NL2032835B1 - Digital core simulation based and computer implemented method for evaluating brittleness of shale oil and gas reservoir - Google Patents
Digital core simulation based and computer implemented method for evaluating brittleness of shale oil and gas reservoir Download PDFInfo
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- 238000000034 method Methods 0.000 title claims abstract description 48
- 238000004088 simulation Methods 0.000 title claims abstract description 16
- 239000003079 shale oil Substances 0.000 title claims abstract description 12
- 229910052500 inorganic mineral Inorganic materials 0.000 claims abstract description 110
- 239000011707 mineral Substances 0.000 claims abstract description 110
- 239000011435 rock Substances 0.000 claims abstract description 54
- 239000012530 fluid Substances 0.000 claims abstract description 32
- 239000011148 porous material Substances 0.000 claims abstract description 19
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- 229910021532 Calcite Inorganic materials 0.000 claims description 6
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- 239000010453 quartz Substances 0.000 claims description 6
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N silicon dioxide Inorganic materials O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims description 6
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- 238000006073 displacement reaction Methods 0.000 claims description 4
- 230000035945 sensitivity Effects 0.000 claims description 2
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- 238000011160 research Methods 0.000 abstract description 15
- 238000011156 evaluation Methods 0.000 abstract description 6
- 238000001914 filtration Methods 0.000 description 4
- 230000009466 transformation Effects 0.000 description 4
- 239000004927 clay Substances 0.000 description 2
- 229910052570 clay Inorganic materials 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 230000002349 favourable effect Effects 0.000 description 2
- 239000005416 organic matter Substances 0.000 description 2
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 229910001748 carbonate mineral Inorganic materials 0.000 description 1
- 238000012512 characterization method Methods 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 229910000514 dolomite Inorganic materials 0.000 description 1
- 239000010459 dolomite Substances 0.000 description 1
- 239000008398 formation water Substances 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 229910052683 pyrite Inorganic materials 0.000 description 1
- 239000011028 pyrite Substances 0.000 description 1
- NIFIFKQPDTWWGU-UHFFFAOYSA-N pyrite Chemical compound [Fe+2].[S-][S-] NIFIFKQPDTWWGU-UHFFFAOYSA-N 0.000 description 1
- 238000012795 verification Methods 0.000 description 1
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Abstract
Disclosed is a digital core simulation based and computer implemented method for evaluating brittleness of a shale oil and gas reservoir. The method includes: step 1, 5 constructing a three-dimensional digital core model of shale on the basis of a two-dimensional core image; step 2, constructing three-dimensional digital core models of shale having different micropore development, fluid characteristics, fracture occurrences and mineral component features, step 3, using digital rock physical experiments to carry out single-factor analysis on influences of porosities, the fluid 10 characteristics, the fracture occurrences and minerals on rock brittleness, and step 4, establishing a brittleness indeX model comprehensively reflecting mineral components, the fracture occurrences, pores and the fluid characteristics. The present invention provides a new method for shale rock physics and rock mechanics research, and uses the digital core based digital rock physical experiments, to provide a basic theoretical 15 support for shale brittleness logging evaluation.
Description
DIGITAL CORE SIMULATION BASED AND COMPUTER IMPLEMENTED
METHOD FOR EVALUATING BRITTLENESS OF SHALE OIL AND GAS
RESERVOIR
[01] Shale gas is an unconventional oil and gas resource having great potential.
The exploitation of shale gas depends on effective volume fracturing transformation.
The better the brittleness of shale is, the stronger the fracture making ability is, and the better the transformation effect is. Therefore, brittleness is one of the important contents of shale reservoir geological evaluation and an important parameter to select favorable areas and favorable intervals.
