A method for calculating saturation of volcanic rock considering altered clay

Abstract

The application discloses a method for calculating the saturation of volcanic rocks by considering altered clay, which is based on natural volcanic plug rock electrical data and whole rock analysis measurement data, and applies the Simandoux model to evaluate the conductive contribution of altered clay minerals, so that a saturation model more suitable for altered volcanic rocks is obtained. The formation conductivity calculated by the new altered volcanic rock saturation model is consistent with the real measurement data, and when applied in a well, more reliable saturation evaluation results of the altered volcanic rocks are obtained. The model experiment effect is good, and the model is more suitable for the study of the rock conductivity of volcanic rocks.

Description

Technical Field

[0001] This invention is a method for calculating the saturation of volcanic rocks that takes altered clay into account. Specifically, it relates to a new method for calculating the water saturation of altered volcanic rock reservoirs through well logging, and belongs to the field of downhole exploration and development technology. Background Technology

[0002] Compared to argillaceous sandstone, volcanic rocks have more complex lithology and pore structure, with relatively lower porosity and permeability. Establishing saturation equations for volcanic rocks is challenging, and simpler models such as the Archie formula or argillaceous sandstone models, like the typical Hossin model, Simondu model, and WS model, have been used. This difficulty is further exacerbated when volcanic rocks exhibit altered clay content, making suitable models unavailable. Therefore, establishing saturation models for volcanic rocks is essential to understanding their reservoir characteristics, particularly the impact of alteration on conductivity.

[0003] Wang Di et al., in "Application of Improved Simondu Model in Saturation Logging Interpretation" (Offshore Oil, Vol. 36, No. 3, September 2016), optimized the Simondu model based on clay-added conductivity. They used a calibration model to calculate the clay mass fraction and substituted it into the saturation model for calculation. At the same time, they used the fitting of rock electrical parameters to eliminate the influence of clay-added conductivity on the m and n values, which can improve the accuracy of the Simondu formula.

[0004] The invention patent with publication number CN102454401A discloses a method for obtaining the logging saturation of low-porosity and permeability reservoirs. First, the cementation index m corresponding to different resistivities in the reservoir is determined. Then, a functional relationship between resistivity and cementation index m is established. Finally, an approximate solution for the gas saturation of the reservoir is obtained, thereby achieving an accurate judgment of the reservoir type.

[0005] However, neither of the above two methods takes into account the effect of alteration on the reservoir characteristics of volcanic rocks. Therefore, it is necessary to establish a new method for calculating the water saturation of altered volcanic rock reservoirs through well logging. Summary of the Invention

[0006] The purpose of this invention is to provide a method for calculating the saturation of volcanic rocks that takes altered clay into account. This method is based on electrical data from natural volcanic rock plug samples and whole-rock analysis measurements. It applies the Simondus model to evaluate the electrical conductivity contribution of altered clay minerals, resulting in a saturation model more suitable for altered volcanic rocks. The formation conductivity calculated by the new altered volcanic rock saturation model matches the actual measured data. Its application in wells has yielded more reliable saturation evaluation results for altered volcanic rocks. The model experiments show good results and are more suitable for studying the electrical conductivity of volcanic rocks.

[0007] This invention is achieved through the following technical solution: a method for calculating the saturation of volcanic rocks considering altered clay, comprising the following steps:

[0008] A. Based on well logging data or core analysis data, establish at least one alteration index and determine the degree of alteration;

[0009] B. Based on whole-rock analysis data, establish a model relating alteration index to altered clay minerals;

[0010] C. Based on rock electrical experiments, the altered clay content calculated using the Siemens model and alteration index was used to correct for formation factors;

[0011] D. Establish the alteration saturation equation and complete the well logging process.

[0012] In step A, the alteration index is established. IA and alteration index I s,

[0013] The alteration index IA The formula is as follows:

[0014]

[0015] in, AC These are the values ​​from sonic transit time logging. CNL This is the neutron logging value. DEN This is the density logging value; Rt These are deep lateral resistivity logging values;

[0016] The alteration index I The formula for s is as follows:

[0017]

[0018] in, AC min This represents the minimum acoustic transit time across the entire well. CNL min This is the minimum value for a neutron.

