Method for correcting absolute adsorption capacity in isothermal adsorption experiment in supercritical state

Abstract

The invention discloses a method for correcting absolute adsorption capacity in an isothermal adsorption experiment in a supercritical state, which comprises the following steps: 1, carrying out the isothermal adsorption experiment by adopting a static volumetric method, and calculating the amount of substance of methane in a sample cylinder at each test pressure point; 2, calculating the adsorption probability of methane molecules in the sample cylinder at each test pressure point based on the amount of substance of methane and the excess adsorption amount, and drawing a probability density curve according to the adsorption probability; 3, performing hypothesis verification on the probability density curve to obtain a probability density function; and 4, integrating the probability density function to obtain an absolute adsorption capacity curve, and obtaining the corrected absolute adsorption capacity according to the absolute adsorption capacity curve. According to the method, the absolute adsorption quantity is corrected on the basis of the adsorption probability of the methane molecules in the sample cylinder, the whole correction method does not need to modify the experimental process and does not involve any assumption on the physical properties of the methane molecules, and a more accurate and reliable result can be obtained with lower cost.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas field development, and in particular to a method for correcting absolute adsorption amount in an isothermal adsorption experiment under a supercritical state. Background Art

[0002] Shale gas is natural gas existing in shale layers, and its main component is methane. Shale gas mainly exists in shale in the form of adsorbed phase and free phase. The shale adsorbed gas content is a key parameter to characterize the quality of shale reservoirs. Previous studies have shown that the total amount of adsorbed gas accounts for 30%-80% of the total shale gas reserves. Therefore, the accurate amount of adsorbed gas in shale gas reservoirs is the key to calculating shale gas reserves, evaluating shale gas reservoirs and formulating effective development strategies.

[0003] For a long time, volumetric and gravimetric isothermal adsorption experiments under shale gas reservoir temperature and pressure conditions have been used to estimate the amount of adsorbed gas in shale gas. After obtaining the amount of adsorbed gas of shale samples at different pressure points through experiments, the Langumir equation is used to calculate the adsorbed gas content of shale reservoirs at formation temperature and pressure. This method was first used to calculate the gas content in coal seams, and is also suitable for calculating the amount of adsorbed gas in shale gas reservoirs with shallow burial depths, low formation temperatures and pressures. However, most shale formations are under high temperature and high pressure. The Longmaxi Formation shale in the southwest of the Sichuan Basin in China is generally buried at a depth of 2000m, with a formation pressure of 30MPa and a formation temperature of 70℃. Shale gas is usually in a supercritical state, because the critical point of methane is 4.6MPa and 190.4K. Since the physical properties of supercritical gas are different from those of subcritical gas, the theory describing the adsorption behavior of subcritical gas is difficult to describe the adsorption behavior of supercritical gas, which affects the estimation of the amount of adsorbed gas in shale reservoirs. Under supercritical conditions, the adsorption amount measured by isothermal adsorption experiments is the Gibbs excess adsorption amount, not the real adsorption amount (i.e., the absolute adsorption amount). This results in a decrease in the excess adsorption amount measured by shale isothermal adsorption experiments under high pressure. Many shale isothermal adsorption experiments show that the isothermal adsorption characteristics under high pressure are different from those under low pressure conditions. The isothermal adsorption curve will reach a maximum value at a certain pressure point, and then the isothermal adsorption curve will decrease as the pressure increases. The classic Langumir equation can no longer describe this phenomenon. This phenomenon leads to how to convert the excess adsorption amount measured by isothermal adsorption experiments into absolute adsorption amount, which has become an important problem to be solved urgently. This is because the absolute adsorption amount reflects the total amount of gas molecules in the adsorption phase in the adsorption system, while the excess adsorption amount only reflects the total amount of adsorbed gas molecules in the adsorption phase that exceeds the free phase gas density. This problem is particularly important when using isothermal adsorption experiments to estimate the adsorbed gas volume of shale gas reservoirs with high formation pressure, because the free phase gas density is quite large under high pressure. Directly using excess adsorption to estimate the adsorbed gas volume in shale will greatly underestimate shale gas reserves and EUR.

