Petroleum-fluid property prediction from gas chromatographic analysis of rock extracts or fluid samples

Abstract

This method allows for prediction of subsurface fluid properties (e.g., phase or API gravity) using gas chromatogram data of a small-volume extract. Small volume equates to microliter scale volume (or milligram scale weight) from a subsurface rock sample, where a fluid test may not be available for analysis. The method may also be applied to petroleum liquid samples where drilling fluid or other contaminants preclude accurate direct property measurement. Gas chromatographic data is calibrated to measured petroleum properties; preferably local oils from the same petroleum system, however, a general global calibration can also be used.

Claims

1. A method of analyzing a reservoir fluid property of a small volume rock extract or fluid sample, comprising: a) obtaining a rock extract or fluid sample from a subterranean reservoir, wherein a volume of the rock extract or fluid sample is in the microliter range; b) analyzing the rock extract or fluid sample by gas chromatography to provide a gas chromatogram; c) calculating chromatographic peak area for three or more alkanes in the gas chromatogram; d) selecting a representative series of three or more alkane peaks for analysis; e) optionally, normalizing the alkane peak areas across the representative series; f) optionally, converting the chromatographic peak areas of the selected series to mole percent or weight percent; g) obtaining a plot of the concentration in terms of peak area, mole percent or weight percent, against carbon number or molecular weight; h) fitting an equation to plotted concentrations obtained from (g); i) determining goodness of fit (R.sup.2) for the equation to the plotted peaks; j) re-selecting the representative series of alkane peaks and repeating (e), (f), (g), (h), (i) and (j), k) identifying calibration petroleum sample or samples with similar plot; l) assigning one or more fluid properties from the calibration petroleum sample or samples to the rock extract or fluid sample with unknown property; m) determining the unknown property of the rock extract or fluid sample based on the assigning; and n) adjusting a field development operation based on the unknown property determined in step m).

2. The method of claim 1, wherein the peak area in step (g) is normalized.

3. The method of claim 1, wherein the mole percent or weight percent in step (g) is normalized.

4. The method of claim 1, wherein said selected representative series of alkanes or pseudocomponents comprises peaks or areas between C.sub.10 and C.sub.35.

