CN111624233A - Shale gas saturation calculation method based on resistivity method - Google Patents
Shale gas saturation calculation method based on resistivity method Download PDFInfo
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- G—PHYSICS
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
- G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
- G01N27/041—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a solid body
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- E—FIXED CONSTRUCTIONS
- E21—EARTH DRILLING; MINING
- E21B—EARTH DRILLING, e.g. DEEP DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B49/00—Testing the nature of borehole walls; Formation testing; Methods or apparatus for obtaining samples of soil or well fluids, specially adapted to earth drilling or wells
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume, or surface-area of porous materials
- G01N15/08—Investigating permeability, pore-volume, or surface area of porous materials
- G01N15/088—Investigating volume, surface area, size or distribution of pores; Porosimetry
Abstract
The invention provides a shale gas saturation calculation method based on a resistivity method, wherein an equivalent water conductivity item in an effective pore is added after a traditional calculation model formula to obtain an improved calculation model formula; the term equivalent water conductivity in the effective pore is expressed as
Wherein: EPOR-equivalent Water porosity, V/V; m is the cementation index corresponding to the effective pore without dimension; a-lithology coefficient associated with effective pore space, dimensionless; b-lithologic saturation coefficient, dimensionless; rwFormation water resistivity, Ω. Compared with the prior art, the method is based on the recognition of the low resistance cause of the shale gas layer, and realizes the transmission statistics calculation model by supplementing the equivalent water conductivity itemAnd correcting the high conductivity to obtain an improved calculation model formula, so that the problem of accurate calculation of the saturation of the free gas in the low-resistivity shale gas can be solved.
Description
Technical Field
The invention relates to the technical field of unconventional natural gas, in particular to a shale gas saturation calculation method based on a resistivity method.
Background
There are many well logging computational saturation models, and conventional electrical saturation models include Archie and Watermann-Smith, Simmon, etc. with muddiness correction. However, the problem of accurate calculation of the saturation of free gas in low-resistance shale gas cannot be solved by various traditional saturation models, because the main cause of low resistance of the shale gas layer is not recognized, and the influence of various low-resistance causes cannot be described; the traditional saturation model is based on a volume model, the smaller the porosity is, the smaller the conductivity is, the non-Archie phenomenon that the water saturation is not matched with the conductivity occurs in the low-resistance shale gas layer, and the clay content or the clay additional conductivity cannot describe the high conductivity generated when a small amount of water or a conductive mineral net distribution state in the microcracks; therefore, the original argillaceous correction saturation model is not applicable to the low-resistance shale gas layer. In conclusion, providing a method for calculating the free gas logging saturation of the low-resistivity shale gas layer becomes a technical problem to be solved urgently by those skilled in the art.
Disclosure of Invention
In view of this, the invention aims to provide a shale gas saturation calculation method based on a resistivity method, which can solve the problem of accurate calculation of free gas saturation in low-resistivity shale gas.
The invention provides a shale gas saturation calculation method based on a resistivity method, wherein an equivalent water conductivity item in an effective pore is added after a traditional calculation model formula to obtain an improved calculation model formula;
the term equivalent water conductivity in the effective pore is expressed as
Wherein:
EPOR-equivalent Water porosity, V/V;
m is the cementation index corresponding to the effective pore without dimension;
a-lithology coefficient associated with effective pore space, dimensionless;
b-lithologic saturation coefficient, dimensionless;
Rwformation water resistivity, Ω.
Preferably, the traditional calculation model formula is an Archie formula; the modified Archie's formula is:
wherein:
Rt-rock resistivity, Ω.m;
POR-effective porosity, V/V;
Sw-water saturation, V/V;
n-index of saturation, dimensionless.
Preferably, the traditional calculation model formula is a Indonesia formula; the improved Indonesia formula is:
wherein:
Vsh-argillaceous content, V/V;
Rshmudstone resistivity, Ω.
Preferably, the formula of the traditional calculation model is a siemens formula; the modified siemens formula is:
preferably, the traditional calculation model formula is a double-water model formula; the improved double water model formula is as follows:
wherein:
Qvcation exchange capacity of argillaceous sandstone, mmol/cm3；
PORsh-100% mudstone porosity, V/V;
PORt-total porosity, V/V.