[0019] Using well logging data, based on the alteration index I s determines the degree of alteration:

[0020] Mild alteration: 0% ≤ alteration index I s<10%;

[0021] Moderate alteration: 10% ≤ alteration index I s<30%;

[0022] Severe alteration: 30% ≤ alteration index I s.

[0023] In step B, the alteration index I The relationship model between s and altered clay minerals is as follows:

[0024] Altered clay mineral content %=

[0025] Where a and b are both fitting coefficients.

[0026] In step A, the alteration index is established. I e, its formula is as follows:

[0027]

[0028] in, a For lithological parameters, W Al , W Fe , W Gd and W Si These represent the weight percentage content of Al, Fe, Gd, and Si elements, respectively.

[0029] Core analysis data were obtained using ECS ​​logging, and based on the alteration index I e. Determine the degree of alteration:

[0030] Mild alteration: 0% ≤ alteration index I e<10%;

[0031] Moderate alteration: 10% ≤ alteration index I e<30%;

[0032] Severe alteration: 30% ≤ alteration index I e.

[0033] In step B, the alteration index I The relationship model between e and altered clay minerals is as follows:

[0034] Altered clay mineral content %=

[0035] Where a and b are both fitting coefficients.

[0036] In step C, based on rock electrical experiments, the rock conductivity under different saturation states is obtained. Then, the Siemens model and altered clay content are used to correct the formation factors. The corrected formation factors F * The formula is as follows:

[0037]

[0038] in, C w For the electrical conductivity of the formation water, C 0sh The electrical conductivity of the rock under saturation conditions. Csh The electrical conductivity of the clay fraction in a rock under saturated conditions. V sh This represents the mass fraction of clay.

[0039] In step D, the alteration saturation equation is as follows:

[0040]

[0041] in, C t For the deep resistivity of the well, C w For the electrical conductivity of the formation water, S w Water saturation V 蚀变 To calculate the altered clay content, C 蚀变 The electrical conductivity is the content of altered clay. n This is the saturation index.

[0042] A computer system is used to implement the model establishment, calibration, and equation calculation in steps A, B, C, and D.

[0043] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0044] (1) The alteration phenomenon commonly found in volcanic oil and gas reservoirs makes well logging evaluation complex and difficult. Current research on volcanic reservoirs is mostly conducted in unaltered volcanic rocks, with few studies on well logging evaluation of altered volcanic reservoirs. Therefore, this invention optimizes the alteration index based on the well logging response characteristics of altered volcanic reservoirs. I A new alteration index was defined by combining ECS ​​logging data with core clay content and performing correlation analysis. I e, using I s and I This method quantitatively evaluates the degree of alteration and establishes the relationship between the alteration index and the content of altered clay, which is a new method for quantitatively calculating the content of altered clay minerals in well logging.

[0045] (2) Compared with argillaceous sandstone, volcanic rocks have complex lithology, complex pore structure, and relatively low porosity and permeability. In existing technologies, it is difficult to establish a saturation equation for volcanic rocks, and simple Archie formulas or models for argillaceous sandstone, such as the typical Hossin model, Simondu model, and WS model, have been used. This difficulty is exacerbated when altered clay is present, and no suitable model is available. Therefore, it is necessary to establish a saturation model for volcanic rocks to address the characteristics of volcanic rock reservoirs. Therefore, this invention, based on the electrical conductivity data of natural volcanic rock plug samples and whole-rock analysis measurement data, applies the Simondu model to evaluate the electrical conductivity contribution of clay, obtaining a new saturation model more suitable for altered volcanic rocks. The rock conductivity calculated by the new alteration saturation model as a function of saturation is consistent with the actual experimental measurement data.

[0046] (3) The present invention applies the above-mentioned new model in wells and obtains more reliable saturation evaluation results for altered volcanic rocks. The model experiment results are good and it is more suitable for the study of the electrical conductivity of volcanic rocks. Attached Figure Description

[0047] Figure 1 The cross-plot of clay content and element weight percentage in the core is shown in (a).

