Method of assessing at least one petroleum characteristic of a rock sample

Abstract

The invention is a method of assessing at least one petroleum characteristic of a rock sample. Starting from a temperature ranging between 50 C. and 120 C., the temperature of a rock sample is raised to a temperature ranging between 180 C. and 220 C. Temperature (T2) is maintained for a predetermined time duration. The temperature of the sample is increased to a temperature (T3) ranging 330 C. and 370 C. Temperature (T3) is maintained for a predetermined time duration. The temperature of the sample is then raised to a temperature (T4) ranging 630 C. and 670 C. Three quantities S.sub.h0, S.sub.h1 and S.sub.h2, representative of the amount of hydrocarbon compounds released during the temperature change stages, are measured and at least one petroleum characteristic of the sample is deduced from these quantities.

Claims

1. A method of assessing at least one petroleum characteristic of a rock sample from a geological formation, wherein a sample is heated in an inert atmosphere according to a temperature sequence, comprising: a) raising a temperature of the sample from a first temperature ranging between 50 C. and 120 C., according to a first temperature gradient ranging between 1 C./min and 50 C./min, to a second temperature ranging between 180 C. and 220 C., and maintaining the sample at the second temperature for a first predetermined time duration; b) raising the temperature of the sample from the second temperature according to a second temperature gradient ranging between 1 C./min and 50 C./min to a third temperature ranging between 330 C. and 370 C., and maintaining the sample at the third temperature for a second predetermined time duration; c) raising the temperature of the sample from the third temperature according to a third temperature gradient ranging between 1 C./min and 50 C./min to a fourth temperature ranging between 630 C. and 670 C.; d) determining three quantities representative of the measured amount of hydrocarbon compounds released in steps a), b) and c) respectively; and e) determining, from at least one of the three determined quantities, at least one petroleum characteristic of the sample.

2. The method as claimed in claim 1, wherein the rock sample comprises a hydrocarbon source rock.

3. The method as claimed in claim 2, wherein the first and second time durations range between 2 and 4 minutes.

4. The method as claimed in claim 1, wherein the first temperature ranges between 80 C. and 120 C.

5. The method as claimed in claim 4, wherein the first temperature ranges between 90 C. and 110 C.

6. The method as claimed in claim 4, wherein the second temperature ranges between 190 C. and 210 C.

7. The method as claimed in claim 4, wherein the third temperature ranges between 340 C. and 360 C.

8. The method as claimed in claim 4, wherein the fourth temperature ranges between 640 C. and 660 C.

9. The method as claimed in claim 1, wherein the first temperature ranges between 90 C. and 110 C.

10. The method as claimed in claim 1 wherein, at a start of step a), the sample is maintained at the first temperature.

11. The method as claimed in claim 1, wherein the second temperature ranges between 190 C. and 210 C.

12. The method as claimed in claim 1, wherein the third temperature ranges between 340 C. and 360 C.

13. The method as claimed in claim 12, wherein the fourth temperature ranges between 640 C. and 660 C.

14. The method as claimed in claim 1, wherein the fourth temperature ranges between 640 C. and 660 C.

15. The method as claimed in claim 1, wherein the first, second and third temperature gradients range between 20 C./min and 30 C./min.

16. The method as claimed in claim 1, wherein the petroleum characteristic of the sample calculated in e) is selected from a free hydrocarbon content index denoted by a quality index of hydrocarbons, a production index, an American Petroleum Institute (API) degree, a Gas to Oil Ratio (GOR) parameter and a Gas to Condensate Ratio (GCR) parameter.

17. The A method as claimed in claim 16, wherein the free hydrocarbon content index is calculated according to a formula as follows:
HC.sub.cont=S.sub.h0+S.sub.h1 wherein S.sub.h0, S.sub.h1 and HC.sub.cont are expressed in milligrams of hydrocarbon compound per gram of rock.

