CN110554064B - Method for accurately estimating hydrate saturation in marine sediment based on dielectric properties - Google Patents
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Abstract
The invention relates to a method for accurately estimating hydrate saturation in marine sediments based on dielectric properties, which comprises the following steps: step S1, combining a three-dimensional finite difference (3D-FDM) model with Data Constraint Modeling (DCM) software to obtain dielectric constants under different hydrate saturations, hydrate micro-distribution modes and sediment porosities; s2, establishing a reliable relation for estimating the saturation of the hydrate by using the dielectric constant under the condition of 1GHz frequency; and step S3, according to the porosity and the dielectric constant of the marine sediment, accurately estimating the saturation of the hydrate in the marine sediment by using the obtained reliable relation between the dielectric constant and the porosity and the saturation of the hydrate of the marine sediment. The method can quickly and accurately estimate the hydrate saturation by using the dielectric constant of the hydrate-containing sediment under the conditions of different hydrate micro distribution modes and sediment porosities, and can effectively avoid the influence of seawater conductivity on the estimation result.
Description
Technical Field
The invention relates to a method for accurately estimating hydrate saturation in marine sediments based on dielectric properties.
Background
Natural gas hydrate, an ice-like substance composed of water and natural gas (mainly methane) under low temperature and high pressure conditions, is widely distributed in marine continental shelf marginal sediments and permafrost (Kvenvolden and Lorenson, 2001). Gas hydrates are considered as a very potential alternative energy source due to their huge energy resources (Kvenvolden, 1988; Collett,2002), on the other hand, changes in subsea temperature and pressure can cause gas hydrates to break down, thereby inducing subsea landslides, threatening ocean engineering safety, exacerbating global warming (Dickens, 2004; Brown et al, 2006; Pecher et al, 2008). The quantitative estimation of the saturation of the hydrate in the sediment is an effective method for accurately estimating the resource reserve of the hydrate and the stable state of the hydrate. Therefore, it is important to develop a method that can accurately estimate hydrate saturation in hydrate reservoirs.
Up to now, geophysical methods remain the primary method for identifying and evaluating hydrate resources in field survey methods. Among them, seismic exploration is one of the most widely used investigation methods. Wang et al (2017) studied the formation of hydrates in the new zealand chikurony nose-down edge using seismic data and indicated that anisotropy of permeability plays an important role in hydrate distribution. Hillman et al (2016) accurately plots a three-dimensional submarine-like reflector from seismic data of different seismic source frequencies and acquisition systems, and can effectively determine whether a natural gas hydrate exists. Furthermore, geophysical logging techniques are the most straightforward and efficient method for identifying and estimating hydrates in an in situ reservoir. The Schenkian et al (2017) estimate hydrate saturation in fractured hydrate reservoirs by using acoustic logging and effectively estimate the change characteristics of the hydrate saturation in the borehole stratum. Liu Jie et al (2017) quantitatively calculated the hydrate saturation of the SH2 well in the Haichou area of south China Hicishi fox based on resistivity and acoustic logging, which shows that a corrected resistivity method can more accurately estimate the hydrate saturation. Although the above method plays an important role in identification of hydrate resources, there is still a deficiency in accurate estimation of the amount of hydrate resources, which mainly depends on the relationship between various geophysical properties and hydrate saturation.
In order to establish the relationship between various geophysical properties and hydrate saturation, a large number of laboratory studies have been conducted by researchers. There are two main methods currently used for determining hydrate saturation of experimental samples, including Time Domain Reflectometry (TDR) (Wright et al, 2002) and pressure-volume-temperature methods (P-V-T) (Zhang et al, 2011; chenyufeng et al, 2013). The above method has been widely used as a tool in the study of geophysical properties (such as resistivity and sonic velocity) versus hydrate saturation (Spangenberg and Kulenkampff, 2006; Hu et al, 2010; chenyufeng et al, 2013). However, the P-V-T method requires precise measurement of parameters such as porosity, gas volume and density of the deposit to ensure accuracy of calculating the hydrate saturation, which greatly increases the difficulty of the experiment. TDR technology is relatively more straightforward and reliable, and is based primarily on measuring the dielectric constant of a sample containing hydrates, and thus obtaining the residual water content in the sample pores to determine hydrate saturation. Therefore, the TDR technique has become the primary method of estimating hydrate saturation in the laboratory.