[0048] Figure 2 The cross-plot of clay content and element weight percentage in the core is shown in (b).

[0049] Figure 3 Alteration index I The relationship between s and clay content.

[0050] Figure 4 Alteration index I The relationship between e and clay content.

[0051] Figure 5 This is a cross-plot of corrected and uncorrected formation factors and porosity.

[0052] Figure 6 This is a comparison between laboratory measurements and Simendo model calculations.

[0053] Figure 7 Comparison of laboratory measurements, Siemens model, and new alteration saturation model calculation results.

[0054] Figure 8 This is a diagram illustrating the overall interpretation of well logging results.

[0055] Figure 9 This is a flowchart of the method of the present invention. Detailed Implementation

[0056] The invention's objective, technical solution, and beneficial effects will be further explained in detail below.

[0057] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the claimed invention. Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0058] This embodiment presents a method for calculating the saturation of volcanic rocks that takes altered clay into account. The specific method is as follows:

[0059] Step 1: Based on core analysis data and well logging data, assess the degree of alteration in this area, establish multiple alteration factors, and determine the degree of alteration.

[0060] Step 1: Establish the alteration index IA .

[0061] Since altered clay minerals increase neutron and sonic transit time values ​​while decreasing density and resistivity values, this alteration index is defined using sonic transit time (AC), neutron (CNL), density (DEN), and deep lateral resistivity (Rt) logging values, as shown in the following formula:

[0062] (1)

[0063] Step 2: Establish the alteration index I s.

[0064] Based on alteration index IA Define the alteration index I s, the formula is as follows:

[0065] (2)

[0066] To amplify the effects of alteration, we assume a minimum sonic transit time across the entire well ( ACmin ) and neutron minimum ( CNLmin The ) represents the acoustic transit time and neutron value of the unaltered rock strata, expressed as the difference between the acoustic transit time and neutron measurement values ​​and their corresponding minimum values. IA In the replacement AC and CNL Then, using the normalization method, compared to the previous AC min and CNLmin To reduce the error caused by large differences in different well logging values, the alteration index was finally obtained. I s.

[0067] Step 3: Alteration Index I e.

[0068] Existing research has found that the elemental composition of rock formations changes during alteration because the migration of mobile elements during alteration leads to either "dilution" or "concentration" of elemental composition. Therefore, a new alteration index can be defined based on this principle and well logging data of formation elements. I e.

[0069] First, based on the clay content obtained from experimental tests and the elemental weight percentages obtained from ECS logging, a cross-plot of core clay content versus elemental weight percentages was constructed, as follows: Figure 1 , Figure 2 As shown, the study found that the content of aluminum and iron was directly proportional to the clay content of the core, while the content of silicon and gadolinium was inversely proportional to the clay content of the core.

[0070] By combining the four elements with relatively stable correlations to core clay content obtained from cross-plots, an alteration index was defined using their weight percentage content. I e, the formula is as follows:

[0071] (3)

[0072] In the formula, a For lithological parameters, W Al , W Fe , W Gd and W Si These represent the weight percentage content of Al, Fe, Gd, and Si elements, respectively.

[0073] Step 4: Select the alteration index based on the actual situation and determine the degree of identification.

[0074] Due to the alteration index IA This method has certain limitations, therefore this embodiment uses the alteration index. Is and alteration index Ie To quantitatively analyze the degree of alteration.

[0075] For example, in a specific implementation, when only conventional logging data is available, the alteration index Is is calculated according to the methods shown in Step 1 and Step 2, and then based on the alteration index... I s determines the degree of alteration:

[0076] Mild alteration: 0% ≤ alteration index I s<10%;

[0077] Moderate alteration: 10% ≤ alteration index I s<30%;

[0078] Severe alteration: 30% ≤ alteration index I s.