[0004] At present, it is still unclear how to estimate the actual adsorption amount (absolute adsorption amount) of methane in shale under supercritical conditions (formation conditions). Accurately estimating the absolute adsorption amount requires the establishment of the state equation of the adsorption phase, that is, it is necessary to clarify how the volume and density of the adsorption phase change with changes in temperature and pressure. However, the state equation of the adsorption phase under the supercritical state has not yet been established because the density and volume of the adsorption phase under the supercritical state cannot be measured experimentally. Therefore, many different methane supercritical adsorption models have been established to convert the excess adsorption amount measured by isothermal adsorption experiments under supercritical conditions into absolute adsorption amount. Existing technical solutions usually regard the adsorption phase methane as a liquid or quasi-liquid, and assume that the density of the adsorption phase methane is the density of liquid methane or van der Waals density, and then use the Adanson formula to convert the excess adsorption amount into absolute adsorption amount. However, these densities are not related to temperature, pressure, and surface properties of the adsorbate, and lack theoretical support. There are also some technologies that use the extended Langmuir model, Dubinin-Radushkevich (DR) model, and Dubinin-Astakhov (DA) model to estimate the density of the adsorbed phase methane, and then convert the excess adsorption amount into the absolute adsorption amount according to the Gibbs equation. However, the above formulas do not establish the relationship between the adsorption amount and the density of the adsorbed phase, and the (DA) formula and the (DR) formula implicitly assume that the adsorbate gas is an ideal gas and the adsorbed phase is a saturated liquid, which does not conform to the physical law that gases cannot be liquefied under supercritical conditions. There is still uncertainty in the process of directly applying it to converting the excess adsorption amount into the absolute adsorption amount. There are few comparative studies on these adsorption models, and there is no recognized opinion on the effectiveness of each model.

[0005] In addition, the patent document with the publication number CN108446526A also discloses a method for predicting the adsorption amount of shale based on the adsorption characteristic curve and the characteristic equation, which includes collecting organic shale samples, vacuuming them; collecting methane isothermal adsorption data; calculating the adsorption potential and the excess adsorption amount; correcting the excess adsorption amount to the absolute adsorption amount, etc. This method can correct the excess adsorption amount obtained at multiple sets of temperatures to the absolute adsorption amount, thereby realizing the effective prediction of the adsorbed gas content of the organic shale reservoir. But in fact, the patent document uses the supercritical DA equation to fit the methane isothermal adsorption data, and the DA equation contains the following assumptions, namely, assuming that the adsorbed phase methane is a saturated liquid, which is contrary to the physical law that gas cannot be liquefied under the supercritical state. Moreover, it also assumes that the free phase methane gas still has a saturated vapor pressure after exceeding the critical temperature and pressure. This assumption does not conform to the basic characteristics of supercritical gas at all and is not practical. In addition, the supercritical state DA equation proposed in the patent document does not give a calculation method for the characteristic adsorption energy, and its operability is insufficient. Summary of the invention

[0006] The object of the present invention is to provide a method for calibrating the absolute adsorption amount in an isothermal adsorption experiment under a supercritical state. The method realizes the correction of the absolute adsorption amount based on the probability of adsorption of methane molecules in a sample cylinder. The entire correction method does not require modification of the experimental process, nor does it involve any assumptions about the physical properties of methane molecules, thereby being able to obtain more accurate and reliable results at a lower cost.

