Method for characterizing the water vapor sorption behavior of a rock

Abstract

The application discloses a method for characterizing rock water vapor adsorption behavior, and relates to the field of unconventional oil and gas characterization. The method comprises the following steps: providing a rock powder sample after drying treatment; performing a water vapor adsorption experiment on the rock powder sample to obtain adsorption data of the rock powder sample, wherein the adsorption data comprises water vapor adsorption amounts under different relative humidities; fitting the adsorption data in logarithmic form to obtain a target curve, determining a target relative humidity corresponding to a maximum change point of a slope of the target curve, and the target relative humidity is a relative humidity when capillary condensation of the rock powder sample starts; and determining monolayer adsorption amounts, multilayer adsorption amounts and capillary condensation adsorption amounts of the rock powder sample under different relative humidities according to a Dent adsorption model and the target relative humidity. The application can obtain the monolayer adsorption amounts, the multilayer adsorption amounts and the capillary condensation adsorption amounts of the rock powder sample under different relative humidities, respectively.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of unconventional oil and gas characterization, in particular to a method for characterizing water vapor adsorption behavior of rock. BACKGROUND

[0002] The water adsorption behavior of unconventional reservoirs is of great significance for oil and gas resource potential evaluation, exploration and development. In addition, in the field of CO2 geological storage, the water adsorption behavior of rock also has a great impact on the efficiency and safety of CO2 storage. However, the current research on the water adsorption behavior of unconventional reservoirs mainly focuses on the total adsorption amount, which has limitations, and lacks attention to further subdivision of adsorption.

[0003] The traditional water vapor adsorption experiment can only test the change of total adsorption amount with relative humidity, and cannot reflect the contribution of various adsorption effects in the total adsorption amount. Because the mechanisms of different adsorption effects are different, different adsorption effects have different meanings for the water adsorption capacity of rock samples. However, the current water vapor adsorption experiment of rock cannot measure the adsorption amount produced by different adsorption effects in detail. SUMMARY

[0004] The purpose of the present application is to provide a method for characterizing water vapor adsorption behavior of rock, which determines the monolayer adsorption amount, multilayer adsorption amount and capillary condensation adsorption amount of rock powder sample under different relative humidities, and can solve the problem that the current water vapor adsorption experiment of rock cannot measure the adsorption amount produced by different adsorption effects in detail.

[0005] To achieve the above purpose, the present application provides the following solutions:

[0006] The present application provides a method for characterizing water vapor adsorption behavior of rock, which comprises:

[0007] providing a rock powder sample after drying treatment;

[0008] performing a water vapor adsorption experiment on the rock powder sample to obtain adsorption data of the rock powder sample, the adsorption data including water vapor adsorption amounts under different relative humidities;

[0009] fitting the adsorption data in logarithmic form to obtain a target curve, and determining a target relative humidity corresponding to a maximum change point of the slope of the target curve, the target relative humidity being the relative humidity when capillary condensation of the rock powder sample starts;

[0010] determining the monolayer adsorption amount, multilayer adsorption amount and capillary condensation adsorption amount of the rock powder sample under different relative humidities according to the Dent adsorption model and the target relative humidity.

[0011] The application provides a method for characterizing rock water vapor adsorption behavior. First, a water vapor adsorption experiment is performed on a rock powder sample to obtain the water vapor adsorption amount of the rock powder sample at different relative humidities. Then, the adsorption data are converted into logarithmic form and fitted. The slope of the fitted curve represents the adsorption strength. Since the adsorption strength of the rock powder sample at different adsorption stages (the first stage is monolayer adsorption and multilayer adsorption, and the second stage is capillary condensation) is obviously different, the maximum change point of the slope of the target curve obtained by fitting is the critical point of the start of capillary condensation. The relative humidity corresponding to this point is the relative humidity at the start of capillary condensation, i.e. the target relative humidity. Finally, by combining the target relative humidity with the Dent adsorption model, the multilayer adsorption saturation value of the rock powder sample can be calculated. Based on the adsorption saturation value, the multilayer adsorption amount and capillary condensation amount in the Dent adsorption model calculation result can be further subdivided. Finally, the monolayer adsorption amount, multilayer adsorption amount and capillary condensation adsorption amount of the rock powder sample at different relative humidities can be obtained respectively. Therefore, the method for characterizing rock water vapor adsorption behavior provided in the above embodiment solves the problem that the current rock water vapor adsorption experiment cannot measure the adsorption amount generated by different adsorption effects in detail. BRIEF DESCRIPTION OF DRAWINGS

[0012] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed in the embodiments. Obviously, the drawings in the following description are only some embodiments of the application, and for those skilled in the art, other drawings can also be obtained without creative labor.

