CN114705591A - A method for calculating the wettability of coal/shale in high pressure environment - Google Patents

Abstract

The present invention provides a method for calculating the wettability of coal/shale in a high-pressure environment, including: step 1. In a high-pressure CO ₂ environment, adsorbed water and CO ₂ adsorbed gas are adsorbed on the surface of the coal/shale, and the coal/shale is adsorbed on the surface. The surface has the wettability characteristics of the mixed surface; step 2, from the wettability relationship of the liquid on the mixed surface, combined with the conditions of step 1, obtain the wettability expression of the gas-adsorbed coal/shale surface; step 3. From the Young's equation of wetting angle combined with the "sharp-kink" approximation theorem, the expression of the wetting angle of the coal/shale surface that is not adsorbed by gas is obtained _; Expression for the wetting angle of the coal/shale surface of the gas. The invention solves the problem that the wettability changing factor of the interaction between gas adsorption and water-gas-coal/shale is not considered in the prior art coal/shale wettability measurement process.

Description

A method for calculating the wettability of coal/shale in high pressure environment

Field of study

The invention belongs to the technical field of reservoir wettability measurement and evaluation in the process of coalbed methane and shale gas exploration and development and CO ₂ geological storage, and in particular relates to a calculation method of coal/shale wettability in high pressure environment.

Background technique

Wettability refers to the ability of a liquid to spread on a solid surface. Judgment of the wettability of coal/shale in high pressure gas environments is critical for CBM and shale gas development, and CO ₂ sequestration in coal or shale reservoirs. important. The wettability of coal/shale in high-pressure gas environment is an important factor affecting the distribution of gas and water in the formation and the two-phase seepage process of gas and water in the process of natural gas development. At the same time, for shale layers, the judgment of wettability is an important index to evaluate it as a CO ₂ sequestration caprock. In the high-pressure _CO2 environment, if the water-wet shale is changed to be wetted by _CO2 gas, the capillary force of the caprock is reduced, and the sealing effect of the caprock is lost, resulting in _CO2 leakage.

The methods for quantitative determination of wettability in conventional environments mainly include: (1) Centrifuge method (USBM, United States Bureau of Mines), Amott method (imbibition and expulsion method), both of which are based on gas under normal pressure, In the water flooding experiment, because the coal/shale samples are mostly low porosity and low permeability, the flooding experiment is difficult to operate. (2) Contact angle measurement method, the measurement results are intuitive and accurate, and it is the main method for measuring the wettability of coal or shale.

In the high pressure _CO2 environment, the wettability of the coal/shale surface changes due to the adsorption of _CO2 gas on the coal/shale surface and the change of the gas-water interfacial tension. At present, the measurement technology of water wettability of coal/shale in high pressure gas environment is relatively slow. The main measurement technique is to place the sample in a high pressure gas chamber and measure the water contact angle. The biggest problem with such methods is that due to the short measurement time, it is difficult to take into account the changes in wettability caused by gas adsorption and interactions with water-gas-coal/shale. Therefore, it is urgent to establish a new wettability measurement method in a high-pressure gas environment that takes into account gas adsorption, the interaction between water-gas-coal/shale, and the change of gas-water interfacial tension.

SUMMARY OF THE INVENTION

The invention provides a method for calculating the wettability of coal/shale in a high pressure environment, which solves the problem of the lack of consideration of gas adsorption and water-gas-coal/shale wettability measurement process in the prior art. Interactions between wettability altering factors.

A method for calculating the wettability of coal/shale in a high pressure environment, comprising:

Step 1. In a high-pressure _CO2 environment, _CO2 gas is adsorbed on part of the coal/shale surface to replace part of the adsorbed water adsorbed on the coal/shale surface, and the _CO2 gas adsorbed on the coal/shale surface is _CO2 adsorbed gas, Adsorbed water and the CO ₂ adsorbed gas are adsorbed on the surface of the coal/shale, the outer surface of the CO ₂ adsorbed gas is in contact with water, and the surface of the coal/shale has the wettability characteristics of a mixed surface;

Step 2. From the wettability relationship of the liquid on the mixed surface, combined with the conditions of step 1, obtain the wettability expression of the surface of coal/shale with CO ₂ gas adsorbed in the high-pressure CO ₂ environment;

Among them, the relationship between the wettability of the liquid on the mixed surface is:

In the formula, θ is the _wetting angle of the mixed surface; _{γ LG} is the gas-liquid surface tension; material; f _i represents the proportion of the contact area of each material and the mixing surface in the mixing surface, f ₁ +f ₂ +…f _n =1, n represents the total number of materials in the mixing surface; θ _i represents the liquid and Wetting angle between each material;