[0007] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0008] A method for correcting the absolute adsorption amount in an isothermal adsorption experiment under a supercritical state, characterized by comprising the following steps:

[0009] Step 1: The isothermal adsorption experiment of the shale sample is carried out by using the static volume method. First, multiple different test pressure points are set, and the excess adsorption amount of methane in the sample cylinder, the methane gas pressure in the reference cylinder before adsorption, and the methane gas pressure in the reference cylinder after adsorption are tested when the adsorption equilibrium is reached at each test pressure point. Then, based on the principle that the volume of the reference cylinder remains unchanged and the pressure of the methane gas in the reference cylinder changes before and after adsorption, Boyle's law is used to calculate the amount of methane in the sample cylinder at each test pressure point;

[0010] Step 2: Based on the amount of methane and the excess adsorption amount, the probability of methane molecules being adsorbed in the sample cylinder at each test pressure point is calculated respectively, and then the probability density curve of the probability of methane molecules being adsorbed in the sample cylinder is drawn with the amount of methane in the sample cylinder at each test pressure point as the horizontal axis and the probability of methane molecules being adsorbed at each test pressure point as the vertical axis;

[0011] Step 3: Take the amount of methane in the sample cylinder as a random variable to verify the hypothesis of the probability density curve, determine the function type of the probability density curve and obtain the probability parameters, and then obtain the probability density function corresponding to the probability density curve based on the probability parameters;

[0012] Step 4: According to the definition of adsorption and the definition of probability distribution, the probability density function is integrated. After the integration is completed, the absolute adsorption amount curve of the shale sample under the condition of the increase of the total number of methane molecules in the sample cylinder is obtained, and then the corrected absolute adsorption amount can be obtained according to the absolute adsorption amount curve.

[0013] In step 1, the amount of methane in the sample cylinder is calculated as follows:

[0014] n f i=p 1 V r / RTZ 1 -p 2 V r / RTZ2

[0015] Where n f i is the amount of methane in the sample cylinder at any test pressure point, in mol;

[0016] p 1 is the methane gas pressure in the reference cylinder before adsorption at any test pressure point, in MPa;

[0017] p 2 is the methane gas pressure in the reference cylinder after adsorption at any test pressure point, in MPa;

[0018] V r is the reference cylinder volume in cm 3 ;

[0019] Z 1 For p 1 Deviation coefficient of methane gas in the reference cylinder under the condition;

[0020] Z 2 For p 2 Deviation coefficient of methane gas in the reference cylinder under the condition;

[0021] T is the experimental temperature of the current isothermal adsorption setting, in K.

[0022] In step 2, the probability of methane molecules being adsorbed is calculated as:

[0023] P i =n ex i / n f i

[0024] Where P i is the probability of methane molecules being adsorbed in the sample cylinder at the test pressure point;

[0025] n ex i is the excess adsorption amount, in mol.

[0026] In step 3, the probability density function obtained is:

[0027]

[0028] Where F(Pi) is the probability density function of the adsorption of methane molecules in the sample cylinder at any test pressure point.

[0029] In step 4, the calculation method of the absolute adsorption amount curve is:

[0030]

[0031] In step 4, the definition of adsorption refers to: methane molecules are adsorbed on the surface of the shale sample. Since the methane adsorption sites provided by the shale sample surface are limited, as the adsorption gradually proceeds, the previously adsorbed methane molecules occupy more and more adsorption sites, and subsequent methane molecules gradually find it difficult to find adsorption sites. When the methane adsorption sites on the shale sample surface are completely occupied, the adsorption amount of methane molecules tends to a constant, which is the saturated adsorption amount.

[0032] The definition of the probability distribution is: the probability interval of methane molecules being adsorbed in the sample cylinder is [0, 1], and the corresponding adsorption amount is [0, saturated adsorption amount]. When the adsorption of methane molecules in the sample cylinder reaches saturation, the corresponding saturated adsorption amount is the adsorption amount corresponding to the sum of the probabilities of methane molecules being adsorbed.

[0033] In step 1, the pressure range of the test pressure point is 0-60MPa.

[0034] In step 1, the number of test pressure points is no less than 6.

[0035] In step 1, the pressure interval between two adjacent test pressure points is not less than 3 MPa.

[0036] The isothermal adsorption experiment was carried out based on the standard of GB / T 35210.1-2017.