[0013] Figure 1 A flowchart of a method for characterizing rock water vapor adsorption behavior in an embodiment of the application;

[0014] Figure 2 A detailed flowchart of step 120 in the method for characterizing rock water vapor adsorption behavior in an embodiment of the application; Figure 1

[0015] Figure 3 A fitting result graph of adsorption data in logarithmic form for Freundlich isotherm model in an embodiment of the application;

[0016] Figure 4 A detailed flowchart of step 140 in the method for characterizing rock water vapor adsorption behavior in an embodiment of the application; Figure 1

[0017] A fitting result graph of adsorption data for Dent adsorption model in an embodiment of the application. Figure 5 Reference signs:

[0018] ​​DETAILED DESCRIPTION

[0019] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative effort belong to the protection scope of the present application.

[0020] The above purposes, features and advantages of the present application will be more apparent and understandable. The present application will be described in further detail below with reference to the drawings and specific embodiments.

[0021] In one exemplary embodiment, as shown in Figure 1 A method for characterizing rock water vapor adsorption behavior is provided, including the following steps 110 to 140. Wherein:

[0022] Step 110, providing a rock powder sample after drying treatment.

[0023] In order to accurately measure the water vapor adsorption behavior of the rock, it is necessary to avoid the interference of the initial moisture in the rock, so it is necessary to dry the rock powder sample.

[0024] First, a rock powder sample needs to be prepared. Exemplarily, a sample of a certain mass can be taken on the target rock, which is crushed into a rock powder of 60-80 mesh, i.e. a rock powder sample is obtained.

[0025] Secondly, the rock powder sample continues to be dried. Exemplarily, the rock powder sample can be placed in a drying oven and dried at 60℃ for 48 hours.

[0026] Step 120, performing a water vapor adsorption experiment on the rock powder sample to obtain adsorption data of the rock powder sample, the adsorption data including water vapor adsorption amounts at different relative humidities.

[0027] Wherein, with reference to Figure 2 The water vapor adsorption experiment on the rock powder sample specifically includes steps 121 and 122.

[0028] Step 121, iteratively recording the equilibrium mass of the rock powder sample at the current relative humidity in a test environment at a target temperature; wherein the current relative humidity is iteratively increased by a preset step size, and the equilibrium mass is the mass when the mass change is less than a preset threshold and maintained for a preset time.

[0029] Exemplarily, the target temperature can be set between 20℃-60℃, the relative humidity can be iteratively increased in a preset step size in the range of 0-0.95, the preset step size can be 0.1, the preset threshold can be 0.002% per minute, and the preset time can be 10 minutes. It should be noted that according to different test ranges and test accuracies of experiments, the above parameters can also be set to other values.

[0030] Step 122, determining the water vapor adsorption amount of the rock powder sample at each relative humidity according to the equilibrium mass of the rock powder sample at each relative humidity.

[0031] wherein for any relative humidity, the equilibrium mass of the rock powder sample at the humidity is subtracted from the initial mass of the rock powder sample, and the mass of the water vapor adsorbed by the rock powder sample at the humidity can be obtained. In the embodiment, the ratio between the mass of the water vapor adsorbed and the initial mass of the rock powder sample is used to represent the water vapor adsorption amount.

[0032] Exemplarily, the water vapor adsorption amount of the rock powder sample at any relative humidity S is A S determined by a target formula, and the target formula is:

[0033]

[0034] wherein, M S and M O respectively represent the equilibrium mass of the rock powder sample at the relative humidity S and the initial state. It should be noted that in the embodiment, the unit of the water vapor adsorption amount is mg / g, and the numerical value represents the mg number of water vapor absorbed per g of rock powder. In other embodiments, the unit of the water vapor adsorption amount can also be defined as other, such as g / g, and the constant 1000 in the target formula can be omitted.