[02] The research on shale brittleness is still in the exploratory stage in China, and the current research mainly focuses on characterizing shale brittleness by means of shale mineral components or elastic parameters. However, the problem of mineral method characterization of rock brittleness 1s mainly shown as follows: (1) brittle minerals are vaguely defined, and it is widely believed that quartz is a brittle mineral, but whether the carbonate mineral (calcite and dolomite) and pyrite belong to brittle minerals is controversial; (2) the influence of porosity, fracture and fluid brittleness lacks of consideration; and (3) the degree of influence of the content and distribution of brittle minerals on the brittleness is not fully reflected in the brittleness index of minerals.
[03] In addition, the research on the brittle failure features, fracture mechanism and brittleness mechanism of shale is still limited to the macroscopic core scale, and the macroscopic fracture of rock is closely related to the microstructure features such as microcracks, pores and laminae inside the rock. It is of great significance to carry out the experimental research on the mechanical properties of shale on the microscopic scale and establish the relation between the microstructure features and macroscopic mechanical properties for deeply understanding the brittleness mechanism of shale.
However, the sample analysis and rock mechanics experiment have a long period and high cost. For rock having strong heterogeneity and anisotropy such as shale gas reservoir, it 1s difficult to obtain representative samples due to the limited factors such as core source and yield rate. With limitation by the sample quantity and individual difference, the experiment results are not universal. Therefore, it is extremely difficult to carry out traditional rock physical experiment for the shale gas reservoir.
[04] The technical problem to be solved by the present invention is:
[05] (1) difficulty of a traditional physical experiment of shale is great;
[06] (2) brittle minerals are vaguely defined;
[07] (3) influences of pores, fractures and fluids on brittleness lack of consideration; and
[08] (4) influences of the content and distribution of brittle minerals on brittleness is not reflected in a mineral brittleness index.
[09] The present invention provides a digital core simulation based and computer implemented method for evaluating brittleness of a shale oil and gas reservoir. The method includes:
[10] step 1, constructing a three-dimensional digital core model of shale on the basis of a two-dimensional core image; [HI] step 2, constructing three-dimensional digital core models of shale having different micropore development, fluid characteristics, fracture occurrences and mineral component features on the basis of a core embedment technology,
[12] step 3, using digital rock physical experiments to carry out single-factor analysis on influences of porosities, the fluid characteristics, the fracture occurrences and minerals on rock brittleness on the basis of the three-dimensional digital core models of shale; and
[13] step 4, determining main influence factors and a weight factor of each of influence factors on the basis of an influence degree of each of the influence factors, so as to establish a brittleness index model comprehensively reflecting mineral components, the fracture occurrences, pores and the fluid characteristics.
[14] Further, step 1 includes:
[15] step 1.1, determining main mineral components of a shale core according to geological data and core mineral analysis data;
[16] step 1.2, aiming at the obtained two-dimensional core image of shale, identifying the main mineral components, and using a multi-threshold segmentation method to carry out image segmentation according to an image gray level data histogram to obtain distribution of main minerals in the two-dimensional core image;
[17] step 1.3, on the basis of the two-dimensional core image after mineral component identification, using a Markov chain-Monte Carlo based method to reconstruct a three-dimensional digital core of shale to establish the three-dimensional digital core model of shale; and
[18] step 1.4, verifying the three-dimensional digital core model of shale, and comparing types and relative contents of mineral components of the two-dimensional core image with types and relative contents of mineral components of the constructed three-dimensional digital core.
[19] Further, step 2 includes:
[20] step 2.1: determining core porosities according to geological data and core analysis data, and then using a random method to construct the three-dimensional digital core models having different porosities;
[21] step 2.2, using a lattice Boltzmann method to carry out two-phase displacement simulation in a pore space to obtain three-dimensional digital core models having different oil and gas saturations;
[22] step 2.3, obtaining basic information of the fracture occurrences according to the geological data, then extracting fractures developed in the core by means of an image filtering technology and an image segmentation technology on the basis of the two-dimensional core image, computing parameters of fracture widths, fracture dips, etc, and finally, using a slab model to establish the three-dimensional digital core models having different fracture occurrences;
[23] step 2.4, determining components and contents of main minerals according to the core analysis data or experimental data, and then using a random method to construct three-dimensional digital core models having different mineral types and different mineral contents; and
[4] step 2.5, using the core embedment technology to embed the three-dimensional digital core models having pores, fluid characteristics, fracture occurrences and minerals established from steps 2.1 to 2.4 into the three-dimensional digital core model of shale established in step 1.3, so as to establish three-dimensional digital core models of shale having different porosities, fluid characteristics, fracture occurrences, mineral components and distributions.