In the high pressure _CO2 environment, the coal/shale surface is composed of the coal/shale surface not adsorbed by gas and the coal/shale surface adsorbed by gas, and the wettability expression of the coal/shale surface adsorbed by _CO2 gas The formula is:

cosθ=f ₁ cosθ ₁ +f ₂ cosθ ₂ (2)

In the formula, θ represents the wetting angle of the coal/shale surface in the high-pressure _CO2 environment, and f1 represents the proportion of the coal/shale surface without adsorbed gas, that is, the contact area between the coal/shale surface and the adsorbed water accounts for The ratio of coal/shale surface; _θ1 represents the wetting angle between the coal/shale surface not adsorbed by gas and the water; f2 represents the proportion of coal surface adsorbed by gas, that is, the _CO2 adsorbed gas The contact area with water accounts for the proportion of coal/shale surface; θ ₂ represents the wetting angle between the coal surface adsorbed by the gas and water, that is, the wetting angle between the CO ₂ adsorbed gas and water is π.

Step 3. From the Young's equation of the wetting angle combined with the "sharp-kink" approximation theorem, the expression of the wetting angle between the surface of coal/shale that is not adsorbed by gas and water is obtained:

where θ ₁ is the wetting angle between the coal/shale surface not adsorbed by gas and water, I is the integral of the van der Waals potential energy; Δρ is the gas-liquid density difference; γ _LG is the gas-liquid interfacial tension, and ρ _g is the gas density ; ρ _lf indicates that the coal/shale surface liquid film density is equal to the liquid density.

Step 4. Combining steps 2 and 3, update the expression of the wetting angle of the coal/shale surface with _CO gas adsorbed in the high pressure CO ₂ environment as:

是常数，f₁表示未被吸附气体的煤/页岩表面所占比例，即煤/页岩表面和所述吸附水的接触面积占煤/页岩表面的比例，ρ_g表示高压CO₂环境中的气体密度；ρ_lf表示煤/页岩表面的液体膜密度等于水溶液密度。where θ is the wetting angle of the coal/shale surface in the high pressure _CO environment; for the constant temperature of the coal/shale surface and the aqueous solution, it is known that

is a constant, f ₁ represents the proportion of coal/shale surface without adsorbed gas, that is, the contact area of coal/shale surface and the adsorbed water accounts for the proportion of coal/shale surface, ρ _g represents high pressure _CO environment gas density in ; ρ _lf means that the density of the liquid film on the coal/shale surface is equal to the density of the aqueous solution.

Further, in the updated high-pressure CO ₂ environment described in step 4, the method for measuring the parameters in the expression of the wetting angle of coal/shale with CO ₂ gas adsorbed is:

Step 401, using low-field nuclear magnetic resonance technology to measure the T2 spectrum of water in the water-saturated coal sample under different _CO2 pressures;

Step 402 , analyze the data of the T2 spectrum, obtain the contact area between the coal/shale surface and water under different CO ₂ pressures, and calculate the contact between the coal/shale surface and water under different CO ₂ pressures Area as a percentage of coal/shale surface area;

值；Step 403: Measure the wetting angle of the coal/shale surface under standard conditions, and import it into the updated high-pressure CO ₂ environment, in the expression (5) of the wetting angle of the coal/shale with CO ₂ gas adsorbed, to obtain

value;

值导入所述更新的高压CO₂环境中，吸附有CO₂气体的煤/页岩润湿角的表达式(5)内，得到不同CO₂压力下煤/页岩表面润湿角。Further, the calculation method of the wetting angle of the coal/shale surface under different CO ₂ pressures is as follows: the contact area between the coal/shale surface and the water under different CO ₂ pressures described in step 402 is accounted for the coal/shale surface. The ratio of the area and the one obtained in step 403 as described

The value is imported into the expression (5) of the wetting angle of coal/shale adsorbed with CO ₂ gas in the updated high-pressure CO ₂ environment, and the surface wetting angle of coal/shale under different CO ₂ pressures is obtained.

Further, the contact area between the coal/shale surface and water under different CO ₂ pressures described in step 402 is based on the T2 spectrum data of water in the water-saturated coal sample under standard conditions.

Further, the formula for the ratio of the contact area between the coal/shale surface and water to the coal/shale surface area under different CO ₂ pressures described in step 402 is:

In the formula, f ₁ represents the ratio of the contact area between the coal/shale surface and water to the coal/shale surface area, P _1-0 represents the spectral peak signal amplitude of the first peak on the left in the T2 spectrum under standard conditions, P _1-n represents the spectral peak signal amplitude of the first peak on the left in the T2 spectrum under different pressures.