However, the key to the successful quantitative estimation of natural gas hydrates by applying the TDR method is to establish a reliable relationship between the saturation of natural gas hydrates and the dielectric constant of hydrate-containing deposits. Sun et al (2005, 2011) propose a high resolution dielectric method based on the combination of equivalent medium theory and in situ dielectric logging data for estimation of hydrate saturation. Although this method has met with initial success in situ hydrate saturation estimation, the physical significance of the equivalent medium theory on which it relies remains to be elucidated. On the other hand, Wight et al (2002) conducted experimental studies in the laboratory on a sample containing a hydrate, which is mainly composed of quartz, obtained the relationship between the water content and the dielectric constant in the test sample under giga-frequency conditions, and compared with the P-V-T method, verified the validity of the obtained relationship. Furthermore, Kliner and Grozic (2006) determine the relationship between hydrate saturation and dielectric constant in sandstone samples and describe the volume expansion caused by hydrates. Lee et al (2008) measured the dielectric constant of hydrate-containing deposits from the northern gulf of mexico over the microwave frequency range, indicating that the dielectric method has potential application value for accurate estimation of hydrate saturation. However, none of the above studies have considered the effect of frequency, hydrate micro-distribution and deposit porosity on hydrate saturation estimation, which may significantly affect the dielectric properties of hydrate-containing deposits. Therefore, the correspondence obtained by the above studies has not been widely used.
Disclosure of Invention
The invention provides a method for accurately estimating hydrate saturation in marine sediments based on dielectric properties, which can quickly and accurately estimate the hydrate saturation by using the dielectric constant of the hydrate-containing sediments under the conditions of different hydrate micro-distribution modes and sediment porosities and can effectively avoid the influence of seawater conductivity on the estimation result under the condition of 1GHz frequency, and the adopted technical scheme is as follows:
a method for accurately estimating hydrate saturation in marine sediments based on dielectric properties is characterized by comprising the following steps:
step S1, combining a three-dimensional finite difference (3D-FDM) model with Data Constraint Modeling (DCM) software, and performing numerical simulation on a digital core of a hydrate-containing sediment sample to obtain dielectric constants under different hydrate saturation, hydrate micro-distribution and sediment porosity;
step S2, fitting to obtain a relation between a dielectric constant and a hydrate saturation according to the influence rule of the hydrate saturation on the dielectric constant of the hydrate-containing sediment in different sediment porosity and hydrate micro distribution modes, and establishing a reliable relation for estimating the hydrate saturation by using the dielectric constant, wherein the sediment porosity and the dielectric constant are input parameters of the relation, and the hydrate saturation is an output parameter of the relation;
and step S3, according to the porosity and the dielectric constant of the marine sediment, accurately estimating the saturation of the hydrate in the marine sediment by using the obtained reliable relation between the dielectric constant and the porosity and the saturation of the hydrate in the hydrate-containing sediment.
Based on the above technical solution, the step S1 is specifically implemented in a 3D-FDM model, assuming that the parallel plate capacitor has n3A cubic system of grid points, the current parallel to the electric field through the four sides of the system is set to zero as a boundary condition, and then the total current flowing through the system to which the electric potential is applied can be calculated at a given frequency to obtain the equivalent dielectric constant of the system, which is applied to the system containing the hydrate deposit and can obtain the equivalent dielectric constant of the hydrate deposit.
Based on the above technical solution, in step S2, the DCM software takes the X-CT Image as an input object, outputs a three-dimensional structure having micro volume distribution of each group of materials and pores under CT resolution, and may regard it as a three-dimensional digital core, in order to reconstruct the three-dimensional structure of an experimental sample by the DCM software, Image J software is used to process CT scan images of a group of artificial core samples into five groups of core images, which may be regarded as CT scan images of five core samples with different porosities and different hydrate micro distribution patterns and hydrate saturations, and these core samples are composed of only quartz matrix, hydrate and seawater, the processed images are input into the DCM software, and the output result is a complete three-dimensional structure, where each grid point includes volume fraction information of all three components, on the basis, data constraint modeling software carrying a 3D-FDM model algorithm plug-in carries out numerical simulation on the digital core containing hydrate sediments, and dielectric constants under different hydrate saturations, hydrate micro distribution modes and sediment porosities are obtained.