[0035] Based on the above example explanation for steps 121 and 122, an operation example of a water vapor adsorption experiment is provided as follows:

[0036] 1. Start the water vapor adsorption instrument, keep the sample chamber open, and use dry nitrogen to purge the sample chamber for 10 minutes to dry the sample chamber. After calibrating the microbalance of the device, a rock powder sample with a mass of M 0 about 20 mg) is loaded into the sample chamber and the sample chamber is closed.

[0037] 2. Set the test environment temperature (20-60℃) according to the experimental requirements. Set the relative humidity of each test level at an interval of 0.1, and the test relative humidity increases from 0 to 0.95 step by step. The equilibrium time is set to be less than 0.002% of the change in the mass of the rock powder sample per minute, and maintained for 10 minutes before entering the next relative humidity level.

[0038] 3. After the settings are completed, start the test. The instrument automatically increases the relative humidity from 0 step by step according to the settings and records the mass of the rock powder sample at the end of each level, i.e. obtains the relative humidity and the corresponding mass of the rock powder sample at each relative humidity.

[0039] 4. Calculate the water adsorption amount of the rock powder sample at each relative humidity using the target formula.

[0040] Step 130, fit the adsorption data in logarithmic form to obtain the target curve, and determine the target relative humidity corresponding to the maximum change point of the slope of the target curve. The target relative humidity is the relative humidity at which the capillary condensation of the rock powder sample begins.

[0041] In this step, a coordinate system is established with the logarithm of relative humidity as the horizontal coordinate and the logarithm of water vapor adsorption amount as the vertical coordinate. Different data points in the coordinate system are adsorption data in logarithmic form. Then, these data points are fitted to obtain a target curve, and the slope of the curve represents the water vapor adsorption strength of the rock powder sample. Under different adsorption actions, the adsorption strength of the rock powder sample has significant differences. Specifically, during the single-layer adsorption and multi-layer adsorption stages, the rock powder sample can basically maintain an adsorption strength, while during the capillary condensation stage, the rock powder sample will maintain another adsorption strength, and the two adsorption strengths are different. Therefore, in the target curve, the target curve is basically composed of two straight line segments (approximately) connected. At this time, the junction of the two straight line segments is the maximum change point of the slope of the target curve, which is the transition point from the capillary condensation stage to the capillary condensation stage, i.e. the critical point of the beginning of capillary condensation. The relative humidity corresponding to the critical point is the relative humidity at which the capillary condensation of the rock powder sample begins.

[0042] Among them, the data fitting can be realized by using the Freundlich isotherm model (Freundlich adsorption isotherm model). Specifically, fitting the adsorption data in logarithmic form to obtain the target curve includes: establishing a coordinate system with the logarithm of relative humidity as the horizontal coordinate and the logarithm of water vapor adsorption amount as the vertical coordinate, and mapping the adsorption data in logarithmic form into the coordinate system; linearly fitting the in the coordinate system by the linear form of the Freundlich isotherm model to obtain the target curve, and the target curve is composed of two straight line segments with different slopes, and the junction of the two straight line segments is the maximum change point of the slope of the target curve.

[0043] The linear expression of Freundlich isotherm model is:

[0044]

[0045] wherein, A represents water vapor adsorption amount, k and n both represent constant coefficients, n can represent adsorption strength ，S represents relative humidity. n The greater the value represents the stronger interaction between the pore surface and water molecules, and the stronger the adsorption strength. For the linear fitting results of Freundlich isotherm model, 1 / n represents the slope of the fitting straight line, therefore, the greater the slope represents n the smaller the value, and the weaker the adsorption strength.

[0046] It should be noted that the two straight line segments in the target curve have different linear expressions. The two linear expressions have different constant coefficients.

[0047] Specifically, referring to Figure 3 , according to the difference in the slope of the linear fitting straight line, the data points in the coordinate system can be divided into two regions of low relative humidity and high relative humidity. Two straight line formulas are obtained by fitting the two regions respectively according to Freundlich isotherm model. The straight line slope in the low relative humidity region is smaller, corresponding to a larger n value (meaning stronger adsorption strength), representing monolayer and multilayer adsorption of pore surface to water molecules. The straight line slope in the high relative humidity region is larger, corresponding to a smaller n value (meaning weaker adsorption strength), representing capillary condensation.