[0037] The advantages of adopting the present invention are:

[0038] 1. The method of the present invention regards all the methane molecules in the sample cylinder as a system, and then converts the excess adsorption amount measured in the experiment into the absolute adsorption amount according to the probability of adsorption of methane molecules in the system. The entire correction method does not require modification of the experimental process, nor does it require additional experiments. At the same time, it does not involve any assumptions about the physical properties of methane molecules in the supercritical state, avoiding the limitation of the application scope of the technical solution caused by the establishment of assumptions and the limitation of the assumptions not being established, so that more accurate and reliable results can be obtained at a lower cost. The problem that the classical Langumir equation cannot describe the situation that the excess adsorption amount measured in the isothermal adsorption experiment gradually decreases with the continuous increase of the adsorption pressure under the supercritical state is solved, and the excess adsorption curve can be corrected to a form that conforms to the classical Langumir equation, and a good foundation is laid for further calculation of the Langumir volume and Langumir pressure.

[0039] 2. The present invention regards the states of methane molecules in the sample cylinder during the isothermal adsorption experiment as having only two states: adsorbed state and free state, regards the adsorption of methane molecules on the surface of the shale sample as a random event, regards the saturated adsorption amount as the sum of the probabilities of methane molecules being adsorbed under a certain pressure, and gives probabilistic meanings to the excess adsorption amount and saturated adsorption amount measured by isothermal adsorption, ultimately greatly improving the reliability and accuracy of the absolute adsorption amount data obtained compared with the prior art.

[0040] 3. The present invention first calculates the probability of methane molecules being adsorbed in the sample cylinder at each test pressure point based on the amount of methane substance and the excess adsorption amount, and then plots the probability density curve of the probability of methane molecules being adsorbed in the sample cylinder with the amount of methane substance as the horizontal axis and the probability of methane molecules being adsorbed as the vertical axis. The advantage is that it can clearly determine the change in the probability of methane molecules being adsorbed in the sample cylinder under the condition that the number of methane molecules in the sample cylinder changes, and it is also helpful to grasp the change in the probability of methane molecules being adsorbed or the adsorption increment under the condition that the adsorption pressure gradually increases.

[0041] 4. The present invention verifies the probability density curve of methane molecules adsorbed in the sample cylinder at different test pressure points, which can clarify the function type of the probability density curve and derive the probability parameter, and then obtain the probability density function. On the one hand, it is conducive to improving the accuracy and reliability of the results, and on the other hand, it can make the probability density function predict the absolute adsorption amount under higher test pressure.

[0042] 5. The present invention sets the pressure range of the test pressure points to 0-60MPa, sets the number of test pressure points to no less than 6, and makes the pressure interval between two adjacent test pressure points no less than 3MPa. On the one hand, it is convenient to obtain test data with uniform distribution intervals and eliminate the influence of pressure fluctuations on the test results as much as possible, thereby improving the accuracy of the correction results. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a flowchart of the present invention;

[0044] Figure 2 It is the probability density curve diagram obtained in step 2 of the present invention;

[0045] Figure 3 is a graph of the absolute adsorption amount obtained in step 4 of the present invention;

[0046] Figure 4 This is a verification comparison diagram of the present invention. DETAILED DESCRIPTION

[0047] Example 1

[0048] The present invention provides a method for correcting the absolute adsorption amount in an isothermal adsorption experiment under supercritical conditions, such as Figure 1 As shown, it includes the following steps:

[0049] Step 1: The isothermal adsorption experiment of the shale sample is carried out using the static volumetric method. The experiment is based on the standard GB / T35210.1-2017 "Isothermal adsorption determination method for shale methane Part 1: Volumetric method". It should be noted that the isothermal adsorption experiment involves the reference cylinder and sample cylinder of the isothermal adsorption instrument, and the reference cylinder and the sample cylinder can be controlled by a valve. The shale sample is placed in the sample cylinder during the experiment. In the static volumetric isothermal adsorption experiment process, all the methane adsorbed in the shale sample comes from the reference cylinder of the isothermal adsorption instrument, and the methane gas pressure in the reference cylinder will decrease after adsorption compared to before adsorption, but the volume of the reference cylinder remains unchanged. Therefore, during the test, multiple different test pressure points can be set first, and the excess adsorption of methane in the sample cylinder, the methane gas pressure in the reference cylinder before adsorption, and the methane gas pressure in the reference cylinder after adsorption can be tested when the adsorption equilibrium is reached at each test pressure point. The excess adsorption of methane in the sample cylinder can be calculated based on the experimental standard according to the methane gas pressure in the sample cylinder before adsorption and the methane gas pressure in the sample cylinder after adsorption. Then, based on the principle that the volume of the reference cylinder remains unchanged and the pressure of the methane gas in the reference cylinder changes before and after adsorption, Boyle's law is used to calculate the amount of methane in the sample cylinder at each test pressure point.

