CN117055114B - Quantitative analysis method for free gas saturation of reservoir sediment - Google Patents
Quantitative analysis method for free gas saturation of reservoir sediment Download PDFInfo
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Abstract
The invention belongs to the field of geophysical well logging application, and relates to a quantitative analysis method for free gas saturation of reservoir sediments, which considers the strong inhibition effect of free gas in sediments on acoustic wave signals, namely, the free gas is contained to quickly reduce the acoustic wave speed, so that the attenuation of the acoustic wave signals is obviously increased, and a free gas effect enhanced rock physical model is constructed; according to the logging data of the working area, calculating the bulk modulus and the shear modulus of the solid phase, substituting the bulk modulus and the shear modulus of the solid phase into a free gas action enhanced rock physical model to obtain the longitudinal wave speed and the attenuation coefficient of the sediment of the working area, constructing a least square error objective function of a theoretical value and the measured data of the acoustic logging, and solving the free gas saturation of the sediment of the working area. The method calculates the free gas saturation based on the acoustic parameters and the constructed free gas action enhanced rock physical model, has high accuracy, and provides technical basis for quantitative evaluation of the free gas saturation.
Description
Technical Field
The invention belongs to the field of geophysical well logging, and particularly relates to a quantitative analysis method for free gas saturation of reservoir sediments.
Background
Free form natural gas exists widely in the bottom of a lake in the nature, in submarine sediments or in a permafrost zone, free form natural gas also exists in an underground rock natural gas reservoir, and accurate assessment of reservoir free gas saturation is one of the important tasks in the field of geophysical well logging. Currently, the free gas saturation is mainly calculated by using an Alqi formula or an improved petrophysical model thereof, and the petroelectricity coefficient of the petrophysical model needs to be determined by combining a petroelectricity experiment, and for the submarine and lake bottom sediments such as underdiagenetic or weak cementing, as a complete plunger rock sample cannot be obtained, the petroelectricity parameter is directly set to a typical value (for example, a and b take values of 1, m and n take values of 2) in the saturation calculation, so that the saturation calculation result and the reservoir saturation true value have a large difference. For this reason, the calculation of free gas saturation by acoustic petrophysical models is an important choice, which has an important role in the qualitative identification of gas content and the quantitative calculation of saturation. However, existing acoustic petrophysical models cannot accurately characterize the control mechanism of reservoir free gas to sound velocity and waveform decay at small free gas saturation.
Free gas is a main cause of elastic wave dispersion and attenuation in saturated water sediments, and a small amount of free gas contained in a reservoir can have obvious influence on the reduction of sound wave speed and the attenuation of sound wave signals. This means that free gas saturation is a key factor affecting the sound wave velocity and the attenuation of the sound wave signal. Although the existing models (such as Biot-Gassmann theory by Lee, abbreviated as BGTL model) obtain a certain application effect in quantitative evaluation of the velocity and attenuation of the sediment containing free gas, the models do not consider the strong inhibition effect of free gas on the sound velocity, and the change of the sound velocity and the attenuation rule of the sound wave signal in the sediment containing free gas can not be accurately described, so that the longitudinal wave velocity and the attenuation characteristic of the sediment containing free gas can not be accurately represented, and the accurate calculation of the saturation degree of free gas in a reservoir is restricted and influenced.
Disclosure of Invention
Aiming at the problem that the longitudinal wave speed and the attenuation characteristic of the sediment containing the free gas cannot be accurately represented in the prior art, the invention provides a quantitative analysis method for the free gas saturation of the sediment in a reservoir. The free gas action enhanced rock physical model constructed by the method can provide technical basis for quantitative interpretation of the saturation of the free gas.