On the basis of the technical scheme, under the condition of 1GHz frequency, DCM software carrying a 3D-FDM model algorithm plug-in unit performs numerical simulation on the digital core containing hydrate deposits.
On the basis of the technical scheme, the five different porosities of the hydrate deposit-containing digital core are respectively 25.8%, 40.1%, 49.2%, 59.2% and 72.9%.
On the basis of the above technical solution, the step S3 is to obtain dielectric constants of the hydrate-containing deposit at five porosities and different hydrate saturations respectively by a numerical simulation method under a frequency condition of 1GHz, and fit the dielectric constants to obtain corresponding exponential formulas, which are respectively expressed as:
porosity 25.8%:
R2＝0.9934
porosity 40.1%:
R2＝0.9891
porosity 49.2%:
R2＝0.9975
porosity 59.2%:
R2＝0.9905
porosity 72.9%:
R2＝0.9932
it can be expressed uniformly as the following relationship:
meanwhile, the parameters a and b of the above formula (1) have dependence on the porosity of the deposit, so that the relationship between the two coefficients and the porosity is obtained by further fitting, which can be respectively expressed as:
in the formula, epsilon is the dielectric constant of the hydrate-containing sediment, ShIn order to obtain the saturation of the hydrate,
for deposit porosity, R2The degree of fit, i.e. the degree of correlation between the fitted curve and the data used for fitting,
further, the relationship between the dielectric constant of the hydrate-containing deposit and the saturation degree and porosity of the hydrate under the frequency condition of 1GHz is obtained, and the equations (1) to (3) are specifically shown.
The invention has the beneficial effects that: according to the method, through reconstruction of X-CT images of hydrate-containing sediments and numerical simulation calculation, dielectric responses of the hydrate-containing sediments under different sediment porosities, hydrate micro-distribution modes and hydrate saturations can be comprehensively obtained, further, the relation between the dielectric constant and the hydrate saturations is obtained, the dielectric constant-hydrate saturations relation under different porosities and hydrate micro-distribution modes is established, and quantitative evaluation of the hydrate saturations is achieved. The dielectric constant-hydrate saturation relation established by the invention under different porosities and hydrate micro-distribution modes fully utilizes the dielectric properties of hydrate-containing sediments, particularly utilizes the dielectric constant to calculate the hydrate saturation, and is independent of the salinity of seawater, so that the physical significance of the relation is more definite, the accuracy is higher, and the application range is wider.
Drawings
FIG. 1 is a graph of the dielectric constant of the hydrate-containing deposits of varying porosity obtained in accordance with the present invention as a function of hydrate saturation at a frequency of 1 GHz.
FIG. 2 is a fitting curve diagram of the relationship between parameters a and b and deposit porosity in a fitting formula of the dielectric constant and hydrate saturation of the hydrate-containing deposit obtained by the invention.
FIG. 3 is a graph comparing the results of the relationship calculations (point values in the graph) obtained by the present invention with actual well log data;
FIG. 4 is a graph of the dielectric response of hydrate-containing deposits having different micro-distribution patterns over different ranges of hydrate saturation;
Detailed Description
The invention is further illustrated by the following examples:
as shown in figures 1 and 2, the invention provides a method for accurately estimating the saturation degree of hydrate in marine sediments based on dielectric properties, which is characterized by comprising the following steps:
step S1, combining a three-dimensional finite difference (3D-FDM) model with Data Constraint Modeling (DCM) software, and performing numerical simulation on a digital core of a hydrate-containing sediment sample to obtain dielectric constants under different hydrate saturation, hydrate micro-distribution and sediment porosity;
step S2, fitting to obtain a relation between a dielectric constant and a hydrate saturation according to the influence rule of the hydrate saturation on the dielectric constant of the hydrate-containing sediment in different sediment porosity and hydrate micro distribution modes, and establishing a reliable relation for estimating the hydrate saturation by using the dielectric constant, wherein the sediment porosity and the dielectric constant are input parameters of the relation, and the hydrate saturation is an output parameter of the relation;
and step S3, according to the porosity and the dielectric constant of the marine sediment, accurately estimating the saturation of the hydrate in the marine sediment by using the obtained reliable relation between the dielectric constant and the porosity and the saturation of the hydrate in the hydrate-containing sediment.