5. The method of claim 1, wherein said representative series of alkanes or pseudocomponents comprises peaks or areas corresponding to an alkane molecular weight of approximately C.sub.8-C.sub.12, C.sub.10-C.sub.14, C.sub.10-C.sub.15, C.sub.10-C.sub.16, C.sub.10-C.sub.17, C.sub.10-C.sub.18, C.sub.10-C.sub.19, C.sub.10-C.sub.20, C.sub.10-C.sub.21, C.sub.10-C.sub.22, C.sub.10-C.sub.23, C.sub.10-C.sub.24, C.sub.10-C.sub.25, C.sub.10-C.sub.26, C.sub.10-C.sub.27, C.sub.10-C.sub.30, C.sub.10-C.sub.32, C.sub.10-C.sub.34, C.sub.10-C.sub.36, C.sub.10-C.sub.37, C.sub.12-C.sub.14, C.sub.12-C.sub.15, C.sub.12-C.sub.16, C.sub.12-C.sub.17, C.sub.12-C.sub.18, C.sub.12-C.sub.19, C.sub.12-C.sub.20, C.sub.12-C.sub.21, C.sub.12-C.sub.22, C.sub.12-C.sub.23, C.sub.12-C.sub.24, C.sub.12-C.sub.25, C.sub.12-C.sub.26, C.sub.12-C.sub.27, C.sub.12-C.sub.30, C.sub.12-C.sub.32, C.sub.12-C.sub.34, C.sub.12-C.sub.36, C.sub.12-C.sub.37, C.sub.14-C.sub.16, C.sub.14-C.sub.17, C.sub.14-C.sub.18, C.sub.14-C.sub.19, C.sub.14-C.sub.20, C.sub.14-C.sub.21, C.sub.14-C.sub.22, C.sub.14-C.sub.23, C.sub.14-C.sub.24, C.sub.14-C.sub.25, C.sub.14-C.sub.26, C.sub.14-C.sub.27, C.sub.14-C.sub.30, C.sub.14-C.sub.32, C.sub.14-C.sub.34, C.sub.14-C.sub.36, C.sub.14-C.sub.37C.sub.15-C.sub.17, C.sub.15-C.sub.18, C.sub.15-C.sub.19, C.sub.15-C.sub.20, C.sub.15-C.sub.21, C.sub.15-C.sub.22, C.sub.15- C.sub.23, C.sub.15-C.sub.24, C.sub.15-C.sub.25, C.sub.15-C.sub.26, C.sub.15-C.sub.27, C.sub.15-C.sub.30, C.sub.15-C.sub.32, C.sub.15-C.sub.34, C.sub.15-C.sub.36, C.sub.15-C.sub.37, C.sub.17-C.sub.19, C.sub.17-C.sub.20, C.sub.17-C.sub.21, C.sub.17-C.sub.22, C.sub.17-C.sub.23, C.sub.17-C.sub.24, C.sub.17-C.sub.25, C.sub.17-C.sub.26, C.sub.17-C.sub.27, C.sub.17-C.sub.30, C.sub.17-C.sub.32, C.sub.17-C.sub.34, C.sub.17-C.sub.36, C.sub.17-C.sub.37, C.sub.21-C.sub.16, C.sub.21-C.sub.18, C.sub.21-C.sub.19, C.sub.21-C.sub.23, C.sub.21-C.sub.24, C.sub.21-C.sub.25, C.sub.21-C.sub.26, C.sub.21-C.sub.27, C.sub.21-C.sub.30, C.sub.21-C.sub.32, C.sub.21-C.sub.34, C.sub.21-C.sub.36, C.sub.21-C.sub.37, C.sub.22-C.sub.16, C.sub.22-C.sub.18, C.sub.22-C.sub.19, C.sub.22-C.sub.20, C.sub.22-C.sub.24, C.sub.22-C.sub.25, C.sub.22-C.sub.26, C.sub.22-C.sub.27, C.sub.22-C.sub.30, C.sub.22-C.sub.32, C.sub.22-C.sub.34, C.sub.22-C.sub.36, C.sub.22-C.sub.37, C.sub.23-C.sub.16, C.sub.23-C.sub.18, C.sub.23-C.sub.19, C.sub.23-C.sub.20, C.sub.23-C.sub.25, C.sub.23-C.sub.26, C.sub.23-C.sub.27, C.sub.23-C.sub.30, C.sub.23-C.sub.32, C.sub.23-C.sub.34, C.sub.23-C.sub.36, C.sub.23-C.sub.37, and any combination thereof.

6. The method of claim 1, wherein a chromatographic spectra is stored in a non-transitory computer medium utilizing column separated value, tab separated value, hypertext machine language, relational database, text or other format of storing chromatographic spectra.

7. A method of analyzing a reservoir fluid property of a small volume rock extract or fluid sample, comprising: a) obtaining a rock extract or fluid sample from a subterranean reservoir, wherein a volume of the rock extract or fluid sample is in the microliter range; b) analyzing the rock extract or fluid sample by gas chromatography to provide a gas chromatogram; c) calculating chromatographic area for three or more alkane pseudocomponents in the gas chromatogram; d) selecting a representative series of three or more pseudocomponent areas for analysis; e) optionally, normalizing the pseudocomponent peak areas across the representative series; f) optionally, converting the pseudocomponent peak areas of the selected series to mole percent or weight percent; g) obtaining a plot of the concentration in terms of pseudocomponent peak area, mole percent or weight percent against carbon number or molecular weight; h) fitting an equation to plotted concentrations obtained from (g); i) determining goodness of fit (R.sup.2) for the equation to the pseudocomponent areas; j) re-selecting the representative series of pseudocomponent areas and repeating (e), (f), (g), (h), (i) and (j) as needed to obtain suitable goodness of fit; k) identifying a calibration petroleum sample with a similar plot; l) assigning one or more fluid properties selected from the group consisting of API gravity, fluid phase, and any combination thereof, from the calibration petroleum sample to the rock extract or fluid sample with unknown properties; m) determining petroleum API gravity or petroleum phase of the rock extract or fluid sample based on the assigning; and n) adjusting a field development operation based on the petroleum API gravity or petroleum phase of the rock extract or fluid sample determined in step m).

8. The method of claim 7, wherein the peak area in step (g) is normalized.

9. The method of claim 7, wherein the mole percent or weight percent in step (g) is normalized.

10. The method of claim 7, wherein said representative series of alkanes or pseudocomponents comprises peaks or areas between C.sub.10 and C.sub.35.

