An improved method for determining a reservoir fluid composition

Abstract

Method for determining the fluid composition of a reservoir, in particular of a petroleum reservoir, by using at least one rock sample of the reservoir, said method comprising: - a first analysis of first released gasses, that are released following to the application of a mechanical action, on a first rock sub-sample of said rock sample, so as to determine the molecular quantities of a first hydrocarbons subgroup, - a second analysis of second released gasses that are released following to the application of a mechanical action, on a second rock sub-sample of said rock sample, so as to determine the molecular quantities of a second hydrocarbons subgroup overlapping at least partially with said first hydrocarbons subgroup, - determining a combination of the molecular quantities of an hydrocarbons group comprising said first hydrocarbons subgroup and second hydrocarbons subgroup by using the molecular quantities so determined by means of said first analysis and said second analysis.

Description

[0001] AN IMPROVED METHOD FOR DETERMINING A RESERVOIR FLUID COMPOSITION.

[0002] FIELD OF INVENTION

[0003] The present invention relates to the technical field of oil and gas reservoirs. The method according to the invention is configured and intended to determine a reservoir fluid composition in terms of component quantities, in particular is configured and intended to determine the fluid composition of a petroleum reservoir in terms of hydrocarbon and / or nonhydrocarbon quantities.

[0004] STATE OF THE ART - PRIOR ART

[0005] Petroleum present within a typical subsurface reservoir is a complex mixture of many thousands of different components, ranging from methane to macromolecules containing hundreds or thousands of carbon atoms. A robust understanding of the composition of the petroleum mixture in an accumulation is necessary to assess the viability of economic development, to estimate the number of producer and injector wells required for production, and to design adequate well completions and surface fluid handling facilities.

[0006] The known industry practice involves the collection of petroleum fluid samples from the reservoir using down-hole sampling devices, or at surface during well testing; usually dedicated metered oil and gas flow lines on a separator vessel are used. These fluid samples are then analyzed in a laboratory to determine their physical properties and molecular composition. The resulting compositional data (often mathematically combined) are used by reservoir and petroleum engineers as input to thermodynamic equations of state in numerical simulation software to predict fluid flow and phase behavior within the reservoir and in the production facilities.

[0007] It is not possible, both from analytical and computational perspectives, to characterize every individual molecular component within a petroleum fluid. Therefore, equations of state calculations use lumped pseudo-components as input. The thermodynamic properties of the pseudo-components are then mathematically adjusted (“tuned”) to provide the best match between calculated and observed fluid properties. The known current analysis methods typically quantify hydrocarbon components and pseudocomponents up to C36 with higher molecular weight material reported as a C36+ fraction.

[0008] A significant drawback of the known technology is the limited availability of physical fluid samples. The very high cost of down-hole sampling and well testing means that relatively few samples can be taken for PVT (pressure-volume-temperature) compositional analysis during the appraisal and development stages of a hydrocarbon accumulation. Moreover, in many cases samples of individual reservoirs can not be taken because of technical issues related to sampling equipment, low permeability of the rock matrix, or the complexity of the reservoir architecture prohibiting subsampling of each individual flow zone. This causes significant uncertainty in spatial reservoir fluid composition and consequently in the prediction of fluid phase behavior in reservoirs in which reservoir fluids are not at thermodynamic equilibrium. Such non-equilibrium conditions may occur naturally as a consequence of incomplete fluid mixing caused by multiple mechanisms, for example, heterogeneous reservoir rocks, active fluid migration or in-reservoir biodegradation.

[0009] US2018 / 195383 discloses methods for determining the hardness and / or ductility of a material by compression of a geologic material sample, and also for the analysis of compounds contained in such sample, which can indicate the presence of valuable materials, such as petroleum-associated hydrocarbons.

[0010] OBJECTS OF THE INVENTION

[0011] The object of the invention is to propose a method for determining the reservoir fluid composition, in particular for determining fluid composition in terms of hydrocarbons quantities, which overcomes, at least in part, the drawbacks of known solutions.

[0012] Another object of the invention is to provide a high-resolution fluid characterization at much reduced cost in comparison to the known solutions.

[0013] Another object of the invention is to propose a method that allows to reduce uncertainty of fluid composition and phase behavior.

[0014] Another object of the invention is to propose a method that allows to provide detailed compositional analysis data suitable for use in known industry standard workflows.

[0015] Another object of the invention is to propose a method that uses samples collected as part of routine drilling operations.

[0016] Another object of the invention is to propose a method that is of easy, quick and low-cost implementation.

[0017] Another purpose of the invention is to propose a method that is precise, reliable and accurate.

[0018] Another object of the invention is to propose a method that can be used to create easily and quickly a profile of the fluid composition at various depths, preferably so as to create a Reservoir Fluid Composition (RFC) Log.

[0019] Another object of the invention is to propose a method that can be used to create easily and quickly a detailed 3D representation of spatial fluid distributions including vertical and lateral fluid discontinuities and vertical and lateral fluid gradients.

[0020] Another object of the invention is to propose a method that has an alternative and / or improved characterization, both in constructive and functional terms, with respect to the traditional ones.

[0021] Another object of the invention is to propose a method that is alternative to the traditional ones. Another object of the invention is to propose a method which meets high functional standards, and at the same time has an affordable cost, thus allowing the possibility of its utilization on a large scale.

[0022] SUMMARY OF THE INVENTION

[0023] All these objects, both alone and in any combination thereof, and others which will result from the following description are achieved, according to the invention, with a method with the features of claim 1 .

[0024] The present invention relates to a method for determining the fluid composition of a reservoir, in particular of a petroleum reservoir, by using at least one rock sample of the reservoir, said method comprising:

[0025] - a first analysis of first released gasses, that are released following to the application of a mechanical action, preferably a crushing action, on a first rock sub-sample of said rock sample, so as to determine the molecular quantities of a first hydrocarbons subgroup,

[0026] - a second analysis of second released gasses and / or a further analysis of further released gasses, that are released following to the application of a mechanical action, preferably a crushing action, on a second rock sub-sample of said rock sample, so as to determine the molecular quantities of a second hydrocarbons subgroup overlapping at least partially with said first hydrocarbons subgroup, and / or a further analysis of further released gasses, that are released following to the application of a mechanical action, preferably a crushing action, on a further rock sub-sample of said rock sample, so as to determine the molecular quantities of non-hydrocarbons and also of at least one hydrocarbon belonging to said first hydrocarbons subgroup,

[0027] - determining the molecular quantities of an hydrocarbons group comprising said first hydrocarbons subgroup and second hydrocarbons subgroup by using the molecular quantities so determined by means of said first analysis and said second analysis and / or determining the molecular quantities of non-hydrocarbons by using the molecular quantities so determined by means of said first analysis and by said further analysis.

[0028] The present invention relates to a method for determining the fluid composition of a reservoir, in particular of a petroleum reservoir, the method using at least one rock sample of the reservoir for determining the molecular quantities of a hydrocarbons group having the carbon atoms within a given range, said method comprising:

[0029] - a first analysis of first released gasses, that are released following to the application of a mechanical action, preferably a crushing action, on a first rock sub-sample of said rock sample, so as to determine the molecular quantities of a first hydrocarbons subgroup having the carbon number within a first subrange of said given range, - a second analysis of second released gasses, that are released following to the application of a mechanical action, preferably a crushing action, on a second rock subsample of said rock sample, so as to determine the molecular quantities of a second hydrocarbons subgroup having the carbon atoms within a second subrange of said range, wherein said second hydrocarbons subgroup overlaps partially with said first hydrocarbons subgroup,

[0030] - determining the molecular quantities of the hydrocarbons group by using the molecular quantities so determined of said first hydrocarbons subgroup and of said second hydrocarbons subgroup.