[25] Further, step 3 includes:
[26] step 3.1, on the basis of the three-dimensional digital core models of shale, using a finite element method to simulate and compute elastic parameters of rock, i.e.,
Young's modulus and Poisson's ratio;
[27] step 3.2: using the following formula to compute a rock brittleness index:
EE Ein VV gin
BI=50 4 rer)
[28] Er En Vax TV min
[29] where BI is a brittleness index, E is a Young's modulus, v is a Poisson's ratio,
Emax represents a maximum Young's modulus of the rock, Emin represents a minimum
Young's modulus, Vmax is a maximum Poisson's ratio of the rock, and Vmin is a minimum Poisson's ratio of the rock; and
[30] step 3.3, using a single factor method to research influences of the porosities, the fluid characteristics, the fracture occurrences and the mineral components on rock brittleness respectively, and determining the influence degree of each of the influence factors. 31] Further, step 4 includes:
[32] step 4.1, selecting fractures and mineral components as main factors, and taking corresponding sensitive degrees in sensitive factor analysis as weight factors
Weiracture and W minerat;
[33] step 4.2, establishing a new brittleness index model according to the main influence factors and the weight factors:
[34] BI new = Wfacte*BL fracture+Wminera<BL mineral,
[35] where BI new represents the new brittleness index model, BI fracture 5 represents a computed brittleness index of the fracture, BI mineral represents a computed brittleness index of a mineral, Wiacture represents the weight factor of the fracture, and Wminerat represents the weight factor of the mineral; and
[36] the brittleness index BI facture of the fracture is determined according to the fracture porosity, and
[37] the brittleness index BI mineral of the mineral is determined by using the following formula: son tin a ore Poe Veere 100
BI mineral nar Pon Voer Vonn Pew “Lown Vas Vonn Vane
[38] - 2
[39] where Vquanz is the content of quartz, Veateite 1s the content of calcite, and
Vminerat is the content of all the minerals.
[40] The present invention has the beneficial effects:
[41] (1) aiming at the problem of great difficulty of a traditional physical experiment of shale, the present invention uses a digital core technology to construct the multi-component digital core of the shale reservoir for a digital rock physical experiment;
[42] (2) aiming at the problem of vague definition of brittle minerals, the present invention uses a digital rock physical experiment method to research influences of different mineral components on the elastic parameters and the brittleness indexes of the rock;
[43] (3) aiming at the lack of consideration of influences of pores, fractures and fluids on brittleness, the present invention uses the digital core technology to construct digital cores having different pores, fracture features and fluid saturations to research influences of the above factors on the elastic parameters and brittleness indexes of rock;
[44] (4) aiming at the fact that influences of the content and the distribution form of brittle minerals on brittleness are not reflected in the mineral brittleness index, the present invention selects the main influence factors to establish the new brittleness index model on the basis that a digital rock physical experiment researches an influence of each of the factors on the brittleness index; and
[45] (5) the present invention may replace the traditional physical experiment extremely difficult to complete in practice, to provide a new method and idea for shale rock physics and rock mechanics research, and moreover, uses a digital core based digital rock physical experiment, to carry out exploratory research on a shale brittleness mechanism, and provide a basic theoretical support for shale brittleness logging evaluation and even fracturing transformation research of the shale reservoir.
Along with development of shale gas exploration and development in China, the requirement for transformation and fracturing of the shale reservoir is continuously increased, and an increasing number of reservoir brittleness evaluation is required, and the present invention has an important effect of brittleness evaluation and fracturability evaluation of the shale reservoir at present and in the future.