Preferably, the conventional calculation model formula is a Wolfman-Smith model formula; the improved Wolsman-Smith model formula is as follows:
wherein:
m*-Waxman-Smits cementation index, dimensionless;
n*-the Waxman-Smits saturation index, dimensionless;
b-equivalent conductivity of exchange cation, S.cm3/(mmol·m)。
Preferably, in the term of equivalent water conductivity in the effective pores, the EPOR utilizes experimental analysis water saturation data back calculation or utilizes resistivity back-stepping of the adjacent normal shale gas layer.
Preferably, the improved calculation model formula can reversely calculate the true resistivity of the gas layer; the calculation formula of the true resistivity of the gas layer is as follows:
wherein:
Rtgand (4) calculating the resistivity of the low-resistance shale gas layer, omega.
The invention provides a shale gas saturation calculation method based on a resistivity method, wherein an equivalent water conductivity item in an effective pore is added after a traditional calculation model formula to obtain an improved calculation model formula; the term equivalent water conductivity in the effective pore is expressed as
Wherein: EPOR-equivalent Water porosity, V/V; m is the cementation index corresponding to the effective pore without dimension; a-lithology coefficient associated with effective pore space, dimensionless; b-lithologic saturation coefficient, dimensionless; rwFormation water resistivity, Ω. Compared with the prior art, the method is based on the recognition of the low resistance cause of the shale gas layer, realizes the high conductivity correction of the traditional calculation model by supplementing the equivalent water conductivity item, further obtains an improved calculation model formula, and can solve the problem of accurate calculation of the saturation of the free gas in the low-resistivity shale gas.
In addition, the shale gas saturation calculation method based on the resistivity method provided by the invention is also a traditional calculation model formula of various saturations when the equivalent water porosity is 0, so that the improved calculation model formula is suitable for any resistivity condition of the shale gas layer and has a wider application prospect.
Drawings
FIG. 1 is a graph of the results of a logging process for a low resistivity shale gas formation in an embodiment of the present invention;
FIG. 2 is a schematic diagram of the comparison between the "Archie" model and the "improved Archie" model in the embodiment of the present invention.
Detailed Description
The technical solutions of the present invention will be described clearly and completely with reference to the following embodiments of the present invention, and it should be understood that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The invention provides a shale gas saturation calculation method based on a resistivity method, wherein an equivalent water conductivity item in an effective pore is added after a traditional calculation model formula to obtain an improved calculation model formula;
the term equivalent water conductivity in the effective pore is expressed as
Wherein:
EPOR-equivalent Water porosity, V/V;
m is the cementation index corresponding to the effective pore without dimension;
a-lithology coefficient associated with effective pore space, dimensionless;
b-lithologic saturation coefficient, dimensionless;
Rwformation water resistivity, Ω.
The shale gas saturation calculation method provided by the invention is based on the knowledge of the low resistance cause of the shale gas layer, researches are carried out on the low resistance cause of the shale gas layer, the equivalent water conductivity of a small amount of water or conductive minerals in the effective pores of the gas layer in the microcracks and the conductivity of clay bound water are the root causes causing the low resistivity of the reservoir, and a new saturation model is established according to the equivalent water conductivity.
By analyzing a plurality of blocks of shale gas layers, the main reason why the shale gas layers form low resistance is analyzed as follows:
(1) micro-crack development: the shale casting body thin slice can clearly observe the development of the shale and has a large amount of interlayer seams in a macroscopic view; the shale has a shale seam which is mainly a seam between parallel layer texture layer surfaces with stripping line texture formed in the shale deposition process and consists of a series of thin layers of shale; the page is an interface with weak mechanical property and is easy to peel; due to the huge pressure in the shale gas generation process, complex micro-cracks develop in the shale layer; the interlayer seams are generally parallel to the interlayer surfaces, and mainly slide due to the extrusion of the stratum by external force, and are filled or semi-filled with pyrite, clay, authigenic minerals and the like at the later stage; the microcracks can cause low resistance of an air layer, or high conductivity caused by the network distribution of a small amount of bound water or conductive minerals in the microcracks; in addition, invasion of the water-based drilling fluid may cause a more pronounced decrease in resistivity of the interval in which the microcracks develop.