[0031] The present invention relates to a method for determining the fluid composition of a reservoir, in particular of a petroleum reservoir, the method using at least one rock sample of the reservoir for determining the molecular quantities of hydrocarbons and also nonhydrocarbons, said method comprising:

[0032] - a first analysis of first released gasses, that are released following to the application of a mechanical action, preferably a crushing action, on a first rock sub-sample of said rock sample, so as to determine the molecular quantities of a first hydrocarbons subgroup,

[0033] - a further analysis of further released gasses, that are released following to the application of a mechanical action, preferably a crushing action, on a further rock subsample of said rock sample, so as to determine the molecular quantities of nonhydrocarbons and also of at least one hydrocarbon belonging to said first hydrocarbons subgroup,

[0034] - determining, by using the molecular quantities so determined by means of said first analysis and by said further analysis, the molecular quantities of non-hydrocarbons and hydrocarbons belonging to the first hydrocarbons subgroup.

[0035] Preferably, the method provides the distribution of the molecular quantities of the hydrocarbons group subdivided by the number of carbon atoms, i.e. the pseudocomponents of the hydrocarbons group. More preferably, according to the standard petroleum reservoir engineering practice, the pseudo-components comprise all components occurring between normal alkanes (e.g. pseudo-component P12 represents all material eluting from gas chromatographic analysis of an oil after nC11 up to, and including, nC12).

[0036] Preferably, said first hydrocarbons subgroup comprises C1 - C5 hydrocarbons (i.e. wherein the carbon atoms are in the first sub range 1 - 5). Preferably, the first analysis provides the distribution of C1 - C5 alkanes.

[0037] Preferably, the pseudo-components of the first hydrocarbons subgroup C1 - C5 (i.e. the total composition of all hydrocarbons subdivided by carbon number in the first subrange 1 - 5) are derived directly by the integration of the molecular quantities obtained by said first analysis, i.e. obtained by the analysis of first released gasses, that are released following to the application of a mechanical action, preferably a crushing action, on a first rock sub-sample of said rock sample. Preferably, the first analysis provides the distribution of C1 - C5 pseudo-components.

[0038] Preferably, said second hydrocarbons subgroup comprises at least C4 - C6 hydrocarbons (i.e. wherein the carbon atoms are in the second subrange 4 - 6). Preferably, the second analysis provides the distribution of at least C4 - C6 alkanes.

[0039] Preferably, the pseudo-components of the second hydrocarbons subgroup C4 - C6 (i.e. the total composition of hydrocarbons subdivided by carbon number in the subrange C4

[0040] - C6) are derived directly by the integration of the molecular quantities obtained by said second analysis, i.e. obtained by the analysis of second released gasses, that are released following to the application of a mechanical action, preferably a crushing action, on a second rock sub-sample of said rock sample. Preferably, the second analysis provides at least the distribution of C4 - C6 pseudo-components.

[0041] Preferably, said second hydrocarbons subgroup comprises (at least) or may consist only of C4 - C8 hydrocarbons (i.e. wherein the carbon atoms are in the second subrange 4

[0042] - 8). Preferably, the second analysis provides the distribution of C4 - C8 alkanes.

[0043] Preferably, the pseudo-components of the second hydrocarbons subgroup C4 - C8 (i.e. the total composition of hydrocarbons subdivided by carbon number in the subrange C4

[0044] - C8) are derived directly by the integration of the molecular quantities obtained by said second analysis, i.e. obtained by the analysis of second released gasses, that are released following to the application of a mechanical action, preferably a crushing action, on a second rock sub-sample of said rock sample. Preferably, the second analysis provides the distribution of C4 - C8 pseudo-components.

[0045] Preferably, the method provides the combination of molecular quantities of the first hydrocarbons subgroup and of said second hydrocarbons subgroup by using the relative quantity of the hydrocarbons that are common / overlapped between the first hydrocarbons subgroup and the second hydrocarbons subgroup, so as to construct the relative quantities of all hydrocarbons of the first hydrocarbons subgroup and also of the second hydrocarbons subgroup.

[0046] Preferably, the hydrocarbons that are common / overlapped between the first hydrocarbons subgroup and the second hydrocarbons subgroup comprise C4 and C5 hydrocarbons, and more preferably comprise butane, isobutane, pentane and isopentane.

[0047] Ideally, the relative quantities of the hydrocarbons of the first and second subgroup, that together comprises the hydrocarbons at least in the range C1 to C6, more ideally at least in the range C1 to C8, are calculated by normalizing the quantity of the hydrocarbons C4 to C5 that are quantified in both analyses.

[0048] Preferably, said hydrocarbons group comprises the hydrocarbons at least in the range C1 - C6 (i.e. wherein the carbon atoms are at least in the range 1 - 6). Preferably, said hydrocarbons group comprises the hydrocarbons in the range C1 - C8 (i.e. wherein the carbon atoms are in the range 1 - 8).

[0049] Advantageously, the method allows to reconstruct a distribution of the molecular quantities of the group of at least C1 - C6 hydrocarbons for the reservoir fluid; in particular, the group of hydrocarbons so determined has the carbon atoms within at least the given range of 1 - 6. More advantageously, the method allows to reconstruct a full distribution at least of C1 - C6 pseudo-components in the fluid reservoir, representing the total composition of the C1 - C6 hydrocarbons subdivided by carbon number. Preferably, the method provides the combination of the distribution of C1 - C5 alkanes (obtained by the first analysis) and of the distribution of at least C4 - C6 (obtained by the second analysis) using the molecular quantities C4 and C5 hydrocarbons as common / overlapping reference, so as to obtain a first distribution of the molecular quantities of at least C1 - C6 alkanes. Advantageously, the method allows to reconstruct a full distribution of at least C1 - C6 alkanes in the fluid reservoir.

[0050] Advantageously, the method allows to reconstruct a distribution of the molecular quantities of the group of C1 - C8 hydrocarbons for the reservoir fluid; in particular, the group of hydrocarbons so determined has the carbon atoms within the given range of 1 - 8. More advantageously, the method allows to reconstruct a full distribution of C1 - C8 pseudocomponents in the fluid reservoir, representing the total composition of the C1 - C8 hydrocarbons subdivided by carbon number. Preferably, the method provides the combination of the distribution of C1 - C5 alkanes (obtained by the first analysis) and of the distribution of C4 - C8 alkanes, and in particular the first aromatics elute in this range, (obtained by the second analysis) using the molecular quantities C4 and C5 hydrocarbons as common / overlapping reference, so as to obtain a first distribution of the molecular quantities of C1 - C8 alkanes. Advantageously, the method allows to reconstruct a full distribution of C1 - C8 alkanes in the fluid reservoir.

[0051] Preferably, said second hydrocarbons subgroup may comprise, in addition to C4 - C6 hydrocarbons, also hydrocarbons larger than C6. Preferably, said second hydrocarbons subgroup may comprise, in addition to C4 - C8 hydrocarbons, also hydrocarbons larger than C8 (for example, up to C11 or even higher).

[0052] Preferably, the method further comprises: - a third analysis of oil extracted from a third rock sub-sample of said rock sample or of third released fluids, that are released by chemical treatment or thermal treatment or physical / mechanical action on a third rock sub-sample of said rock sample, so as to determine the molecular quantities of a third hydrocarbons subgroup having the carbon atoms within a third subrange of said given range,

[0053] - determining the molecular quantities of a wider hydrocarbons group by using the molecular quantities so determined of said first hydrocarbons subgroup, of said second hydrocarbons subgroup and of said third hydrocarbons subgroup.

[0054] Preferably, the fluids for the third analysis are obtained by solvent extraction of a third rock sub-sample of said rock sample.

[0055] Preferably, the method provides the distribution of the molecular quantities of the wider hydrocarbons group subdivided by the number of carbon atoms.

[0056] Preferably, said third hydrocarbons subgroup comprises C19 - C36 hydrocarbons (i.e. wherein the carbon atoms are in the third subrange 19 - 36).

[0057] Preferably, the third analysis provides the distribution of C19 - C36 n-alkanes (i.e. linear alkanes or straight-chain alkanes).

[0058] Preferably, the pseudo-components of the third subgroup C19 - C36 (i.e. the total composition of hydrocarbons subdivided by carbon number in the third subrange 19 - 36) are derived directly from the molecular quantities obtained by said third analysis, i.e. obtained by the gas chromatography analysis of oil extracted from a third rock sub-sample that is released by chemical, physical or thermal extraction, preferably a solvent extraction, on a third rock sub-sample of said rock sample. Preferably, the third analysis provides the distribution of C19 - C36 pseudo-components.