11. The method of claim 7, wherein said representative series of alkanes or pseudocomponents comprises peaks or areas corresponding to an alkane molecular weight of approximately C.sub.8-C.sub.12, C.sub.10-C.sub.15, C.sub.10-C.sub.16, C.sub.10-C.sub.17, C.sub.10-C.sub.18, C.sub.10-C.sub.19, C.sub.10-C.sub.20, C.sub.10-C.sub.21, C.sub.10-C.sub.22, C.sub.10-C.sub.23, C.sub.10-C.sub.24, C.sub.10-C.sub.25, C.sub.10-C.sub.26, C.sub.10-C.sub.27, C.sub.10-C.sub.30, C.sub.10-C.sub.32, C.sub.10-C.sub.34, C.sub.10-C.sub.36, C.sub.10-C.sub.37, C.sub.12-C.sub.14, C.sub.12-C.sub.15, C.sub.12-C.sub.16, C.sub.12-C.sub.17, C.sub.12-C.sub.18, C.sub.12-C.sub.19, C.sub.12-C.sub.20, C.sub.12-C.sub.21, C.sub.12-C.sub.22, C.sub.12-C.sub.23, C.sub.12-C.sub.24, C.sub.12-C.sub.25, C.sub.12-C.sub.26, C.sub.12-C.sub.27, C.sub.12-C.sub.30, C.sub.12-C.sub.32, C.sub.12-C.sub.34, C.sub.12-C.sub.36, C.sub.12-C.sub.37, C.sub.14-C.sub.16, C.sub.14-C.sub.17, C.sub.14-C.sub.18, C.sub.14-C.sub.19, C.sub.14-C.sub.20, C.sub.14-C.sub.21, C.sub.14-C.sub.22, C.sub.14-C.sub.23, C.sub.14-C.sub.24, C.sub.14-C.sub.25, C.sub.14-C.sub.26, C.sub.14-C.sub.27, C.sub.14-C.sub.30, C.sub.14-C.sub.32, C.sub.14-C.sub.34, C.sub.14-C.sub.36, C.sub.14-C.sub.37C.sub.15-C.sub.17, C.sub.15-C.sub.18, C.sub.15-C.sub.19, C.sub.15-C.sub.20, C.sub.15-C.sub.21, C.sub.15-C.sub.22, C.sub.15-C.sub.23, C.sub.15- C.sub.24, C.sub.15-C.sub.25, C.sub.15-C.sub.26, C.sub.15-C.sub.27, C.sub.15-C.sub.30, C.sub.15-C.sub.32, C.sub.15-C.sub.34, C.sub.15-C.sub.36, C.sub.15-C.sub.37, C.sub.17-C.sub.19, C.sub.17-C.sub.20, C.sub.17-C.sub.21, C.sub.17-C.sub.22, C.sub.17-C.sub.23, C.sub.17-C.sub.24, C.sub.17-C.sub.25, C.sub.17-C.sub.26, C.sub.17-C.sub.27, C.sub.17-C.sub.30, C.sub.17-C.sub.32, C.sub.17-C.sub.34, C.sub.17-C.sub.36, C.sub.17-C.sub.37, C.sub.21-C.sub.16, C.sub.21-C.sub.18, C.sub.21-C.sub.19, C.sub.21-C.sub.23, C.sub.21-C.sub.24, C.sub.21-C.sub.25, C.sub.21-C.sub.26, C.sub.21-C.sub.27, C.sub.21-C.sub.30, C.sub.21-C.sub.32, C.sub.21-C.sub.34, C.sub.21-C.sub.36, C.sub.21-C.sub.37, C.sub.22-C.sub.16, C.sub.22-C.sub.18, C.sub.22-C.sub.19, C.sub.22-C.sub.20, C.sub.22-C.sub.24, C.sub.22-C.sub.25, C.sub.22-C.sub.26, C.sub.22-C.sub.27, C.sub.22-C.sub.30, C.sub.22-C.sub.32, C.sub.22-C.sub.34, C.sub.22-C.sub.36, C.sub.22-C.sub.37, C.sub.23-C.sub.16, C.sub.23-C.sub.18, C.sub.23-C.sub.19, C.sub.23-C.sub.20, C.sub.23-C.sub.25, C.sub.23-C.sub.26, C.sub.23-C.sub.27, C.sub.23-C.sub.30, C.sub.23-C.sub.32, C.sub.23-C.sub.34, C.sub.23-C.sub.36, C.sub.23-C.sub.37, and any combination thereof.