[0050] Preferably, the amount of methane in the sample cylinder is calculated as follows:

[0051] n f i=p 1 V r / RTZ 1 -p 2 V r / RTZ 2

[0052] Where n f i is the amount of methane in the sample cylinder at any test pressure point, in mol;

[0053] p 1 is the methane gas pressure in the reference cylinder before adsorption at any test pressure point, in MPa;

[0054] p 2 is the methane gas pressure in the reference cylinder after adsorption at any test pressure point, in MPa;

[0055] V r is the reference cylinder volume in cm 3 ;

[0056] Z 1 For p 1 Deviation coefficient of methane gas in the reference cylinder under the condition;

[0057] Z 2 For p 2 Deviation coefficient of methane gas in the reference cylinder under the condition;

[0058] T is the experimental temperature of the current isothermal adsorption setting, in K.

[0059] Further preferably, the pressure range of the test pressure points in this step is 0-60MPa, the number of test pressure points is not less than 6, and the pressure interval between two adjacent test pressure points is not less than 3MPa. Taking the number of test pressure points as 8 as an example, the pressures at the test pressure points can be: 0MPa, 5MPa, 10, 20MPa, 30MPa, 35MPa, 40MPa, 50MPa and 60MPa.

[0060] Step 2: When adsorption equilibrium occurs, the sample cylinder and the reference cylinder are not connected to each other. At this time, the sample cylinder and the methane gas therein can be regarded as a system. The methane molecules in the system are either in a free state or in an adsorbed state. From the perspective of probability theory, for a methane molecule in the system, being in an adsorbed state is a random event. For all methane molecules in the system, the amount of methane gas adsorbed at different adsorption equilibrium pressures is a random variable. According to the definition of probability statistics theory, the amount of methane in the sample cylinder is defined as the number of experimental occurrences, or the total amount of statistical samples, and the excess adsorption amount is defined as the probability of an adsorption event occurring in a statistical sample. Based on the definition of probability, the probability of methane molecules being adsorbed in the sample cylinder at each test pressure point can be calculated based on the known amount of methane and the excess adsorption amount.

[0061] Preferably, the probability of methane molecules being adsorbed is calculated as follows:

[0062] P i =n ex i / n f i

[0063] Where P i is the probability of methane molecules being adsorbed in the sample cylinder at the test pressure point;

[0064] n ex i is the excess adsorption amount, in mol.

[0065] Furthermore, after obtaining the probability of methane molecules being adsorbed, according to the definition of probability statistics theory, the amount of methane in the sample cylinder is combined with the probability of methane molecules being adsorbed, and the change in the probability of methane molecules being adsorbed as the amount of methane in the sample cylinder increases can be obtained. That is, with the amount of methane in the sample cylinder at different test pressure points as the horizontal axis and the probability of methane molecules being adsorbed at different test pressure points as the vertical axis, a probability density curve of the probability of methane molecules being adsorbed in the sample cylinder at different pressure points can be drawn.

[0066] Step 3: Take the amount of methane in the sample cylinder as a random variable to verify the hypothesis of the probability density curve, determine the function type of the probability density curve and obtain the probability parameters, and then obtain the probability density function corresponding to the probability density curve based on the probability parameters.