Hydrate-containing deposits can be considered heterogeneous materials that undergo electric polarization when subjected to an alternating electric field (Asami, 2002). The electric polarization of a material is caused by various polarization mechanisms, and the polarization can be measured by measuring the dielectric constant in a certain time range. In the case of hydrate-containing deposits, interfaces exist between constituent elements having different electrical properties (e.g., the deposit framework, pore fluids, and hydrates), and thus the characteristic polarization of this material is Maxwell-Wagner polarization, which is attributed to the accumulation of charge at the interfaces at frequencies of about 1MHz to 1GHz (Von Hippel, 1952; Schwan, 1957; Foster and Schwan, 1995; Asami, 2002). Three-dimensional finite difference methods (3D-FDM) have been developed for calculating the equivalent dielectric constant of heterogeneous systems (Calame, 2003; Asami, 2005, 2006). Preferably, the step S1 is embodied in a three-dimensional finite difference model, assuming that the parallel plate capacitor has n3The cubic system of the individual grid points, the current parallel to the electric field through the four sides of the system is set to zero as a boundary condition, and then the total current flowing through the system applying the electric potential can be calculated at a given frequency, thereby obtaining the equivalent dielectric constant of the system containing the hydrate deposit.
Preferably, the step S2 is implemented by taking an X-CT Image as an input object by Data Constraint Modeling (DCM) software, outputting a three-dimensional structure with microscopic volume distribution of each material and pore at CT resolution, and regarding the three-dimensional structure as a three-dimensional digital core, in order to reconstruct the three-dimensional structure of an experimental sample by the DCM software, processing CT scan pictures of a group of artificial core samples into five groups of core pictures by Image J software, wherein the five groups of core pictures can be regarded as five core samples with different porosities and CT scan pictures with different micro distribution patterns of hydrates and saturation of hydrates, and the core samples only consist of quartz matrix, hydrates and seawater, inputting the processed pictures into the DCM software, outputting a complete three-dimensional structure, wherein each grid point contains volume fraction information of all three components (i.e. quartz matrix, hydrates and seawater), and on the basis, DCM software carrying a 3D-FDM model algorithm plug-in performs numerical simulation on the digital core containing hydrate sediments to obtain the dielectric constants under different hydrate saturations, hydrate micro-distribution and sediment porosities. The plug-in may be an existing plug-in, i.e. a C + + program executing a 3D-FDM algorithm, which follows the detailed procedure outlined in Asami (2006), and has been successfully run as a plug-in to DCM software (Yang et al 2015, 2016, 2017). The DCM software takes the X-CT image as an input object, outputs a three-dimensional structure with microscopic volume distribution of materials and pores at CT resolution, and can be considered as a three-dimensional digital core. The numerical simulation method combining the 3D-FDM model and DCM software is successfully applied to the dielectric property research of carbonate rocks and porous rocks containing cracks (Han and Yang, 2018 and 2019).
Preferably, the digital core containing the hydrate deposits is numerically simulated by data constraint modeling software carrying a 3D-FDM model algorithm plug-in under the frequency condition of 1 GHz. To demonstrate the applicability of the 3D-FDM model in hydrate-containing deposits, we compared the results of the numerical simulation with experimental data obtained from Wright et al (2002). Wright et al (2002) conducted experimental studies of dielectric properties at gigahertz conditions on samples of hydrate-containing deposits consisting primarily of silica sand and having a porosity of 50.6%. Accordingly, we simulated the above-described digital core with a porosity of 49.2% at a frequency of 1MHz to 1 GHz. At 1GHz frequency, the two results agree quite well, which fully demonstrates that the 3D-FDM model can be applied to the study of dielectric properties of hydrate-containing deposits.