12. The method of claim 7, wherein a chromatographic spectra is stored in a non-transitory computer medium utilizing column separated value, tab separated value, hypertext machine language, relational database, text or other format of storing chromatographic spectra.

13. A method of analyzing a reservoir fluid property of a contaminated, degraded, or evaporated rock extract or fluid sample, comprising: a) obtaining a contaminated, degraded or evaporated rock extract or fluid sample from a subterranean reservoir, wherein a volume of the contaminated, degraded or evaporated rock extract or fluid sample is in the microliter range; b) analyzing the contaminated, degraded or evaporated rock extract or fluid sample by gas chromatography to provide a gas chromatogram; c) calculating chromatographic areas for three or more alkane areas in the gas chromatogram; d) selecting a representative series of three or more alkane areas for analysis; e) optionally, normalizing the alkane peak areas across the representative series; f) optionally, converting the chromatographic peak areas of the selected series to mole percent or weight percent; g) obtaining a plot of the concentration in terms of peak area, mole percent or weight percent against carbon number or molecular weight; h) fitting an equation to plotted concentrations obtained from (g); i) determining goodness of fit (R.sup.2) for the equation to the plotted peaks; j) re-selecting the selected series of alkane areas and repeating (e), (f), (g), (h), (i) and (j) as needed to obtain suitable goodness of fit; k) identifying a calibration petroleum sample with similar plot; l) assigning one or more fluid properties from the calibration petroleum sample to the contaminated, degraded or evaporated rock extract or fluid sample with unknown property; m) determining the unknown property of the rock extract or fluid sample based on the assigning; and n) adjusting a field development operation based on the unknown property determined in step m).

14. The method of claim 13, wherein said selected representative series of alkanes or pseudocomponents comprises peaks or areas between C.sub.10 and C.sub.35.

15. The method of claim 13, wherein said selected representative series of alkanes or pseudocomponents comprises peaks or areas corresponding to an alkane molecular weight of approximately C.sub.8-C.sub.12, C.sub.10-C.sub.14, C.sub.10-C.sub.15, C.sub.10-C.sub.16, C.sub.10-C.sub.17, C.sub.10-C.sub.18, C.sub.10-C.sub.19, C.sub.10-C.sub.20, C.sub.10-C.sub.21, C.sub.10-C.sub.22, C.sub.10-C.sub.23, C.sub.10-C.sub.24, C.sub.10-C.sub.25, C.sub.10-C.sub.26, C.sub.10-C.sub.27, C.sub.10-C.sub.30, C.sub.10-C.sub.32, C.sub.10-C.sub.34, C.sub.10-C.sub.36, C.sub.10-C.sub.37, C.sub.12-C.sub.14, C.sub.12-C.sub.15, C.sub.12-C.sub.16, C.sub.12-C.sub.17, C.sub.12-C.sub.18, C.sub.12-C.sub.19, C.sub.12-C.sub.20, C.sub.12-C.sub.21, C.sub.12-C.sub.22, C.sub.12-C.sub.23, C.sub.12-C.sub.24, C.sub.12-C.sub.25, C.sub.12-C.sub.26, C.sub.12-C.sub.27, C.sub.12-C.sub.30, C.sub.12-C.sub.32, C.sub.12-C.sub.34, C.sub.12-C.sub.36, C.sub.12-C.sub.37, C.sub.14-C.sub.16, C.sub.14-C.sub.17, C.sub.14-C.sub.18, C.sub.14-C.sub.19, C.sub.14-C.sub.20, C.sub.14-C.sub.21, C.sub.14-C.sub.22, C.sub.14-C.sub.23, C.sub.14-C.sub.24, C.sub.14-C.sub.25, C.sub.14-C.sub.26, C.sub.14-C.sub.27, C.sub.14-C.sub.30, C.sub.14-C.sub.32, C.sub.14-C.sub.34, C.sub.14-C.sub.36, C.sub.14-C.sub.37, C.sub.15-C.sub.17, C.sub.15-C.sub.18, C.sub.15-C.sub.19, C.sub.15-C.sub.20, C.sub.15-C.sub.21, C.sub.15-C.sub.22, C.sub.15-C.sub.23, C.sub.15-C.sub.24, C.sub.15-C.sub.25, C.sub.15-C.sub.26, C.sub.15-C.sub.27, C.sub.15-C.sub.30, C.sub.15-C.sub.32, C.sub.15-C.sub.34, C.sub.15-C.sub.36, C.sub.15-C.sub.37, C.sub.17-C.sub.19, C.sub.17-C.sub.20, C.sub.17-C.sub.21, C.sub.17-C.sub.22, C.sub.17-C.sub.23, C.sub.17-C.sub.24, C.sub.17-C.sub.25, C.sub.17-C.sub.26, C.sub.17-C.sub.27, C.sub.17-C.sub.30, C.sub.17-C.sub.32, C.sub.17-C.sub.34, C.sub.17-C.sub.36, C.sub.17-C.sub.37, C.sub.21-C.sub.16, C.sub.21-C.sub.18, C.sub.21-C.sub.19, C.sub.21-C.sub.23, C.sub.21-C.sub.24, C.sub.21-C.sub.25, C.sub.21-C.sub.26, C.sub.21-C.sub.27, C.sub.21-C.sub.30, C.sub.21-C.sub.32, C.sub.21-C.sub.34, C.sub.21-C.sub.36, C.sub.21-C.sub.37, C.sub.22-C.sub.16, C.sub.22-C.sub.18, C.sub.22-C.sub.19, C.sub.22-C.sub.20, C.sub.22-C.sub.24, C.sub.22-C.sub.25, C.sub.22-C.sub.26, C.sub.22-C.sub.27, C.sub.22-C.sub.30, C.sub.22-C.sub.32, C.sub.22-C.sub.34, C.sub.22-C.sub.36, C.sub.22-C.sub.37, C.sub.23-C.sub.16, C.sub.23-C.sub.18, C.sub.23-C.sub.19, C.sub.23-C.sub.20, C.sub.23-C.sub.25, C.sub.23-C.sub.26, C.sub.23-C.sub.27, C.sub.23-C.sub.30, C.sub.23-C.sub.32, C.sub.23-C.sub.34, C.sub.23-C.sub.36, C.sub.23-C.sub.37, and any combination thereof.

