Mercury injection experiment data correction method based on apparent density and true density of coal

Abstract

The invention relates to the technical field of coal bed gas development, in particular to a mercury injection experiment data correction method based on apparent density and true density of coal. The invention relates to a coal apparent density and true density-based mercury injection experimental data correction method, which comprises the following steps of: identifying and correcting a pressure interval by adopting a consistent volume and intrusive volume identification method, and determining actual initial mercury injection pressure and closing pressure; the consistency effect stage is that the mercury injection pressure is smaller than or equal to the actual initial mercury injection pressure, the compression effect stage is that the mercury injection pressure is larger than or equal to the closing pressure, and only mercury injection experiment data of the consistency effect stage and the compression effect stage are corrected, and in the consistency effect stage, the corrected mercury injection experiment pore volume is equal to 0; and in the compression effect stage, the corrected stage pore volume is equal to the apparent mercury feeding amount-the coal matrix volume compression amount-the mercury volume compression amount. The method is simple in parameter value taking and operation, accurate and reliable in correction result and capable of providing reference for accurate characterization of the coal pore structure.

Description

Technical Field

[0001] This invention relates to the field of coalbed methane development technology, and more specifically to a method for correcting mercury intrusion porosimetry experimental data based on the apparent density and true density of coal. Background Technology

[0002] The formation and types of coalbed methane pores are complex, with a wide range of pore sizes, a high degree of development of nanoscale pores, and strong heterogeneity, which increases the difficulty of characterizing the pore structure of coal seams. Accurate characterization of the pore structure and connectivity of coal seams is fundamental to understanding the occurrence, migration, and production of fluids in coal seams.

[0003] Mercury intrusion porosimetry (MIP) is a method for characterizing the pore structure of coal seams. It measures the volume of mercury intruding into the pore structure of a coal seam under specific intrusion pressures (up to 60,000 psi, or 413.7 MPa) to determine pore throat volume, porosity, pore size distribution, pore specific surface area, pressure-mercury saturation curves, and to estimate sample permeability or relative permeability. As one of the most commonly used methods for characterizing pore structure, MIP offers advantages such as fast measurement speed, low requirements on sample shape, low cost, ease of use, and mature technology.

[0004] However, mercury intrusion porosimetry (MIP) for characterizing coal seam pore structure has two types of errors: (1) MIP experiments usually use broken samples to shorten the mercury injection time. During the MIP experiment, a consistency effect occurs, meaning that before mercury enters the sample pores, it first fills the gaps between sample particles, the voids on the particle surface, and compensates for the irregular parts of the broken sample surface, resulting in an overestimation of the sample pore volume. Moreover, as the mercury injection pressure increases, the actual pore size of the sample decreases, and a higher pressure is needed to fill all the voids and voids before mercury can penetrate the actual pores. (2) High mercury injection pressure can compress the sample and lead to additional mercury injection, causing the sample pore volume to be overestimated, i.e., the compression effect. Both the consistency effect and the compression effect can lead to an overestimation of the sample pore volume, so it is necessary to correct the MIP experimental data to obtain a more realistic pore structure.

[0005] Currently, the pressure range for mercury intrusion porosimetry (MIP) data correction mainly employs the fractal dimension method and the consistency volume / intrusion volume identification method. The fractal dimension method calculates the fractal dimension of the pores in coal using MIP data to determine the actual initial mercury intrusion pressure and closure pressure. The consistency volume / intrusion volume identification method compares the pore volume compressibility coefficient calculated based on MIP data with the linear portion of the mercury intrusion pressure curve to determine the actual initial mercury intrusion pressure and closure pressure. The portion of the calculated pore volume compressibility coefficient that deviates from the linear fitting point at lower and higher mercury intrusion pressures is considered the actual initial mercury intrusion pressure and closure pressure, respectively. For mercury intrusion correction, the direct correction method is mainly used, or other experimental methods are employed to quantify the compressibility of the sample under high pressure, thereby correcting the actual mercury intrusion amount and pore volume at high mercury intrusion pressures. The direct correction method substitutes the determined actual initial mercury intrusion pressure and closure pressure into the corrected pore volume compressibility coefficient expression to obtain the corrected mercury intrusion amount and pore volume. This method is simple to operate but lacks theoretical basis. Calibration methods based on other experimental approaches typically employ mercury intrusion porosimetry (MIP) experiments under identical conditions using epoxy resin-coated samples to obtain the volume compressibility coefficient of the samples under the same experimental conditions, thereby correcting the influence of compression on mercury intrusion and pore volume. While this correction method has a sound theoretical basis, it requires repeated mercury intrusion experiments, significantly increasing the research workload.