Further, the method for measuring the wetting angle of the coal/shale surface under the standard conditions described in step 403 is:

Take the coal/shale sample and crush it to a size larger than 200 mesh, and take the coal powder to make a test piece with a smooth plane; in the constant humidity chamber, drop the distilled water droplets of less than 4 mg on the test piece; take a picture, and measure the length of the contact between the water droplet and the test piece surface and the water droplet radius to obtain the contact angle; and then obtain the wetting angle of the coal/shale surface under standard conditions.

Further, the production process of the saturated water-coal sample in the step 401 is:

Pulverized coal/shale samples were made into samples, and after drying to constant weight, quantitative samples were taken into vacuum-saturated water to obtain saturated water-saturated coal samples.

Compared with the prior art, the present invention has the following beneficial effects:

1. The calculation method provided by the present invention is suitable for high pressure CO ₂ environment, comprehensively considering factors such as gas adsorption and the change of wettability caused by the interaction between water-gas-coal/shale, etc., to obtain a more optimized coal. / Shale wettability measurements.

2. The present invention realizes the calculation and measurement of the wettability of coal/shale under different gas pressures, and is a method for measuring the wettability of coal/shale in real time, in situ and dynamically.

Description of drawings

In order to illustrate the embodiments of the present invention or the technical solutions in the prior art more clearly, the following briefly introduces the accompanying drawings that are required to be used in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only exemplary, and for those of ordinary skill in the art, other implementation drawings can also be obtained according to the extension of the drawings provided without creative efforts.

Fig. 1 is a schematic diagram of adsorption sites on coal/shale pore surface and adsorption of molecular simulated water molecules on coal/shale surface;

Figure 2 is a schematic diagram of the displacement of water molecules by _CO adsorption in a _CO environment.

Figure 3 is a coal surface showing a mixed surface of solid and adsorbed gas composition in a _CO2 environment.

4 is a schematic diagram of the change of the T spectrum of the sample in the 0-6MPa _CO gas in the embodiment of the present invention _;

FIG. 5 is a measurement diagram of the wetting angle of the sample under standard conditions in the embodiment of the present invention.

Labels in the figure:

1 is adsorption site; 2 is adsorbed water; 3 is coal/shale surface; 4 is free water; 5 is CO ₂ .

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

There are a series of adsorption sites on the pore surface of coal/shale. These adsorption sites are the main sites for moisture attachment on the coal surface. As shown in Figure 1, the schematic diagram of the adsorption sites on the coal/shale pore surface and the molecular simulation of the adsorption of water on the coal/shale surface. When _CO2 is adsorbed on the surface of coal/shale, due to the stronger interaction between _CO2 and the non-adsorbed sites on the surface of coal/shale, the _CO2 gas adsorbed on the surface of coal/shale replaces some Adsorbed water, as shown in Fig. 2, the displaced adsorbed water migrates to the middle of the pores to form free water, the blank spaces in Fig. 2 are all free water, and the free water covers the _CO gas adsorbed on the surface of coal/shale. However, the adsorbed water directly adsorbed on the adsorption site is difficult to be replaced due to the strong force between them, and will still be adsorbed on the surface of coal/shale pores. Therefore, in a high-pressure _CO2 environment, _CO2 is adsorbed to the coal/shale surface, displacing a part of the adsorbed water on the surface, as shown in Figure 3. The displacement capacity of _CO2 for water molecules is mainly related to the adsorption capacity of _CO2 on the coal surface.

In the present invention, a method for calculating the wettability of coal/shale in a high-pressure CO ₂ environment is provided by synthesizing the change factors of gas adsorption and the interaction between water-gas-coal/shale on wettability,

Step 1. In a high-pressure CO ₂ environment, CO ₂ gas is adsorbed on the surface of coal/shale to replace part of the adsorbed water, and part of the CO ₂ gas adsorbed on the surface of coal/shale is CO ₂ adsorbed gas. A heterogeneous mixed surface is formed on the coal/shale surface, and the mixed surface consists of the coal/shale surface that is not adsorbed by gas and the coal/shale surface that is adsorbed by gas, as shown in Figure 3. Therefore, when coal/shale is placed in a high-pressure _CO2 environment, the surface moisture distribution of coal/shale changes under the action of _CO2 adsorption, and the surface of coal/shale is adsorbed with adsorbed water and the _CO2 adsorbed gas, the The outer surface of the _CO2 adsorbed gas is in contact with free water, and the coal/shale surface has the wettability characteristics of a mixed surface.

