Method for testing gas composition of closed pores of shale under in-situ condition

Abstract

The invention relates to the technical field of shale oil and gas geochemical analysis, and discloses a method for testing gas composition of closed pores of shale under in-situ conditions, and the method comprises the following steps: crushing a shale sample to 3-8 meshes; carrying out thermal desorption treatment on the sample at the temperature of 100 DEG C so as to empty gas in the communicating pores; placing the treated sample in a closed ball milling tank filled with helium in advance, and carrying out ball milling for 2 hours at a rotating speed of 600r / min so as to release gas in closed pores; and finally, collecting gas by using a closed gas collecting needle, and quantifying the gaseous hydrocarbon of each component through gas chromatographic analysis and an n-butane external standard method. According to the invention, through a two-stage treatment strategy of first purification and second release, closed pore gas components capable of representing in-situ conditions are effectively obtained, and the technical problem of result distortion caused by difficulty in accurately distinguishing gases in different occurrence states in the prior art is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of shale oil and gas geochemical analysis, and particularly to a method for testing gas composition of sealed pores under in-situ conditions of shale. BACKGROUND

[0002] Accurate evaluation of shale oil and gas content is the basis for resource potential assessment and sweet spot prediction, and plays a decisive role in shale oil and gas geological theory research and exploration and development. However, after the shale sample is separated from the formation environment, due to the sudden drop of pressure, the light hydrocarbon components, especially the gaseous hydrocarbons, will be irreversibly dispersed. This dispersion leads to the fact that the oil and gas composition data obtained by subsequent laboratory analysis cannot truly reflect the in-situ state of the shale reservoir, thereby restricting the accuracy of geological evaluation.

[0003] In order to inhibit gas dispersion, although the closed pressure maintaining coring technology has been developed in the field, the technology has strict requirements on engineering conditions and high implementation cost, and it often needs to be assisted by liquid nitrogen for low-temperature preservation in operation, and the fixing efficiency of liquid nitrogen on light components such as methane is limited, so the accuracy of the analysis results is still questionable. In comparison, the gas dispersion problem of the sample obtained by using the conventional coring method is more serious, and the representativeness of the analysis results on the in-situ components is also lower.

[0004] There are a large number of physical sealed pores in the shale reservoir. Unlike the gas in the connected pore network which is easy to disperse, the hydrocarbon components existing in the sealed pores are not affected by the later reconstruction because they are wrapped by the dense rock matrix, and thus constitute a high-fidelity geological record of the original geochemical information during oil and gas accumulation. SUMMARY

[0005] In view of the shortcomings of the prior art, the present application provides a method for testing gas composition of sealed pores under in-situ conditions of shale, which solves the problem that the prior art cannot accurately distinguish different occurrence state gases, resulting in distorted results.

[0006] To achieve the above object, the present application is implemented by the following technical scheme: the present application provides a method for testing gas composition of sealed pores under in-situ conditions of shale, the core of the method is to adopt a two-stage processing strategy: first, through selective thermal desorption treatment, remove the connected pore gas which is easy to disperse and has been distorted in the sample; second, in a controlled inert atmosphere, release and capture analysis of the original gas sealed in the sealed pores by high-energy mechanical crushing.

[0007] Specifically, the technical scheme provided by the present application includes: Select a blocky shale sample, measure the density of the shale sample, and crush the sample to a specific target particle size range and then weigh the total mass; The sample is subjected to thermal desorption treatment to selectively remove gaseous hydrocarbon components located on the outer surface of the particles and in the interconnected pore network. The temperature of the thermal desorption treatment is preferably 90℃-110℃ and the treatment time is 1-3 hours. The thermally desorbed sample is placed in a sealed ball mill jar with an internal total volume and subjected to high-energy ball milling in an inert gas environment. The shale matrix is ​​broken by mechanical crushing, and the gaseous hydrocarbon components stored in the sealed pores are released into the headspace of the sealed ball mill jar. The ball milling speed is preferably 500 r / min-700 r / min and the treatment time is 1-3 hours. The headspace gas was extracted and analyzed by gas chromatography to obtain the chromatographic peak area of ​​the target component compound; Finally, the content of gaseous hydrocarbons in the closed pores of the sample was calculated.

[0008] In a preferred embodiment of the present invention, the inert gas is helium.

[0009] In another preferred embodiment of the present invention, the content of gaseous hydrocarbons in the closed pores of the sample ( It can be calculated using the following formula: ; In the formula, This indicates the absolute content of a certain component of gaseous hydrocarbons in a sample, expressed in mg / g. This indicates the chromatographic peak area of ​​the component compound in the sample, expressed in pA·s; This indicates the content of injected standard gas (n-butane) calculated according to the above formula, in μg; This represents the chromatographic peak area obtained when measuring standard gas, expressed in pA·s; This indicates the total internal volume of the sealed ball mill jar module, in cm. 3 ; This indicates the total mass of the ball-milled shale sample, expressed in grams. This indicates the density of the shale sample, expressed in g / cm³. 3 .

[0010] In the above formula, the standard gas content ( It can be obtained through the following formula: ; In the formula, This indicates the volume of the quantitative loop in a gas chromatograph, in units of... ; The concentration of the standard gas is expressed in ppm; 58 represents the relative molecular mass of n-butane; 22.4 represents the molar volume of the gas under standard conditions. .