[0059] Preferably, the pseudo-components of the third subgroup are calculated by considering the normal alkane of the third subgroup.

[0060] Preferably, the method further comprises the calculation step of the molecular quantities of a fourth hydrocarbons subgroup having the carbon atoms within a fourth subrange, said calculation step is based on the molecular quantities so determined of said third hydrocarbons subgroup.

[0061] More preferably, the molecular quantities of a further (fourth) hydrocarbons subgroup are calculated by extrapolation using the molecular quantities of said third hydrocarbons subgroup and assuming that in reservoir oil - as demonstrated in the published literature about petroleum fluids - the molar quantity of normal alkanes (or pseudo-components) with respect to their carbon atom numbers follow an exponential relationship. Preferably, the method further comprises the calculation step of the molecular quantities of a fourth hydrocarbons subgroup having the carbon atoms within a fourth subrange, said calculation step is based on the molecular quantities so determined of said third and / or second hydrocarbons subgroup. Preferably, said fourth hydrocarbons subgroup comprises C8 - C18 alkanes. Preferably, said fourth hydrocarbons subgroup comprises C8 - C18 pseudo-components.

[0062] Preferably, said calculated fourth hydrocarbons subgroup comprises the hydrocarbons having the carbon numbers within an intermediate subrange that is between the second subrange and the third subrange. Ideally, said calculated fourth hydrocarbons subgroup comprises C8 - C18 hydrocarbons (i.e. wherein the carbon atoms are in the intermediate subrange 8 - 18). Preferably, the pseudo-components of the fourth subgroup are calculated by the regression of the pseudo-component or normal alkane distribution of the third subgroup.

[0063] Preferably, the pseudo-components of the third and / or of the fourth subgroup C8 - C36 (i.e. the total composition of hydrocarbons subdivided by carbon number in the subrange 8 - 36) are derived directly by the integration of the molecular quantities obtained by said third analysis, i.e. obtained by the analysis of oil extracted from a third rock subsample of said rock sample or obtained by the analysis of third released hydrocarbons, that are released following extraction, preferably a solvent extraction, on a third rock sub-sample of said rock sample.

[0064] Preferably, in a possible embodiment, the pseudo-components in the fourth hydrocarbons subgroup C8 - C18 are calculated by extrapolation using the pseudocomponents of the third subgroup C19 - C36 obtained by said third analysis and / or using the pseudo-components of the second subgroup C4 - C8 obtained by said second analysis.

[0065] Preferably, in a possible embodiment, the pseudo-components in the fourth hydrocarbons subgroup are obtained by a fourth analysis of fourth released hydrocarbons, that are released following to the application of a mechanical action, preferably a crushing action, on a fourth rock sub-sample of said rock sample, so as to determine the molecular quantities of the fourth hydrocarbons subgroup.

[0066] Preferably, the method reconstructs a further distribution of the hydrocarbons subdivided by carbon numbers belonging to the third and fourth subrange, wherein said further distribution is determined by using the molecular quantities of the third hydrocarbons subgroup obtained by said third analysis and by extrapolation of data resulting from the molecular quantities of said third and / or second hydrocarbons subgroup.

[0067] In a possible embodiment, wherein the second hydrocarbons subgroup may comprise only C4 - C6 hydrocarbons, the calculated fourth hydrocarbons subgroup may comprise C6 - C18 hydrocarbons (i.e. wherein the carbon atoms are in the intermediate subrange 6 - 18).

[0068] Preferably, said wider hydrocarbons group comprises the hydrocarbons in the range C1 - C36 (i.e. wherein the carbon atoms are in the range 1 - 36).

[0069] Preferably, the method provides:

[0070] - the combination of molecular quantities distributions of the first hydrocarbons subgroup and of said second hydrocarbons subgroup, so as to determine a first distribution of the hydrocarbons subdivided by carbon numbers belonging to the first and second subrange,

[0071] - the calculation of molecular quantities of the fourth hydrocarbons subgroup by using the molecular quantities of the third hydrocarbons subgroup, so as to determine a further distribution of the hydrocarbons subdivided by carbon numbers belonging to the third and the fourth subrange, wherein said further distribution has at least one carbon component that is common / overlapped with the first distribution,

[0072] - the further combination of the first distribution with the further distribution so determined so as to obtain a total distribution of the hydrocarbons subdivided by carbon numbers belonging to the full range.

[0073] Preferably, the first distribution is a distribution of C1 - C8 hydrocarbons, while the further distribution is a distribution of C8 - C36 hydrocarbons. Preferably, the common hydrocarbon - working as common / overlapping reference for the further combination of first distribution with the further distribution - is a C8 hydrocarbon. Preferably, the total distribution is a distribution of C1 - C36 hydrocarbons.

[0074] Preferably, the merging of the first distribution with the further distribution so determined, so as to obtain a total wider distribution of the hydrocarbons subdivided by carbon numbers belonging to the full wider range, is performed:

[0075] - by rescaling the first distribution and the further distribution at the common carbon component, and

[0076] - by renormalizing, preferably to 100 mole%, the distribution so rescaled to obtain said total distribution of the hydrocarbons subdivided by carbon numbers belonging to the full range.

[0077] Advantageously, the method allows to reconstruct a full distribution of the molecular quantities of the wider group of C1 - C36 hydrocarbons for the reservoir fluid; in particular, the group of hydrocarbons so determined has the carbon atoms within the given range of 1

[0078] - 36. More advantageously, the method allows to reconstruct a full distribution of C1 - C36 pseudo-components in the fluid reservoir, representing the total composition of the C1 - C36 hydrocarbons subdivided by carbon number. The reconstruction of the full C1 - C36 hydrocarbon distribution may be obtained by using C1 - C36 pseudo-components and / or by using C1 - C36 normal alkanes.

[0079] Preferably, the method comprises a further analysis of further released gasses, that are released following to the application of a mechanical action, preferably a crushing action, on a further rock sub-sample of said rock sample, so as to determine molecular quantities about non-hydrocarbon components. Advantageously, this allows to get information also on the non-hydrocarbon components present in the reservoir fluid, preferably on carbon dioxide and hydrogen sulphide.

[0080] Preferably, the gasses so released are analysed so as to determine the molecular quantity of a further subgroup comprising non-hydrocarbon gases, preferably such as carbon dioxide and hydrogen sulphide, and also at least one of C1-C4 hydrocarbons.

[0081] Preferably, the gasses so released are analysed by way of gas chromatography with thermal conductivity detection.

[0082] Preferably, the abundance of non-hydrocarbon components carbon dioxide and hydrogen sulphide within the petroleum reservoir fluid can be calculated by using the relative quantity of at least one hydrocarbon that is common / overlapped between said further subgroup and the first hydrocarbons subgroup C1 - C5 or the full distribution of C1 - C36 hydrocarbons. Ideally, as common / overlapping hydrocarbon it is used the abundance of C1 hydrocarbon in the further subgroup and in full C1-C36 hydrocarbon distribution. More preferably, the resulting combined distribution containing the distribution of C1-C36 hydrocarbons, of carbon dioxide and of hydrogen sulphide is then renormalized to 100mole%.

[0083] Preferably, said at least one rock sample comprises rock samples collected by core and / or by drill cuttings. Ideally, said at least one rock sample comprises samples of drill cuttings, side wall cores and / or conventional core chips / samples.

[0084] Preferably, said at least one rock sample is a non-preserved (i.e. not retrieved to surface under confining pressure or refrigerated using dry ice or liquid nitrogen) rock sample of the reservoir.

[0085] Preferably, said at least one rock sample has a known subsurface origin.

[0086] Preferably, the first rock sub-sample, the second rock sub-sample, the third rock sub-sample, the fourth rock sub-sample and / or the further rock sub-sample (or other subsamples that may be used for analysis) are obtained from the same rock sample or by rock samples having the same origin. More preferably, the rock sample / s - ideally after disaggregation - is / are divided into four or more sub-samples.

[0087] Preferably, the first released gasses, the second released gasses and the further released gases are released by applying a mechanical crushing to the corresponding rock subsamples of the rock sample. Preferably, in a possible embodiment, also the third released hydrocarbons and the fourth released components may be released by applying a mechanical crushing to the corresponding rock subsamples of the rock sample or by way of solvent extraction, vacuum extraction, centrifuge extraction or thermal extraction.