[0067] This step verifies the hypothesis of the probability density curve based on the fact that adsorption is a continuous process, that is, the chances of methane molecules being adsorbed and not being adsorbed are equal, and the probability of methane molecules being adsorbed under different pressures is also continuous. Therefore, the probability of methane being adsorbed under different pressures must obey a certain law, that is, when the amount of methane molecules in the sample cylinder changes, the law of methane molecules being adsorbed is certain. Therefore, in order to clarify the law of methane molecules being adsorbed, it is necessary to verify the probability density curve in order to master its mathematical expression. Specifically, an assumption can be made in advance about the overall parameters or distribution form of the probability density curve, and then the type of probability density curve can be determined using the two sample information of the amount of methane in the sample cylinder and the excess adsorption obtained from the experiment. The verification method is to use the amount of methane in the sample cylinder under different test pressures as the X column, and the probability of methane molecules being adsorbed in the sample cylinder as the Y column, input them into SPSS software for verification, and select the significant level parameter α as the judgment standard. When α is closest to 0.05, determine the function type corresponding to the probability density curve of the α value, and express the probability density curve of methane molecules being adsorbed using the function to obtain the probability parameter.

[0068] It should be further explained that the distribution types of continuous probability density functions include normal distribution, exponential distribution, logarithmic distribution and so on. Each type has its own unique mathematical expression. To determine which type the probability density curve of methane adsorption belongs to, it is mainly necessary to see the probability density curve of methane adsorption listed through hypothesis testing and then make a judgment. The probability distribution it meets is determined by which type of distribution it belongs to. The probability parameter is obtained by substituting the probability density curve obtained from the real experiment into the probability distribution mathematical expression.

[0069] Furthermore, since the adsorption process of methane molecules is continuous and uninterrupted, the function expression of the probability density curve of methane molecules adsorption is an integrable function. Based on this, the probability density function obtained after hypothesis verification is:

[0070]

[0071] Where F(Pi) is the probability density function of the adsorption of methane molecules in the sample cylinder at any test pressure point.

[0072] Step 4: According to the definition of adsorption, methane molecules are adsorbed on the surface of shale samples. Since the methane adsorption sites provided by the surface of shale samples are limited, as the adsorption proceeds, the previously adsorbed methane molecules occupy more and more adsorption sites, and subsequent methane molecules gradually find it difficult to find adsorption sites. When the methane adsorption sites on the surface of the shale sample are completely occupied, the adsorption amount of methane molecules tends to a constant, which is the saturated adsorption amount. According to the definition of probability distribution, the probability interval of methane molecules adsorbed in the sample cylinder is [0,1], and the corresponding adsorption amount is [0, saturated adsorption amount]. When the adsorption of methane molecules in the sample cylinder reaches saturation, the corresponding saturated adsorption amount is the adsorption amount corresponding to the sum of the probabilities of methane molecules adsorbed. Therefore, according to the definition of adsorption and the definition of probability distribution, the probability density function is integrated. After the integration is completed, the absolute adsorption amount curve of the shale sample under the condition of the increase of the total number of methane molecules in the sample cylinder is obtained, and then the corrected absolute adsorption amount can be obtained based on the absolute adsorption amount curve.

[0073] Preferably, the calculation method of the absolute adsorption amount curve is:

[0074]

[0075] From the above, it can be seen that the present invention regards all the methane molecules in the sample cylinder as a system, and regards the states of the methane molecules in the system as having only two states, namely, adsorbed state and free state, and regards the adsorption of methane molecules on the shale surface as a random event, thereby converting the experimentally measured excess adsorption amount into an absolute adsorption amount according to the probability of adsorption of methane molecules in the system. This not only has higher accuracy and reliability, but also reduces costs and is more suitable for wide promotion.

[0076] Example 2

[0077] This example verifies the method described in Example 1, as follows:

[0078] Step 1: Conduct isothermal adsorption experiments on shale samples using the static volume method.

