HYDROCARBON EXPLORATION METHOD

Abstract

A method of exploring for hydrocarbons in a region, including the steps of obtaining seismic data for the region corresponding to two or more different times and analyzing the seismic data corresponding to the two or more different times to determine whether there are any changes in the seismic data.

Claims

1. A method of exploring for hydrocarbons in a region, the method comprising: (a) obtaining seismic data for the region corresponding to two or more different times; (b) analysing the seismic data corresponding to two or more different times to determine whether there are any changes in the seismic data; and (c) determining whether there are any changes in the seismic data which are indicative of the presence of hydrocarbons, wherein changes in the seismic data which are indicative of the presence of hydrocarbons comprise changes which are indicative of gas coming out of an oil phase solution and/or a gas cap expansion or compression.

2. (canceled)

3. (canceled)

4. A method as claimed in claim 1, wherein the seismic data for the region corresponding to two or more different times comprises at least a first seismic data set corresponding to a first time and a second seismic data set corresponding to a second time, the method further comprising preconditioning at least one of the first and second seismic data sets.

5. A method as claimed in claim 1, wherein the seismic data for the region corresponding to two or more different times are measured in a sufficiently similar way such that the seismic data for the region corresponding to two or more different times can be compared.

6. A method as claimed in claim 1, wherein the seismic data for the region corresponding to two or more different times comprises at least a first seismic data set corresponding to a first time and a second seismic data set corresponding to a second time, and wherein analysing the seismic data comprises subtracting data of the first seismic data set from data of the second seismic data set or subtracting data of the second seismic data set from data of the first seismic data set to determine a difference between the data of the first and second seismic data sets.

7. A method as claimed in claim 6, further comprising displaying the difference between the seismic data graphically.

8. A method as claimed in claim 6, further comprising determining at least partially from the difference whether there is an indication of a possible presence of hydrocarbons in the region.

9. A method as claimed in claim 8, further comprising, if there is an indication of a possible presence of hydrocarbons in the region, making a decision about whether to explore for such hydrocarbons.

10. A method as claimed in claim 9, further comprising exploring for such hydrocarbons.

11. A method as claimed in claim 1, further comprising, prior to step (a), deciding whether a region is a candidate for further analysis.

12. A method as claimed in claim 11, wherein deciding whether a region is a candidate for further analysis comprises: (i) checking whether there is seismic data recorded at two or more different times for the region; (ii) checking whether there is a sufficient amount of time between the two or more different times; (iii) checking whether the seismic data recorded at two or more different times is comparable, or may be preconditioned or transformed such that it is comparable; (iv) checking whether an estimated initial pressure of the region is sufficiently close to an estimated bubble point pressure; (v) checking whether there is a sufficient pressure depletion in the region; and/or (vi) checking whether a gas cap may be present in the region.

13. A method as claimed in claim 12, further comprising assigning to a region an indicator indicating how good a candidate for further analysis the region is.

14. A method as claimed in claim 13, further comprising displaying the indicator on a map.

15. A computer program product comprising computer readable instructions that, when run on a computer, is configured to cause one or more processers to perform the method of claim 1.

16. A system for exploring for hydrocarbons, the system comprising one or more software elements arranged to perform the method of claim 1.

17. A method as claimed in claim 1, further comprising, if there are changes in the seismic data, identifying potential new locations of hydrocarbons based on the changes in the seismic data.

Description

[0061] FIG. 1 is a flow chart illustrating the key steps of an embodiment of the method;

[0062] FIG. 2(a) is a plot illustrating a model of a subsurface structure with water, oil and gas cap regions;

[0063] FIG. 2(b) is a plot illustrating simulated seismic data for the model of FIG. 2(a);

[0064] FIG. 3(a) is a plot illustrating a model of a subsurface structure with water, oil and gas cap regions where gas has come out of solution in the oil phase resulting in 10% of the pore volume available for fluid fill now being filled with gas compared to that in FIG. 2(a);

[0065] FIG. 3(b) is a plot illustrating simulated seismic data for the model of FIG. 3(a); and

[0066] FIG. 4 is a comparison plot showing the difference between the plots of FIG. 2(b) and FIG. 3(b).

[0067] The present invention provides a method of exploring for hydrocarbons by analysing seismic data for a region collected at different times.

[0068] FIG. 1 is a flow chart illustrating an embodiment of a method of the present invention. The method comprises five steps 1-5 as shown in the chart.

[0069] At step 1, it is determined whether a particular region is a candidate for further analysis. If it is determined that the region is a candidate, then the further steps of the method are performed. If not, then the method stops at step 1 in such a case.