[0088] For example, the rock sub-samples obtained from drill cuttings, side wall core chips and / or conventional core chips are placed into a sealed vessel and are mechanically crushed so as to liberate the entrained gas, thus obtaining said released gasses.

[0089] Preferably, the rock samples are hand-picked to differentiate between different lithofacies.

[0090] Preferably, said first analysis is performed via gas chromatography. More preferably, said first analysis is performed via gas chromatography with a flame ionization detector (GC-FID). Ideally, said first analysis is performed via gas chromatography to assess molecular quantity of the C1 - C5 hydrocarbons, and in particular of the hydrocarbons comprising methane, ethane, propane, isobutane, normal butane, neopentane, isopentane, normal pentane.

[0091] Preferably, said second analysis is performed via gas chromatography. More preferably, said second analysis is performed via gas chromatography with a flame ionization detector (GC-FID). Ideally, said second analysis is performed via gas chromatography to assess molecular quantity of at least the C4 - C6 hydrocarbons, more ideally of at least the C4 - C8 hydrocarbons, and in particular of all isomers of the alkanes, cycloalkanes and aromatic compounds at least in the range C4 - C6, more ideally in the range C4 - C8.

[0092] Preferably, said second analysis is performed with analytical conditions that are different in respect to the analytical condition of the first analysis. Ideally, the second analysis is performed with modified temperature and chromatographic conditions in respect to the first analysis.

[0093] Preferably, the oil for the third analysis is extracted from a third rock sub-sample by solvent extraction. Ideally, the oil for the third analysis is extracted from a third rock subsample by using a non-polar organic solvent.

[0094] Preferably, the oil for the third analysis is extracted from a third rock sub-sample by thermal extraction.

[0095] Preferably, said third analysis is performed via gas or liquid chromatography. More preferably, said third analysis is performed via chromatography with a flame ionization detector.

[0096] Ideally, said third analysis is performed via gas chromatography to assess molecular quantity of the C19 - C36 hydrocarbons. Preferably, said third analysis is performed via gas chromatography with a flame ionization detector (GC-FID). Ideally, said third analysis is performed via gas chromatography to assess molecular quantity of the C19 - C36 hydrocarbons, and in particular of all isomers of the alkanes, cycloalkanes and aromatic compounds in the range C19 - C36.

[0097] Preferably, when said fourth analysis for determining the fourth sub-group of hydrocarbons comprises the analysis of further released gasses, said fourth analysis is performed via gas chromatography. More preferably, said fourth analysis is performed via gas chromatography with a flame ionization detector (GC-FID). Ideally, said further analysis is performed via gas chromatography to assess molecular quantity of the C8 - C18 hydrocarbons, and in particular of all isomers of the alkanes, cycloalkanes and aromatic compounds in the range C8 - C18.

[0098] Preferably, said further analysis for determining non-hydrocarbons components is performed via gas chromatography. Preferably, said further analysis is performed with analytical conditions that are different in respect to the analytical condition of the first and second analysis. Ideally, the further analysis is performed with modified temperature and chromatographic conditions in respect to the first and second analysis.

[0099] Preferably, the analyses of the method are performed for each rock sample of a plurality of rock samples collected at different known depths of the reservoir fluid, so as to generate a profile of the fluid composition of the reservoir for each different known depth.

[0100] More preferably, the method comprises the analyses for each rock sample coming from a plurality of different known depths / origins in the reservoir.

[0101] Preferably, said method is characterized in that the molecular quantities of the hydrocarbons group, more preferably the distribution of the molecular quantities of the wider hydrocarbons group subdivided by the number of carbon atoms, is associated to the reservoir origin / depth of the corresponding rock samples used for the analyses, so as to define / build a reference profile (for example organized as a look-up table, a log or other organized dataset wherein to each input is associated the related output) comprising for each reservoir origin / depth of the rock sample used for the analyses the corresponding molecular quantities of the wider hydrocarbons group, more preferably the distribution of the molecular quantities of C1 - C36 hydrocarbons subdivided by the number of carbon atoms. More preferably, the C1 - C36 hydrocarbon distribution is supplemented with relative molecular abundance of non-hydrocarbon species, ideally carbon dioxide and hydrogen sulphide.

[0102] Ideally, the analyses of the method are performed for each rock sample coming from a plurality of different known depths / origins in the reservoir so as to determine for each rock sample the corresponding molecular quantities of the hydrocarbons and possibly also of non-hydrocarbons group, more preferably to determine the distribution of the molecular quantities of C1 - C36 hydrocarbons subdivided by the number of carbon atoms, and possibly also of carbon dioxide and hydrogen sulphide, thus generating corresponding logs of reservoir fluid composition at a given known depth / origi n.

[0103] Preferably, the method provides the combination of a plurality of reservoir fluid composition logs to generate a 3D representation of spatial fluid distributions in the reservoir, ideally comprising also vertical and lateral fluid discontinuities and vertical and lateral fluid gradients.

[0104] The skilled person will appreciate that all preferred or optional features of the invention described with reference to only some aspects or embodiments of the invention may be applied to all aspects of the invention.

[0105] It will be appreciated that optional features applicable to one aspect of the invention can be used in any combination, and in any number. Moreover, they can also be used with any of the other aspects of the invention in any combination and in any number. This includes, but is not limited to, the dependent claims from any claim being used as dependent claims for any other claim in the claims of this application.

[0106] DESCRIPTION OF THE FIGURES

[0107] The present invention is hereinafter further clarified in some of its preferred embodiments shown for purely exemplifying and non-limiting purposes with reference to the accompanying drawings, in which:

[0108] Figure 1 A shows a diagram of the steps in the method according to the invention,

[0109] Figure 1 B shows a diagram of the steps in a further embodiment of the method according to the invention,

[0110] Figure 1C shows a diagram of the steps of another further embodiment of the method according to the invention,

[0111] Figure 2A shows an example of gas chromatograms of a first sub-group of hydrocarbons, and in particular of C1 - C5 hydrocarbons, in first gasses released from a first rock sub-sample,

[0112] Figure 2B shows an example of gas chromatograms of a second sub-group of hydrocarbons, and in particular of 04 - 08 hydrocarbons, in second gasses released from a second rock sub-sample,

[0113] Figure 20 shows the gas chromatograms of Fig. 2A and Fig. 2B highlighting the area of their overlap that is used to compare and combine data from the two respective analyses, Figure 3 shows the normalized molecular composition (i.e. the first distribution) resulting from the combination of the analysis of Fig. 2A and Fig. 2B,

[0114] Figure 4 shows an example of gas chromatograms of a third sub-group of hydrocarbons in an oil extracted from a rock sample,

[0115] Figure 5 shows a reconstruction of the normalized molecular composition of a hydrocarbons group, and in particular of the group containing a plurality or all C1 - C36 hydrocarbons, by using the normal alkane data resulting from the analysis of the first, second and third sub-groups of hydrocarbons,

[0116] Figure 6 shows a reconstruction of the normalized molecular composition of a hydrocarbons group, and in particular of the group containing a plurality or all C1 - C36 hydrocarbons, by using the pseudo-component (PC) data resulting from the analysis of the first, second and third sub-groups of hydrocarbons,

[0117] Figure 7 shows an example of a gas chromatogram of the further sub-group comprising hydrocarbons and non-hydrocarbons, and in particular of C1 - C5 hydrocarbons, carbon dioxide and hydrogen sulphide as resulting from the further analysis of the gasses released from a further rock sub-sample,

[0118] Figure 8 shows a general schematic view of an apparatus used to release gas from rock subsamples.

[0119] DETAILED DESCRIPTION OF THE INVENTION AND OF ITS PREFERRED AND EXEMPLARY EMBODIMENTS

[0120] The method according to the invention is used for determining the fluid composition of a reservoir, in particular of a petroleum reservoir, and uses at least one rock sample of the reservoir for determining the molecular quantities of a group of hydrocarbons having the carbon atoms within a given range.