Step 2. From the relational expression of the wettability of the liquid on the mixed surface, combined with the conditions of step 1, the wettability expression of the surface of the coal/shale with CO ₂ gas adsorbed in the high pressure CO ₂ environment is obtained.

Among them, the relationship between the wettability of the liquid on the mixed surface is:

In the formula, θ represents the wetting angle of the mixed surface; L represents liquid, G represents gas, S represents solid, and γ represents surface tension; γ _LG , γ _SL , γ _SG represent the surface tension of gas-liquid, solid-liquid, and gas-solid, respectively; γ _{i, SG} , γ _{i, SL} represent the interfacial tension between each material in the mixed surface and gas and liquid, respectively; i represents the material in the mixed surface; f _i represents the contact area between each material and the mixed surface in the mixed surface. Proportion, f ₁ +f ₂ +...f _n =1, n represents the total number of materials in the mixed surface; θ _i represents the wetting angle between the liquid and each material.

Combined with the high-pressure CO ₂ environment in step 1, the surface of coal/shale has adsorbed water and the CO ₂ adsorbed gas, and the wettability expression of the surface of coal/shale adsorbed with CO ₂ gas is:

cosθ=f ₁ cosθ ₁ +f ₂ cosθ ₂ (2)

In the formula, θ represents the wetting angle of the coal/shale surface in the high-pressure _CO2 environment, and f1 represents the proportion of the coal/shale surface without adsorbed gas, that is, the contact area between the coal/shale surface and the adsorbed water accounts for The ratio of coal/shale surface; _θ1 represents the wetting angle between the coal/shale surface not adsorbed by gas and water; f2 represents the proportion of coal surface adsorbed by gas, that is, the _CO2 adsorbed gas and coal The ratio of the contact area to the surface of the shale/shale. Since the CO ₂ adsorbed gas is adsorbed on the coal/shale surface, and the outer surface of the CO ₂ adsorbed gas is in contact with free water, the CO ₂ adsorbed gas and coal / The contact area of the shale surface is equal to the contact area of the CO ₂ adsorbed gas and water; θ ₂ represents the wetting angle between the coal surface adsorbed by the gas and water, that is, the wetting angle between the CO ₂ adsorbed gas and water Wet angle, π.

Since f ₁ +f ₂ =1, the calculation formula of the wetting angle of coal/shale surface in high pressure CO ₂ environment is:

cosθ=f ₁ cosθ ₁ -f ₂ =f ₁ (1+cosθ ₁ )-1 (3)

式中γ_GS表示气固表面张力；γ_LS表示液固表面张力；γ_LG表示气液界面张力)结合“sharp-kink”近似定理(由文献《Roshan,H.,A.Z.Al-Yaseri,M.Sarmadivalehand S.Iglauer(2016),On wettability of shale rocks.Journal of Colloid andInterface Science,475,104-111.》可知)，得到未考虑吸附气体作用下煤/页岩表面与水之间润湿角θ₁的表达式，也就是本发明中未被气体吸附的煤/页岩表面与水之间润湿角θ₁的表达式：Step 3. Under different gas pressures, under the influence of gas concentration, the gas-liquid interfacial tension changes, and the wetting angle _θ1 between the surface of coal/shale that is not adsorbed by gas and water also changes with the change of pressure, as shown by Wetting angle Young's equation (

In the formula, γ _GS represents the gas-solid surface tension; γ _LS represents the liquid-solid surface tension; γ _LG represents the gas-liquid interfacial tension) combined with the "sharp-kink" approximation theorem (by the literature "Roshan, H., AZAl-Yaseri, M. Sarmadivalehand"). S.Iglauer (2016), On wettability of shale rocks. Journal of Colloid and Interface Science, 475, 104-111.”), the expression of wetting angle θ ₁ between the surface of coal/shale and water is obtained without considering the effect of adsorbed gas formula, that is, the expression of the wetting angle θ ₁ between the coal/shale surface that is not adsorbed by gas and water in the present invention:

In the formula, I is the integral of van der Waals potential energy; Δρ is the gas-liquid density difference; γ _LG is the gas-liquid interfacial tension, ρ _g is the gas density; ρ _lf is the liquid film density on the solid surface, which is generally equal to the liquid density.