[46] FIG. 1 is a flow chart of a digital core simulation based and computer implemented method for evaluating brittleness of a shale oil and gas reservoir;
[47] FIG. 2 is a core image (b) after different mineral components are obtained by means of multi-threshold segmentation on the basis of a two-dimensional core image (a);
[48] FIG. 3 1s construction of a multi-component three-dimensional digital core (a) of shale and a verification (b) of mineral components and contents of a three-dimensional digital core by using a Markov chain-Monte Carlo based method on the basis of the two-dimensional core image after mineral identification;
[49] FIG. 4 is use of image segmentation to extract pores (d-f) and use of image filtering to obtain fractures (g-i) for core images (a-c) of xoy, yoz and xoz planes;
[50] FIG. 5 is an establishment of three-dimensional digital core models of shale having different microscopic features by using a core embedment technology to embed three-dimensional digital core models having different fracture occurrences (a)-(c), different porosities (d)-(f), different fluid characteristics (g)-(1), mineral contents and distribution features (j)-(I) into a multi-component three-dimensional digital core model (m) of shale;
[51] FIG. 6 is determination of a sensitive degree of each of influence factors by using single-factor analysis; and
[52] FIG. 7 is obtainment of a new brittleness index model (Brittleness index new) for a shale reservoir on the basis of a three-dimensional digital core technology for log interpretation.
[53] The present invention provides a digital core simulation based and computer implemented method for evaluating brittleness of a shale oil and gas reservoir. The method includes:
[54] step 1, construct a three-dimensional digital core model of shale on the basis of a two-dimensional core image.
[59] Step 1.1, determine main mineral components of a shale core according to geological data and core mineral analysis data;
[56] step 1.2, aiming at the obtained two-dimensional core image of shale, identify the main mineral components, and use a multi-threshold segmentation method to carry out image segmentation according to an image gray level data histogram to obtain distribution of main minerals in the two-dimensional core image;
[57] step 1.3, on the basis of the two-dimensional core image after mineral component identification, use a Markov chain-Monte Carlo based method to reconstruct a three-dimensional digital core of shale to establish the three-dimensional digital core model of shale; and
[58] step 1.4, verify the three-dimensional digital core model of shale, and compare types and relative contents of mineral components of the two-dimensional core image with types and relative contents of mineral components of the constructed three-dimensional digital core.
[59] Step 2, construct three-dimensional digital core models of shale having different micropore development, fluid characteristics, fracture occurrences and mineral component features on the basis of a core embedment technology.
[60] Step 2.1: determine core porosities according to geological data and core analysis data, and then use a random method to construct the three-dimensional digital core models having different porosities;
[61] step 2.2, use a lattice Boltzmann method to carry out two-phase displacement simulation in a pore space to obtain three-dimensional digital core models having different oil and gas saturations;
[62] step 2.3, obtain basic information of the fracture occurrences according to the geological data, then extract fractures developed in the core by means of an image filtering technology and an image segmentation technology on the basis of the two-dimensional core image, compute parameters of fracture widths, fracture dips, etc., and finally, use a slab model to establish the three-dimensional digital core models having different fracture occurrences;
[63] step 2.4, determine components and contents of main minerals according to the core analysis data or experimental data, and then use a random method to construct three-dimensional digital core models having different mineral types and different mineral contents; and
[64] step 2.5, use the core embedment technology to embed the three-dimensional digital core models having pores, fluid characteristics, fracture occurrences and minerals established from steps 2.1 to 2.4 into the three-dimensional digital core model of shale established in step 1.3, so as to establish three-dimensional digital core models of shale having different porosities, fluid characteristics, fracture occurrences, mineral components and distributions.
[65] Step 3, use digital rock physical experiments to carry out single-factor analysis on influences of porosities, the fluid characteristics, the fracture occurrences and minerals on rock brittleness on the basis of the three-dimensional digital core models of shale.