(2) And (3) conductive mineral development: the content of the pyrite is low, and imaging data are in a dispersed state if displayed, so that the low resistance of a shale gas reservoir is not greatly influenced; even though there may be connected states along the shale bedding distribution, resulting in low reservoir resistivity, it may be described by the high conductivity of small amounts of water.
(3) Clay mineral additional conductivity: the total content of clay minerals in the shale gas target layer sections is high, the clay mineral components are mainly in the illite-montmorillonite mixed layer, and the content of the illite-montmorillonite mixed layer is high; when the content of the illite-montmorillonite mixed layer of the reservoir is high, the additional conductivity of the montmorillonite and the illite-montmorillonite mixed layer is most obvious, and the additional conductivity causes the resistivity of the reservoir to be remarkably reduced; because the specific surface of the clay mineral is higher than that of sandstone, the saturation of the bound water is higher, and a low-resistivity reservoir is formed; for the same shale reservoir, although the clay content is higher, the shale reservoir is low in resistance, but the influence is an overall background influence, particularly for a high-quality shale reservoir, the clay content is obviously reduced, the mineral content of the quartz or carbonate rock is increased, and the high conductivity cannot be expressed by the lower clay content or the additional conductivity of the clay.
(4) Graphitization: the maturity of organic matters in the shale gas production layer is greatly changed, and the shale gas production layer can be divided into 3 types of over-mature shale gas, over-low maturity mixed shale gas and low maturity shale gas; the shale reservoirs with different organic matter maturity have obvious difference in pore structure characteristics, so that the resistivity is obviously different; in the thermal evolution process of the source rock, as the thermal maturity is increased, organic matters are degraded into kerogen, and the kerogen generates methane gas in the subsequent change process; with increasing temperature, kerogen changes gradually, gradually turning into low hydrogen carbonaceous residue and finally into graphite (i.e. graphitization), and the conversion from ion to electron conduction occurs, possibly exhibiting a phenomenon of resistivity reduction, and the low resistance formed by the increased graphitization of electrons is also an indication of high conductivity.
In conclusion, the low resistance formation of the shale gas is mainly caused by high conductivity generated by the network distribution of a small amount of water or conductive minerals in the microcracks, the high conductivity is equivalent to the conductivity of all or part of effective pore volumes of water, the high conductivity generated by the small amount of water in the shale gas layer due to the network distribution of the microcracks, namely, the communication state can be expressed by the equivalent water conductivity, for example, in an experiment, the conductivity generated by the small amount of conductive aluminum with the same volume in the crude oil is different from that generated by the small amount of conductive aluminum in the crude oil in the network distribution, the conductivity influence of the crude oil with the cluster aluminum distribution is smaller, and the conductivity of the crude oil with the network aluminum distribution is equivalent to the conductivity of all or part of the crude oil in the network distribution, and the phenomenon shows that the; if the low resistance cause of part of the shale gas layer is caused by the invasion influence of the microcracks, namely the conductivity increased by the invaded part of the microcracks can be expressed by equivalent water conductivity; even if the high content illite-montmorillonite mixed layer in the low content clay can cause low resistance, namely the high conductivity of the bound water of a small amount of clay can be equivalent to the conductivity of the high content clay water; the increased conductivity of electrons generated by graphitization cannot be described by clay conductivity, but can be described by equivalent water conductivity. Therefore, no matter what cause causes the low resistance of the shale gas layer, the high conductivity correction is needed by adopting the electrical method saturation model.
On the basis, the invention provides a shale gas saturation calculation method based on a resistivity method, wherein an equivalent water conductivity item in effective pores is added after a traditional calculation model formula to obtain an improved calculation model formula;
the term equivalent water conductivity in the effective pore is expressed as
Wherein:
EPOR-equivalent Water porosity, V/V;
m is the cementation index corresponding to the effective pore without dimension;
a-lithology coefficient associated with effective pore space, dimensionless;
b-lithologic saturation coefficient, dimensionless;
Rwformation water resistivity, Ω.