16. The method of claim 13, wherein a chromatographic spectra is stored in a non-transitory computer medium utilizing column separated value, tab separated value, hypertext machine language, relational database, text or other format of storing chromatographic spectra.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings in which:

(2) FIG. 1: Flow chart for calibration and application.

(3) FIG. 2: A) Gas chromatographic profile of an oil sample; B) Gas chromatographic profile of a rock extract from a frozen sandstone core sample from a reservoir zone of an oil field; C) Gas chromatographic profile that defines pseudocomponents for an oil.

(4) FIG. 3: Plot of normalized percent molar n-alkane concentration on a log scale versus n-alkane carbon number. Plots as shown are constructed for oils or condensates. Slope is a function of API gravity.

(5) FIG. 4: A) Calibration curve using the exponential coefficient B for Basin A with two end member oil types and a mixed oil type of the two end member oil types. B) Calibration curve using the coefficient A for Basin A with two end member oil types and a mixed oil type of the two end member oil types. A best fit equation is constructed for both A and B factors for each source type. For unknowns the predicted API is calculated from these equations using the A and B factors.

(6) FIG. 5: A) Petroleum phases may be discriminated by coefficient B magnitude. B) Petroleum phases may be discriminated by coefficient A magnitude. Illustration of the phase cutoffs of coefficients A and B for low-sulfur (low asphaltene) shale source for a specific area, Basin A (see FIG. 4).

(7) FIG. 6: A) Example gas chromatogram of a Soxhlet extract from a sidewall core sample from a well that was not tested until 5 years later. Predicted API=38.1 for a core extract, the measured API of DST oil overlapping the depth range of the core was 37.5. B) Plot of Normalized Molar Percent n-Alkane vs. n-Alkane Carbon Number for SWC Extract. Predicted API=38.1. Well was tested over 5 years later & the measured API for flowing oil was found to be 37.5.

(8) FIG. 7: Example calculations showing actual data for a SWC extract from an oil stained sandstone.

(9) FIG. 8: Middle East Sandstone Core Extract; Altered by Evaporation.

(10) FIG. 9: Global Calibration Curve for Gas Condensates.