[0121] A first embodiment of the method according to the invention is shown in Fig. 1 A. In particular, this method comprises:

[0122] - a first analysis 11 of first released gasses, that are released following to the application of a mechanical action, preferably a crushing action 10, on a first rock sub-sample of said rock sample 1 , to determine the molecular quantities of a first hydrocarbons subgroup having the carbon number within a first subrange of said given range, in particular to determine / obtain a distribution 12 of the molecular quantities of said first hydrocarbons subgroup,

[0123] - a second analysis 21 of second released gasses, that are released following to the application of a mechanical action, preferably a crushing action 10, on a second rock sub-sample of said rock sample 1 , so as to determine the molecular quantities of a second hydrocarbons subgroup having the carbon atoms within a second subrange of said range, in particular to determine / obtain a distribution 22 of the molecular quantities of said second hydrocarbons subgroup overlapping partially with said first hydrocarbons subgroup,

[0124] - determining the molecular quantities of the hydrocarbons group by using the molecular quantities so determined of said first hydrocarbons subgroup and of said second hydrocarbons subgroup; in particular, said step provides the combination 30 of the distribution 12 of the molecular quantities of said first hydrocarbons subgroup with the the distribution 22 of the molecular quantities of said second hydrocarbons subgroup so as to determine a first distribution 31 of the molecular quantities of the hydrocarbons group comprising the first subgroup and the second subgroup.

[0125] Preferably, said first hydrocarbons subgroup comprises C1 - C5 hydrocarbons. Preferably, said second hydrocarbons subgroup comprises C4 - C8 hydrocarbons. Preferably, said hydrocarbons group comprises C1 - C8 hydrocarbons. Preferably, the hydrocarbons that are common / overlapped between the first hydrocarbons subgroup and the second hydrocarbons subgroup comprise C4 and C5 hydrocarbons.

[0126] Advantageously, the present invention provides the combination 30 of the C1 - C5 hydrocarbons distribution 12, obtained by the first analysis 11 , with the C4 - C8 hydrocarbons distribution 22, obtained by the second analysis 21 , to determine the C1 - C8 hydrocarbons distribution 31 .

[0127] In a possible embodiment not illustrated in the figures, said second hydrocarbons subgroup may consist only of C4-C6 hydrocarbons and, consequently, the C1-C6 hydrocarbons distribution 31 is determined.

[0128] In a possible embodiment not illustrated in the figures, said second hydrocarbons subgroup may comprise, in addition to C4 - C8 hydrocarbons, also hydrocarbons larger than C8 (for example, up to C11 or even higher) and, consequently, the hydrocarbons distribution 31 that is determined may comprise hydrocarbons from C1 to those larger than C8.

[0129] A further preferred embodiment of the method according to the invention is shown in Fig. 1 B. In particular, in this further preferred embodiment, in addition to the steps of Fig. 1A and to the combination 30 of molecular quantities distribution of the first hydrocarbons subgroup and of said second hydrocarbons subgroup, so as to determine a first distribution 31 of the hydrocarbons subdivided by carbon numbers belonging to the first and second subrange, the method also comprises:

[0130] - a third analysis 41 of oil extracted from a third rock sub-sample of said rock sample, to determine the molecular quantities of a third hydrocarbons subgroup having the carbon atoms within a third subrange of said given range, in particular to determine the distribution 42 of the molecular quantities of said third hydrocarbons subgroup,

[0131] - the calculation 45 of molecular quantities of the fourth hydrocarbons subgroup by using the molecular quantities of the third hydrocarbons subgroup, to determine a further distribution 46 of the hydrocarbons subdivided by carbon numbers belonging to the third and the fourth subrange, wherein said further distribution 46 has at least one carbon component that is common / overlapped with the first distribution 31 ,

[0132] - the further combination 50 of the first distribution 31 with the further distribution 46 so determined so as to obtain a total distribution 51 of the hydrocarbons subdivided by carbon numbers belonging to the wider full range.

[0133] Ideally, said total distribution 51 is then normalized.

[0134] Preferably, said third hydrocarbons subgroup comprises C19 - C36 hydrocarbons. Preferably, said fourth hydrocarbons subgroup comprises C8 - C18 hydrocarbons. Preferably, said first distribution 31 of hydrocarbons comprises the distribution of molecular quantities of C1 - C8 hydrocarbons. Preferably, said further distribution 46 of hydrocarbons comprises the distribution of molecular quantities of C8 - C36 hydrocarbons. Preferably, C8 hydrocarbon is the hydrocarbon that is common / overlapped between the first hydrocarbons distribution 31 and the further hydrocarbons distribution 46. Preferably, said wider / total distribution 51 of hydrocarbons comprises the distribution of molecular quantities of C1 - C36 hydrocarbons.

[0135] Advantageously, the present invention comprises a combination of the C1 - C5 hydrocarbons distribution 12, obtained by the first analysis 11 , with the C4 - C8 hydrocarbons distribution 22, obtained by the second analysis 21 , so as to determine the C1 - C8 hydrocarbons distribution 31 , a third analysis 41 to determine the C19 - C36 hydrocarbons distribution and a calculation 45 by extrapolation, using the C19 - C36 hydrocarbons distribution 42 so as to determine a further distribution 46 of C8 - C36 hydrocarbons, a further combination 50 of the C1 - C8 hydrocarbons distribution 31 with the C8 - C36 hydrocarbons distribution 46 to provide a normalized C1 - C36 hydrocarbons wider / ful I distribution 51 for a given rock sample.

[0136] A further preferred embodiment of the method according to the invention is shown in Fig. 1C. In particular, in this further preferred embodiment, in addition to the steps of Fig. 1A and Fig. 1 B and to the combination 51 of molecular quantities distribution of the C1 - C36 hydrocarbons, the method also comprises:

[0137] - a further analysis 61 of further released gasses, that are released following to the application of a mechanical action, preferably a crushing action 10, on a further rock subsample of said rock sample, to determine the molecular quantities of a further subgroup comprising hydrocarbons and also non-hydrocarbons, in particular to determine an additional distribution 62 of the molecular quantities of at least one of C1 - C5 hydrocarbons and also of carbon dioxide and hydrogen sulphide as non-hydrocarbons,

[0138] - determining a combined distribution 64 of C1 - C36 hydrocarbons with carbon dioxide and hydrogen sulphide as non-hydrocarbons by using the total distribution 51 of C1 - C36 and the additional distribution 62 of the molecular quantities of C1 - C5 hydrocarbons with carbon dioxide and hydrogen sulphide as non-hydrocarbons; in particular, said step provides the combination 63 of the total distribution 51 of the molecular quantities of C1 - C36 hydrocarbons with the additional distribution 62 of the molecular quantities of C1-C5 hydrocarbons with carbon dioxide and hydrogen sulphide as non-hydrocarbons by using the relative abundance of C1 hydrocarbon as common reference, so as to obtain the combined distribution 64 of C1-C36 hydrocarbons with carbon dioxide and hydrogen sulphide as non-hydrocarbons.

[0139] Ideally, said combined distribution 64 is then normalized.

[0140] More in detail, in a possible embodiment (see figure 1 B), the method comprises the following steps of: a) mechanically crushing (step 10) a first sub-sample of a rock sample of the reservoir to release the first gasses (i.e. to liberate first occluded gasses), b) performing the first analysis 11 by analyzing the first released gasses, preferably via gas chromatography with flame ionization detector, to determine the molecular quantities of C1-C5 hydrocarbons (comprising methane, ethane, propane, isobutane, normal butane, neopentane, isopentane, normal pentane), in particular to obtain the distribution 12 of molecular quantities of C1-C5 hydrocarbons, c) repeating step a) on a second sub-sample of a rock sample of the reservoir to release the second gasses (i.e. to liberate second occluded gasses) and performing the second analysis 21 of said second released gasses using different temperature and chromatographic conditions in respect to the first analysis 11 in order to determine the molecular quantities of all isomers of the alkanes, cycloalkanes and aromatic compounds of C4 - C8 hydrocarbons, in particular to obtain the distribution 22 of molecular quantities of C4 - C8 hydrocarbons, d) combining (step 30) the molecular distributions obtained in steps b) and c) using as common / overlapping reference the relative quantities of C4 - C5 hydrocarbons to construct a first distribution 31 of relative molecular quantities of C1 - C8 hydrocarbons, e) extracting oil from a third sub-sample of a rock sample of the reservoir, preferably using a non-polar organic solvent, and analyzing (step 41) the oil so extracted, preferably by gas chromatography with flame ionization detector, f) integrating all normal alkanes or pseudo-components in the oil chromatogram obtained in step e), g) determining the distribution 42 of the molecular quantities of C19 - C36 normal alkanes or pseudo-components (or of components that are not affected by evaporation), h) calculating (step 45) an exponential curve fitting through the C19 - C36 normal alkanes or pseudo-components or through normal alkanes / pseudo-components that are not affected by evaporation as determined in step g), i) extrapolating the molecular quantities of normal alkanes or pseudo-components C8 - C18 using the exponential equation obtained in step h) so as to determine a further distribution 46 with the molecular quantities of C8 - C36 hydrocarbons, j) further combining (step 50) the further distribution of C8 - C36 hydrocarbons obtained in step i) and the first distribution of C1 - C8 hydrocarbons obtained in step d) by using the quantity of C8 hydrocarbon that is a common reference for both distributions, so as to obtain a full distribution 51 of C1 - C36 hydrocarbons, k) re-normalizing to 100% the full distribution 51 of C1 - C36 hydrocarbons of step j).