Ecker (2001) preliminarily states that the state of attachment of hydrates in the pores of sediments can be divided into three microscopic distribution modes, including: a contact cementation mode, a pore filling mode and an encapsulation cementation mode, wherein the three microcosmic distribution modes are sequentially transformed along with the increase of the saturation degree of the hydrate. In this numerical simulation work, we preliminarily corresponded the following three microscopic distribution patterns (contact cementation, pore filling, and pack cementation with pore filling) to the hydrate saturation ranges 0-20%, 20-60%, and 60-90%, respectively. The hydrate micro-distribution has a significant effect on the dielectric response of the hydrate-containing deposit, as shown in FIG. 4, particularly as a characteristic of the change in dielectric behavior with respect to frequency. In the low frequency range, there is a significant difference in the relative dielectric constant value and the variation trend thereof, and the influence of the hydrate micro distribution mode on the dielectric behavior gradually decreases with the increase of the frequency. Since the mainstream dielectric tool is to operate in the gigahertz frequency range and considering the finding in fig. 4 that the relative permittivity changes most smoothly with hydrate saturation at gigahertz frequency, we will focus on the measurement frequency of 1GHz to provide a uniform correlation between relative permittivity and hydrate saturation for reliable estimation of natural gas hydrate saturation.
Further, the five different porosities of the hydrate deposit-containing digital cores were 25.8%, 40.1%, 49.2%, 59.2%, and 72.9%, respectively.
Further, step S3 is to obtain dielectric constants of the hydrate-containing deposit at five porosities and different hydrate saturations by a numerical simulation method, and fit the obtained dielectric constants at 1GHz frequency to obtain corresponding exponential equations, which are respectively expressed as:
porosity 25.8%:
R2＝0.9934
porosity 40.1%:
R2＝0.9891
porosity 49.2%:
R2＝0.9975
porosity 59.2%:
R2＝0.9905
porosity 72.9%:
R2＝0.9932
it can be expressed uniformly as the following relationship:
meanwhile, the parameters a and b of the above formula (1) have dependence on the porosity of the deposit, so that the relationship between the two coefficients and the porosity is obtained by further fitting, which can be respectively expressed as:
in the formula, epsilon is the dielectric constant of the hydrate-containing sediment, ShIn order to obtain the saturation of the hydrate,
for deposit porosity, R2The degree of fit, i.e. the degree of correlation between the fitted curve and the data used for fitting,
further, the relationship between the dielectric constant of the hydrate-containing deposit and the saturation degree and porosity of the hydrate under the frequency condition of 1GHz is obtained, and specifically the formulas (1) to (3).
As shown in FIG. 3, the actual log data was from an Erbert mountain hydrate test well located on the North slope of Alaska, USA (Collett et al, 2011). A dielectric logging tool known as an Electromagnetic Propagation Tool (EPT) is deployed in the ebert mountains hydrate test well. The propagation time (phase) and attenuation (amplitude) of a sinusoidal electromagnetic wave can be measured by EPT (Schlumberger, 1989) and the relative permittivity can be calculated using maxwell's equations (Sun et al, 2011).
The relative permittivity obtained by EPT and the porosity obtained by Combining Magnetic Resonance (CMR) (Collett et al, 2011) are used as input parameters for equations (1) - (3). Comparing the hydrate saturation calculated by the obtained formula with the hydrate calculated by the CMR, fig. 3 shows the input parameters (i.e. porosity and relative permittivity) and the comparison of the hydrate saturation estimated by the formula and CMR, it can be found that the hydrate saturation calculated by the present study based on the formula proposed by numerical simulations fits well with the saturation derived by the CMR, indicating that the relationship between the relative permittivity and hydrate saturation given by us can be used to reliably estimate the hydrate saturation in hydrate-containing deposits.
While there have been shown and described what are at present considered the fundamental principles and essential features of the invention and its advantages, it will be apparent to those skilled in the art that the invention is not limited to the details of the foregoing exemplary embodiments, but is capable of other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. Any reference sign in a claim should not be construed as limiting the claim concerned.
Furthermore, it should be understood that although the present description refers to embodiments, not every embodiment may contain only a single embodiment, and such description is for clarity only, and those skilled in the art should integrate the description, and the embodiments may be combined as appropriate to form other embodiments understood by those skilled in the art.