[66] Step 3.1, on the basis of the three-dimensional digital core models of shale, use a finite element method to simulate and compute elastic parameters of rock, i.e,
Young's modulus and Poisson's ratio;
[67] step 3.2: use the following formula to compute a rock brittleness index:
E-E VV nis
BI=50( ZL 4 lj)
[68] Far =F Vows Vo (1)
[69] where Bl is a brittleness index, E is a Young's modulus, v is a Poisson's ratio,
Emax represents a maximum Young's modulus of the rock, Emin represents a minimum
Young's modulus, vmax is a maximum Poisson's ratio of the rock, and Vmin is a minimum Poisson's ratio of the rock; and
[70] step 3.3, use a single factor method to research influences of the porosities, the fluid characteristics, the fracture occurrences and the mineral components on rock brittleness respectively, and determine the influence degree of each of the influence factors.
[71] Step 4, determine main influence factors and a weight factor of each of influence factors on the basis of an influence degree of each of the influence factors, so as to establish a brittleness index model comprehensively reflecting mineral components, the fracture occurrences, pores and the fluid characteristics.
[72] Step 4.1, determine weight factors of the main influence factors on the basis of single-factor analysis; and
[73] step 4.2, establish a new brittleness index model according to the main influence factors and the weight factors.
[74] The specific implementation of the present invention will be further described below in combination with the drawings and particular embodiments:
[75] Embodiment 1
[76] With a new brittleness index model provided on the basis of a digital core technology for a shale reservoir of 4050 m-4125 m of a well Z of an oilfield in China as an example, FIG. 1 is a digital core simulation based and computer implemented method for evaluating brittleness of a shale oil and gas reservoir. The method specifically includes:
[77] step 1, construct a three-dimensional digital core model of shale on the basis of a two-dimensional core image.
[78] Step 1.1, determine main mineral components of a shale core as an organic matter, clay, quartz and calcite according to geological data and core mineral analysis data;
[79] step 1.2, for the obtained two-dimensional core image of the shale (FIG. 2(a and b)), use multi-threshold segmentation to obtain different mineral distributions, i.e.,
O-organic matter, 1-clay, 2-quartz and 3-calcite; and
[80] step 1.3, on the basis of the two-dimensional image of the shale core after mineral component identification, use a Markov chain-Monte Carlo based method to establish a three-dimensional digital core model of shale, which is shown in FIG. 3(a); and then compare types and relative contents of mineral components of the two-dimensional core image with types and relative contents of mineral components of the constructed three-dimensional digital core, which is shown in FIG. 3(b), to verify correctness of the constructed three-dimensional digital core of shale.
[81] Step 2, construct three-dimensional digital core models of shale having different micropore development, fluid characteristics, fracture occurrences and mineral component features on the basis of a core embedment technology.
[82] Step 2.1:
[83] firstly; on the basis of core images of three planes of xoy(FIG. 4a), yoz(FIG. 4b) and xoz(FIG. 4c), extract fractures developed in the core (FIG. 4(g-1)) by means of an image filtering technology (FIG. 4(d-f)) and an image segmentation technology;
[84] secondly, define a fracture width as the number of voxels occupied by the fractures, where voxels having widths of 8.2, 19.2 and 7.2 are obtained respectively;
define a fracture length as a ratio of the fracture length to a core length in the two-dimensional core image, where the fracture length is mainly 38.3%; and define a fracture dip as an angle between the fracture and a horizontal direction in the core image, where an average fracture dip angle 1s 66.8°; and
[85] finally, use a slab model to establish three-dimensional digital core models having different fracture occurrences, which is shown in FIG. 5(a-c);
[86] step 2.2, determine a core porosity according to geological data and core analysis data, and then use a random method to construct three-dimensional digital core models having different porosities, which is shown in FIG. 5(d-f);
[87] step 2.3, use a lattice Boltzmann method to carry out two-phase displacement simulation in a pore space to obtain three-dimensional digital core models having different oil and gas saturations, which is shown in FIG. 5(h-1), where dark black is gas, and light grey is formation water;
[88] step 2.4, use a random method to construct three-dimensional digital core models having different mineral types and different mineral contents, which is shown in FIG. 5 (1-1); and
[89] step 2.5, use the core embedment technology to embed the three-dimensional digital core models having pores, fluid characteristics, fracture occurrences and minerals established from steps 2.1 to 2.4 into the three-dimensional digital core model of shale (FIG. 5m) established in step 1.3, so as to establish three-dimensional digital core models of shale having different porosities, fluid characteristics, fracture occurrences, mineral components and distributions.