18. The method as claimed in claim 16, wherein a quality index HQI of the hydrocarbons is calculated according to a formula as follows: $\mathrm{HQI}=\frac{S_{h0}}{S_{h0}+S_{h1}}100$ wherein S.sub.h0 and S.sub.h1 are expressed in milligrams of hydrocarbon compound per gram of rock and HQI is expressed as percentage by mass.

19. The method as claimed in claim 16, wherein a production index is calculated according to the formula as follows: ${\mathrm{PI}}_{\mathrm{Shale}}=\frac{\left(S_{h0}+S_{h1}\right)}{\left(S_{h0}+S_{h1}+S_{h2}\right)}100$ wherein S.sub.h0, S.sub.h1 and S.sub.h2 an expressed in milligrams of hydrocarbon compound per gram of rock, and PI.sub.Shale is expressed as a percentage by mass.

20. The method as claimed in claim 19, wherein calculation of the production index is repeated for the samples from different sedimentary layers of the formation, and at least one sedimentary layer of the formation being considered for exploration and/or exploitation is determined when the production index is greater by a factor A than the average of all of the production indices PI.sub.Shale measured for the formation.

21. The method as claimed in claim 20, wherein the factor A ranges between 1.1 and 1.5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages of the method according to the invention will be clear from reading the description hereafter of embodiments given by way of non-limitative example, with reference to the accompanying figures wherein:

(2) FIGS. 1A and 1B show the evolution of the amount of hydrocarbon compounds (Q) over time (t) during a pyrolysis, established according to the Basic method and to the Reservoir method respectively for the same hydrocarbon source rock sample. It can be noted that the X-axis representing time is given on an indicative basis: the position of the peaks in relation to the X-axis cannot be directly compared because the temperature sequences are different from one method to the other;

(3) FIG. 2 illustrates the temperature sequence of the method according to the invention; and

(4) FIG. 3 illustrates the evolution of the amount of hydrocarbon compounds (Q) over time (t) during a pyrolysis, established according to the method of the invention on a hydrocarbon source rock sample, the sample being identical to the one used in FIGS. 1A and 1B. It can be noted that the X-axis representing time is given on an indicative basis: the position of the peaks in relation to the X-axis cannot be directly compared because the temperature sequences are different from one method to the other.

DETAILED DESCRIPTION OF THE INVENTION

(5) The present invention is a method of assessing at least one petroleum characteristic of a rock sample from a geological formation, wherein the sample is heated in an inert atmosphere according to a particular temperature sequence.

(6) The present invention is applicable to any rock type such as, for example, a source rock, a reservoir rock or a hydrocarbon source rock. The advantages of the method according to the invention are shown hereafter within the context of an application to a rock sample from a hydrocarbon source rock.

(7) Thus, the present invention comprises at least the following stages:

(8) a) from a first temperature value (T1) ranging between 50 C. and 120 C., raising the temperature of the sample, according to a first temperature gradient ranging between 1 C./min and 50 C./min, to a second temperature value (T2) ranging between 180 C. and 220 C., and maintaining the sample at the second temperature value (T2) for a first predetermined time duration;

(9) b) from the second temperature value (T2), raising the temperature of the sample according to a second temperature gradient ranging between 1 C./min and 50 C./min to a third temperature value (T3) ranging between 330 C. and 370 C., and maintaining the sample at the third temperature value (T3) for a second predetermined time duration;

(10) c) from the third temperature value (T3), raising the temperature of the sample according to a third temperature gradient ranging between 1 C./min and 50 C./min to a fourth temperature value (T4) ranging between 630 C. and 670 C.;

(11) d) determining quantities S.sub.h0, S.sub.h1 and S.sub.h2 representative of the measured amount of hydrocarbon compounds released in stages a, b and c respectively;

(12) e) determining, from at least one of the three quantities S.sub.h0, S.sub.h1 and S.sub.h2, at least one petroleum characteristic of the sample.

(13) FIG. 2 illustrates the temperature sequence of the pyrolysis operation in an inert atmosphere according to the method of the invention.

