reduces sample processing problems and makes it possiole for a technician to keep up with the drilling rate, which is generally not possible with currently available methods using the normal alkanes, largely due to the time required for washing and air drying the samples.

2 1 9909 i 2) The method provides enhanced hydrocarbon detection due to the fact that the samples are analyzed before the volatile hydrocarbons are lost through the drying step. Field tests show up to a 50% or greater drop in QFT'M intensities over a 3 day period S in drill cuttings containing condensates.

3) In the present invention a short chain alcohol having dual functionalities, such as isopropyl alcohol(2-propanol), more selectively extracts the components of commercial interest, providing a better indication of producibility in contrast to the commonly used heptane which is not useable in the presence of water and which also extracts portions of the tar, asphaltic and other nonproducible components.

4) The method of this invention provides a safer solvent system with a lower flash point, higher ignition temperature, lower heat of combustion, boiling point, and vapor density than heptane or hexane.

5) The method of this invention allows for as much as a 46~ reduction in chemical costs compared to other solvents.
The following example(s) will further illustrate the method of solvating wet samples or cuttings. These examples are given by way of illustration and not as limitations on the scope of the invention. Thus, it should be understood that the steps of the invention method may be varied to achieve similar results.

E~AMPLE
A series of cuttings were taken from different depths in a formation known to contain hydrocarbons. Equal amounts of each sample were used for three tests for samples from each depth. In the first test _gm. of the sample cuttings were washed and dried according to procedures presently known in the art and the air dried sample was solvated in _ ml. of heptane.
In the second test _gm. of the cuttings were washed, air dried, and solvated in _ml. isopropanol.
Finally, _ gm. of the cuttings were solvated in _ml.
isopropanol without being washed and air dried.
All solvated samples were e~cited by a fluorometer at a fi~ed, relatively narrow e~Ycitation wavelength and the results are recorded in TABLE I.
TABLE II records values for standard deviation.~It might be good to briefly explain this.) The fluorometer used to determine wavelength emissions consisted basically of an ultraviolet light source, and excitation radiation filter between the light source and the sample, a photomultiplier tube which reads the intensity of radiation emitted by the sample at right angles to excitation radiation, and an emission filter placed between the sample and the photomultiplier tube. A reference light path between the light source and the 2~ 990q~

photomultiplier was also provided so the difference between emitted radiation and exciting radiation can be easily determined. One suitable fluorometer which fits this description is commercially available from Turner Designs, 845 W. Maude Ave., Sunnyvale, CA
594086, under the name 10-AU-015.
The light source employed was a far ultraviolet source U
tube having a Turner Model No. 110-8S1, GE No. G4T4/1 or equivalent. 95% of the radiation from this light source is a 254nm, with some output at 297, 313, 405, 436, and 546.
10The excitation radiation filter employed was a Turner No.
7-54 filter which has a bell-shaped radiation transmission curve.
This filter transmits about 80% of the radiation which strikes it from about 290 to about 360nm, and 40% or more of incident radiation from about 250nm to about 390nm. Only 10% of incident 15radiation is transmitted at 236 and 400nm. The end result of this combination of light source and excitation radiation filter is that 99% of the excitation radiation used in the examples was at 254nm.
The emission filter employed was a 320nm narrow band filter. The transmission curve of this emission filter allows 25%
20transmittance of incident radiaiton at 320 nm, dropping steeply to 20% transmittance at 313 nm and 327nm. Transmittance is only 4% at 310nm and 330nm.
The invention method is by no means limited to the combination of filters and light source employed. Other !~

fluorometers and light sources, including lasers and filters, may be employed with the invention method with equal success. What the invention requires is that the solvated samples be radiated at an excitation wavelength at which most petroleum compounds fluoresce, generally below about 400 nm, preferably within the region of about 250nm to about 400nm. ~lthough these examples were run with an excitation radition of 254 nm and emission radiation measured at 320nm, it may be desirable to change the wavelengths employed to better eliminate the effect of fluorescence from other components present in the drill cutting, such as mineral, pipe d o p e , o r f i l t r a t e o f o i l b a s e m u d s .

Figure 1 is a bar graph representation of the information in TABLE I.
Many other variations and modifications may be made in S the concepts described by those skilled in the art without departing from the concepts of the present invention. Accordingly, it should be clearly understood that the concepts disclosed in the description are illustrative only and are not intended as limitations on the scope of the invention.