(11) FIG. 10: Application to a contaminated sample. A) Example whole oil gas chromatogram of oil sample contaminated by oil-based drilling mud. B) Plot of uncorrected and corrected normalized percent molar n-alkane concentration on a log scale versus n-alkane carbon number.

(12) FIG. 11: Mixed Fluids with Different Slopes. This petroleum is a mixture of light condensate with small amounts of black oil present in the reservoir. The C.sub.7-C.sub.20 hydrocarbons dominate the fluid, thus the method correctly predicts (+1) the flowing fluid API gravity when the correct range of n-alkanes is used in the method. A) gas chromatogram of API 60 condensate, b) molar slopes plot with carbon number for mixed fluid shows two slopes representing the light and heavier black oil components.

(13) FIG. 12A-12E illustrates various steps of determining fluid property as described in the Example.

DETAILED DESCRIPTION

(14) Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow.

(15) The present invention provides an inexpensive, rapid, and easily interpreted analysis of petroleum fluid quality. This analysis can be used to speed decisions and improve the quality and quantity of information available for reservoir engineers. Other advantages of the present invention will be apparent from the disclosure herein.

(16) In one embodiment, a general flow chart for calibration and application of the method is shown in FIG. 1. The present invention uses small volume extracts of rock samples, and determines the gas chromatographic profile of n-alkanes in the rock extract or oil sample (FIGS. 2A and 2B). The pattern is compared with patterns of petroleums of known fluid properties (i.e., API gravity or petroleum phase) to determine the properties of the fluid in the rock sample.

(17) A rock extract or contaminated fluid is analyzed by gas chromatography. In the simplest method, peak areas are measured for the alkanes of interest (usually n-alkanes) and a carbon range is selected that is appropriate to the sample. The raw data is entered into an automated spreadsheet to facilitate fast and efficient processing of data for large or small numbers of samples. However, pseudocomponents may be used instead of a single n-alkane. Each pseudocomponent is the sum all peaks between n-alkanes and including the highest carbon number n-alkane, (See FIG. 2C). In other embodiments, other analytical techniques may be used to obtain concentration information in lieu of gas chromatography.

(18) The data for a range of n-alkanes is normalized (e.g., C.sub.17-C.sub.27 is normally used to avoid evaporation in the rock extract of light alkanes and capture information on major contributors to petroleum, but for some samples ranges of C.sub.8-C.sub.20, C.sub.12-C.sub.22 or C.sub.22-C.sub.32 may be advantageous. For light oils and condensates an n-alkane range like C.sub.12-C.sub.22 may be advantageous (the range can be adjusted to lower or higher carbon number range to fit petroleum characteristics). Extracts of rocks that have been stored under less than optimal conditions and that have experienced extensive evaporation of light hydrocarbons, may be best evaluated by an alkane range like C.sub.22-C.sub.32. The normalized data is converted to molar percent in a spreadsheet using industry accepted molecular weights (Katz and Firoozabadi, 1978). The molar percent of each alkane is plotted on a log scale vs. the carbon number for that alkane (FIG. 3) within the spreadsheet. The result should be a straight line and the best fit equation through that line may be of the form of equation 1, where A and B=coefficients and x=carbon number or carbon number molecular weight.
n-Alkane Mole %=A e.sup.Bx(1)

(19) If the goodness of fit (R.sup.2) is less than 0.98 because of contamination, peak co-elutions, specious measurement, component loss, or some other unknown reason; then the spreadsheet allows for editing of the n-alkane or alkanes normalized molar percent affected by the contaminant to improve the mathematic fit and the most representative A and B coefficients. This editing feature allows for prediction of properties of contaminated oil samples.

(20) Coefficients A and B are proportional to fluid properties, thus, calibration plots of each coefficient versus measured fluid property (e.g., API gravity) of known oils can be plotted such that the best fit of these later plots produces calibration equations. The optimum calibration plot is constructed from data for oils from the same petroleum system as the unknown sample (rock or oil), usually from the same geologic area or basin. FIGS. 4A and 4B show examples of calibration curves for geologic basin A with two oil types and a mixed oil type of the two end members. A more general series of global calibration curves can also be constructed for different petroleum types (see example in FIG. 9). Once calibration equations are available, then coefficients for unknowns may be substituted into the calibration equations and a predicted fluid property calculated (e.g., API gravity).