In the present invention, equivalent water conductivity is introduced to describe the high conductivity resulting from various low resistance causes, especially small amounts of water or conductive minerals in the microcracks, which is equivalent to all or part of the effective pore volume being the conductivity of water; the invention considers the small amount of water as a single part, considers the influence of the distribution state of the water on the conductivity of a reservoir, adopts the equivalent water porosity (EPOR) size to represent the size of the pore of which the distribution state of cracks and conductive minerals has influence on the conductivity of the reservoir, and is not always existed due to the influence of factors such as the pore structure of shale, the micro-crack distribution, namely the communication state of the water or the conductive minerals, the graphitized electron distribution and the like, and the equivalent water conductivity is also the traditional various saturation models when the equivalent water porosity is 0, so the modified model is suitable for any resistivity condition of the shale gas layer.
In the invention, the traditional calculation model formula is preferably an Archie formula; on the basis, the improved Archie formula is as follows:
wherein:
Rt-rock resistivity, Ω.m;
POR-effective porosity, V/V;
m is the cementation index corresponding to the effective pore without dimension;
Sw-water saturation, V/V;
n-saturation index, dimensionless;
a-lithology coefficient associated with effective pore space, dimensionless;
b-lithologic saturation coefficient, dimensionless;
Rw-formation water resistivity, Ω.m;
EPOR-equivalent Water porosity, V/V.
In the invention, the traditional calculation model formula is preferably Indonesia formula; on the basis, the improved Indonesia formula is as follows:
wherein:
Rt-rock resistivity, Ω.m;
POR-effective porosity, V/V;
m is the cementation index corresponding to the effective pore without dimension;
a-lithology coefficient associated with effective pore space, dimensionless;
b-lithologic saturation coefficient, dimensionless;
Rw-formation water resistivity, Ω.m;
Vsh-argillaceous content, V/V;
Rshmudstone resistivity, Ω.m;
Sw-water saturation, V/V;
n-saturation index, dimensionless;
EPOR-equivalent Water porosity, V/V.
In the invention, the traditional calculation model formula is preferably a Siemens formula; on the basis, the improved siemens formula is as follows:
wherein:
Rt-rock resistivity, Ω.m;
POR-effective porosity, V/V;
m is the cementation index corresponding to the effective pore without dimension;
a-lithology coefficient associated with effective pore space, dimensionless;
b-lithologic saturation coefficient, dimensionless;
Rw-formation water resistivity, Ω.m;
Sw-water saturation, V/V;
n-saturation index, dimensionless;
Vsh-argillaceous content, V/V;
Rshmudstone resistivity, Ω.m;
EPOR-equivalent Water porosity, V/V.
In the invention, the traditional calculation model formula is preferably a double-water model formula; on this basis, the improved double-water model formula is as follows:
wherein:
Rt-rock resistivity, Ω.m;
POR-effective porosity, V/V;
m is the cementation index corresponding to the effective pore without dimension;
Sw-water saturation, V/V;
n-saturation index, dimensionless;
a-lithology coefficient associated with effective pore space, dimensionless;
b-lithologic saturation coefficient, dimensionless;
Rw-formation water resistivity, Ω.m;
Qvcation exchange capacity of argillaceous sandstone, mmol/cm3；
PORsh-100% mudstone porosity, V/V;
Rshmudstone resistivity, Ω.m;
EPOR-equivalent Water porosity, V/V
Vsh-argillaceous content, V/V;
PORt-total porosity, V/V.
In the present invention, the conventional calculation model formula is preferably a Wolsman-Smith model formula; on the basis, the improved Wolfman-Smith model formula is as follows:
wherein:
Rt-rock resistivity, Ω.m;
POR-effective porosity, V/V;
m is the cementation index corresponding to the effective pore without dimension;
Sw-water saturation, V/V;
n-saturation index, dimensionless;
a-lithology coefficient associated with effective pore space, dimensionless;
b-lithologic saturation coefficient, dimensionless;
Rw-formation water resistivity, Ω.m;
b-equivalent conductivity of exchange cation, S.cm3/(mmol·m)；
QvCation exchange capacity of argillaceous sandstone, mmol/cm3；
m*-Waxman-Smits cementation index, dimensionless;
n*-the Waxman-Smits saturation index, dimensionless;
EPOR-equivalent Water porosity, V/V.
A table showing a comparison of the improved calculation model formula provided by the present invention with the conventional calculation model formula is shown in table 1.