(14) At the beginning of the process, the sample is fed into the oven heated at a first temperature (T1). This initial temperature value (T1) ranges between 50 C. and 120 C. According to a preferred embodiment of the invention, first temperature (T1) ranges between 80 C. and 120 C. According to another embodiment of the present invention, first temperature (T1) ranges between 90 C. and 110 C.

(15) According to an embodiment of the present invention, the sample is maintained at first temperature (T1) for a non-zero-time duration. This preliminary stage of maintaining the sample at first temperature (T1) allows at least one of warming up of the sample and release of the very light hydrocarbon compounds present in a slightly damaged or undamaged sample.

(16) The sample is then subjected to a programmed heating phase where the temperature rise ranges between 1 C./min and 50 C./min (segment A), up to a temperature corresponding to a second temperature (T2) ranging between 180 C. and 220 C. Advantageously, a second temperature (T2) ranging between 190 C. and 210 C. is selected. The temperature gradient preferably ranges between 20 C./min and 30 C./min.

(17) The sample is maintained at second temperature (T2) for a first predetermined non-zero-time duration (segment B) greater than a half-minute for example and preferably ranging between 2 and 4 minutes. This second temperature (T2) substantially corresponds to the end of the thermovaporization stage of the lighter hydrocarbons contained in the rock sample and to the start of the stage of cracking through pyrolysis of the heavier hydrocarbons.

(18) Then, from second temperature (T2), the temperature is raised (segment C) up to a temperature corresponding to a third temperature (T3) according to a temperature gradient ranging between 1 C./min and 50 C./min. The value of third temperature (T3) ranges between 330 C. and 370 C. Advantageously, a third temperature value between 340 C. and 360 C. is selected. According to a preferred embodiment, the temperature gradient ranges between 20 C./min and 30 C./min.

(19) Third temperature (T3) is maintained (segment D) for a predetermined non-zero-time duration greater than a half-minute for example and preferably ranging between 2 and 4 minutes. This third temperature (T3) substantially corresponds to the end of the thermovaporization stage of the heavy hydrocarbons contained in the rock sample and to the start of the stage of cracking through pyrolysis of the very heavy hydrocarbons.

(20) Pyrolysis is continued (segment E) so as to reach a fourth temperature (T4) according to a temperature gradient ranging between 1 C./min and 50 C./min. The value of fourth temperature (T4) ranges between 630 C. and 670 C., preferably between 640 C. and 660 C. The temperature gradient advantageously ranges between 20 C./min and 30 C./min. This fourth temperature (T4) substantially corresponds to the end of the pyrolysis stage, that is the end of the thermal cracking of the organic matter present in the rock sample.

(21) Thus, the temperature sequence of the method according to the invention comprises a succession of three heating stages (ramps illustrated by segments A, C and E in FIG. 2) separated by two temperature maintenance stages (isothermal stages illustrated by segments B and D in FIG. 2).

(22) FIG. 3 shows the pyrogram resulting from the application of the method according to the invention to the same hydrocarbon source rock sample as the one considered for establishing FIGS. 1A and 1B. It can be observed in FIG. 3 that the pyrogram is characterized by the presence of three peaks: a first peak referred to as peak S.sub.h0, a second peak referred to as peak S.sub.h1 and a third peak referred to as peak S.sub.2.

(23) The surface area of peak S.sub.h0, given for example in milligram of hydrocarbon compounds per gram of rock, corresponds to the amount of hydrocarbon compounds obtained between first temperature (T1) and second temperature (T2), more precisely obtained during segments A and B of FIG. 2. This quantity, referred to as quantity S.sub.h0, is representative of the lightest thermovaporizable hydrocarbons.

(24) The surface area of peak S.sub.h1, given for example in milligrams of hydrocarbon compounds per gram of rock, corresponds to the amount of hydrocarbon compounds obtained between second temperature (T2) and third temperature (T3), more precisely obtained during segments C and D of FIG. 2. This quantity, referred to as quantity S.sub.h1, is representative of the heavy thermovaporizable hydrocarbons.