[0070] At step 2, the seismic data sets on which the analysis is to be performed are obtained. The data (or some of the data) could be obtained from memory or it could be measured (e.g. if not already present in a memory). The seismic data is seismic data corresponding to (or recorded at) two or more different times.

[0071] At step 3, the seismic data corresponding to two or more different times is compared and a comparison plot of the seismic data is produced. In some embodiments, e.g. if 3D data is being compared, a difference cube is produced. In other embodiments, e.g. if 2D data is being compared, a difference section is produced. In other embodiments, alternative means of displaying the comparison are produced.

[0072] At step 4, the comparison plot (or other means of displaying the comparison) is analysed and it is determined whether the plot (or other means of displaying the comparison) indicates the possibility of the presence of hydrocarbons.

[0073] At step 5, based on the outcome of step 4 (and possibly further studies or checks), it is decided whether to physically explore and/or drill for hydrocarbons in the region.

[0074] Each of the steps 1-5 will now be described in more detail.

[0075] Step 1 involves determining whether a particular region is a candidate for further analysis.

[0076] Step 1 involves a number of sub-steps.

[0077] First, step 1 involves checking whether there is seismic data recorded at two or more different times for the region. The seismic data could comprise four-dimensional seismic data spanning a (sufficiently long) time period, four-dimensional seismic data and three-dimensional seismic data recorded at a different time to the four-dimensional seismic data, or two or more three-dimensional seismic data sets (seismic surveys), i.e. taken at different times.

[0078] If such seismic data recorded at two or more different times is found, then it is checked whether the seismic data is comparable, i.e. whether the seismic data are measured in the same or a sufficiently similar way, such that their data can be meaningfully and easily compared. For example, it may be required that source and receiver positions are steered and/or controlled in such a way that the geometrical deviations between e.g. two different seismic acquisitions (surveys) are as small as operationally possible, e.g. in some cases to within 10-20 m.

[0079] If a region is found which fulfils these criteria (i.e. there are two or more comparable seismic data sets), then it is also determined whether there is a sufficient likelihood of there being a signal indicating the presence of hydrocarbons following the rest of the method.

[0080] In order to determine this, it can be checked whether the initial pressure (or an estimated pressure) of the region at the time (or close to the time) at which the earlier/earliest seismic data was recorded is close to the estimated bubble point pressure.

[0081] The initial pressure of the region is an initial pore pressure of the hydrocarbon fluid(s) in the region, which may be obtained from data from nearby wells, where this is estimated while/after drilling.

[0082] The estimated bubble point pressure is determined from laboratory measurements on hydrocarbon fluid samples obtained in nearby wells.

[0083] The pressure difference (e.g. drop) between the times of the later and earlier seismic data is also determined or estimated (e.g. from sources such as exploration wells, pressures in different, but close fields etc.). The greater the pressure difference or drop, the more likely a useful signal could be obtained from analysis of the seismic data.

[0084] If the difference between the pressure corresponding to the time of the earlier/earliest seismic data and the pressure of bubble point is relatively small, e.g. when compared to the pressure difference (drop) between the times of the later and earlier seismic data, then this would indicate the possibility of gas coming out of solution. If the possibility of gas coming out of solution is indicated then this would suggest that it would be worthwhile performing the analysis of the seismic data sets of the region.

[0085] The pressures referred to above can be obtained or estimated from exploration (e.g. from prospect information), petroleum technology development data or production history and well logs, for example.

[0086] As an alternative, or in addition to checking whether the pressure of a region is close to its bubble point, it can be checked whether there is a gas cap present, e.g. by looking at or analysing the initial seismic data. If such a gas cap is present, then a reduction in pressure would result in an increase in the size of the gas cap, and this could indicate the presence of hydrocarbons.

[0087] Thus, a region may be indicated as being a candidate for further analysis if it has a pressure (e.g. a pressure at the time of the earlier/earliest seismic data) close to its bubble point (e.g. sufficiently close that the effect of gas coming out of solution may occur given the pressure drop), an initial gas cap, or both.

[0088] In order to illustrate how it may be decided whether a region is a candidate for further analysis, three examples are presented below. In these examples, P.sub.init is the estimated initial pressure of the hydrocarbons in the region, P.sub.bubblepoint is the estimated bubble point pressure, and the pressure depletion is the difference in the estimated pressure of the region between the earliest and latest (or earlier and later) data sets.

EXAMPLE 1

[0089] P.sub.init P.sub.bubble point=5 bar [0090] (ii) Pressure depletion=10 bar [0091] (iii) No gas cap present
In this example, although there is no gas cap present and the pressure depletion is not that high, the initial pressure is relatively close to the bubble point pressure so the region is indicated as being a (good) candidate for further analysis.