In order to achieve the above purpose, the invention provides a quantitative analysis method for the free gas saturation of reservoir sediment, which comprises the following steps:
s1, establishing a free gas action enhanced rock physical model, which comprises the following specific steps:
s11, obtaining a reservoir sediment frameworkmThe volume ratio, the volume modulus and the shear modulus of each component in the components are obtained according to Hill average equation to obtain the volume modulus of solid particles forming a reservoir sediment frameworkK S Shear modulusμ S Expressed as:
（1）
in the method, in the process of the invention,mto build up the number of components of the reservoir sediment backbone,f i is a componentiIs used in the field of the fuel cell,K i is a componentiIs used for the preparation of a composite material,μ i is a componentiShear modulus of (a);
s12, based on bulk modulusK S Shear modulusμ S Obtaining the bulk modulus of the reservoir sediment dry skeleton according to the dry skeleton cementation modelK m Shear modulusμ m The dry matrix cementation model is expressed as:
（2）
in the method, in the process of the invention,ϕin order to achieve a degree of porosity, the porous material,αto construct the cementation coefficient of the reservoir dry matrix solid particles,γis an intermediate variable;
s13, based on bulk modulusK S Modulus of volumeK m Bulk modulus of reservoir pore waterK w Bulk modulus of reservoir pore free gasK g Free gas action enhancement type and the like calculated by utilizing free gas action enhancement type equivalent bulk modulus formulaThe effective bulk modulus, the free gas action enhanced equivalent bulk modulus formula is:
（3）
in the method, in the process of the invention,K av is the free gas action enhanced equivalent bulk modulus,S g in order to achieve the saturation of the free gas,r g for the enhancement of the index of free gas action,cas an intermediate variable，ϕ S Is the volume fraction of the solid framework;
s14, based on bulk modulusK m Modulus of shearμ m Enhanced equivalent bulk modulus of free gas actionK av According to a Biot biphase wave characteristic equation, obtaining longitudinal wave velocity and attenuation coefficient of saturated water sediment containing free gas, namely a free gas action enhanced rock physical model;
s2, obtaining the volume fraction of the reservoir sediment skeleton component according to natural gamma logging, porosity logging and lithology density logging data of a target horizon of a work area, calculating according to a Hill average equation to obtain the volume modulus and the shear modulus of solid particles of the reservoir sediment skeleton, obtaining a measured value of a longitudinal wave velocity and a measured value of an attenuation coefficient according to a sound logging curve of the work area, substituting initial values of free gas saturation and an enhancement index of free gas action into the physical model of the free gas action enhanced rock to obtain a theoretical value of the longitudinal wave velocity and a theoretical value of the attenuation coefficient of the reservoir of the work area, bringing the measured value of the longitudinal wave velocity, the measured value of the attenuation coefficient, the theoretical value of the longitudinal wave velocity and the theoretical value of the attenuation coefficient into a least square error objective function, and judging that the corresponding free gas saturation is the predicted free gas saturation of the reservoir sediment of the work area when the least square error objective function is smaller than an error threshold.
In some embodiments, in step S14, the Biot biphasic wave feature equation is expressed as:
（4）
in the method, in the process of the invention,det(‧) is a feature root of the matrix;ωrepresenting the wave source frequency;representing complex wave numbers;jis a virtual unit;is a density matrix in which:
，/>，/>，
，/>，ρ S for the density of the particles of the framework,ρ w in order to achieve a water density of the pores,ρ g in order to achieve the free air density,ris the pore space bending coefficient;
is a friction coefficient moment>，η l In order to achieve a coefficient of viscosity of the pore fluid,κeffective permeability for reservoir sediment backbone;
is a bulk modulus coefficient matrix, wherein:
，/>，/>，
；
the calculation formula of the longitudinal wave velocity and the attenuation coefficient is expressed as:
wherein,is the attenuation coefficient;v p is the longitudinal wave velocity of the reservoir containing free gas; re (‧) and Im (‧) take real and imaginary parts, respectively.
In some embodiments, in step S2, the actual value of the attenuation coefficient is calculated from formula (5) based on an acoustic log, where formula (5) is expressed as:
（5）
in the method, in the process of the invention,for the actual measurement of the attenuation coefficient, < >>Is the measured velocity of the longitudinal wave,fin order to be a frequency of the light,X(ω,z 1 ) Is distant from the sound sourcez 1 Is a function of the amplitude spectrum of the receiver of (a),X(ω,z 2 ) Is distant from the sound sourcez 2 Amplitude spectrum of the receiver of (a);
the least squares error objective function is expressed as:
（6）
in the method, in the process of the invention,as a least squares error objective function, +.>Is the maximum value of the longitudinal wave velocity which has been measured, < >>Is the maximum value of the attenuation coefficient which has been calculated.
In some embodiments, in step S2, when the least squares error objective function is determined to be greater than the error threshold, an iterative algorithm is used to update the free air saturation and the free air effect enhancement index until the least squares error objective function is less than the error threshold.