[0141] Preferably, in a further possible embodiment (see figure 1 C), the method comprises also the following steps of: l) repeating step a) on a further sub-sample of a rock sample to release further gasses (i.e. to liberate further occluded gasses) and performing the further analysis 61 of said further released gasses using different temperature and chromatographic conditions in respect to the first analysis 11 in order to determine the molecular quantities of C1 , carbon dioxide and hydrogen sulphide, m) combining (step 63) the molecular distributions obtained in steps k) and I) using as common / overlapping reference the relative quantities of C1 hydrocarbon (methane) to construct a reservoir fluid combined distribution 62 comprising the distribution 51 of C1- C36 hydrocarbons and also of carbon dioxide and hydrogen sulphide, n) re-normalizing to 100% the combined distribution 62 of step m).

[0142] Preferably, in a possible embodiment not shown in the figure, the method may comprise only and specifically the first analysis 11 to determine the molecular quantities of a first hydrocarbons subgroup (for example of C1 - C5 hydrocarbons) and said further analysis 61 so as to determine the molecular quantities of at least one hydrocarbon of said first hydrocarbons subgroup (for example of C1) and also of non-hydrocarbons, for example carbon dioxide and / or hydrogen sulphide, in order to determine then the molecular distributions of the hydrocarbons of said first subgroup and also of non-hydrocarbons, for example carbon dioxide and / or hydrogen sulphide. In particular, such determination step is made by combining the molecular quantities, obtained by the first analysis 11 and by said further analysis 61 , using as common / overlapping reference the quantity - obtained both by the first analysis 11 and by said further analysis 61 - of said at least one hydrocarbon of said first hydrocarbons subgroup (for example of C1).

[0143] Preferably, rock samples from the zones of interest are collected by sampling core plugs or rock chips from conventional cores, side wall cores or drill cuttings. Preferably, cuttings should be gently washed to remove any drilling mud residues. Preferably, samples are disaggregated to 1-3 mm size before homogenization and splitting into three subsamples.

[0144] The released gases from the corresponding rock subsamples (in particular for the first and / or second subsamples) may be collected as follows. A sample of washed and dried rock 1 is placed into a gas tight receptacle 71 provided with a mechanical crushing device 72. The receptacle 71 is connected by means of a flow line 73 to an analytical instrument 76 comprising a gas chromatograph 74 with a detector 75 (typically flame ionization detector or thermal conductivity detector) for determining the gas composition of liberated gas components. The rock sample 1 is crushed to release entrained gas, which is then transferred to the analytical instrument 76 via the flow line 73 (see Figure 8).

[0145] The first analysis 11 of a first sub-sample can achieve quantitative identification of hydrocarbons in the range C1 to C5 (see Fig. 2A). The second analysis 21 is performed as the first analysis but with modified temperature conditions on the second sub-sample and this allows the quantitative identification of hydrocarbons in the range C4 to C8 (see Fig. 2B).

[0146] For example, the first analysis 11 is performed using an Agilent 6890N gas chromatograph (Agilent Technologies, Santa Clara, US) equipped with a 30m 0.32mm Gaspro column (also from Agilent) and a flame ionization detector. Gases released from the crusher apparatus are trapped in liquid nitrogen before being released into a flow of helium onto the GC column and are separated using an oven temperature program that ramps from 50°C to 260°C, while the second analysis 21 is performed using the same apparatus but with a modified chromatographic column (Agilent, DB-1) and oven temperature program, that ramps from 30°C to 250°C.

[0147] For example, the further analysis 61 for determining the molecular quantities of C1 - C5 hydrocarbons (or at least of C1 hydrocarbon) with non-hydrocarbons is performed using the same apparatus but with a modified chromatographic column (for example Agilent, Porabond Q) and an oven temperature program, that ramps from about 35°C to 250°C, with thermal conductivity detector.

[0148] The relative quantities of the separated hydrocarbons in the range C1 to C8 are calculated by normalizing the quantity of components C4 to C5 that are common in both analyses (see fig. 2C). This allows to determine the pseudo-components of the C1 - C8 hydrocarbons, that is the total composition of hydrocarbons subdivided by carbon number 1

[0149] - 8 (see fig 3).

[0150] Preferably, the third sub-sample of rock for the third analysis 41 is soaked in a nonpolar organic solvent, such as cyclohexane, to dissolve crude oil present within the sample. Removal of oil can be enhanced by standard extraction methods involving heating or sonication. An aliquot of the solvent is removed and (if necessary) suspended sediment is removed by filtration. The supernatant liquid may be injected directly into a gas chromatograph for analysis of the extracted oil material, or if oil yield is too low to achieve a good response, then excess solvent is removed by evaporation prior to GC analysis.

[0151] The resulting gas chromatogram of the third analysis (see fig. 4) corresponds to the output of the oil compositional analysis carried out as part of a standard reservoir fluid analysis program, with the exception that some of the more volatile components of the oil are lost to evaporation. In particular, the evaporation of volatile components occurs during sample transport to surface, during storage and during analysis and so is expected to occur in all samples to some extent in components with less than about fifteen carbon atoms.

[0152] Preferably, the output of the oil gas chromatogram of the third analysis (see fig. 4) is integrated to tabulate the areas of all peaks corresponding to the homologous series of normal alkanes and also to determine abundance of pseudo-components. Subsequently, peak areas of normal alkanes and of pseudo-components are converted to molar fractions using estimated molecular weights according to standard engineering practice. Therefore, this allows the quantitative identification of hydrocarbons in the range C19 to C36.

[0153] Preferably, the oil for the third analysis 41 may be extracted from a third rock subsample by solvent extraction. Ideally, the oil for the third analysis 41 may be extracted from a third rock sub-sample by using a non-polar organic solvent. Preferably, the oil for the third analysis 41 may be extracted from a third rock sub-sample by thermal extraction. Preferably, said third analysis 41 may be performed via gas or liquid chromatography. More preferably, said third analysis 41 may be performed via chromatography with a flame ionization detector.

[0154] The published literature in the technical field of analysis of petroleum reservoir fluids

[0155] - see in particular “Compositional regularities common to petroleum reservoir fluids and pyrolysates of asphaltenes and kerogens” of Keith F.M. Thompson, 2002 in “Organic Geochemistry”, Volume 33, Issue 7, July 2002, pages 829-841 and “Catagenesis and composition of petroleum: Origin of n-alkanes and isoalkanes in petroleum crudes” of Y.V. Kissin, 1987 in “Geochimica et Cosmochimica Acta”, Volume 51 , Issue 9, September 1987, pages 2445-2458” - has shown that the molar quantity of normal alkanes in unaltered crude oil displays an exponential relationship to the carbon number of the molecule above C10 and this relationship holds from C6. In particular, according to a pre-defined model derived from the published literature, it results that the molar concentrations of n-alkanes in reservoir fluids decrease exponentially with increasing carbon number. More in detail, two series, C2- n-C5, and n-C6+, are separated by a slope break, the former being steeper; moreover, the pseudo-components, representing the total composition subdivided by carbon number, also decrease in concentration exponentially beyond C10.