[0079] Alternatively, in another specific embodiment, if the elemental content can be directly measured via ECS logging, then the alteration index can be calculated using the ECS data. I e (see Step 3), and then based on the alteration index I e determines the degree of alteration:

[0080] Mild alteration: 0% ≤ alteration index I e<10%;

[0081] Moderate alteration: 10% ≤ alteration index I e<30%;

[0082] Severe alteration: 30% ≤ alteration index I e.

[0083] Step 2: Based on whole-rock analysis data, study the relationship between multiple alteration indices and altered clay minerals in order to establish a model of the relationship between alteration factors and altered clay minerals.

[0084] Step 1: Establish the alteration index I A model relating s to clay mineral content.

[0085] Altered clay mineral content %= (4)

[0086] Where a and b are fitting coefficients, for example: altered clay mineral content % = 1.1694* I s 0.8993 .

[0087] Step 2: Establish the alteration index I Model of the relationship between e and clay mineral content.

[0088] Establish alteration index I Model of the relationship between e and clay mineral content.

[0089] Altered clay mineral content %= (5)

[0090] Where a and b are fitting coefficients, for example: altered clay mineral content % = 0.0727* I e 1.2583 .

[0091] In one specific embodiment, clay mineral content and alteration index were obtained based on whole-rock analysis, conventional well logging data, and lithological scanning well logging, respectively. I s, and then based on clay content, rock electrical parameters, sonic logging values, density logging values, neutron logging values, and deep resistivity values, the following are obtained: Figure 3 Alteration index shown I The curve showing the relationship between s and clay mineral content.

[0092] Meanwhile, clay mineral content and alteration index were obtained based on whole-rock analysis and ECS logging data, respectively. I e, and then based on the clay content and the weight percentage of elements, we obtain, as follows: Figure 4 Alteration index shown I The curve showing the relationship between e and clay mineral content.

[0093] Depend on Figure 3 and Figure 4 It can be seen that using the alteration index I The correlation coefficient between e and the clay mineral content in the core is higher than that of the alteration index. I The correlation coefficient between s and the clay mineral content of the core is used; therefore, in this embodiment, it is recommended to use the alteration factor. I e is used to calculate the content of altered clay minerals.

[0094] Step 3: Based on rock electrical experiments, the altered clay content calculated using the Siemens model and alteration factor is used to correct for formation factors.

[0095] Step 1: Rock electrical experiment measurement.

[0096] This embodiment uses the "wetting method" to measure the effect of the prepared brine entering the rock on the change in electrical conductivity. Generally, rock electrical conductivity increases with increasing water content. The rock electrical conductivity is denoted as Ct. Several brine wetting cycles are performed, the process being dripping-soaking-complete soaking-evacuation saturation-pressurization saturation, until the pores are saturated with formation water. The method for calculating the water saturation degree measured each time is as follows:

[0097] (6)

[0098] in, m i This represents the weight of the core plug obtained in the i-th measurement, in grams; m 0 represents the weight of the dry sample obtained after the rock is dried, in grams; m ws The value in grams represents the weight of formation water within the pores of a rock when it is fully saturated. Based on the above experiments, the electrical conductivity of rocks saturated with formation water and under different saturation conditions can be obtained.

[0099] Step 2: Correct formation factor F by using the altered clay content calculated using the Simondu model and alteration factor.

[0100] In rock electrical experiments, the formation factor F is calculated using Archie's formula, as shown below:

[0101] (7)

[0102] In the formula, a It is the lithology coefficient; the m value is called the cementation index. C w and C 0 represents the electrical conductivity of formation water and the electrical conductivity of rock saturated with formation water, respectively. It is worth noting that Equation (7) is only applicable to pure sandstone formations or formations without clay. For actual formations containing altered clay minerals, the lithology and mineral composition are complex, so there will be a large error in calculating formation factors using Archie's formula.