[0079] Among them, the shale sample comes from the Longmaxi Formation in the Sichuan Basin. The TOC content of the sample is 3.69%. The sample is crushed into particles of 180μm to 250μm, weighed about 100g, evenly spread on a clean transparent paper and moved into a drying oven. Dry at 105℃ for at least 48h. The ISO300 HPHT isothermal adsorption instrument is used. The experimental temperature is set to the formation temperature, i.e. 90℃. The sample cylinder volume is 214ml, the reference cylinder volume is 90ml, and 8 test pressure points are set, namely: 5MPa, 10MPa, 15MPa, 20MPa, 25MPa, 30MPa, 35MPa, and 40MPa. The shale sample is loaded into the sample cylinder, and after checking that the sealing of the isothermal adsorption instrument is intact, the isothermal adsorption experiment is started. The methane gas pressure in the sample cylinder and the reference cylinder before and after adsorption obtained in the experiment, and then according to Boyle's law, the excess adsorption amount of the sample and the amount of methane in the sample cylinder are calculated as shown in the following table:

[0080]

[0081] Step 2: Based on the calculation results of step 1 above, draw a probability distribution curve of the adsorption probability of methane molecules in the sample cylinder. The specific probability distribution curve is as follows: Figure 2 shown.

[0082] Step 3: Perform a probability hypothesis test on the probability distribution curve to determine that the function type corresponding to the probability distribution curve obeys the exponential distribution, and determine that the probability parameter is 2.7. Based on this, the probability density function obtained is:

[0083] f(x)=2.7l -2.7x ,x>0.

[0084] Step 4: After integrating the probability density function obtained in step 3, the mathematical expression of the absolute adsorption amount is obtained as follows:

[0085]

[0086] After the integration is completed, the absolute adsorption curve of the shale sample under the condition of the increase of the total number of methane molecules in the sample cylinder is obtained, as shown in the following figure: Figure 3 As shown, finally the corrected absolute adsorption amount can be obtained according to the absolute adsorption amount curve.

[0087] Finally, the applicant uses the DA equation to verify the technical solution of the present invention, and the verification results are as follows: Figure 4 As shown. Figure 4It can be seen that compared with the absolute adsorption amount curve calculated by the DA equation, the absolute adsorption amount obtained by the present invention is larger, which is more consistent with the understanding of the gas content test results in shale exploration and development. In addition, the present invention requires fewer parameters for calculation, does not require the estimation of the supercritical methane saturated vapor pressure in the DA equation, and does not assume that the adsorbed phase methane is liquid. The calculation method is simpler and more reliable. Therefore, the present invention is more suitable for calculating the adsorbed gas content in shale gas reservoirs with high formation pressure and high formation temperature.

[0088] The above description is only a specific implementation mode of the present invention. Any feature disclosed in this specification, unless otherwise stated, can be replaced by other alternative features that are equivalent or have similar purposes; all the disclosed features, or all the steps in the methods or processes, except for mutually exclusive features and / or steps, can be combined in any way.

Claims

1. A method for correcting the absolute adsorption amount in an isothermal adsorption experiment under supercritical conditions, characterized in that The following steps are involved: Step 1: The isothermal adsorption experiment of the shale sample is carried out by using the static volume method. First, multiple different test pressure points are set, and the excess adsorption amount of methane in the sample cylinder, the methane gas pressure in the reference cylinder before adsorption, and the methane gas pressure in the reference cylinder after adsorption are tested when the adsorption equilibrium is reached at each test pressure point. Then, based on the principle that the volume of the reference cylinder remains unchanged and the pressure of the methane gas in the reference cylinder changes before and after adsorption, Boyle's law is used to calculate the amount of methane in the sample cylinder at each test pressure point; Step 2: Based on the amount of methane and the excess adsorption amount, the probability of methane molecules being adsorbed in the sample cylinder at each test pressure point is calculated respectively, and then the probability density curve of the probability of methane molecules being adsorbed in the sample cylinder is drawn with the amount of methane in the sample cylinder at each test pressure point as the horizontal axis and the probability of methane molecules being adsorbed at each test pressure point as the vertical axis; Step 3: Take the amount of methane in the sample cylinder as a random variable to verify the hypothesis of the probability density curve, determine the function type of the probability density curve and obtain the probability parameters, and then obtain the probability density function corresponding to the probability density curve based on the probability parameters; Step 4: According to the definition of adsorption and the definition of probability distribution, the probability density function is integrated. After the integration is completed, the absolute adsorption amount curve of the shale sample under the condition of the increase of the total number of methane molecules in the sample cylinder is obtained, and then the corrected absolute adsorption amount can be obtained according to the absolute adsorption amount curve.