[0048] Step 140, according to the Dent adsorption model (Dent multi-component adsorption model) and the target relative humidity, the monolayer adsorption amount, the multilayer adsorption amount and the capillary condensation adsorption amount of the rock powder sample under different relative humidities are determined.

[0049] The Dent adsorption model can represent two different adsorption effects of the rock powder sample, i.e., quantifying the relationship between the water vapor adsorption amount and the relative humidity under different effects. The first can represent the monolayer adsorption effect, i.e., the monolayer adsorption amount under any relative humidity can be directly calculated. The second can represent the combined effect of the multilayer adsorption effect and the capillary condensation effect, but cannot further subdivide the multilayer adsorption effect and the capillary condensation effect, i.e., the sum of the multilayer adsorption amount and the capillary condensation amount under any relative humidity can be directly calculated, but the multilayer adsorption amount and the capillary condensation amount cannot be subdivided. Considering that the capillary condensation effect occurs when the multilayer adsorption effect is saturated, the target relative humidity calculated in the previous step can be substituted into the Dent adsorption model to calculate the multilayer adsorption saturation value. If the relative humidity to be calculated is less than the target relative humidity, the adsorption amount calculated at this time is entirely the multilayer adsorption amount. If the relative humidity to be calculated is greater than the target relative humidity, the adsorption amount calculated at this time is the sum of the multilayer adsorption amount and the capillary condensation amount. The sum minus the multilayer adsorption saturation value can obtain the corresponding capillary condensation amount. Therefore, by combining the Dent adsorption model and the target relative humidity, the monolayer adsorption amount, the multilayer adsorption amount, and the capillary condensation adsorption amount under any relative humidity can be calculated.

[0050] With reference to Figure 4 In this embodiment, step 140 includes step 141 and step 142.

[0051] Step 141: fitting the water vapor adsorption amount under different relative humidities by the Dent adsorption model to obtain a first model and a second model of the rock powder sample. The first model is used to represent the monolayer adsorption effect, and the second model is used to represent the combined effect of the multilayer adsorption effect and the capillary condensation effect.

[0052] The first model is:

[0053]

[0054] wherein, A 1, P M and Q 1 represent the monolayer adsorption amount, the monolayer adsorption maximum value, and the strength of the monolayer adsorption effect of the rock powder sample, respectively, S RH represents the relative humidity, Q 2 represents the total strength of the multilayer adsorption effect and the capillary condensation effect.

[0055] The second model is:

[0056]

[0057] wherein, A2 represents the sum of the multilayer adsorption amount and the capillary condensation amount of the rock powder sample, P M 、 Q 1 and 2 respectively represent the monolayer adsorption maximum, the strength of monolayer adsorption, and the total strength of multilayer adsorption and capillary condensation of the rock powder sample, Q S represents the relative humidity.

[0058] With reference to Figure 5 , specifically, a coordinate system is established with the relative humidity as the horizontal coordinate and the water vapor adsorption amount as the vertical coordinate, the water vapor adsorption amounts under different relative humidities are mapped into the coordinate system, and each data point in the coordinate system is fitted by the Dent adsorption model, and the fitting result is:

[0059]

[0060] wherein, A represents the total adsorption amount of the rock powder sample, A = A 1+ 2. A Based on the above fitting result, the Dent adsorption model can be divided into two types, and the first model and the second model are obtained. The first model is a model of the monolayer adsorption amount changing with the relative humidity, and the second model is a model of the sum of the multilayer adsorption amount and the capillary condensation amount changing with the relative humidity.

[0061] Step 142, determining the monolayer adsorption amount of the rock powder sample under different relative humidities according to the first model, and determining the multilayer adsorption amount and the capillary condensation adsorption amount of the rock powder sample under different relative humidities according to the second model and the target relative humidity. This step specifically includes:

[0062] 1, substituting the target relative humidity into the second model to obtain the maximum value of the multilayer adsorption amount of the rock powder sample.

[0063] 2, for any first relative humidity, substituting the first relative humidity into the second model to obtain the multilayer adsorption amount of the rock powder sample under the first relative humidity, and the first relative humidity is less than the target relative humidity.

[0064] 3, for any second relative humidity, substituting the second relative humidity into the second model and subtracting the maximum value of the multilayer adsorption amount from the calculation result of the second model to obtain the capillary condensation adsorption amount of the rock powder sample under the second relative humidity, and the second relative humidity is greater than the target relative humidity.