[0156] Hence, by using the normal alkanes or pseudo-components distribution C19 - C36 and by using a predefined model (that is based on the above-mentioned published literature in the technical field of petroleum reservoir fluids analysis) according to which the molar quantities of normal alkanes and pseudo-components in unaltered crude oil displays an exponential relationship to the carbon number of the molecules above C6, the molecular quantities of at least C8 - C18 normal alkanes are calculated by extrapolation. More in detail, a curve fit through normal alkanes distribution C19 - C36 - i.e. normal alkanes that are not affected by evaporation - allows to then calculate by extrapolation the molar quantity of all normal alkanes C8 - C18. Therefore, the further distribution 46 of molar quantities of C8 - C36 normal alkane is obtained. This allows to determine the pseudo-components of the C8 - C36 hydrocarbons, that is the total composition of hydrocarbons subdivided by carbon number 8 - 36.

[0157] Advantageously, the published literature in the technical field of analysis of petroleum reservoir fluids - see in particular “Compositional regularities common to petroleum reservoir fluids and pyrolysates of asphaltenes and kerogens” of Keith F.M. Thompson, 2002 in “Organic Geochemistry”, Volume 33, Issue 7, July 2002, pages 829- 841 - also shows that there is a direct relationship between the regression of normal alkanes data and lumped pseudo-components data in the same oil, which means that n-alkanes may, in a first approximation, act as a proxy for true pseudo-components data.

[0158] Once the molar quantities of C8 - C36 normal alkanes has been determined, the full C1 - C36 hydrocarbon composition is determined by rescaling the C1 - C8 distribution and the C8 - C36 distribution at the common / overlapping reference defined by of C8, and renormalizing to 100 mole% so as to generate the C1 - C36 distribution (see fig. 5). The same procedure can be carried out using pseudo-components data (see fig. 6).

[0159] Advantageously, the method may be extended to further characterize the abundance of non-hydrocarbon gases carbon dioxide and hydrogen sulphide in the reservoir fluid, by way of analysis of a further rock sub-sample. Gases from said further sub-sample are released by crushing and analyzed using gas chromatography with thermal conductivity detection to quantify the abundance of C1 - C5 hydrocarbons and non-hydrocarbons carbon dioxide and hydrogen sulphide (see figure 7). The relative abundance of the non- hydrocarbon components within the total reservoir fluid may be obtained by reference to C1 hydrocarbon (methane) abundance derived from analysis of the further released gases with abundance in the C1 - C36 hydrocarbon distribution.

[0160] In the method according to the invention, the analysis of gas released from samples of rock material collected at surface during the drilling of the well or nearby wells are used. Preferably, cuttings are washed to remove drilling mud and air dried prior to storage. Preferably, they are typically collected at about 10 meters intervals and so offer the opportunity to construct “logs” of gas characteristics throughout the drilled section. Ideally, cuttings from the same well may be analyzed, but samples from nearby wells in the same field can be used if such samples are not available. Core chip samples and side wall core samples may also be used for the analysis.

[0161] The present invention has been illustrated and described in some of its preferred embodiments, but it is understood that executive variants can be applied to them in practice, without however departing from the scope of protection of the present patent for industrial invention.

[0162] In relation to the detailed description of the different embodiments of the invention, it will be understood that one or more technical features of one embodiment can be used in combination with one or more technical features of any other embodiment where the transferred use of the one or more technical features would be immediately apparent to a person of ordinary skill in the art to carry out a similar function in a similar way on the other embodiment.

[0163] In the preceding discussion of the invention, unless stated to the contrary, the disclosure of alternative values for the upper or lower limit of the permitted range of a parameter, coupled with an indication that one of the values is more highly preferred than the other, is to be construed as an implied statement that each intermediate value of the parameter, lying between the more preferred and the less preferred of the alternatives, is itself preferred to the less preferred value and also to each value lying between the less preferred value and the intermediate value.

[0164] The features disclosed in the foregoing description or the following drawings, expressed in their specific forms or in terms of a means for performing a disclosed function, or a method or a process of attaining the disclosed result, as appropriate, may separately, or in any combination of such features be utilized for realizing the invention in diverse forms thereof.

Claims

C L A I M S1. Method for determining the fluid composition of a reservoir, in particular of a petroleum reservoir, by using at least one rock sample (1) of the reservoir, said method comprising:- a first analysis (11) of first released gasses, that are released following to the application of a mechanical action, preferably a crushing action (10), on a first rock sub-sample of said rock sample (1), so as to determine the molecular quantities of a first hydrocarbons subgroup (12),- a second analysis (21) of second released gasses, that are released following to the application of a mechanical action, preferably a crushing action (10), on a second rock sub-sample of said rock sample (1), so as to determine the molecular quantities of a second hydrocarbons subgroup (22) overlapping at least partially with said first hydrocarbons subgroup (12),- determining (30) the molecular quantities of an hydrocarbons group (31 ) comprising said first hydrocarbons subgroup (12) and second hydrocarbons subgroup (22) by using the molecular quantities so determined by means of said first analysis (11) and said second analysis (21) wherein:- said first hydrocarbons subgroup (12) comprises C1 - C5 hydrocarbons,- said second hydrocarbons subgroup (22) comprises at least C4 - C6 hydrocarbons,- said hydrocarbons group (31) comprises at least C1 - C6 hydrocarbons,- the hydrocarbons that are common between the first hydrocarbons subgroup (12) and the second hydrocarbons subgroup (22) comprise C4 and C5 hydrocarbons,- pseudo-components of the first hydrocarbons subgroup C1 - C5, comprising total composition of all said hydrocarbons subdivided by carbon number, are derived directly by the integration of the molecular quantities obtained by said first analysis (11),- pseudo-components of the second hydrocarbons subgroup of at least C4 - C6, comprising total composition of all said hydrocarbons subdivided by carbon number, are derived directly by the integration of the molecular quantities obtained by said second analysis (21).

2. Method according to claim 1 , wherein:- the first analysis (11) reconstructs a distribution of the molecular quantities of the first hydrocarbons subgroup (12),- the second analysis (21) reconstructs a distribution of the molecular quantities of the second hydrocarbons subgroup (22),- said step of determining (30) the molecular quantities of the hydrocarbons group (31) by using the molecular quantities so determined of said first hydrocarbons subgroup and of said second hydrocarbons subgroup comprises the combination of the distribution of the molecular quantities of the first hydrocarbons subgroup (12) with the distribution of the molecular quantities of the second hydrocarbons subgroup (22) by using the molecular quantities of common / overlapping hydrocarbons as reference, so as to obtain a first distribution (31 ) of the molecular quantities of the hydrocarbons group.

3. Method according to one or more of the previous claims, wherein:- said second hydrocarbons subgroup (22) comprises at least C4 - C8,- said hydrocarbons group (31) comprises at least C1 - C8,- pseudo-components of the second hydrocarbons subgroup of at least C4 - C8, comprising total composition of all said hydrocarbons subdivided by carbon number, are derived directly by the integration of the molecular quantities obtained by said second analysis.

4. Method according to one or more of the previous claims, wherein said method comprises:- a further analysis (61) of further released gasses, that are released following to the application of a mechanical action, preferably a crushing action (10), on a further rock sub-sample of said rock sample, so as to determine the molecular quantities of nonhydrocarbons and also of at least one hydrocarbon belonging to said first hydrocarbons subgroup (12),- determining (63) the molecular quantities of non-hydrocarbons, preferably carbon dioxide and hydrogen sulphide, by using the molecular quantities so determined by means of said first analysis (11) and by said further analysis (61).