Step 4. For a constant temperature, the determined coal/shale surface and aqueous solution, combined with steps 2 and 3, update the expression of the wetting angle of the coal/shale surface with CO ₂ gas adsorbed in the high-pressure CO ₂ environment as:

是常数(由文献《Al-Yaseri,A.Z.,H.Roshan,M.Lebedev,A.Barifcani,and S.Iglauer(2016),Dependence of quartz wettability on fluiddensity.Geophysical Research Letters,43(8),3771-3776.》可知)，f₁表示未被吸附气体的煤/页岩表面所占比例，即煤/页岩表面和所述吸附水的接触面积也就是煤/页岩表面和水的接触面积占煤/页岩表面的比例，ρ_g表示高压CO₂环境中的气体密度；ρ_lf表示煤/页岩表面的液体膜密度，一般等于水溶液密度。where θ is the wetting angle of the coal/shale surface in the high pressure _CO environment; for the constant temperature of the coal/shale surface and the aqueous solution, it is known that

is a constant (from the literature "Al-Yaseri, AZ, H. Roshan, M. Lebedev, A. Barifcani, and S. Iglauer (2016), Dependence of quartz wettability on fluiddensity. Geophysical Research Letters, 43(8), 3771- 3776.”), f ₁ represents the proportion of the coal/shale surface without adsorbed gas, that is, the contact area between the coal/shale surface and the adsorbed water, that is, the contact area between the coal/shale surface and water. The ratio of coal/shale surface, ρ _g is the gas density in high pressure _CO2 environment; ρ _lf is the liquid film density on the coal/shale surface, which is generally equal to the density of the aqueous solution.

Therefore, according to formula 5, the wetting angle of coal/shale surface under different gas pressures can be obtained by obtaining f ₁ , ρ _g , ρ _lf and the surface wetting angle of coal/shale under standard conditions (ie, normal pressure).

In order to accurately obtain the wetting angle of coal/shale surface in high pressure CO ₂ environment, according to the above calculation principle of high pressure wetting angle, the present invention provides a method for measuring the wettability of coal/shale in high pressure environment. The value of f ₁ under different pressures is obtained from the resonance spectrum, and the wetting angle of the coal/shale surface under normal pressure is obtained at the same time. Using the above calculation formula, the quantitative determination of the wettability of coal/shale under different gas pressures can be obtained. A new method for instant, in situ, and dynamic measurement of coal/shale wettability.

For the unknowns in the above formula 5, the present invention provides a measurement method, comprising:

Step 401, using low-field nuclear magnetic resonance technology to measure the T2 spectrum of water in the water-saturated coal sample under different _CO2 pressures;

Step 402: Analyze the T2 spectrum data to obtain the contact area between the coal/shale surface and water under different CO ₂ pressures, and calculate the contact area between the coal/shale surface and water under different CO ₂ pressures percentage of coal/shale surface area;

值；Step 403: Measure the wetting angle of the coal/shale surface under standard conditions, import it into the coal/shale wettability formula 5, and obtain

value;

值导入所述煤/页岩润湿性公式5内，得到不同压力下煤/页岩表面润湿角。Step 404: Calculate the ratio f1 _of the contact area between the coal/shale surface and the liquid to the coal/shale surface area under different _CO

The value is imported into the coal/shale wettability formula 5 to obtain the coal/shale surface wettability angle under different pressures.

In the measurement method provided by the present invention, the low-field nuclear magnetic resonance technology is combined with the coal/shale surface water contact wetting angle measurement technology, and the scale of the wettability of the coal/shale and the nuclear magnetic resonance T2 spectrum signal is established. A quantitative determination of the wettability of coal/shale under different gas pressures is achieved.

结合T2谱图得到的f₁值及不同压力下CO₂气体密度，和标况下的煤/页岩表面润湿角，从而求得设定气体压力下煤/页岩的水润湿角。In this application, low-field NMR measurements were performed on water in water-saturated coal samples under different CO ₂ pressures, and the T ₂ spectrum of water in coal was obtained. According to the amplitude of the first peak (P1) on the left of the T ₂ spectrum Represents the adsorbed water on the pore surface of coal. Under different gas pressures, the area of the P1 peak changes, which means that _CO2 gas in the coal is adsorbed to the coal pore surface and replaces the adsorbed water. This part of the water is no longer in contact with the pore surface, and becomes free water and migrates to the middle of the large pores. Therefore, the quantitative measurement of the remaining adsorbed water on the pore surface by NMR technology (nuclear magnetic resonance) can obtain the contact area between the solid and the liquid under different gas pressures, that is, f ₁ . According to the formula

Combined with the f ₁ value obtained from the T2 spectrum, the CO ₂ gas density under different pressures, and the surface wetting angle of coal/shale under standard conditions, the water-wetting angle of coal/shale under the set gas pressure was obtained.

In a possible embodiment, the steps of the measurement method are as follows:

1. Sample preparation and loading

Pulverize the coal/shale sample and take a number of coal sample particles with a diameter of about 1 cm, put it in a drying box to dry to constant weight, and weigh to make a sample; soak the sample in vacuum-saturated water for 48 hours to ensure that the sample is completely Saturated water, that is, saturated water-coal samples. Take the fully saturated water sample, wipe off the excess water on the surface of the fully saturated water sample, and put the fully saturated water sample into the PEEK high pressure resistant sample chamber.