Compared with the prior art, the invention has the beneficial effects that:
(1) According to the quantitative analysis method for the free gas saturation of the reservoir sediment, the elastic modulus (namely the bulk modulus and the shear modulus of the reservoir sediment framework solid particles) of the reservoir sediment framework solid particles are calculated respectively based on a Hill average equation; calculating the bulk modulus and the shear modulus of a dry skeleton of the reservoir sediment by using a dry skeleton cementation model, and calculating the free gas action enhanced equivalent bulk modulus by using a free gas action enhanced equivalent bulk modulus formula; the combined fluid phase elastic modulus (namely the volume modulus of reservoir pore water), the volume modulus and the shear modulus of a reservoir sediment dry skeleton and the free gas action enhanced equivalent volume modulus are used for obtaining the longitudinal wave velocity and the attenuation coefficient of the free gas-containing saturated water sediment according to the Biot biphase fluctuation characteristic equation, namely the free gas action enhanced petrophysical model. The free gas action enhanced rock physical model constructed by the invention considers a strong attenuation mechanism of free gas on sediment sound propagation, and compared with the existing theoretical model, the free gas action enhanced rock physical model is more in line with the actual situation of a reservoir containing free gas, and can be used for describing the acoustic response rule of the sediment containing free gas saturated water, so that the detection and identification work of the sediment containing free gas saturated water can be carried out by utilizing acoustic logging.
(2) The quantitative analysis method for the free gas saturation of the reservoir sediment provided by the invention can describe the two-phase morphology of which the free gas saturation is zero and only contains pore water; pore reservoirs containing free gas and pore water can also be described as conforming to the multiphase morphology of an actual free gas containing reservoir. The quantitative analysis method for the free gas saturation of the reservoir sediment can accurately describe the longitudinal wave speeds and attenuation coefficients of reservoirs with different free gas saturation, and provides technical basis for quantitative interpretation and evaluation of the free gas saturation.
(3) The quantitative analysis method for the free gas saturation of the reservoir sediment can describe the longitudinal wave velocity and the attenuation coefficient of the reservoir with the free gas saturation, and the constructed free gas action enhanced rock physical model is applied to free gas detection, so that a novel method for free gas detection by utilizing acoustic parameters (namely the longitudinal wave velocity and the attenuation coefficient) is provided, and a measurement method can be provided for the improvement and the perfection of acoustic logging.
(4) Compared with the free gas saturation calculation method of the BGTL elastic wave rock physical model and the resistivity Archie formula, the new free gas saturation calculation method based on the acoustic parameters provided by the invention avoids the inapplicability problem when the free gas saturation is lower than 1%, greatly expands the free gas saturation calculation method of the free gas sediment, and solves the evaluation explanation of the free gas saturation of the weak diagenetic stratum to a certain extent.
Drawings
FIG. 1 is a flow chart of a method for quantitative analysis of reservoir sediment free gas saturation according to an embodiment of the present invention;
FIG. 2 is a graph showing the velocity of longitudinal waves of a saturated water deposit containing free gas in pores according to the embodiment of the present invention;
FIG. 3 is a graph showing the variation of attenuation coefficient of the pore gas-containing saturated water deposit with the saturation of free gas according to the embodiment of the present invention.
Detailed Description
The present invention will be specifically described below by way of exemplary embodiments. It is to be understood that elements, structures, and features of one embodiment may be beneficially incorporated in other embodiments without further recitation.
Referring to fig. 1, the embodiment of the invention provides a reservoir sediment free gas saturation quantitative analysis method, which specifically comprises the following steps:
s1, establishing a free gas action enhanced rock physical model, which comprises the following specific steps:
s11, obtaining a reservoir sediment frameworkmThe volume ratio, the volume modulus and the shear modulus of each component in the components are obtained according to Hill average equation to obtain the volume modulus of solid particles forming a reservoir sediment frameworkK S Shear modulusμ S Expressed as:
（1）
in the method, in the process of the invention,mto build up the number of components of the reservoir sediment backbone,f i is a componentiIs used in the field of the fuel cell,K i is a componentiIs used for the preparation of a composite material,μ i is a componentiShear modulus of (c).
S12, based on bulk modulusK S Shear modulusμ S Obtaining the bulk modulus of the reservoir sediment dry skeleton according to the dry skeleton cementation modelK m Shear modulusμ m The dry matrix cementation model is expressed as:
（2）
in the method, in the process of the invention,ϕin order to achieve a degree of porosity, the porous material,αis of the structureThe cementing coefficient of the solid particles of the dry framework of the reservoir,γis an intermediate variable and has no practical meaning.