[0103] Calculation in the Siemens model C The formula for 0 is:

[0104] (8)

[0105] in, C 0sd S*m represents the electrical conductivity of pure rock under saturated conditions. -1 , C 0sh and C sh S*m represents the electrical conductivity of mudstone under saturated conditions (refer to pure mudstone) and the electrical conductivity of the clay component within the rock. -1 Formula (8) is used to calculate the corrected formation factor F*. Substituting the measured values ​​into the formula and simplifying, we get:

[0106] (9)

[0107] In a specific embodiment, the corrected formation factor F* is calculated using formula (9), with porosity as the horizontal axis and the formation factor values ​​F and F* before and after correction as the vertical axes, respectively. The data required for calculating F* include formation water conductivity Cw, rock conductivity C0sh under saturated conditions, clay conductivity Csh in saturated rock, and clay mass fraction Vsh. Then, cross-plots of formation factors F, F*, and porosity are obtained, as shown below. Figure 5 As shown.

[0108] Depend on Figure 5 As shown, compared with the formation factor F calculated by rock electrical experiment, the corrected formation factor F* increases. This is because the presence of conductive minerals such as altered clay in the volcanic rocks leads to an increase in electrical conductivity. In addition, compared with the unaltered volcanic rocks in the region, the corrected altered volcanic rocks have a larger cementation index m, resulting in a larger formation factor value.

[0109] The formula for calculating the corrected value of m is as follows: The m value for unaltered volcanic rocks is: .

[0110] Step 4: Establish the alteration saturation equation and process the actual well data based on the new altered volcanic rock saturation equation.

[0111] Step 1: Establish the relationship between formation electrical conductivity and water saturation.

[0112] Ct is calculated using the Siemens model, where the electrical conductivity of the altered clay is calculated using the same method as the conductivity of the clay component, as shown in the following formula:

[0113] (10)

[0114] in, S w denoted as water saturation, %; and n is the saturation index. n The value can be obtained from rock electrical data. Calculations are performed on multiple rock samples, and the data is processed to obtain the values ​​for each rock sample. S w Corresponding C t value.

[0115] In one specific embodiment, the water saturation of the altered clay was determined using the dichotomy method, and then calculated using the Simondu model. C t Meanwhile, the rock electric experiment method was used to measure the corresponding altered clay. C t Measured values: The Ct calculated by the Siemens model is compared with the Ct measured by the laboratory rock electric experiment method.

[0116] Plot the relationship curve, such as Figure 6 As shown, with C t Compared with the measured values, the Siemens model calculation C t The obtained values ​​are all larger than the measured values, therefore the conductivity of clay minerals in volcanic rocks is higher. C t The effect is lower than that of argillaceous conductive pairs in argillaceous sandstone. C t The influence of these factors indicates that the Simondu model is not applicable to altered volcanic rocks. Therefore, the Simondu model was further improved to obtain a model that is closer to the measured values.

[0117] Step 2: Improve the saturation equation and determine the additional electrical conductivity saturation index k of the altered clay:

[0118] Research has found that saturation models include S wThe more terms related to (water saturation), the greater the complexity. This embodiment considers the parallel relationship between the additional conductivity of altered clay and the conductivity of fluids in the pores, and also assumes that the additional conductivity of clay changes with water saturation. k It is the saturation index of the added electrical conductivity of altered clay. A new alteration saturation model is established as follows:

[0119] (11)

[0120] To determine the additional conductivity saturation index of altered clay in the new model k The measured values ​​from the rock electrical experiment can be substituted into the model to determine the values ​​for each rock core, thereby determining the model's... k The values ​​are determined by the following: The clay mineral content of altered volcanic rocks is obtained from whole-rock analysis data, and C alteration can be obtained from well logging curves. The method for testing samples (…) is calculated. Figure 6 The k value for the medium sample is 2. Figure 7 It was calculated using the new alteration saturation model. C t A comparison chart with the Siemens model and experimental data.

[0121] Step 3: Automatically process the actual well logging data and display the calculation results as graphs.

[0122] Based on well logging data, including logging parameters such as deep lateral resistivity, calculated effective porosity, and calculated clay content, the data is input into a computer system to obtain a comprehensive well logging interpretation diagram, such as... Figure 8 As shown.

[0123] The flowchart in this embodiment can be referred to. Figure 9 As shown.