TABLE 1 comparison of the improved calculation model formulas provided by the present invention with conventional calculation model formulas
In the invention, the improved calculation model formula can reversely calculate the true resistivity of the gas layer; the calculation formula of the true resistivity of the gas layer is as follows:
wherein:
Rtgcalculating the resistivity of the low-resistance shale gas layer in a reverse mode, wherein the resistivity is omega;
Rt-rock resistivity, Ω.m;
EPOR-equivalent Water porosity, V/V;
m is the cementation index corresponding to the effective pore without dimension;
a-lithology coefficient associated with effective pore space, dimensionless;
b-lithologic saturation coefficient, dimensionless;
Rwformation water resistivity, Ω.
In the invention, after the equivalent water conductivity correction is carried out by the traditional calculation model formula, the true resistivity R of the gas layertgThe method can be calculated in a reverse mode, and the original model cannot recover the true resistivity of the gas layer without equivalent water conductivity correction.
In the present invention, EPOR is related to the distribution and connectivity status of small amounts of water and conductive minerals in the equivalent water conductivity term in the effective pore space, preferably by either back-calculation of water saturation data using experimental analysis or back-extrapolation of the resistivity of the adjacent normal shale gas formation. The invention uses the equivalent water porosity to represent the influence volume of the distribution state of a small amount of water or conductive minerals in a reservoir on the conductivity of the reservoir, considers the comprehensive influence of the conductivity of various electric conductors in the stratum formed by parallel connection, series connection, parallel connection plus series connection or net connection, if an adjacent well has experimental analysis water saturation information in the actual process, the known model (formula 2) can be used for reverse calculation, and then the low-resistance gas layer resistivity and the relation between the porosity and the equivalent water porosity are established; because the low resistance of the shale gas is mainly caused by microcracks, the thickness of the low-resistance layer section is small, and an equivalent water porosity regression formula determined by experiments does not have general applicability, the method preferably utilizes the reverse extrapolation of the normal resistivity of the adjacent shale gas layer to determine the equivalent water porosity.
The resistivity of the same shale gas layer is consistent, and the measured resistivity is different due to the difference between the porosity and the pore structure and the difference between the communication conditions of a small amount of water or conductive minerals in a crack, so that the equivalent water porosity EPOR can be determined, namely the higher the resistivity of the shale gas layer is, the smaller the EPOR is, the lower the resistivity of the low-resistance shale gas layer is, and the larger the EPOR is, so that the relative size of the equivalent water porosity can be represented by e, and the calculation formula of e is as follows:
wherein:
Rmax-resistivity of normal resistivity shale gas formation, Ω.m;
Rt-rock resistivity, Ω.m;
Rminminimum resistivity of low-resistance shale gas layer, Ω.
The equivalent water porosity is equivalent to all or part of the effective pore volume, the size of the effective pore volume is related to the size of the effective porosity, and a conversion coefficient c (the numerical value range is 0-1) exists when the EPOR is calculated by adopting a resistivity method, so that the equivalent water porosity EPOR can be expressed as:
EPOR ═ cxe × POR (formula 9),
wherein:
c, calculating an equivalent porosity conversion coefficient by using a resistivity method, wherein the equivalent porosity conversion coefficient is a constant value for the same interval and ranges from 0 to 1;
POR-effective porosity, V/V.
C is 0 in normal resistivity shale gas interval, c (constant value) is determined in low resistivity shale gas interval, and the determination of c can be determined according to the processed water saturation SwAnd back-calculated shaleResistivity of gas layer RtgThe method is compared with an adjacent normal resistivity layer for monitoring, the value of the regional low-resistance characteristic layer parameter c can be determined under the condition of actual saturation value calibration, just like the calculation of various stresses by array acoustic logging data, the calculated result is only changed correspondingly, and accurate numerical values need to be calibrated by the adjacent normal resistivity layer or experimental analysis data.
Since e is less than or equal to 1, emDecreases rapidly as m increases, resulting in c having to increase rapidly and be greater than 1, and saturation data analysis is analyzed according to closed core experiments, although resistivity method determines emThe correlation of (a) is improved, but since c is in the range of 0 to 1, the index of e is not as large as possible, which is also the case when e is used in the calculation of the equivalent water conductivity by the resistivity methodmInstead of e3、e4Or e5The rationality of equation 9 was also verified.