S13, based on bulk modulusK S Modulus of volumeK m Bulk modulus of reservoir pore waterK w Bulk modulus of reservoir pore free gasK g Calculating a free gas action enhanced equivalent bulk modulus using a free gas action enhanced equivalent bulk modulus formula, the free gas action enhanced equivalent bulk modulus formula expressed as:
（3）
in the method, in the process of the invention,K av is the free gas action enhanced equivalent bulk modulus,S g in order to achieve the saturation of the free gas,r g for the enhancement of the index of free gas action,cis an intermediate variable, has no practical meaning,ϕ S is the volume fraction of the solid framework.
S14, based on bulk modulusK m Modulus of shearμ m Enhanced equivalent bulk modulus of free gas actionK av And obtaining longitudinal wave velocity and attenuation coefficient of the saturated water sediment containing the free gas according to the Biot biphase wave characteristic equation, namely the free gas action enhanced rock physical model.
Specifically, the Biot biphasic wave characteristic equation is expressed as:
（4）
in the method, in the process of the invention,det(‧) is a feature root of the matrix;ωrepresenting the wave source frequency;representing complex wave numbers;jis a virtual unit;is a density matrix in which:
，/>，/>，
，/>，ρ S for the density of the particles of the framework,ρ w in order to achieve a water density of the pores,ρ g in order to achieve the free air density,ris the pore space bending coefficient;
is a friction coefficient moment>，η l In order to achieve a coefficient of viscosity of the pore fluid,κeffective permeability for reservoir sediment backbone;
is a bulk modulus coefficient matrix, wherein:
，/>，/>，；
the calculation formula of the longitudinal wave velocity and the attenuation coefficient is expressed as:
wherein,is the attenuation coefficient;v p is the longitudinal wave velocity of the reservoir containing free gas; re (‧) and Im (‧) take real and imaginary parts, respectively.
S2, obtaining the volume fraction of the reservoir sediment skeleton component according to natural gamma well logging, porosity well logging and lithology density well logging data of a target horizon of a work area, calculating according to a Hill average equation to obtain the volume modulus and the shear modulus of solid particles of the reservoir sediment skeleton, and obtaining a longitudinal wave velocity actual measurement value and an attenuation coefficient actual measurement value according to a sound wave well logging curve of the work area; substituting initial values of free gas saturation and free gas action enhancement index into the free gas action enhancement petrophysical model to obtain a longitudinal wave speed theoretical value and an attenuation coefficient theoretical value of the work area reservoir, bringing the measured longitudinal wave speed value, the measured attenuation coefficient value, the longitudinal wave speed theoretical value and the attenuation coefficient theoretical value into a least square error objective function, and judging that the corresponding free gas saturation is the predicted free gas saturation of the work area reservoir sediment when the least square error objective function is smaller than an error threshold value.
Specifically, in the step S2, the actual measurement value of the attenuation coefficient is calculated from formula (5) based on an acoustic logging, where formula (5) is expressed as:
（5）
in the method, in the process of the invention,for the actual measurement of the attenuation coefficient, < >>Is the measured velocity of the longitudinal wave,fin order to be a frequency of the light,X(ω,z 1 ) Is distant from the sound sourcez 1 Is a function of the amplitude spectrum of the receiver of (a),X(ω,z 2 ) Is distant from the sound sourcez 2 Amplitude spectrum of the receiver of (a);
the least squares error objective function is expressed as:
（6）
in the method, in the process of the invention,as a least squares error objective function, +.>Is the maximum value of the longitudinal wave velocity which has been measured, < >>Is the maximum value of the attenuation coefficient which has been calculated.
Specifically, in the step S2, when the least square error objective function is determined to be greater than the error threshold, the free air saturation and the free air effect enhancement index are updated by using an iterative algorithm until the least square error objective function is less than the error threshold.
In some embodiments, the error threshold is set to 0.001, and when the least squares error objective function is less than the error threshold of 0.001, the calculated free gas saturation and free gas effort enhancement index meet the accuracy requirement, otherwise, the iterative algorithm is continued to calculate the free gas saturation and free gas effort enhancement index until the accuracy requirement is met.