-16-~

,_vc,~l FOR wET CUTTlr~lGS ANAIYSIS 21 99091 ORILL CU I TI~GS
~o~ ION ORY DRY DRY W-cT W-cT
HE, TANrc HEPTANE -- HEPTANE AVER~AGE IPA IPA Av~RAGc Oc?T~ INTcNSiTY INTcNSlTYINT-NslTy INTcNSlTY - INT-c~lSlr INT,NSITY INTcNSl 2890 t 84 157 189-~ - 177 162 206 l 31 2980 22 ¢ ô 7 236-- i 76 185 ~ 84 1 ~ _ 3050 413 230 3~0328 358 438 3g8 3160 5~ 313 5~-7:. . - 47Z 556 413 sas 3110 .25 260 439375 385 368 3/7 3180 253 355 35031q 393 C05 3~9 31 gO ag 25o 35 l367 375 350 360 32~0 306 289 cl o335 425 ~--7 30 3320 117 50 106 91 135 136 l 30 3350 237 233 2Q3254 318 3 ' 331 SOL~/E~IT FOR WET CUTTINGS ANALYSIS
PURPCSc: C~T-RMINE ST,~NOARO OCVIATION OF ~ACH SO~V-~T ON ORILL CUTTiNGS
SAMP~ GLASSCOC:C rEE T3 CC)NOITlaN ORY
SOL\/ENT HEPTAN~
rRlAL INT'NSITY

2 1210 1126 = Average C~FT Intensity Values 3 1000 8;~ = Standard Deviation 41080 1000- 1 2ao = Range of QFT Values 1240 __ SAMP~':CRIL! CUTTINGS
CONO N ION ORY ORY WET
SO~VENT HEPTANE IPA IPA
OE?TH INT'NSITY INTcNSlTY INTcNSlTY
28~0 177 1 i8 187 2g8C 176 137 185 3100 472 ~ô4 ~85 3~ 70 375 329 377 3320 91 1 â4 130 3350 25~ 327 331

Claims (9)

1. In a method for evaluating samples of underground formation to determine the hydrocarbon content of the formation by solvating a known volume of the washed and dried formation sample in a known volume of a solvent, quantitatively measuring with a fluorometer the emission fluorescence of the solvated sample, determining the hydrocarbon content of any hydrocarbon present in the sample by comparing the emission fluorescence of said solvated sample to the emission fluorescence of known samples, the improvement which makes it possible to evaluate unwashed, wet cuttings which comprises:
adding said wet cuttings to a polar oxygenated hydrocarbon solvent exhibiting hydrophilic and hydrophobic properties, mixing the solvent and wet cuttings, filtering the solvent/wet cuttings solution, and quantitatively measuring with a fluorometer the emission fluorescence of the solvated sample below about 400 nm at an excitation wavelength at which most petroleum compounds fluoresce, and determining the hydrocarbon content of any hydrocarbon present in the sample by comparing the emission fluorescence of said solvated sample to the emission fluorescence of known samples, wherein correlations are drawn between the emission fluorescence of said solvated samples and the emission fluorescence of known samples.

2. The method of Claim 1 wherein the polar oxygenated hydrocarbon solvent is selected from the group consisting of methanol, ethanol, 1-propanol, 2-propanol(isopropanol), 2-methyl-2-propanol and allyl alcohol.

3. The method of Claim 2 wherein the solvent is isopropyl alcohol.

4. The method of Claim 1 wherein the samples are from the group consisting of wet soil samples, rock samples, drill cuttings, core samples, and water samples.

5. The method of Claim 4 wherein the samples are wet drill cuttings.

6. The method of Claim 5 wherein the emission fluorescence is measured between about 300 and 400 nanometers.

7. The method of Claim 5 wherein the sample extract is excited between about 245 and 310 nanometers.

8. A method of evaluating samples from an underground formation to determine the hydrocarbon content of the portion of the formation from which the sample came, which comprises:
solvating a known volume of sample in a known volume of solvent selected from the group consisting of methanol, ethanol, 1-propanol, 2-propanol, 1-propyl alcohol, 2-methyl-2-propanol, and allyl alcohol) to extract the oil from the sample, mixing the wet drill cuttings and solvent, filtering the solution, quantitatively measuring with a fluorometer the emission fluorescence of the solvated cuttings between about 250 and about 400 nanometers, determining the hydrocarbon content of any hydrocarbon present in the sample by comparing the emission fluorescence of said sample to the emission fluorescence of known samples, wherein correlations are drawn between the emission fluorescence of known hydrocarbon samples and the emission fluorescence of the samples in said solvent at said excitation wavelength band.

9. In any method for evaluating samples by quantitative fluorescence wherein the samples are dried before solvating them to extract hydrocarbons, the improvement of solvating the wet cuttings in polar oxygenated solvent having dual functionalities.