[0006] Based on the identification of the pressure range, this invention proposes a method for correcting mercury intrusion porosimetry data based on the apparent density and true density of coal. The method is simple to operate and does not require additional workload, aiming to obtain more accurate pore structure distribution characteristics in coal.

[0007] The purpose of this invention is to provide a method for correcting mercury intrusion porosimetry experimental data based on the apparent density and true density of coal, so as to reduce the workload of coal pore structure characterization research and improve accuracy.

[0008] The technical solution of this invention is as follows: A method for correcting mercury intrusion test data based on apparent density and true density of coal is proposed. The method is as follows: The consistency volume and intrusion volume identification method is used to identify the correction pressure range and determine the actual initial mercury intrusion pressure. P c and closing pressure P i ; Mercury inlet pressure P Hg ≤ Actual initial mercury inlet pressure P c For the consistency effect stage, the mercury ingress pressure P Hg ≥ Closing pressure P i For the compression effect stage, only the mercury intrusion porosimetry experimental data from the consistency effect stage and the compression effect stage are corrected: In the consistency effect stage, the corrected mercury intrusion test pore volume = 0; in the compression effect stage, the corrected stage pore volume = apparent mercury intrusion amount - coal matrix volume compression amount - mercury volume compression amount.

[0009] The correction pressure range is determined by comparing the pore volume compressibility coefficient. Fitted pore volume compressibility coefficient The deviation is obtained; where the fitted pore volume compressibility coefficient is... By pore volume compressibility Obtained by fitting a linear curve to the mercury inlet pressure; The specific identification process for the correction pressure range is as follows: (The rest of the text is incomplete and cannot be translated.) The starting point is the initial mercury inlet pressure. P c ,by The endpoint is the closing pressure. P i Less than the initial mercury inlet pressure P c Greater than the closing pressure P i The portion that is defined is the correction pressure range.

[0010] Wherein, the pore volume compressibility coefficient The calculation process is as follows: (1) In the formula: The pore volume compressibility coefficient is expressed in MPa. -1 ; Mercury intake volume, unit: cm³ 3 / g; This refers to the inlet pressure of mercury, expressed in MPa. Mercury injection volume per stage, in cm³ 3 / g; The pressure at which mercury enters the stage is expressed in MPa.

[0011] Wherein, the fitted pore volume compressibility coefficient The calculation process is as follows: (2) In the formula: To fit the pore volume compressibility coefficient, MPa -1 ; This represents the intercept of the linear curve of pore volume compressibility coefficient versus mercury ingress pressure. The slope is dimensionless.

[0012] The specific calculation process for the corrected stage pore volume is as follows: Define a pore volume compression ; get ; In the formula: The corrected stage pore volume is expressed in cm³. 3 / g; The corrected pore volume is expressed in cm³. 3 / g; The amount of mercury introduced is expressed in cm³. 3 / g; This refers to the volumetric compression of the coal matrix, expressed in cm³. 3 / g; This is the mercury volume compression, in cm. 3 / g; where The volume compressibility coefficient of the coal sample is expressed in MPa. -1 ; The volumetric compressibility coefficient of coal matrix, in MPa. -1 ; Mercury's bulk compressibility is expressed in MPa. -1 ; The pressure at which mercury enters the stage is expressed in MPa. (3) Assuming mercury volume compressibility A fixed value of 4.0 × 10 -5 MPa -1 Then equation (3) can be transformed into: (4) In the formula: Mercury intake volume, unit: cm³ 3 / g; Mercury intake volume per stage, in cm³ 3 / g; This refers to the volume of the coal sample, in cm³. 3 ; This refers to the volume of a staged coal sample, in cm³. 3 ; Mercury inlet pressure P Hg The change in mercury volume caused by compression, in cm³. 3 / g.