[0065]

[0066] ​​After determining the first model and the second model, any relative humidity is substituted into the first model to directly calculate the monolayer adsorption amount of the rock powder sample at the relative humidity. For any relative humidity less than the target relative humidity, the relative humidity is substituted into the second model to directly calculate the multilayer adsorption amount of the rock powder sample at the relative humidity, at which time the capillary condensation amount is 0. For any relative humidity greater than the target relative humidity, the relative humidity is substituted into the second model to calculate the sum of the multilayer adsorption amount and the capillary condensation amount of the rock powder sample at the relative humidity, since the multilayer adsorption amount is saturated at the target relative humidity, the calculated result is subtracted by the saturated value of the multilayer adsorption amount to obtain the capillary condensation amount of the rock powder sample at the relative humidity, at which time the multilayer adsorption amount is the saturated value thereof.

[0067] As above, it is a characterization method of rock water vapor adsorption behavior provided in the embodiment. In one example: assuming that the water vapor adsorption amount of the rock powder sample at a relative humidity of 0.9 needs to be solved, the target relative humidity calculated by step 130 is 0.7827, 0.7827 is substituted into the second model fitted to obtain the saturated value of the multilayer adsorption amount of the rock powder sample at this time, 0.9 is substituted into the first model to obtain the monolayer adsorption amount of the rock powder sample at this time, 0.9 is substituted into the second model to obtain the sum of the multilayer adsorption amount and the capillary condensation amount of the rock powder sample at this time, and 26.78 mg / g is subtracted by 13.25 mg / g to obtain the capillary condensation amount of the rock powder sample at this time. Therefore, referring to Figure 4 , at the relative humidity of 0.9, the monolayer adsorption amount of the rock powder sample is 4.87 mg / g, the multilayer adsorption amount is 13.25 mg / g, and the capillary condensation amount is 13.53 mg / g, the total adsorption amount is 31.65 mg / g, and the monolayer adsorption amount, the multilayer adsorption amount, and the capillary condensation amount respectively account for 15.39%, 41.86%, and 42.75% of the total adsorption amount.

[0068] In addition, for a plurality of rock powder samples tested under the same conditions, the Q 1 parameter can also compare the relative size of the adsorption strength of water molecules on the surface of different rock powder samples. The strength of multilayer adsorption gradually decreases with the increase of the number of adsorption layers. The capillary condensation mainly depends on the interaction between water molecules, and the adsorption strength is basically the same.

[0069] In summary, the method for characterizing the water vapor adsorption behavior of a rock provided in the embodiment is as follows: first, a water vapor adsorption experiment is performed on a rock powder sample to obtain the water vapor adsorption amount of the rock powder sample at different relative humidities; then, the adsorption data are converted into logarithmic form and fitted, and the slope of the fitted curve represents the adsorption strength; since the adsorption strength of the rock powder sample at different adsorption stages (the first stage is monolayer adsorption and multilayer adsorption, and the second stage is capillary condensation) is obviously different, the maximum change point of the slope of the target curve obtained by fitting is the critical point at which capillary condensation begins, and the relative humidity corresponding to the point is the relative humidity at which capillary condensation begins, i.e., the target relative humidity; finally, by combining the target relative humidity with the Dent adsorption model, the multilayer adsorption saturation value of the rock powder sample can be calculated, and based on the adsorption saturation value, the multilayer adsorption amount and the capillary condensation amount in the Dent adsorption model calculation result can be further subdivided, and finally the monolayer adsorption amount, the multilayer adsorption amount and the capillary condensation adsorption amount of the rock powder sample at different relative humidities can be obtained. Therefore, the method for characterizing the water vapor adsorption behavior of a rock provided in the above embodiment solves the problem that the current rock water vapor adsorption experiment cannot measure the adsorption amount generated by different adsorption actions in detail.

[0070] Any combination of the technical features of the above embodiments can be made, and to make the description concise, not all possible combinations of the technical features in the above embodiments are described, however, as long as the combination of the technical features does not exist, it should be considered as the scope of the present application.

[0071] The principles and implementation modes of the present application are described by using specific examples in the present application, and the above embodiment descriptions are only used to help understand the method and its core idea; meanwhile, for those skilled in the art, according to the idea of the present application, the specific implementation modes and application ranges will be changed. In summary, the content of the present application should not be understood as a limitation.