(25) The surface area of peak S.sub.h2, given for example in milligrams of hydrocarbon compounds per gram of rock, corresponds to the amount of hydrocarbon compounds obtained between third temperature (T3) and fourth temperature (T4), more precisely obtained during segment E of FIG. 2. This quantity, referred to as quantity S.sub.h2, is representative of the very heavy thermovaporizable hydrocarbons.

(26) It can be observed in FIG. 3 that the method according to the invention allows obtaining a first complete, non-truncated peak (peak S.sub.h0), unlike the Basic (peak S.sub.1 in FIG. 1A) and Reservoir (peak S.sub.1r in FIG. 1B) methods. The method according to the invention thus allows more exhaustive recording of the free hydrocarbons contained in a sample and therefore, more generally, better quantification of the hydrocarbon compounds contained in a sample. Furthermore, the method according to the invention allows better separation of the last peak (peak S.sub.h2) than in the case of the Reservoir method (peak S.sub.2b in FIG. 1B) or in the case of the Basic method (peak S.sub.2 in FIG. 1A).

(27) Thus, the advantages of the method according to the invention shown in the above example are explained by the temperature sequence characteristic of the method according to the invention. Indeed, as the temperature sequence starts at a first low temperature (T1) in relation to the prior art methods, the method according to the invention allows measuring more completely the amount of free hydrocarbon compounds present in a sample. Furthermore, the method of the invention comprises, between two heating stages (ramps A, C and E in FIG. 2), temperature maintenance stages (isothermal stages B and D in FIG. 2) of a duration at least greater than one half-minute. These isothermal stages ensure the end of the thermovaporization of the hydrocarbon compounds of interest in the temperature range being considered.

(28) From at least one of these three quantities, at least one petroleum characteristic of the source rock sample being considered is calculated.

(29) According to an embodiment of the present invention, a free hydrocarbon content index HC.sub.cont is calculated according to the formula as follows:
HC.sub.cont=S.sub.h0+S.sub.h1

(30) with S.sub.h0, S.sub.h1 and HC.sub.cont expressed in milligram of hydrocarbons per gram of rock.

(31) According to another embodiment of the present invention, a quality index HQI of the hydrocarbons is calculated according to the formula as follows:

(32) $\mathrm{HQI}=\frac{S_{h0}}{S_{h0}+S_{h1}}100$

(33) with S.sub.h0 and S.sub.h1 expressed in milligram per gram of rock. HQI represents the proportion, expressed as percentage by mass, of very light hydrocarbons in relation to the thermovaporizable fraction.

(34) Advantageously, a production index PI.sub.Shale is calculated according to the formula as follows:

(35) ${\mathrm{PI}}_{\mathrm{Shale}}=\frac{\left(S_{h0}+S_{h1}\right)}{\left(S_{h0}+S_{h1}+S_{h2}\right)}100$

(36) with S.sub.h0, S.sub.h1 and S.sub.h2 expressed in milligram per gram of rock. Index PI.sub.Shale, expressed as percentage by mass, represents the quantity relative to the light hydrocarbon fraction in relation to all of the pyrolyzable hydrocarbons (thermovaporizable hydrocarbons plus those starting to be thermocracked).

(37) According to an embodiment of the present invention wherein the assessment of index PI.sub.Shale is repeated for rock samples from different sedimentary layers of a geological formation, the sedimentary layer(s) of the geological formation of interest with a view to at least one of oil exploration and/or exploitation are determined when their production index PI.sub.Shale is greater by a factor than the average of all the production indices PI.sub.Shale measured for the geological formation being considered. According to a preferred embodiment of the invention, a value ranging between 1.1 and 1.5 is selected for factor A.