[0124] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications or equivalent changes made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for calculating the saturation of volcanic rocks considering altered clay, characterized in that: Includes the following steps: A. Based on well logging data or core analysis data, establish at least one alteration index and determine the degree of alteration; B. Based on whole-rock analysis data, establish a model relating alteration index to altered clay minerals; C. Based on rock electrical experiments, the rock electrical conductivity under different saturation states was obtained. Then, the Siemens model and altered clay content were used to correct the formation factors. The corrected formation factors F * The formula is as follows: in, C w For the electrical conductivity of the formation water, C 0sd The electrical conductivity of pure rock under saturation conditions. C 0sh The electrical conductivity of the rock under saturation conditions. C sh The electrical conductivity of the clay fraction in a rock under saturated conditions. V sh The mass fraction of clay content; D. Establish the alteration saturation equation and complete the well logging process. The alteration saturation equation is as follows: in, C t For the deep resistivity of the well, C w For the electrical conductivity of the formation water, S w This represents the water saturation level. V 蚀变 To calculate the altered clay content, C 蚀变 The electrical conductivity is the content of altered clay. n The saturation index. k The saturation index that imparts electrical conductivity to altered clay.

2. The method for calculating the saturation of volcanic rocks considering altered clay according to claim 1, characterized in that: In step A, the alteration index is established. IA and alteration index I s, The alteration index IA The formula is as follows: in, AC These are the values ​​from sonic transit time logging. CNL This is the neutron logging value. DEN This is the density logging value; Rt These are deep lateral resistivity logging values; The alteration index I The formula for s is as follows: in, AC min This represents the minimum acoustic transit time across the entire well. CNL min This is the minimum value for a neutron.

3. The method for calculating the saturation of volcanic rocks considering altered clay according to claim 2, characterized in that: Using well logging data, based on the alteration index I s Determines the degree of alteration: Mild alteration: 0% ≤ alteration index I s <10%; Moderate alteration: 10% ≤ alteration index I s <30%; Severe alteration: 30% ≤ alteration index I s.

4. The method for calculating the saturation of volcanic rocks considering altered clay according to claim 3, characterized in that: In step B, the alteration index I The relationship model between s and altered clay minerals is as follows: Altered clay mineral content %= Where a and b are both fitting coefficients.

5. The method for calculating the saturation of volcanic rocks considering altered clay according to claim 1, characterized in that: In step A, the alteration index is established. I e, its formula is as follows: in, a For lithological parameters, W Al , W Fe , W Gd and W Si These represent the weight percentage content of Al, Fe, Gd, and Si elements, respectively.

6. The method for calculating the saturation of volcanic rocks considering altered clay according to claim 5, characterized in that: Core analysis data were obtained using ECS ​​logging, and based on the alteration index Ie Determine the degree of alteration: Mild alteration: 0% ≤ alteration index I e <10%; Moderate alteration: 10% ≤ alteration index I e<30%; Severe alteration: 30% ≤ alteration index I e.

7. The method for calculating the saturation of volcanic rocks considering altered clay according to claim 1, characterized in that: In step B, the alteration index I The relationship model between e and altered clay minerals is as follows: Altered clay mineral content %= Where a and b are both fitting coefficients.

8. The method for calculating the saturation of volcanic rocks considering altered clay according to claim 1, characterized in that: In step C, the rock electrical experiment uses the wetting method to measure the rock conductivity, specifically including: performing several salt water wetting processes, the wetting process being dripping, soaking, complete soaking, vacuum saturation, and pressurized saturation, until the pores are saturated with formation water; the water saturation is measured each time. S w Calculate using the following formula: in, m i This represents the weight of the core plug obtained in the i-th measurement; m 0 represents the weight of the dry sample obtained after the rock is dried; m ws This represents the weight of formation water within the pores of a rock when it is fully saturated.

9. The method for calculating the saturation of volcanic rocks considering altered clay according to claim 1, characterized in that: The alteration saturation equation k The value was determined through rock electric experiments, and k The value of is 2.

10. The method for calculating the saturation of volcanic rocks considering altered clay according to claim 1, characterized in that: A computer system is used to implement the model establishment, calibration, and equation calculation in steps A, B, C, and D.

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