2. Select the low-field NMR measurement parameters and set the ambient temperature

Select the CPMG (Carr-Purcell-Meiboom-Gill) measurement sequence; the specific measurement parameters include: echo time (TE), stacking times (SCANS), waiting time (TW), echo number (NECH); set the test ambient temperature to appropriate temperature and maintain a constant temperature.

The NMR instrument adopts the CPMG sequence, and the specific measurement parameters are preferably as follows: echo time (TE)=0.3ms, stacking times (SCANS)=64, waiting time (TW)=3s, echo number (NECH)=18000; The appropriate ambient temperature is 25°C.

3. Obtain f ₁ values under different pressures

Measure the T2 spectrum of the fully saturated water sample under standard conditions, denoted as T _2-0 , and the signal amplitude of the P1 spectrum peak is P _1-0 . Inject the CO ₂ gas of nMPa pressure into the sample chamber, let it stand for 10 hours, and wait until the CO ₂ is fully adsorbed into the coal. Test the T2 spectrum of the fully saturated water sample at this time, denoted as T _2-n , in which the P1 spectrum The peak signal amplitude is P _1-n . Therefore, the value of f1 at _n MPa pressure is:

4. Measure the wetting angle of coal/shale surface under standard conditions.

求出接触角θ.即标况下煤/页岩表面润湿角。利用公式4及已知的标况下煤/页岩表面润湿角和气液密度，此处液体为蒸馏水，固体表面液体膜密度ρ_lf为水密度取1g/cm³，求得

值。Take the coal sample/shale and pulverize it to more than 200 mesh, take the pulverized coal and press it into a test piece with a smooth plane under a pressure of 50MPa, which can be made into a cylinder or ellipsoid test piece. In the constant humidity chamber, drop a drop of distilled water less than 4 mg onto the test piece with a syringe, take a picture with a fast camera, measure the length a of the contact between the water drop and the surface through image analysis, and obtain the circle radius r by fitting the shape of the water drop with a single circle. formula

Obtain the contact angle θ. That is, the wetting angle of the coal/shale surface under standard conditions. Using formula 4 and the known wetting angle and gas-liquid density of the coal/shale surface under standard conditions, where the liquid is distilled water, the liquid film density ρ _lf on the solid surface is the water density and takes 1g/cm ³ to obtain

value.

5. Using formula 5, calculate and obtain the wetting angle of coal/shale surface under different pressures.

值，即可获得不同压力下煤/页岩样品的表面润湿角。Using the obtained values of f1 at different pressures and _CO2 gas densities at different pressures and the values in step ₄

value, the surface wetting angles of coal/shale samples under different pressures can be obtained.

Example 1

For demonstration, the present invention provides a specific application embodiment, and the operation steps are as follows:

1. Sample preparation and loading

Select the middle-rank coal sample in the Ordos Basin, pulverize the coal sample to obtain coal sample particles with a diameter of about 1 cm, put it in a drying oven to dry to a constant weight, and weigh 19.88 g; vacuum the sample with saturated water for 48 hours to ensure that the sample is Fully saturated water. Take the fully saturated water sample, wipe off the excess water on the surface of the fully saturated water sample, and put the fully saturated water sample into the PEEK high pressure resistant sample chamber.

2. Select the low-field NMR measurement parameters and set the ambient temperature

The CPMG (Carr-Purcell-Meiboom-Gill) measurement sequence is selected, and the specific parameters of the measurement are as follows: echo time (TE)=0.3ms, stacking times (SCANS)=64, waiting time (TW)=3s, echo number (NECH) =18000; set the appropriate ambient temperature to 25℃.

3. Obtain f ₁ values under different pressures

继续向煤样中注入CO₂气体，压力达到2MPa，静置10个小时，然后测试完全饱和水样品的T2谱图，记做T_2-2,其中P1谱峰信号幅度为P_1-2，2MPa压力下的f₁值为：

依次类推直至压力增大到6MPa，6MPa压力下的f₁值为：

依次计算得到f₁数值如下：As shown in Figure 4, the ordinate in the figure is "T2 spectral signal amplitude (au), and the T2 spectral diagram of the fully saturated water sample under standard conditions is measured, marked as T _2-0 , and the P1 spectral peak signal amplitude is P _{1- 0.} Inject CO ₂ gas with a pressure of 1MPa into the sample cavity, let it stand for 10 hours after pressurization, and then test the T2 spectrum of the fully saturated water sample, denoted as T _2-1 , where the P1 spectrum peak signal amplitude is P _{1 -1} , at this time, the value of f ₁ under 1MPa pressure is:

Continue to inject CO ₂ gas into the coal sample, the pressure reaches 2MPa, let it stand for 10 hours, and then test the T2 spectrum of the fully saturated water sample, denoted as T _2-2 , where the P1 spectrum peak signal amplitude is P _1-2 , _The value of f1 under 2MPa pressure is:

And so on until the pressure increases to 6MPa, the value of f ₁ under 6MPa pressure is:

The f ₁ values are calculated sequentially as follows:

Table 1 P1 peak amplitude and f ₁ under different pressures

4. Measure the wetting angle of coal/shale surface under standard conditions.

求出表面接触角78.46°.利用公式5及已知的标况下煤/页岩表面润湿角和气液密度求得

值为1.2079。The coal sample was pulverized to a size larger than 200 mesh, and the pulverized coal was pressed into a cylindrical test piece with a smooth plane under a pressure of 50 MPa with a pressurized molding die. In the constant humidity chamber, drop the distilled water droplet of less than 4 mg on the test piece with a syringe, and take a picture with a fast camera to obtain Figure 5. Through image analysis, the length a=1.94mm and the radius r=0.99mm of the contact between the water droplet and the surface are measured. by formula

Obtain the surface contact angle of 78.46°. Use formula 5 and the known standard conditions of coal/shale surface wetting angle and gas-liquid density to obtain

The value is 1.2079.

5. Using formula 5, calculate and obtain the surface wetting angle of coal/shale samples under different pressures.

值，即可获得不同压力下煤/页岩样品表面润湿角，如表2所示。Using the obtained values _of f1 at different pressures and _CO2 gas density at different pressures and the above steps

value, the surface wetting angles of coal/shale samples under different pressures can be obtained, as shown in Table 2.

Table 2 Surface wetting angles of coal/shale samples under different pressures

Pressure(MPa) 0 1 2 3 4 5 6 Wetting angle (°) 78 86.6 92.9 96.4 99.6 102.2 106.3

The above embodiments are only exemplary embodiments of the present application, and are not intended to limit the present application. The protection scope of the present application is defined by the claims. Those skilled in the art can make various modifications or equivalent replacements to the present application within the spirit and protection scope of the present application, and such modifications or equivalent replacements should also be regarded as falling within the protection scope of the present application.

Claims (7)

1. A method for calculating coal/shale wettability in a high-pressure environment is characterized by comprising the following steps:

step 1, high pressure CO₂In the environment, CO₂The gas is adsorbed on part of the coal/shale surface to replace part of the adsorbed water adsorbed on the coal/shale surface, and the CO adsorbed on the coal/shale surface₂The gas being CO₂Adsorbing gas, adsorbing water and the CO on the surface of the coal/shale₂Adsorbing gas, said CO₂With gas-adsorbing external surfaces in contact with water and coal/shale surfaces having mixed surfacesWettability characteristics;

step 2, obtaining high-pressure CO by combining the relation of the wettability of the liquid on the mixed surface and the conditions of the step 1₂In the environment, CO is adsorbed₂A wettability expression of the coal/shale surface of the gas;

wherein the wettability relation of the liquid on the mixing surface is as follows:

wherein θ represents a mixing surface wetting angle; gamma ray_LGRepresents the gas-liquid surface tension; gamma ray_i，SG、γ_i，SLRespectively representing the interfacial tension of each material in the mixing surface with the gas and the liquid; i represents the material in the mixing surface; f. of_iRepresenting the proportion of the contact area of each material with the mixing surface in the mixing surface, f₁+f₂+…f_nN denotes the total number of materials in the mixing surface as 1; theta_iIndicating the wetting angle between the liquid and each material;

high pressure CO₂In the environment, the coal/shale surface is composed of coal/shale surface which is not adsorbed by gas and coal/shale surface which is adsorbed by gas, CO is adsorbed₂The wettability expression of the coal/shale surface of the gas is:

cosθ＝f₁cosθ₁+f₂cosθ₂ (2)

in the formula, theta represents high-pressure CO₂The wetting angle of the coal/shale surface in the environment, f1 represents the proportion of the coal/shale surface which is not adsorbed with gas, namely the proportion of the contact area of the coal/shale surface and the adsorbed water in the coal/shale surface; theta₁Represents the wetting angle between the coal/shale surface and water that is not adsorbed by the gas; f2 represents the ratio of the surface of coal adsorbed by gas, i.e., the CO₂The contact area of the adsorption gas and the water accounts for the proportion of the surface of the coal/shale; theta.theta.₂Indicating the wetting angle between the surface of the coal adsorbed by the gas and the water, i.e. the CO₂The wetting angle between the adsorbed gas and water is pi.