Aiming at the situation that after various electrical saturation formulas commonly adopted in the petroleum industry are added with equivalent water conductivity, an improved saturation model (formula 1-formula 6) can be used for calculating the free gas saturation in the conventional resistivity and low resistivity shale gas layer under the condition that the shale gas layer is qualitatively identified or tested to be low-resistivity; due to the fact that the clay content of the shale reservoir is high, the AlcLett model applicable to pure sandstone has no argillaceous correction, and therefore the application effect of the AlcLett model and the improved AlcLett model in the shale gas layer is poor. The model considering the shale correction and the equivalent water conductivity is suitable for calculating the saturation of high and low resistivity reservoirs of the shale gas layer in China and even the world, and can improve the calculation result of the gas saturation of the low resistivity shale gas layer.
In summary, the shale gas saturation calculation method based on the resistivity method provided by the invention is characterized in that the equivalent water conductivity of the effective pore is newly increased on the basis of the traditional calculation model formula, and when the original model is expressed in a conductivity mode, the equivalent water conductivity item can be expressed as
Electronic molecule due to pore structure, micro-crack distribution, i.e. water or conductive mineral communication state and graphitizationAnd under the influence of factors such as cloth and the like, the equivalent water conductivity does not always exist, and when the equivalent water porosity is 0, the model is also a traditional various saturation model, so that the modified model is suitable for any resistivity condition of the shale reservoir.
The invention provides a shale gas saturation calculation method based on a resistivity method, wherein an equivalent water conductivity item in an effective pore is added after a traditional calculation model formula to obtain an improved calculation model formula; the term equivalent water conductivity in the effective pore is expressed as
Wherein: EPOR-equivalent Water porosity, V/V; m is the cementation index corresponding to the effective pore without dimension; a-lithology coefficient associated with effective pore space, dimensionless; b-lithologic saturation coefficient, dimensionless; rwFormation water resistivity, Ω. Compared with the prior art, the method is based on the recognition of the low resistance cause of the shale gas layer, realizes the high conductivity correction of the traditional calculation model by supplementing the equivalent water conductivity item, further obtains an improved calculation model formula, and can solve the problem of accurate calculation of the saturation of the free gas in the low-resistivity shale gas.
In addition, the shale gas saturation calculation method based on the resistivity method provided by the invention is also a traditional calculation model formula of various saturations when the equivalent water porosity is 0, so that the improved calculation model formula is suitable for any resistivity condition of the shale gas layer and has a wider application prospect.
To further illustrate the present invention, the following examples are provided for illustration.
Examples
As shown in attached figure 1, in the shale section of 2570 meters below, the content of organic carbon is obviously increased, the density is lower, neutrons are smaller, and the neutron-density intersection 'mirror image' at the gas layer is obviously increased, which shows that the gas content of the layer is good and is interpreted as a shale gas layer of type I.
After the low-resistance shale gas layer is identified, carrying out low-resistance shale gas layer model research; taking a traditional 'Archie model' as an example, an improved low-resistance shale gas layer saturation calculation model formula is explained; the Archie model is as follows:
wherein:
Rt-rock resistivity, Ω.m;
POR-effective porosity, V/V;
m is the cementation index corresponding to the effective pore without dimension;
Sw-water saturation, V/V;
n-saturation index, dimensionless;
a-lithology coefficient associated with effective pore space, dimensionless;
b-lithologic saturation coefficient, dimensionless;
Rwformation water resistivity, Ω.