[0013] Wherein, the coal sample volume The specific calculation process is as follows: (5) In the formula: This refers to the weight of the coal sample, in grams. This is the apparent density of the coal sample, in g / cm³. 3 .

[0014] Wherein, the corrected pore volume The specific calculation process is as follows: (6); (7); In the formula: This represents the volume of the solid portion of the coal sample, in cm³. 3 ; This is the true density of the coal sample, in g / cm³. 3 .

[0015] Wherein, the amount of mercury introduced Mercury inlet pressure apparent density of coal samples and the true density of coal samples This was obtained by conducting mercury intrusion porosimetry on coal samples.

[0016] The technical effects of this invention are as follows: In this invention, mercury intrusion rate, mercury intrusion pressure, apparent density, and true density of the coal sample are obtained through mercury intrusion experiments. The calibration pressure range is identified using the consistency volume and intrusion volume identification method. Based on the apparent density and true density of the coal sample, the pore volume under no confining pressure is obtained by correcting the coal matrix volume compression and mercury volume compression. The influence of consistency and compression effects on the calibration results is analyzed. The calibration results of this invention show extremely high agreement with those of commonly used stage calibration methods, and the influence of mercury volume compression on the calibration results under high mercury intrusion pressure is considered. This invention features simple parameter selection and operation, and provides accurate and reliable calibration results, offering a reference for accurately characterizing the pore structure of coal. Attached Figure Description

[0017] Figure 1 It is a calibration pressure range identification diagram.

[0018] Figure 2 This is a comparison chart of the pore volume at any stage under various pore sizes after correction by the new method and stage correction method proposed in this invention, and the mercury intrusion porosimetry experimental data.

[0019] Figure 3 This is a comparison chart of the pore volume after correction using the new method and stage correction method proposed in this invention, and the mercury intrusion porosimetry experimental data. Detailed Implementation Specific Implementation A method for correcting mercury intrusion porosimetry experimental data based on the apparent density and true density of coal is as follows: Step 1: Conduct mercury intrusion porosimetry (MIP) experiments on coal samples to obtain data such as mercury intrusion rate and pressure. Step 2: Using the consistency volume and intrusion volume identification method, identify the correction pressure range and determine the actual initial mercury intrusion pressure.P c and closing pressure P i ; Step 3: Using mercury inlet pressure P Hg ≤ Actual initial mercury inlet pressure P c For the consistency effect stage, the consistency volume is obtained; the consistency volume is used to correct the pore volume of the mercury intrusion porosimetry experiment. In the consistency effect stage, the actual mercury intrusion is 0, and the consistency volume is the error in mercury intrusion caused by the voids. The corrected pore volume of the mercury intrusion porosimetry experiment = apparent mercury intrusion - consistency volume = 0. Step 4: Using mercury inlet pressure P Hg ≥ Closing pressure P i During the compression effect stage, the apparent density of the coal sample was measured according to the methods in "Methods for Determination of Physical and Mechanical Properties of Coal and Rock Part 2: Determination of True Density of Coal and Rock" (GB / T 23561.2-2009) and "Methods for Determination of Physical and Mechanical Properties of Coal and Rock Part 3: Determination of Bulk Density of Coal and Rock" (GB / T 23561.3-2009). True density of coal sample Based on the apparent density of coal samples True density of coal sample Stage pore volume correction is performed. The corrected stage pore volume = apparent mercury intake - coal matrix volume compression - mercury volume compression.