EXAMPLE 2

[0092] (i) P.sub.init P.sub.bubble point=50 bar

[0093] (ii) Pressure depletion=10 bar

[0094] (iii) No gas cap present

In this example, the initial pressure is not that close to the bubble point pressure, the pressure depletion is not that high and there is no gas cap present. As such, this region is not indicated as being a candidate for further analysis.

EXAMPLE 3

[0095] (i) P.sub.init P.sub.bubble point=50 bar

[0096] (ii) Pressure depletion=3 bar

[0097] (iii) Gas cap present In this example, the initial pressure is not that close to the bubble point pressure and the pressure depletion is not that high but there is a gas cap present. As such, this region is indicated as being a (possible) candidate for further analysis.

[0098] In one embodiment, regions are colour-coded (e.g. green for good candidates, yellow for possible candidates and red for not being a candidate) to indicate whether they are a candidate further analysis and the colour coded regions are displayed on a map.

[0099] If a region is indicated as being a candidate for further analysis, as determined at step 1 described above, then, at step 2, the seismic data sets on which the analysis is to be performed are obtained.

[0100] The data (or some of the data) could be obtained from memory or it could be measured (e.g. if new or newer seismic data is required). The seismic data is seismic data corresponding to (or recorded at) two or more different times.

[0101] For example, the seismic data sets could be two (or more) separate 3D seismic data sets. Such sets could all be obtained from memory (i.e. be previously recorded data) or the latest data set could be measured, e.g. for the method of the present invention to be performed.

[0102] The seismic data sets could be seismic data measured from any known or standard method, for example. In some cases, the seismic data could be recorded with air guns.

[0103] In some embodiments where 3D seismic data sets are used, such data sets can be preconditioned before they are analysed. Such preconditioning can help to ensure that the data sets being used are comparable with each other.

[0104] Preconditioning can entail equalising one or more variables such as amplitude levels and/or a spectral bandwidth of the seismic data set(s).

[0105] FIG. 2(a) is a 2D representation of a model of a geological structure comprising a region of water 8, a region of oil 7 and a region of gas (a gas cap) 6. The x-axis represents horizontal position and the y-axis represents depth.

[0106] FIG. 2(b) shows simulated seismic data for the model of FIG. 2(b), such as would be obtained at step 2.

[0107] FIG. 3(a) is a 2D representation of a model of a geological structure corresponding to that of FIG. 2(a) but at a later time, comprising a (unchanged) region of water 8, a region of oil 7′ and a region of gas (a gas cap) 6′. The x-axis represents horizontal position and the y-axis represents depth. In this case, due to a reduction of pressure, gas has come out of solution in the oil phase resulting in 10% of the pore volume available for fluid fill now being filled with gas compared to the case in FIG. 2(a).

[0108] FIG. 3(b) shows simulated seismic data for the model of FIG. 3(b), such as would be obtained at step 2.

[0109] In FIGS. 2(b), 3(b) and 4, the red (darker grey) shading corresponds to negative values and the blue (lighter grey) shading corresponds to positive values.

[0110] After the seismic data has been obtained, at step 3, the seismic data corresponding to two or more different times is compared by determining the difference between the later and earlier seismic data, e.g. by subtracting the later seismic data from the earlier seismic data, or vice versa, and a comparison plot (or other means of displaying the comparison) of the seismic data (i.e. showing this difference) is produced. This is illustrated in the plot in FIG. 4, which shows the difference between the data in FIGS. 2(b) and 3(b).

[0111] Next, at step 4, the comparison plot (or other means of displaying the comparison) is analysed, e.g. by eye, and it is determined whether the plot (or other means of displaying the comparison) indicates the possibility of the presence of hydrocarbons.

[0112] For example, in FIG. 4, the presence of the two bands 9 and 10 clearly indicates that there has been a change between the two plots 2(b) and 3(b). This in turn indicates the possible presence of hydrocarbons. On the other hand, if it is found that no such bands or features are present, this would indicate that there had not been any change as a result of a pressure drop, and this would in turn suggest that it is much less likely that hydrocarbons are present.

[0113] Finally, at step 5, based at least partially on the outcome of step 4, it is decided whether to physically explore and/or drill for hydrocarbons in the region. This may, for example, be based on other factors as well as the outcome of step 4, such as the presence or lack of any existing infrastructure, and the size of the region concerned.

[0114] Alternatively or additionally, before performing step 5, any observed effect (e.g. suggesting the presence of gas coming out of solution or gas cap expansion or compression) is ideally studied and it would be attempted to formulate a hypothesis as to why such an effect (e.g. as observed in a difference plot) is observed. Based on that, an assessment of whether the observed effect is a likely hydrocarbon indicator may be performed.