5. Method according to the previous claim, wherein said non-hydrocarbons comprise carbon dioxide and hydrogen sulphide.

6. Method according to one or more of the previous claims, wherein:- the first analysis (11) reconstructs a distribution of the molecular quantities of the first hydrocarbons subgroup (12),- the further analysis (61 ) reconstructs a distribution of the molecular quantities of at least one hydrocarbon belonging to the first hydrocarbons subgroup (12) and also of non- hydrocarbons, preferably carbon dioxide and hydrogen sulphide,- said step of determining (63) the molecular quantities of non-hydrocarbons, preferably carbon dioxide and hydrogen sulphide, comprises the combination of the distribution of the molecular quantities of the first hydrocarbons subgroup (12) with the distribution ofthe molecular quantities of non-hydrocarbons (62), preferably carbon dioxide and hydrogen sulphide, by using as reference the molecular quantities of said at least one hydrocarbon belonging to the first hydrocarbons subgroup (12), so as to obtain an extended distribution of the molecular quantities of the first hydrocarbons subgroup (12) and also of non-hydrocarbons, preferably carbon dioxide and hydrogen sulphide.

7. Method according to one or more of the previous claims, wherein it provides the distribution of the molecular quantities of the hydrocarbons group subdivided by the number of carbon atoms.

8. Method according to one or more of the previous claims, wherein it provides the pseudo-components of the hydrocarbons group and wherein said pseudo-components comprise all components occurring between normal alkanes.

9. Method according to one or more of the previous claims, wherein the method further comprises:- a third analysis (41 ) of oil extracted from a third rock sub-sample of said rock sample (1 ) or of third released fluids, that are released by a chemical treatment or a thermal treatment or by a physical / mechanical action on a third rock sub-sample of said rock sample, so as to determine the molecular quantities of a third hydrocarbons subgroup (42),- determining (50) the molecular quantities of a wider hydrocarbons group by using the molecular quantities so determined of said first hydrocarbons subgroup, of said second hydrocarbons subgroup and of said third hydrocarbons subgroup.

10. Method according to the previous claim, wherein:- said third hydrocarbons subgroup comprises C19 - C36 hydrocarbons,- pseudo-components of the third subgroup are calculated by considering n-alkanes of said third subgroup.

11. Method according to one or more of the previous claims, wherein it further comprises the calculation step of the molecular quantities of a fourth hydrocarbons subgroup.

12. Method according to the previous claim, wherein said calculation step is based on the molecular quantities so determined of said third hydrocarbons subgroup and / or of said second subgroup.

13. Method according to the previous claim, wherein the molecular quantities of said fourth hydrocarbons subgroup are calculated by extrapolation using the molecular quantities of said third hydrocarbons subgroup and assuming that in reservoir oil the molar quantity of normal alkanes or pseudo-components with respect to their carbon atom numbers follow an exponential relationship.

14. Method according to one or more of the claims 11 - 13, wherein:- said calculated fourth hydrocarbons subgroup comprises at least C8 - C18 hydrocarbons, preferably C6 - C18,- pseudo-components of the fourth subgroup are calculated by regression using pseudocomponents or normal alkanes distribution of the third subgroup.

15. Method according to one or more of the previous claims, wherein it also comprises:- the combination (30) of molecular quantities distribution of the first hydrocarbons subgroup obtained by said first analysis (11) and of said second hydrocarbons subgroup obtained by said second analysis (21), so as to determine a first distribution (31) of the hydrocarbons subdivided by carbon numbers belonging respectively to a first subrange and a second subrange,- said third analysis (41), so as to determine the distribution (42) of the molecular quantities of said third hydrocarbons subgroup,- the calculation (45) of molecular quantities of the fourth hydrocarbons subgroup by using the molecular quantities of the third hydrocarbons subgroup, so as to determine a further distribution (46) of the hydrocarbons subdivided by carbon numbers belonging respectively to a third subrange and a fourth subrange, wherein said further distribution (46) has at least one carbon component that is common / overlapped with the first distribution (31),- the further combination (50) of the first distribution (31) with the further distribution (46) so determined so as to obtain a total distribution (51) of the hydrocarbons subdivided by carbon numbers belonging to the full range.

16. Method according to the previous claim, wherein said total distribution (51) is normalized to 100%.

17. Method according to one or more of the previous claims, wherein:- said third hydrocarbons subgroup comprises C19 - C36 hydrocarbons,- said fourth hydrocarbons subgroup comprises C8 - C18 hydrocarbons,- said first distribution (31) comprises the distribution of C1 - C8 hydrocarbons,- said further distribution (46) comprises the distribution of C8 - C36 hydrocarbons,- C8 hydrocarbon is the common reference for the first distribution and the further distribution,- said total hydrocarbons distribution comprises the distribution of C1 - C36 hydrocarbons.

18. Method according to one or more of the previous claims, wherein:- said first hydrocarbons subgroup comprises pseudo-components or normal alkanes of C1 - C5 hydrocarbons,- said second hydrocarbons subgroup comprises pseudo-components or normal alkanes of C4 - C8 hydrocarbons,- said third hydrocarbons subgroup comprises pseudo-components or normal alkanes of C19 - C36 hydrocarbons,- said fourth hydrocarbons subgroup comprises pseudo-components or normal alkanes of C8 - C18 hydrocarbons,- said first distribution (31) comprises the distribution of pseudo-components or normal alkanes of C1 - C8 hydrocarbons,- said further distribution (46) comprises the distribution of pseudo-components or normal alkanes of C8 - C36 hydrocarbons,- said total hydrocarbons distribution comprises the distribution of pseudo-components or normal alkanes of C1 - C36 hydrocarbons, wherein pseudo-components comprise total composition of hydrocarbons subdivided by the corresponding carbon numbers.

19. Method according to one or more of the previous claims, wherein the rock sample (1 ) comprises drill cuttings, side wall core chips and / or conventional core chips.

20. Method according to one or more of the previous claims, wherein it comprises the following steps of: a) mechanically crushing (10) a first sub-sample of a rock sample (1) of the reservoir to release the first gasses, b) performing said first analysis (11) of said first released gasses, preferably via gas chromatography with detector, to determine the distribution (12) of molecular quantities of C1 - C5 hydrocarbons, c) repeating step a) on a second sub-sample of a rock sample of the reservoir to release the second gasses, and performing said second analysis (21) of said second gasses, preferably via gas chromatography with detector, using different temperature and chromatographic conditions in respect to the first analysis (11) to determine the distribution (22) of molecular quantities of C4 - C8 hydrocarbons, d) combining (30) the molecular distributions obtained in steps b) and c) using as common reference the relative quantities of C4 - C5 hydrocarbons to construct a first distribution (31) of relative molecular quantities of C1 - C8 hydrocarbons.

21. Method according to the previous claim, wherein it further comprises: e) extracting oil from a third sub-sample of a rock sample (1 ) of the reservoir and analyzing (41) the oil so extracted, f) integrating all normal alkanes in the oil chromatogram obtained in step e),g) determining the distribution (42) of the molecular quantities of C19 - C36 normal alkanes, h) calculating (45) an exponential curve fitting through the C19 - C36 normal alkanes as determined in step g), i) extrapolating the molecular quantities of normal alkanes C8 - C18 using the exponential equation obtained in step h) so as to obtain a further distribution (46) with the molecular quantities of C8 - C36 hydrocarbons, j) further combining (50) the further distribution of C8 - C36 hydrocarbons obtained in step i) and the first distribution of C1 - C8 hydrocarbons obtained in step d) by using the quantity of C8 hydrocarbon that is a common reference for both distributions, so as to determine a full distribution (51) of C1 - C36 hydrocarbons, k) re-normalizing to 100% the full distribution (51 ) of C1 - C36 hydrocarbons of step j).

22. Method according to the previous claim, wherein it further comprises the following steps for the combination of the two distributions of hydrocarbons with non-hydrocarbons: l) repeating step a) on a further sub-sample of a rock sample (1 ) to release further gasses and performing a further analysis (61) of said further released gasses, preferably via gas chromatography with thermal conductivity detector, using different temperature and chromatographic conditions in respect to the first analysis (11) in order to determine the molecular quantities of C1 hydrocarbon and also of carbon dioxide and hydrogen sulphide as non-hydrocarbon gases, m) combining (63) the molecular distributions obtained in steps k) and I) using as common / overlapping reference the relative quantities of C1 hydrocarbon to construct a combined distribution (64) comprising the full distribution (51 ) of C1 - C36 hydrocarbons and also of carbon dioxide and hydrogen sulphide as non-hydrocarbon gases, n) re-normalizing to 100% said combined distribution (64) of step m).

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