[0021] Specific experimental cases Taking a coal sample from the No. 6 mine of Sair Energy as an example, this invention will be used to explain the method for correcting mercury intrusion porosimetry experimental data based on the apparent density and true density of coal.

[0022] (1) Mercury intrusion porosimetry was performed on the coal sample, and the experimental data are shown in Table 1; Table 1 Mercury porosimetry experimental data

[0023] (2) The consistent volume and intrusion volume identification method is used to identify the correction pressure range and the pore volume compressibility coefficient. and fitted pore volume compressibility coefficient Bias exists on the double logarithmic coordinate axis; the bias in the low-pressure stage originates from the uniformity effect, while the bias in the high-pressure stage is caused by the compressibility effect. This is determined by comparing the pore volume compressibility coefficients. and fitted pore volume compressibility coefficient The deviation between them can be used to obtain the actual initial mercury inlet pressure. P c and closing pressureP i ( Figure 1 );Depend on Figure 1 The pore volume compressibility coefficient can be obtained. and fitted pore volume compressibility coefficient The deviation is quite significant and easy to judge.

[0024] (3) In the consistency effect stage, the corrected pore volume of mercury intrusion porosimetry is 0.

[0025] (4) In the compression effect stage, the corrected stage pore volume = apparent mercury ingress - coal matrix volume compression - mercury volume compression, and the results are as follows: Table 2 Data on the compression effect stage

[0026] The comparison chart of the pore volume (i.e., the increase in pore volume within the corresponding pore size range) at any stage under the new method and stage correction method proposed in this invention with mercury intrusion porosimetry experimental data is shown below. Figure 2 The comparison chart of pore volume and mercury intrusion porosimetry experimental data after correction by the new method proposed in this invention and the stage correction method is shown below. Figure 3 .

[0027] Overall, the trend of the overall correction results of this invention is basically consistent with the mercury intrusion porosimetry experimental data. The differences are: (1) In the consistency effect stage, since the actual mercury intrusion amount is 0 in the consistency effect stage, the corrected pore volume of the mercury intrusion porosimetry experiment is much smaller than that of the mercury intrusion porosimetry experimental data; (2) In the compression effect, the coal matrix volume compressibility coefficient is... It is not a constant value; it increases with increasing mercury injection pressure. Therefore, the corrected stage pore volume is smaller, and the increase in the corrected stage pore volume is slightly smaller than that of mercury intrusion porosimetry data. Figure 3 The slope of the straight line segment in the new method is slightly smaller than that in the mercury intrusion test data.

[0028] Compared with the stage correction method, the two have a higher degree of agreement. However, this invention takes into account the influence of mercury volume compression under high mercury inlet pressure on the correction results, which makes the overall correction results of this invention slightly lower than those of the stage correction method.

[0029] Compared with existing mercury intrusion porosimetry for characterizing coal pore structure, this invention has the following advantages: by collecting coal samples for mercury intrusion experiments, the consistency volume and intrusion volume identification method is used to identify the correction pressure range. Based on this, in the compression effect stage, the stage pore volume under no confining pressure is obtained by subtracting the volume compression of the coal matrix from the apparent mercury intrusion volume to the mercury volume compression. This method can be used for accurate characterization of coal pore structure.

Claims

1. A method for correcting mercury intrusion porosimetry experimental data based on the apparent density and true density of coal, characterized in that, The method is as follows: The consistency volume and intrusion volume identification method is used to identify the correction pressure range and determine the actual initial mercury intrusion pressure. P c and closing pressure P i ; Mercury inlet pressure P Hg ≤ Actual initial mercury inlet pressure P c For the consistency effect stage, the mercury ingress pressure P Hg ≥ Closing pressure P i For the compression effect stage, only the mercury intrusion porosimetry experimental data from the consistency effect stage and the compression effect stage are corrected: In the consistency effect stage, the corrected mercury intrusion test pore volume = 0; in the compression effect stage, the corrected stage pore volume = apparent mercury intrusion amount - coal matrix volume compression amount - mercury volume compression amount.