Specifically, in some embodiments, the iterative algorithm employs a particle swarm algorithm. The initial particle group takes 100 groups of particles, namely in the value rangeAnd->Internal uniform random generation of 100 groups->. Taking the value of each particle into a calculation formula for calculating the longitudinal wave speed and the attenuation coefficient to obtain the theoretical longitudinal wave speed and the attenuation coefficient, and initiallyTaking the measured longitudinal wave velocity at the first position, and initially +.>Taking the attenuation coefficient calculated from the first position and taking the least square error objective function +.>The smallest calculation result is the current optimal particle, the updating direction of each particle is the direction that the particle points to the optimal particle, the updating step length of each particle is 1/100 of the distance between the particle and the optimal particle,and->And (3) taking the maximum value from all the actually measured longitudinal wave speeds and attenuation coefficients obtained by the logging curve, and calculating the current optimal particle again, wherein the updating iteration of the current optimal particle stops at step 50.
In the quantitative analysis method for the free gas saturation of the reservoir deposit, the constructed free gas action enhanced rock physical model considers the strong inhibition effect of the free gas on the acoustic wave propagation, and compared with the existing theoretical model, the method is more in line with the actual condition of the reservoir with the free gas, can more accurately describe the longitudinal wave speeds and attenuation coefficients of reservoirs with different free gas saturation, is more accurate in calculated longitudinal wave speeds and attenuation coefficients, has important significance and value for acoustic detection and identification of the reservoir with the free gas, and provides theoretical basis for quantitative interpretation of the free gas saturation of the reservoir with the free gas. The free gas action enhanced rock physical model is applied to free gas detection, so that a novel method for free gas detection by utilizing acoustic parameters (namely longitudinal wave speed and attenuation coefficient) is provided, and acoustic logging is improved and perfected.
To illustrate the effectiveness of the above-described reservoir sediment free gas saturation quantitative analysis method of the present invention. And calculating the free gas saturation and the free gas action enhancement index in the free gas-containing reservoir by adopting the reservoir sediment free gas saturation quantitative analysis method according to actual test data.
Fig. 2 and 3 show the variation trend of the longitudinal wave velocity and attenuation coefficient in the free gas reservoir with the saturation of the free gas, and compare with the experimental data. Fig. 2 and fig. 3 show that the longitudinal wave velocity and the attenuation coefficient thereof obtained by calculation of the free gas action enhanced petrophysical model constructed in the reservoir sediment free gas saturation quantitative analysis method according to the invention conform to experimental actual measurement data.
Table 1 gives specific measured data including: gas saturation, longitudinal wave velocity, and attenuation coefficient. The acquisition method of the measured data comprises the following steps: in the gradual pressure relief process of sand containing saturated carbon dioxide aqueous solution, the gas saturation, the longitudinal wave speed and the attenuation coefficient are obtained by recording an acoustic response amplitude curve and calculating the longitudinal wave speed and the attenuation coefficient and measuring the gas volume in the sand sample by using a volume variable measuring system.
TABLE 1
| Saturation of gas | Speed [ m/s ]] | Attenuation coefficient [ -dB] |
| 0 | 1749.42 | 0.29 |
| 0.01 | 946.67 | 0.71 |
| 0.02 | 920.00 | 0.93 |
| 0.03 | 913.57 | 0.98 |
| 0.04 | 910.99 | 1.00 |
| 0.05 | 909.73 | 1.00 |
| 0.06 | 909.05 | 0.99 |
| 0.07 | 908.69 | 0.98 |
| 0.08 | 908.52 | 0.97 |
| 0.09 | 908.46 | 0.96 |
The above-described embodiments are intended to illustrate the present invention, not to limit it, and any modifications and variations made thereto are within the spirit of the invention and the scope of the appended claims.