Claims (5)
1. A method for accurately estimating hydrate saturation in marine sediments based on dielectric properties is characterized by comprising the following steps:
step S1, combining the 3D-FDM model with DCM software, and performing numerical simulation on a digital core of a hydrate-containing sediment sample to obtain dielectric constants under different hydrate saturations, hydrate micro distribution modes and sediment porosities;
step S2, fitting to obtain a relation between a dielectric constant and a hydrate saturation according to the influence rule of the hydrate saturation on the dielectric constant of the hydrate-containing sediment in different sediment porosity and hydrate micro distribution modes, and establishing a reliable relation for estimating the hydrate saturation by using the dielectric constant, wherein the sediment porosity and the dielectric constant are input parameters of the relation, and the hydrate saturation is an output parameter of the relation;
step S3, according to the porosity and dielectric constant of the marine sediment, the obtained reliable relation between the dielectric constant and the porosity and hydrate saturation of the hydrate-containing sediment is used for accurately estimating the saturation of the hydrate in the marine sediment;
wherein, the step S2 is to take the X-CT Image as an input object by the DCM software, and output a three-dimensional structure having each group of material and pore microscopic volume distribution under the CT resolution, and may regard it as a three-dimensional digital core, in order to reconstruct the three-dimensional structure of the experimental sample by the DCM software, the CT scan images of a group of artificial core samples are processed by the Image J software into five groups of core images, which may be regarded as CT scan images of five core samples with different porosity having different hydrate microscopic distribution patterns and hydrate saturations, and these core samples are composed of only quartz matrix, hydrate and seawater, the processed images are input in the DCM software, and the output result is a complete three-dimensional structure, wherein each grid point includes volume fraction information of all three components, so as to obtain the porosity and hydrate saturation of the 3D digital core, on the basis, DCM software carrying a 3D-FDM model algorithm plug-in carries out numerical simulation on the digital rock core containing hydrate sediments, and dielectric constants under different hydrate saturation degrees, hydrate micro distribution modes and sediment porosity degrees are obtained.
2. The method of claim 1 for accurate estimation of hydrate saturation in marine sediments based on dielectric properties, wherein: the step S1 is embodied in a 3D-FDM model, assuming that the parallel plate capacitor has n3A cubic system of grid points, the current parallel to the electric field through the four sides of the system is set to zero as a boundary condition, and then the total current flowing through the system to which the electric potential is applied can be calculated at a given frequency to obtain the equivalent dielectric constant of the system, which is applied to the system containing the hydrate deposit and can obtain the equivalent dielectric constant of the hydrate deposit.
3. The method of claim 2, wherein the method comprises the steps of: the DCM software carrying the 3D-FDM model algorithm plug-in was used to numerically simulate the digital core containing hydrate deposits at 1GHz frequency.
4. A method for accurate estimation of hydrate saturation in marine sediments based on dielectric properties as claimed in claim 3 wherein: the five different porosities of the hydrate deposit-containing sample digital cores were 25.8%, 40.1%, 49.2%, 59.2%, and 72.9%, respectively.
5. The method of claim 4, wherein the method comprises the steps of: the step S3 is specifically to obtain dielectric constants of the hydrate-containing deposit at five porosities and different hydrate saturations respectively by a numerical simulation method under the condition of a frequency of 1GHz, and respectively fit to obtain corresponding exponential formulas, which are respectively expressed as:
porosity 25.8%:
R2＝0.9934
porosity 40.1%:
R2＝0.9891
porosity 49.2%:
R2＝0.9975
porosity 59.2%:
R2＝0.9905
porosity 72.9%:
R2＝0.9932
it can be expressed uniformly as the following relationship:
meanwhile, the parameters a and b of the above formula (1) have dependence on the porosity of the deposit, so that the relationship between the two parameters and the porosity is obtained by further fitting, and can be respectively expressed as:
in the formula, epsilon is the dielectric constant of the hydrate-containing sediment, ShIn order to obtain the saturation of the hydrate,
for deposit porosity, R2The degree of fit, i.e. the degree of correlation between the fitted curve and the data used for fitting,
further, the relationship between the dielectric constant of the hydrate-containing deposit and the saturation and porosity of the hydrate, specifically, the formulas (1) to (3), at a frequency of 1GHz was obtained.
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