(38) According to an embodiment of the present invention, it is also possible to calculate, from the three quantities, a petroleum characteristic of the source rock sample considered, such as:

(39) The API degree is a scale known to persons skilled in the art, which is a measure of the gravity of a crude oil. Thus, the lighter a crude oil, the lower the gravity and the higher the API degree thereof. Most crudes have API degrees ranging between 20 (very heavy) and 60 (very light);

(40) The GOR (Gas/Oil Ratio) parameter is a measure is known to persons skilled in the art which expresses the amount of gas produced at the wellhead in relation to the amount of oil;

(41) The GCR (Gas/Condensate Ratio) parameter is a measure is known to persons skilled in the art which expresses the amount of condensates (or light hydrocarbons) in relation to the amount of gas.

APPLICATION EXAMPLES

(42) Table 1 compares the results obtained with the method according to the invention (referred to as Method 1) and the Reservoir method (referred to as Method 2) on three rock samples (referred to as sample A, B and C) from different hydrocarbon source rock types. In the case of the Reservoir method, equivalences denoted by HC*.sub.cont, HQI* and PI*.sub.shale for petroleum characteristics HC.sub.cont, HQI and PI.sub.shale defined above for the method according to the invention were calculated as follows:

(43) $-{\mathrm{HC}}_{\mathrm{cont}}^{*}=S_{1r}+S_{2a};-{\mathrm{HQI}}^{*}=\frac{S_{1r}}{S_{1r}+S_{2a}}100;-{\mathrm{PI}}_{\mathrm{Shale}}^{*}=\frac{\left(S_{1r}+S_{2a}\right)}{\left(S_{1r}+S_{2a}+S_{2b}\right)}100.$

(44) According to this table, it can be observed that, whatever the sample being considered, the value of the petroleum characteristic HC.sub.cont obtained with the method according to the invention is greater than its equivalent HC*.sub.cont obtained with the Reservoir method. Indeed, as shown in Table 1, characteristic HC.sub.cont is 30% greater than its equivalent HC*.sub.cont in the case of sample A, 40% in the case of sample B and 28% in the case of sample C. Thus, the method according to the invention allows measuring a larger amount of free hydrocarbons contained in the sample being considered. As a result, by use of a temperature sequence starting with a lower temperature than the Reservoir method, the method according to the invention allows better assessment of the amount of the free hydrocarbons present in a rock sample from a hydrocarbon source rock than the prior art.

(45) Regards the petroleum characteristics HQI and PI.sub.Shale, no systematic trend can be observed from one sample to the next. This is explained by the fact that these characteristics depend on the proportion of the hydrocarbon types (free, thermovaporizable, thermocrackable) present in the sample being considered respectively.

(46) Table 2 compares the measured quantities S.sub.h0, S.sub.h1 and S.sub.h2 (corresponding to the surface areas of peaks S.sub.h0, S.sub.h1 and S.sub.h2 respectively), as well as some petroleum characteristics (HC.sub.cont, HQI and PI.sub.Shale) obtained with the method of the invention applied to a rock sample containing source rock hydrocarbons, implemented with the following values for the first, second, third and fourth temperatures: case 1: T1=100 C., T2=200 C., T3=350 C. and T4=650 C.; case 2: T1=80 C., T2=200 C., T3=350 C. and T4=650 C.; case 3: T1=100 C., T2=180 C., T3=350 C. and T4=650 C.; case 4: T1=100 C., T2=220 C., T3=350 C. and T4=650 C.

(47) It can be seen in Table 2 that petroleum characteristic HC.sub.cont is equivalent (to within 1.7%, which is the order of magnitude of the measurement uncertainty) in cases 1 or 2. This shows that the free hydrocarbons contained in a rock sample are recovered in an equivalent manner when applying the method according to the invention either with the central temperature or with the minimum temperature of the preferred first temperature range relative to first temperature T1.