Aiming at a common Archie model, when the resistivity of a gas layer is low, the calculated water saturation is high, the real high resistivity of the gas layer cannot be recovered, and the high conductivity of a low-resistance shale gas layer in a micro-crack small-amount water net distribution state cannot be described. According to the Archie formula, formation water is used as an independent part to influence the resistivity of a reservoir, if the condition of the same low-resistance shale gas layer is filled with water, the formation is the same as low-resistivity, namely the gas layer with low water saturation (the water saturation is 23 percent through experimental analysis at 2585.3m in the figure 1) is similar to the water layer with high water saturation (the water saturation is 77 percent through Archie calculation, the water saturation is 63 percent through Siemens calculation, and the water saturation is 64 percent through W-S calculation), obviously equivalent water conductivity exists in the effective pores of the low-resistance shale gas layer, even if the effective pores are influenced by the shale factor, the result of the shale factor is the same, and the difference is also gas and water in the effective pores; the high conductivity existing in the shale gas layer, namely the equivalent water conductivity, is the main reason for influencing the reduction of the resistivity of the reservoir and is consistent with the high conductivity generated in the low resistance cause analysis of the network distribution state of a small amount of electric conductors in the microcracks; a small amount of water is considered as a single part, and the distribution state of water is considered to influence the conductivity of a reservoir, and the conductivity equivalent to that caused by the distribution state of the conductive minerals and all or part of the effective pore volume can be expressed as EPOR, so that the equivalent water conductivity can be expressed as:
this forms an improved Archie's formula:
wherein:
EPOR-equivalent Water porosity, V/V.
The schematic comparison between the "Archie" model and the "modified Archie" model is shown in FIG. 2.
Although conductivity is influenced not by the volume of clay but by the cation exchange capacity of clay in the Wolfman-Smith model, the influence is limited to the argillaceous part and high conductivity generated in a network distribution state of a small amount of water in microcracks cannot be expressed; the improved model after considering the equivalent water conductivity of a small amount of water can therefore also be expressed as:
wherein:
Rt-rock resistivity, Ω.m;
POR-effective porosity, V/V;
m is the cementation index corresponding to the effective pore without dimension;
Sw-water saturation, V/V;
n-saturation index, dimensionless;
a-lithology coefficient associated with effective pore space, dimensionless;
b-lithologic saturation coefficient, dimensionless;
Rw-formation water resistivity, Ω.m;
b-equivalent conductivity of exchange cation, S.cm3/(mmol·m)；
QvCation exchange capacity of argillaceous sandstone, mmol/cm3；
m*-Waxman-Smits cementation index, dimensionless;
n*-the Waxman-Smits saturation index, dimensionless;
EPOR-equivalent Water porosity, V/V.
To two water models, can revise after considering equivalent water conductivity and be:
wherein:
Rt-rock resistivity, Ω.m;
POR-effective porosity, V/V;
m is the cementation index corresponding to the effective pore without dimension;
Sw-water saturation, V/V;
n-saturation index, dimensionless;
a-lithology coefficient associated with effective pore space, dimensionless;
b-lithologic saturation coefficient, dimensionless;
Rw-formation water resistivity, Ω.m;
Qvcation exchange capacity of argillaceous sandstone, mmol/cm3；
PORsh-100% mudstone porosity, V/V;
Rshmudstone resistivity, Ω.m;
EPOR-equivalent Water porosity, V/V
Vsh-argillaceous content, V/V;
PORt-totalPorosity, V/V.
For the siemens du model, the improved model after considering the equivalent water conductivity can be expressed as:
wherein:
Rt-rock resistivity, Ω.m;
POR-effective porosity, V/V;
m is the cementation index corresponding to the effective pore without dimension;
a-lithology coefficient associated with effective pore space, dimensionless;
b-lithologic saturation coefficient, dimensionless;
Rw-formation water resistivity, Ω.m;
Sw-water saturation, V/V;
n-saturation index, dimensionless;
Vsh-argillaceous content, V/V;
Rshmudstone resistivity, Ω.m;
EPOR-equivalent Water porosity, V/V.
After the equivalent water conductivity correction is carried out on the model, the true resistivity R of the gas layertgCan be calculated reversely, and the original model can not recover the true resistivity of the gas layer without equivalent water conductivity correction, RtgThe calculation formula is as follows:
wherein:
Rtgcalculating the resistivity of the low-resistance shale gas layer in a reverse mode, wherein the resistivity is omega;
Rt-rock resistivity, Ω.m;
EPOR-equivalent Water porosity, V/V;
m is the cementation index corresponding to the effective pore without dimension;
a-lithology coefficient associated with effective pore space, dimensionless;
b-lithologic saturation coefficient, dimensionless;
Rwformation water resistivity, Ω.