Claims

1. A method of characterizing the water vapor sorption behavior of a rock, the method comprising: The method comprises: providing a rock powder sample after drying treatment; performing a water vapor adsorption experiment on the rock powder sample to obtain adsorption data of the rock powder sample, the adsorption data comprising water vapor adsorption amounts at different relative humidities; fitting the adsorption data in logarithmic form to obtain a target curve, and determining a target relative humidity corresponding to a maximum change point of a slope of the target curve, the target relative humidity being a relative humidity at which capillary condensation of the rock powder sample begins; fitting the water vapor adsorption amounts at different relative humidities by a Dent adsorption model to obtain a first model and a second model of the rock powder sample, the first model being used to characterize monolayer adsorption, and the second model being used to characterize the combined action of multilayer adsorption and capillary condensation; determining monolayer adsorption amounts of the rock powder sample at different relative humidities according to the first model, and determining multilayer adsorption amounts and capillary condensation adsorption amounts of the rock powder sample at different relative humidities according to the second model and the target relative humidity; the first model is: wherein, A 1, P M and Q 1 represent the monolayer adsorption amount, the maximum value of monolayer adsorption, and the strength of monolayer adsorption of the rock powder sample, respectively, S RH represents the relative humidity, Q 2 represents the total strength of multilayer adsorption and capillary condensation. the second model is: wherein, A 2 represents the sum of the multilayer adsorption and the capillary condensation of the rock powder sample.

2. The method for characterizing the water vapor sorption behavior of rock according to claim 1, characterized in that, the water vapor adsorption experiment on the rock powder sample comprises: iteratively recording an equilibrium mass of the rock powder sample at a current relative humidity in a test environment at a target temperature, wherein the current relative humidity is iteratively increased by a preset step size, and the equilibrium mass is a mass when a mass change is less than a preset threshold and is maintained for a preset time; determining water vapor adsorption amounts of the rock powder sample at different relative humidities according to the equilibrium mass of the rock powder sample at each relative humidity.

3. The method for characterizing the water vapor sorption behavior of rock according to claim 2, characterized in that, the water vapor adsorption amount of the rock powder sample at any relative humidity S A S determined by a target formula, the target formula being: wherein, M S and M O respectively represent the equilibrium mass of the rock powder sample at the relative humidity S and the initial state.

4. The method for characterizing the water vapor sorption behavior of rock according to claim 2, characterized in that, The target temperature is set to be between 20°C and 60°C.

5. The method for characterizing the water vapor sorption behavior of rock according to claim 2, characterized in that, The relative humidity is iteratively increased by a preset step size in a range of 0-0.95, the preset step size is 0.1, the preset threshold is a change of 0.002% per minute, and the preset time is 10 minutes.

6. The method for characterizing the water vapor sorption behavior of rock according to claim 1, characterized in that, The fitting of the adsorption data in logarithmic form to obtain a target curve comprises: establishing a coordinate system with the logarithm of the relative humidity as the abscissa and the logarithm of the water vapor adsorption amount as the ordinate, and mapping the adsorption data in logarithmic form into the coordinate system; linearly fitting the in the coordinate system by a linear form of a Freundlich isotherm model to obtain the target curve, the target curve being composed of two straight line segments with different slopes, and a connection point of the two straight line segments being the maximum change point of the slope of the target curve.

7. The method for characterizing the water vapor sorption behavior of rock according to claim 1, characterized in that, The determination of the multilayer adsorption amounts and capillary condensation adsorption amounts of the rock powder sample at different relative humidities according to the second model and the target relative humidity comprises: substituting the target relative humidity into the second model to obtain a maximum value of the multilayer adsorption amount of the rock powder sample; for any first relative humidity, substituting the first relative humidity into the second model to obtain the multilayer adsorption amount of the rock powder sample at the first relative humidity, the first relative humidity being less than the target relative humidity; For any second relative humidity, the capillary condensation adsorption amount of the rock powder sample at the second relative humidity is obtained by subtracting the maximum value of the multi-layer adsorption amount from the calculation result of the second model after the second relative humidity is substituted into the second model, and the second relative humidity is greater than the target relative humidity.

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