(21) Oil types may be described as containing (1) high asphaltenes with high sulfur; (2) medium asphaltene content with medium sulfur; (3) low asphaltenes with low sulfur content; or (4) very low asphaltene content. Another factor which can affect oil type is thermal maturity (i.e., the highest temperature that a petroleum has experienced or the length of time it has been heated in the subsurface). Multiple geologic and compositional factors may combine to affect the relationship of molecular distribution and petroleum property.

(22) The petroleum fluid phase may be recognized by the magnitude of the coefficients. Thus Basin A petroleum (FIGS. 4A and 4B) from a low-sulfur shale derived subsurface liquid phase will have B>0.185 and A>700. Condensates will have B<0.205 and A<550. The petroleum phase predictions may show slightly different ranges for the A and B factors in other basins, but the cutoffs are derivable from the observed phase of fluids in the specific calibration set. For the low sulfur and low asphaltene family of oils in FIGS. 4A-4B plot of API gravity versus the coefficients (log scale for coefficient A and linear scale for B) allows illustration that the values of the coefficients may be used to predict whether the fluid is an oil or a condensate from a gas phase (FIGS. 5A and 5B).

(23) Note, one sample from a gas field plots with the oil trend in FIG. 5. This occurred because during testing a high pressure drop around the well caused condensate to form as a liquid phase. This liquid phase extracted a residual oil in the small pores of the reservoir rock. The combined condensate+residual oil exists as an oil phase.

(24) Spreadsheet Automation

(25) All calculations were combined into a series of spreadsheets that automate calculations so that large numbers of sample data can be processed. When contaminants or oil constituents other than n-alkanes co-elute in the gas chromatogram with peaks of interest, then plot visualization, and an editing process was also automated into the spreadsheets.

(26) In one embodiment, measured n-alkane peaks and pseudo-component molecular weights are used to calculate molar percent because n-alkanes provide large, easily quantified peaks. In another embodiment, n-alkane areas and n-alkane molecular weights or pseudo-component areas and pseudo-component molecular weights work equally well and any reasonable molecular weight may be used. In yet another embodiment, a correlation using simple weight percent could have also been used if done in a consistent manner.

Example

(27) A gas chromatogram (GC) was analyzed from a Soxhlet extract of a sidewall core (SWC) taken from a fine grained sandstone (FIG. 6). Light hydrocarbons smaller than nC.sub.15 were lost during the extraction. The GC peak areas are proportional to component weight percent (wt %) and are entered into the spreadsheet as raw data. FIG. 7 provides a table showing actual numbers abstracted from the spreadsheet calculator. The first step is to normalize the data so that the sum adds to 100 [Normalized wt %=(peak area*100)/sum of all areas]. The next step converts the normalized wt % to a calculated mole % using EQ. 2.
Mole %=[(100*wt % Cn/MW Cn)/((wt % Cn/MW of Cn)](2)

(28) The mole % are renormalized for the carbon range of interest (last column FIG. 7), in this case nC.sub.17 to nC.sub.27 and the result plotted on a log scale vs. the carbon number for each alkane (FIG. 6B). The resulting equation for the best linear fit line is in the form of EQ. 1 and yields the A factor (coefficient) and the B factor (exponent term). A and B are then entered into the equation derived from calibration data for the oil type that may be in the form of EQ. 3 and 4.
API=m Ln(A)+c(3)
API=xB.sup.2yB+z(4)

(29) Terms m, c, x, y, and z are coefficients determined from the best fit of measured oil API vs. the calculated A or B factors for appropriate oil or condensate samples. The calculated API gravities from the A and B factors for this SWC were 38.0 and 38.1 respectively. The A (190.5855) and B (0.1428) factors indicated this SWC contained oil and not condensate like nearby wells. This well was not tested until over 5 years later. The DST oil across this zone tested oil with API=37.5.

(30) In one embodiment, the process includes the following: Enter the sample data on the Raw Data worksheet of the Mol_Calculator workbook (FIG. 12A). Each sample is given a unique SAMPLE_ID. After entry of one or more sample data on the Raw Data worksheet, select one of the 3 optional calculations (FIG. 12C):

(31) A. NC17-NC27 Calculations & Graphs

(32) B. NC12-NC22 Calculations & Graphs

(33) C. NC22-NC32 Calculations & Graphs The calculation will output graphs of % Molar n-Alkanes vs Carbon Number for the samples. Each sample will have its own worksheet created which contains its graph (FIG. 12E). Graphs Data is accessed in the Calc mol % norm worksheet (FIG. 12B). Each sample graph worksheet cross-references the Calc mol % norm worksheet allowing quick transition from samples to graphs and graphs to samples (FIG. 12D).