[90] Step 3, use digital rock physical experiments to carry out single-factor analysis on influences of porosities, the fluid characteristics, the fracture occurrences and minerals on rock brittleness on the basis of the three-dimensional digital core models of shale.
[91] Step 3.1, on the basis of the three-dimensional digital core models, use a finite element method to simulate and compute elastic parameters of rock, 1.e., Young's modulus and Poisson's ratio;
[92] step 3.2: use the following formula to compute a rock brittleness index:
LEE Vn
BI=50 +) 93] Foor“ Fim Vaar “Vaar (3)
[94] where Bl is a brittleness index, E is a Young's modulus, v is a Poisson's ratio,
Emax and Emin represent a maximum Young's modulus and a minimum Young's modulus of the rock respectively, and vmax and Vmin are a maximum Poisson's ratio and a minimum Poisson's ratio of the rock respectively, and it may be seen by means of analysis of rock mechanics experimental data that a range of the elastic parameters of the rock in a research area is: Vmax = 0.4, Vmin = 0.15, Emax = 55.2 Gpa and Ewin = 6.9
Gpa; and
[99] step 3.3: use a single factor method to obtain a sensitive degree of each of influence factors, which is shown in FIG. 6, sensitivity is sequentially reduced from the fractures, the mineral components, the fluid characteristics to the pores.
[96] Step 4, determine main influence factors and a weight factor of each of influence factors on the basis of an influence degree of each of the influence factors, so as to establish a brittleness index model comprehensively reflecting mineral components, the fracture occurrences, pores and the fluid characteristics.
[97] Step 4.1, select fractures and mineral components as main factors, and take corresponding sensitive degrees in sensitive factor analysis as weight factors W fractre and Wminerat, and
[98] step 4.2, establish a new brittleness index model according to the main influence factors and the weight factors:
[99] BI new = Wfacme*BL fracture+W minera”BL mineral,
[100] where BI new represents the new brittleness index model, BI fracture and
BI mineral represent computed brittleness indexes of the fracture and the mineral, and
Wiacture and Wminera represent weight factors of the fracture and the mineral; and
[101] the brittleness index BI facture of the fracture is related to the fracture porosity, and the brittleness index BI mineral of the mineral is mainly related to types and mechanical parameters of the mineral:
son pr Yn yo Laer Poten oere 190 } Eras Eon Var Vin Vomered
BI mineral = —m™m™mWm ™ mmm
[102] - 2 , (3),
[103] where it may be seen from analysis of rock mechanics experimental data that a range of elastic parameters of the rock in the research area IS: Vmax = 0.4, Vmin = 0.15,
Emax = 55.2 Gpa and Emin = 6.9 Gpa, Vqawtz is the content of quartz, Veit is the content of calcite, and Vminerat 15 the content of all minerals.
[104] The twelfth column in FIG. 7 provides a brittleness index computed on the basis of the new brittleness index.
[105] It may be seen from comparison between Brittleness index new and the brittleness index Brittleness index computed according to mineral types that the new brittleness index is greater than the mineral components based brittleness index when there are fractures and pores, which indicates that the new brittleness index comprehensively reflects an influence of fractures and mineral components on rock brittleness
[106] What is described above is only preferred embodiments of the present invention and is not intended to limit the present invention, which may be modified and changed, for those skilled in the art. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should fall within the scope of protection of the present invention.
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