Claims (4)
1. The quantitative analysis method for the free gas saturation of the reservoir deposit is characterized by comprising the following steps:
s1, establishing a free gas action enhanced rock physical model, which comprises the following specific steps:
s11, obtaining a reservoir sediment frameworkmThe volume ratio, the volume modulus and the shear modulus of each component in the components are obtained according to Hill average equation to obtain the volume modulus of solid particles forming a reservoir sediment frameworkK S Shear modulusμ S Expressed as:
（1）
in the method, in the process of the invention,mto build up the number of components of the reservoir sediment backbone,f i is a componentiIs used in the field of the fuel cell,K i is a componentiIs used for the preparation of a composite material,μ i is a componentiShear modulus of (a);
s12, based on bulk modulusK S Shear modulusμ S Obtaining the bulk modulus of the reservoir sediment dry skeleton according to the dry skeleton cementation modelK m Shear modulusμ m The dry matrix cementation model is expressed as:
（2）
in the method, in the process of the invention,ϕin order to achieve a degree of porosity, the porous material,αto construct the cementation coefficient of the reservoir dry matrix solid particles,γis an intermediate variable;
s13, based on bulk modulusK S Modulus of volumeK m Reservoir holeBulk modulus of interstitial waterK w Bulk modulus of reservoir pore free gasK g Calculating a free gas action enhanced equivalent bulk modulus using a free gas action enhanced equivalent bulk modulus formula, the free gas action enhanced equivalent bulk modulus formula expressed as:
（3）
in the method, in the process of the invention,K av is the free gas action enhanced equivalent bulk modulus,S g in order to achieve the saturation of the free gas,r g for the enhancement of the index of free gas action,c、ϕ S is an intermediate variable;
s14, based on bulk modulusK m Modulus of shearμ m Enhanced equivalent bulk modulus of free gas actionK av According to a Biot biphase wave characteristic equation, obtaining longitudinal wave velocity and attenuation coefficient of saturated water sediment containing free gas, namely a free gas action enhanced rock physical model;
s2, obtaining the volume fraction of the reservoir sediment skeleton component according to natural gamma logging, porosity logging and lithology density logging data of a target horizon of a work area, calculating according to a Hill average equation to obtain the volume modulus and the shear modulus of solid particles of the reservoir sediment skeleton, obtaining a measured value of a longitudinal wave velocity and a measured value of an attenuation coefficient according to a sound logging curve of the work area, substituting initial values of free gas saturation and an enhancement index of free gas action into the physical model of the free gas action enhanced rock to obtain a theoretical value of the longitudinal wave velocity and a theoretical value of the attenuation coefficient of the reservoir of the work area, bringing the measured value of the longitudinal wave velocity, the measured value of the attenuation coefficient, the theoretical value of the longitudinal wave velocity and the theoretical value of the attenuation coefficient into a least square error objective function, and judging that the corresponding free gas saturation is the predicted free gas saturation of the reservoir sediment of the work area when the least square error objective function is smaller than an error threshold.
2. The method of claim 1, wherein in step S14, the Biot biphasic wave signature equation is expressed as:
（4）
in the method, in the process of the invention,det(‧) is a feature root of the matrix;ωrepresenting the wave source frequency;representing complex wave numbers;jis a virtual unit;is a density matrix in which:
，/>，/>，
，/>，ρ S for the density of the particles of the framework,ρ w in order to achieve a water density of the pores,ρ g in order to achieve the free air density,ris the pore space bending coefficient;
is a friction coefficient moment>，η l Is the viscosity coefficient of pore fluid，κEffective permeability for reservoir sediment backbone;
is a bulk modulus coefficient matrix, wherein:
，/>，/>，；
the calculation formula of the longitudinal wave velocity and the attenuation coefficient is expressed as:
wherein,is the attenuation coefficient;v p is the longitudinal wave velocity of the reservoir containing free gas; re (‧) and Im (‧) take real and imaginary parts, respectively.
3. The method of claim 2, wherein in step S2, the actual attenuation coefficient value is calculated from equation (5) based on an acoustic log, and the equation (5) is expressed as:
in the method, in the process of the invention,for the actual measurement of the attenuation coefficient, < >>Is the measured velocity of the longitudinal wave,fin order to be a frequency of the light,X(ω,z 1 ) Is distant from the sound sourcez 1 Is a function of the amplitude spectrum of the receiver of (a),X(ω,z 2 ) Is distant from the sound sourcez 2 Amplitude spectrum of the receiver of (a);
the least squares error objective function is expressed as:
（5）
in the method, in the process of the invention,as a least squares error objective function, +.>Is the maximum value of the longitudinal wave velocity that has been measured,is the maximum value of the attenuation coefficient which has been calculated.
4. A method of quantitative analysis of free gas saturation of a reservoir deposit as claimed in any one of claims 1 to 3 wherein in step S2, when the least squares error objective function is determined to be greater than the error threshold, an iterative algorithm is used to update the free gas saturation and free gas effect enhancement index until the least squares error objective function is less than the error threshold.
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