(34) Optionally, hypothetical values of X may be entered to determine the Y value in the Calc mol % norm worksheet.

(35) Formulas located in the worksheets where calculations were done may be maintained or removed from the worksheets. Buttons have been provided on the Raw Data worksheet to remove formulas on a particular worksheet or all of the worksheets.

(36) In order to run new or additional samples, simply press the reset button provided on the Raw Data worksheet. When it is pressed, it will delete all data on all the worksheets with the exception of the Raw Data worksheet. It will also remove all worksheets with the exception of the original four: Raw Data Renormalized wt % Calc mol % Calc mol % norm
Application to Highly Evaporated Samples

(37) In one embodiment, sandstone core chips from an international well were analyzed by GC analysis (FIGS. 8 and 9). The core had been stored in less than optimal conditions for years and had experienced evaporation of all alkanes below nC.sub.14, and extreme to moderate alteration for nC.sub.14-nC.sub.17 (FIG. 8). These samples were extensively altered, but application of the method provided A and B factors that indicated a gas condensate. Local oils were available for a calibration curve but no local condensates were available for analysis. Global calibration curves (FIG. 9) based upon condensates from North America, Europe, and the Middle East were generated for condensates from carbonate sources (higher sulfur, lower API gravity), marine shale derived condensates (low sulfur, higher API gravity) and condensates with exceptional high API. Other information indicated that the local source was most likely a carbonate, thus, the carbonate curve was used for prediction of API gravity. The predicted API was within one degree of an old PVT report for a test from the well. Present day workers did not trust the integrity of the original data, however, this method was able to confirm the phase as gas plus condensate and the API gravity of the condensate.

(38) The method also works for samples contaminated by drilling fluid materials (FIG. 10). The method can be applied to contaminated rock extracts as well as contaminated oil. The contaminants may preclude accurate direct measurement of some petroleum properties like API gravity.

(39) Application to Contaminated Oils

(40) Many exploration and development wells, worldwide, must be drilled with oil-based muds (OBM), often with hydrocarbon-rich additives or base oils. Fortunately, the drilling fluids and additives usually have a limited carbon range. FIG. 10A shows a light oil from the Gulf of Mexico that has been contaminated by OBM and mud additives where the n-alkanes from nC.sub.15-nC.sub.20 and the pseudocomponents from nC.sub.15-nC.sub.27 are affected. The contaminants preclude direct measurement of a contamination-free API gravity. A best fit trend was calculated for the uncontaminated n-alkanes, nC.sub.21-nC.sub.27. Then, the n-alkane molar per cents were calculated for nC.sub.17-nC.sub.20. The corrected nC.sub.17-nC.sub.27 trend and best fit equation, plus A and B factors is in FIG. 10B. The API gravity can be calculated from the A and B factors.

(41) Application to Mixed Fluids

(42) Fluids will be encountered that are mixtures of two different petroleum charges. If the mixtures are both black oils or both condensates then the normal application will generally provide a reasonable estimate of the combined producible fluid.

(43) If the sample represents a mixture of a small amount of black oil with a large amount of condensate (or retrograde condensate) (FIG. 11A), then the resulting normalized molar percent n-alkanes may be able to split into two regions with two different slopes (FIG. 11B). The results for the fluid in FIG. 11 gave two different estimates; n-alkanes<C.sub.21 predicted a gas condensate with API gravity of 59. However, n-alkanes>C.sub.21 predicted a black oil with a lower API gravity. The gas chromatogram shows that the major fluid constituents were <nC.sub.21, thus a gravity of 59 was predicted. The measured API of producible fluid was 60.

(44) The observation of multiple slopes in the plot of normalized molar percent vs. carbon number confirms mixing of two or more distinct fluids. It also enables prediction of fluid quality for each contributing petroleum. To date, the method has been used extensively tested in oil and gas fields in the United States, the Middle East, and the former Soviet Union.

(45) In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as additional embodiments of the present invention.

(46) Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents.

(47) All of the references cited herein are expressly incorporated by reference. The discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. Incorporated references are listed again here for convenience:

REFERENCES

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