Step 3, combining a Young equation of the wetting angle with the approximate theorem of "sharp-kink" to obtain an expression of the wetting angle between the surface of the coal/shale which is not adsorbed by the gas and the water:

in the formula, theta₁Represents the wetting angle between the coal/shale surface and water that is not adsorbed by the gas, I represents the van der waals potential energy integral; Δ ρ represents a gas-liquid density difference; gamma ray_LGRepresents the gas-liquid interfacial tension, ρ_gRepresents the gas density; rho_lfIndicating that the coal/shale surface liquid film density is equal to the liquid density.

Step 4, combining the steps 2 and 3, updating the high-pressure CO₂In the environment, CO is adsorbed₂The expression for the coal/shale surface wetting angle of the gas is:

in which theta is high pressure CO₂Coal/shale surface wetting angle in the environment; for the constant temperature of the coal/shale surface and aqueous solution, it is known

Is a constant, f₁Represents the proportion of the coal/shale surface which is not adsorbed with gas, namely the proportion of the contact area of the coal/shale surface and the adsorbed water in the coal/shale surface, rho_gRepresenting high pressure CO₂Gas density in the environment; ρ is a unit of a gradient_lfIndicating that the liquid film density at the coal/shale surface is equal to the aqueous solution density.

2. The method of claim 1, wherein the updated high pressure CO of step 4 is used to calculate the coal/shale wettability₂In the environment, CO is adsorbed₂Method for measuring parameters in coal/shale wetting angle expression of gasThe method comprises the following steps:

step 401, measuring different CO's using low-field nuclear magnetic resonance₂T2 spectrum of water in a saturated water coal sample under pressure;

step 402, analyzing the data of the T2 spectrogram to obtain different CO₂The contact area between the coal/shale surface and the water under the pressure is calculated to obtain different CO₂The ratio of the contact area between the coal/shale surface and the water under pressure to the coal/shale surface area;

step 403, measuring the wetting angle of the coal/shale surface under standard conditions, and introducing the updated high-pressure CO₂In the environment, CO is adsorbed₂Within the expression (5) of the coal/shale wetting angle of the gas, the result is

The value is obtained.

3. The method of claim 2, wherein the differential CO is different from the CO₂The method for calculating the surface wetting angle of the coal/shale under pressure comprises the following steps: combining the different COs in step 402₂The ratio of the contact area between the coal/shale surface and the water under pressure to the coal/shale surface area and the ratio obtained in step 403

Value introduction into the renewed high pressure CO₂In the environment, CO is adsorbed₂Within the expression (5) for the coal/shale wetting angle of the gas, different CO's were obtained₂Coal/shale surface wetting angle under pressure.

4. The method of claim 2, wherein the different CO is calculated in step 402 by using the method of calculating the coal/shale wettability in the high pressure environment₂The contact area between the coal/shale surface and the water under pressure is referred to the T2 spectrogram data of the water in a saturated water coal sample under standard conditions.

5. A high voltage as claimed in claim 2Method for calculating coal/shale wettability in environment, characterized in that, in step 402, said different CO₂The formula of the ratio of the contact area between the coal/shale surface and the water under pressure to the coal/shale surface area is:

in the formula (f)₁Representing the ratio of the contact area between the coal/shale surface and the water to the coal/shale surface area, P_1-0Indicating the peak signal amplitude, P, of the first peak to the left in the spectrum of T2 under standard conditions_1-nIndicating the peak signal amplitude of the first left peak in the spectrum of T2 at different pressures.

6. The method for calculating the coal/shale wettability in the high-pressure environment according to claim 2, wherein the method for measuring the wetting angle of the coal/shale surface under the standard condition in the step 403 is as follows:

taking a coal/shale sample, crushing to be larger than 200 meshes, and taking coal powder to prepare a test piece with a smooth plane; dripping less than 4mg of distilled water drops on the test piece in a constant humidity chamber; taking a picture, measuring the contact length of the water drop and the surface of the test piece and the radius of the water drop to obtain a contact angle; and further obtaining the coal/shale surface wetting angle under the standard condition.

7. The method for calculating the coal/shale wettability in the high-pressure environment according to claim 2, wherein the saturated water coal sample in the step 401 is prepared by the following steps:

and crushing the coal/shale sample to prepare a sample, drying the sample to constant weight, and putting a quantitative sample into the vacuumized saturated water to obtain a saturated water-coal sample.

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