2. The method for correcting the absolute adsorption amount in an isothermal adsorption experiment under supercritical state according to claim 1, characterized in that: In step 1, the amount of methane in the sample cylinder is calculated as follows: n f i=p1V r / RTZ1-p2V r / RTZ2 Where n f i is the amount of methane in the sample cylinder at any test pressure point, in mol; p1 is the methane gas pressure in the reference cylinder before adsorption at any test pressure point, in MPa; p2 is the methane gas pressure in the reference cylinder after adsorption at any test pressure point, in MPa; V r is the reference cylinder volume in cm 3 ; Z1 is the deviation coefficient of methane gas in the reference cylinder under p1 condition; Z2 is the deviation coefficient of methane gas in the reference cylinder under p2 condition; T is the experimental temperature of the current isothermal adsorption setting, in K.

3. The method for correcting the absolute adsorption amount in an isothermal adsorption experiment under supercritical state according to claim 2, characterized in that: In step 2, the probability of methane molecules being adsorbed is calculated as: P i =n ex i / n f i Where P i is the probability of methane molecules being adsorbed in the sample cylinder at the test pressure point; n ex i is the excess adsorption amount, in mol.

4. The method for correcting the absolute adsorption amount in an isothermal adsorption experiment under supercritical state according to claim 3, characterized in that: In step 3, the probability density function obtained is: Where F(Pi) is the probability density function of the adsorption of methane molecules in the sample cylinder at any test pressure point.

5. A method for calibrating the absolute adsorption amount in an isothermal adsorption experiment under supercritical state according to claim 4, characterized in that: In step 4, the calculation method of the absolute adsorption amount curve is:

6. A method for calibrating the absolute adsorption amount in an isothermal adsorption experiment under supercritical conditions according to any one of claims 1 to 5, characterized in that: In step 4, the definition of adsorption refers to: methane molecules are adsorbed on the surface of the shale sample. Since the methane adsorption sites provided by the shale sample surface are limited, as the adsorption gradually proceeds, the previously adsorbed methane molecules occupy more and more adsorption sites, and subsequent methane molecules gradually find it difficult to find adsorption sites. When the methane adsorption sites on the shale sample surface are completely occupied, the adsorption amount of methane molecules tends to a constant, which is the saturated adsorption amount. The definition of the probability distribution is: the probability interval of methane molecules being adsorbed in the sample cylinder is [0, 1], and the corresponding adsorption amount is [0, saturated adsorption amount]. When the adsorption of methane molecules in the sample cylinder reaches saturation, the corresponding saturated adsorption amount is the adsorption amount corresponding to the sum of the probabilities of methane molecules being adsorbed.

7. The method for calibrating the absolute adsorption amount in an isothermal adsorption experiment under supercritical state according to claim 1, characterized in that: In step 1, the pressure range of the test pressure point is 0-60MPa.

8. The method for calibrating the absolute adsorption amount in an isothermal adsorption experiment under supercritical state according to claim 1, characterized in that: In step 1, the number of test pressure points is no less than 6.

9. A method for calibrating the absolute adsorption amount in an isothermal adsorption experiment under supercritical state according to claim 8, characterized in that: And the pressure interval between two adjacent test pressure points shall not be less than 3MPa.

10. The method for calibrating the absolute adsorption amount in an isothermal adsorption experiment under supercritical state according to claim 1, characterized in that: The isothermal adsorption experiment was carried out based on the standard of GB / T 35210.1-2017.

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