The experimental result shows that the resistivity of the gas layer section of the high-quality shale is not high, the gas saturation calculated by adopting the original electric method saturation model is lower and is not consistent with the gas saturation of the high-quality shale with high porosity and high organic carbon content; without changing the saturation parameters a, m, b, n, RwUnder the condition, the invention is suitable for low-resistance improved Wolfmann-Smith, improved Simmon and other models to process the water saturation SwObviously reduced and well matched with the result of experimental analysis of gas saturation; the resistivity of the gas layer inversely calculated by the model is much higher than the deep lateral resistance, and the resistivity changes greatly at the place with high porosity, and is consistent with the gas content determined by physical properties and organic carbon in the gas layer; the calculated shale content of the high-quality shale section in the graph is relatively low, and no matter a common traditional shale correction saturation model is used, the fact that the resistivity of a shale gas layer is reduced due to a small amount of shale cannot be explained; the deep and shallow lateral resistivities of the shale gas layer in the section are almost consistent, which shows that no invasion influence exists.
In addition, the traditional and improved Archie models do not perform argillaceous corrections, and the calculated gas saturation is still low, so the improved Archie model is still not suitable for shale, a reservoir with high clay content.
Various formulas and parameters may be queried or calculated in the present invention according to the following references.
[1] The Sunjian well logging saturation interpretation model development and analysis, oil exploration and development, 2008,35(1): 101-;
[2] ouyangjian et al, well logging low contrast oil reservoir causation mechanism and evaluation method, oil industry Press, 2009;
[3] a low-resistance oil and gas reservoir logging identification evaluation method and technology, oil industry publishing agency, 2006.7;
[4] zhang jin et al. New method for calculating gas saturation of low-resistance shale gas layer, natural gas industry, 2017.4, 37(4): 34-41.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims (8)
1. A shale gas saturation calculation method based on a resistivity method is characterized in that an equivalent water conductivity item in effective pores is added after a traditional calculation model formula to obtain an improved calculation model formula;
the term equivalent water conductivity in the effective pore is expressed as
Wherein:
EPOR-equivalent Water porosity, V/V;
m is the cementation index corresponding to the effective pore without dimension;
a-lithology coefficient associated with effective pore space, dimensionless;
b-lithologic saturation coefficient, dimensionless;
Rwformation water resistivity, Ω.
2. The resistivity method based shale gas saturation calculation method according to claim 1, wherein the traditional calculation model formula is an Archie's formula; the modified Archie's formula is:
wherein:
Rt-rock resistivity, Ω.m;
POR-effective porosity, V/V;
Sw-water saturation, V/V;
n-index of saturation, dimensionless.
3. The resistivity method based shale gas saturation calculation method according to claim 1, wherein the traditional calculation model formula is Indonesia formula; the improved Indonesia formula is:
wherein:
Vsh-argillaceous content, V/V;
Rshmudstone resistivity, Ω.
4. The resistivity method based shale gas saturation calculation method according to claim 1, wherein the traditional calculation model formula is a siemens formula; the modified siemens formula is:
5. the resistivity method based shale gas saturation calculation method according to claim 1, wherein the traditional calculation model formula is a double water model formula; the improved double water model formula is as follows:
wherein:
Qvcation exchange capacity of argillaceous sandstone, mmol/cm3；
PORsh-100% mudstone porosity, V/V;
PORt-total porosity, V/V.
6. The resistivity method based shale gas saturation calculation method according to claim 1, wherein the traditional calculation model formula is a Wolfman-Smith model formula; the improved Wolsman-Smith model formula is as follows:
wherein:
m*-Waxman-Smits cementation index, dimensionless;
n*-the Waxman-Smits saturation index, dimensionless;
b-equivalent conductivity of exchange cation, S.cm3/(mmol·m)。
7. The resistivity method based shale gas saturation calculation method according to any one of claims 1 to 6, wherein in the equivalent water conductivity term in the effective pores, EPOR performs back calculation by using experimental analysis water saturation data or performs back calculation by using resistivity of an adjacent normal shale gas layer.
8. The resistivity method based shale gas saturation calculation method according to any one of claims 1 to 6, wherein the improved calculation model formula can inversely calculate true resistivity of a gas layer; the calculation formula of the true resistivity of the gas layer is as follows:
wherein:
Rtgand (4) calculating the resistivity of the low-resistance shale gas layer, omega.
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