2. The method for correcting mercury intrusion porosimetry experimental data based on apparent density and true density of coal according to claim 1, characterized in that, The correction pressure range is determined by comparing the pore volume compressibility coefficient. Fitted pore volume compressibility coefficient The deviation is obtained; where the fitted pore volume compressibility coefficient is... By pore volume compressibility The curve was obtained by fitting the linear curve of the mercury inlet pressure.

3. The method for correcting mercury intrusion porosimetry experimental data based on apparent density and true density of coal according to claim 2, characterized in that, The specific identification process for the correction pressure range is as follows: (The rest of the text is incomplete and cannot be translated.) The starting point is the initial mercury inlet pressure. P c ,by The endpoint is the closing pressure. P i Less than the initial mercury inlet pressure P c Greater than the closing pressure P i The portion that is defined is the correction pressure range.

4. The method for correcting mercury intrusion porosimetry experimental data based on apparent density and true density of coal according to claim 3, characterized in that, The pore volume compressibility coefficient The calculation process is as follows: （1） In the formula: The pore volume compressibility coefficient is expressed in MPa. -1 ; Mercury intake volume, unit: cm³ 3 / g; This refers to the inlet pressure of mercury, expressed in MPa. Mercury injection volume per stage, in cm³ 3 / g; The pressure at which mercury enters the stage is expressed in MPa.

5. The method for correcting mercury intrusion porosimetry experimental data based on apparent density and true density of coal according to claim 3, characterized in that, The fitted pore volume compression coefficient The calculation process is as follows: （2） In the formula: To fit the pore volume compressibility coefficient, MPa -1 ; This represents the intercept of the linear curve of pore volume compressibility coefficient versus mercury ingress pressure. The slope is dimensionless.

6. The method for correcting mercury intrusion porosimetry experimental data based on apparent density and true density of coal according to claim 1, characterized in that, The specific calculation process for the corrected stage pore volume is as follows: Define a pore volume compression ; get ; In the formula: The corrected stage pore volume is expressed in cm³. 3 / g; The corrected pore volume is expressed in cm³. 3 / g; The apparent mercury concentration is expressed in cm³. 3 / g; This refers to the volumetric compression of the coal matrix, expressed in cm³. 3 / g; This is the mercury volume compression, in cm. 3 / g; where The volume compressibility coefficient of the coal sample is expressed in MPa. -1 ; The volumetric compressibility coefficient of coal matrix, in MPa. -1 ; Mercury's bulk compressibility is expressed in MPa. -1 ; The pressure at which mercury enters the stage is expressed in MPa. （3） Assuming mercury volume compressibility A fixed value of 4.0 × 10 -5 MPa -1 Then equation (3) can be transformed into: （4） In the formula: Mercury intake volume, unit: cm³ 3 / g; Mercury injection volume per stage, in cm³ 3 / g; This refers to the volume of the coal sample, in cm³. 3 ; This refers to the volume of a staged coal sample, in cm³. 3 ; Mercury inlet pressure P Hg The change in mercury volume caused by compression, in cm³. 3 / g.

7. The method for correcting mercury intrusion porosimetry experimental data based on apparent density and true density of coal according to claim 6, characterized in that, The volume of the coal sample The specific calculation process is as follows: （5） In the formula: This refers to the weight of the coal sample, in grams. This is the apparent density of the coal sample, in g / cm³. 3 .

8. The method for correcting mercury intrusion porosimetry experimental data based on apparent density and true density of coal according to claim 6, characterized in that, The corrected pore volume The specific calculation process is as follows: （6）； （7）； In the formula: This represents the volume of the solid portion of the coal sample, in cm³. 3 ; This is the true density of the coal sample, in g / cm³. 3 .

9. The method for correcting mercury intrusion porosimetry experimental data based on apparent density and true density of coal according to claim 6, characterized in that, The amount of mercury introduced Mercury inlet pressure apparent density of coal samples and the true density of coal samples This was obtained by conducting mercury intrusion porosimetry on coal samples.

Images