(48) It can also be observed in Table 2 that measured quantity S.sub.h0 varies greatly in cases 3 and 4. Thus, the value of second temperature T2, corresponding to the end of the first heating ramp and to the first temperature stage of the temperature sequence implemented in the present invention, has a significant impact on quantity S.sub.h0. Indeed, at 180 C., all the light free hydrocarbons have not been thermovaporized yet (S.sub.h0 is 1.59 mg/g rock) whereas at 220 C., thermovaporization of part of the heavier free hydrocarbons has started (S.sub.h0 is 2.5 mg/g rock). On the other hand, once again, the value of petroleum characteristic HC.sub.cont is equivalent in cases 3 and 4 (to within 0.01 mg/g rock, which is below the measurement uncertainty). This shows that the free hydrocarbons contained in a rock sample are recovered in an equivalent manner when applying the method according to the invention either with the minimum temperature or with the maximum temperature of the temperature range relative to second temperature T2.

(49) TABLE-US-00001 TABLE 1 S.sub.h0 S.sub.h1 S.sub.h2 HC.sub.cont HQI PI.sub.Shale Method 1 (mg/g) (mg/g) (mg/g) (mg/g) (mass %) (mass %) Sample A 1.72 3.03 2.39 4.75 36.21 66.53 Sample B 5.03 7.50 96.04 12.53 40.14 11.54 Sample C 0.91 2.75 6.33 3.66 24.86 36.64 S.sub.1r S.sub.2a S.sub.2b HC*.sub.cont HQI* HQI* Method 2 (mg/g) (mg/g) (mg/g) (mg/g) (mass %) (mass %) Sample A 1.25 2.04 1.25 3.29 37.99 72.47 Sample B 3.33 4.29 95.70 7.62 43.70 7.38 Sample C 0.70 1.93 6.97 2.63 26.62 27.40

(50) TABLE-US-00002 TABLE 2 Sh0 Sh1 Sh2 HCcont HQI PIShale (mg/g) (mg/g) (mg/g) (mg/g) (mass %) (mass %) Case 1 1.94 2.85 2.51 4.79 40.50 65.62 Case 2 1.89 2.82 2.44 4.71 40.13 65.87 Case 3 1.59 3.31 2.54 4.90 32.45 65.86 Case 4 2.50 2.41 2.53 4.91 50.92 65.99

OTHER EMBODIMENTS

(51) According to a particular embodiment of the present invention, a rock sample from a geological formation is heated in a non-oxidizing atmosphere with the temperature sequence as defined in the method according to the invention, and the amount of hydrocarbon compounds released during the heating stage is continuously measured using a first detector, as well as the amount of CO.sub.2 and of CO contained in the effluent resulting from the heating stage, using a second detector. According to one embodiment of the present invention, from the continuous CO.sub.2 measurements, the amount of CO.sub.2 of organic origin and the amount of CO.sub.2 of mineral origin is determined. According to another embodiment of the present invention, from the continuous CO.sub.2 and CO measurements, the amount of oxygen of organic origin and the amount of oxygen of mineral origin is determined. According to another embodiment of the present invention, the residues resulting from heating in a non-oxidizing atmosphere are placed in another oven where they are heated in an oxidizing atmosphere. According to an embodiment of the present invention, heating in an oxidizing atmosphere can be temperature programmed so as to raise the temperature from approximately 400 C. to approximately 850 C. with a temperature gradient ranging between 10 and 30 C./min. According to an embodiment of the present invention, the amount of at least one of CO.sub.2 and CO contained in the effluent resulting from heating in an oxidizing atmosphere is continuously measured. According to an embodiment of the present invention, from the continuous CO.sub.2 measurements in an oxidizing atmosphere, the amount of CO.sub.2 of organic origin and the amount of CO.sub.2 of mineral origin is determined. According to another embodiment of the present invention, the amount of total organic carbon contained in the sample is determined from the CO.sub.2 and CO measurements obtained after heating sequences in non-oxidizing and oxidizing atmospheres. According to an embodiment of the present invention, the device used for implementing the present invention comprises a single oven allowing heating in an oxidizing atmosphere and heating in a non-oxidizing atmosphere.