[0011] This invention provides a method for testing the composition of gas in closed pores of shale under in-situ conditions. It has the following beneficial effects: 1. The present invention first employs thermal desorption treatment at 90℃-110℃. This specific temperature range can effectively remove the gas in the connected pores and the surface adsorbed gas that are distorted due to sample depressurization. At the same time, it avoids the thermal decomposition of organic matter in the rock to produce new hydrocarbons. This step eliminates interfering components in advance, ensuring that the objects of subsequent analysis are only the original gas in the closed pores that have not been disturbed, thus making the final quantitative results more accurate.

[0012] 2. This invention separates two types of gases with different physical properties and geological significance during the analysis process through a two-stage process of thermal desorption followed by mechanical crushing in a closed environment. This makes the test results no longer the average value of the mixed gas, but rather a highly representative part of the component sealed in the closed pores that best reflects the original gaseous state, providing more targeted information for geological evaluation.

[0013] 3. This invention transfers the thermally desorbed sample to a sealed ball mill jar and crushes, samples, and analyzes it under inert gas protection. This effectively prevents the loss of gas from the sealed pores to the outside during the release process and also eliminates the mixing of environmental impurities such as air. Combined with external standard method for quantification, the final gas content data is accurate and reliable, and can be effectively compared between different samples or different laboratories. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the light hydrocarbon recovery process of the present invention. Figure 2 The chromatogram of gaseous hydrocarbons released in the sealed pores of this invention. Figure 3 This is a comparison diagram of gaseous hydrocarbons released in the closed pores of the present invention and gaseous hydrocarbons released in the open pores by conventional thermal desorption. Detailed Implementation

[0015] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0016] Reference Figure 1 and Figure 3This invention provides a technical solution for restoring the gas composition in shale. The core principle lies in distinguishing between the presence of interconnected and closed pores in shale. When a shale sample is removed from its formation environment, the light hydrocarbon components present in the interconnected pores dissipate in large quantities due to pressure and temperature changes, while the gaseous hydrocarbon components present in the closed pores are physically sealed within the rock matrix, preserving relatively original compositional information.

[0017] The technical solution of this invention employs a two-stage process: First, through a specific pretreatment step (e.g., thermal desorption), residual or surface-adsorbed gaseous components in the interconnected pores are selectively removed to obtain a purified sample; second, in a controlled, sealed, inert atmosphere, the rock matrix is ​​broken by high-energy mechanical crushing (e.g., ball milling) to rupture the sealed pores and release the gaseous hydrocarbons sealed inside into the headspace of the sealed container; finally, the headspace gas is sampled and chromatographically analyzed to restore and quantify the in-situ gas composition.

[0018] The technical solution may specifically include the following steps: Step 1: Select a blocky shale sample and determine its density using a standard method (e.g., a fully automated inclusion density analyzer), and record it as . .

[0019] Step Two: Mechanically crush the block sample and sieve it to obtain particles within a specific particle size range, such as 3-8 mesh (2.36-7.93 mm). Weigh the sample that meets this particle size requirement and record the mass as follows: The choice of this particle size range aims to balance the efficiency of subsequent thermal desorption with avoiding premature destruction of the closed pores during the pretreatment stage.

[0020] Step 3: The mass obtained in Step 2 is The sample is placed in a heat treatment device (e.g., a muffle furnace) and baked at a specific temperature of 90°C-110°C, preferably 100°C in this embodiment, for a duration of 1-3 hours, preferably 2 hours in this embodiment. This step is the purification stage. The temperature and duration are used to allow residual hydrocarbons (especially gaseous hydrocarbons) adsorbed on the interconnected pores and particle surfaces to be fully thermally desorbed and released, while the temperature is insufficient to cause thermal cracking of the rock matrix or large-scale rupture of closed pores.

[0021] Step 4: Quickly transfer the sample after the thermal desorption treatment in Step 3 to a container with a known total internal volume. The grinding jar was placed in a sealed ball mill. The jar was immediately sealed, and a vacuum pump was connected through its valve to evacuate the inside of the jar, removing any pre-existing air. Subsequently, the vacuum pump was disconnected, and an inert gas source (e.g., high-purity helium) was connected to fill the jar to atmospheric pressure. Helium, being chemically inert and producing no interfering signal in gas chromatography analysis, provides a clean background environment for subsequent gas release and detection.

[0022] Step 5: Fix the sealed ball mill jar prepared in Step 4 onto the ball mill and perform mechanical ball milling at a set high speed (e.g., 500 r / min-700 r / min, preferably 600 r / min in this embodiment) for 1-3 hours, preferably 2 hours in this embodiment. This step is the release stage. The mechanical impact and shearing forces provided by the high-energy ball milling refine the sample particle size (e.g., to 200 mesh, <75 μm). This process strongly disrupts the shale matrix, causing the sealed pores to rupture and releasing the gaseous hydrocarbon components sealed inside into the headspace of the ball mill jar.

[0023] Step Six: After ball milling, use a gas-tight syringe to extract a predetermined volume (e.g., 1 mL) of gas sample from the headspace of the milling jar through the injection septum. Inject this gas sample into a thermal evaporative gas chromatograph for gaseous hydrocarbon component detection, and obtain the chromatographic peak area corresponding to each component compound, denoted as . .

[0024] Step 7: Based on the test results of Step 6, quantitative calculations are performed using the external standard method.

[0025] First, calculate the amount of the injected standard gas (e.g., n-butane). (Unit: μg) Subsequently, based on standard gas content and their corresponding chromatographic peak areas Calculate the absolute content of each gaseous hydrocarbon component released from the sealed pores in the sample. (Unit: mg / g).

[0026] Example 1: Complete Test Procedure for Gas Composition in Closed Pores of Shale Sample preparation and characterization: Sample selection and density measurement: This invention uses representative massive shale samples for analysis. The representativeness of the samples is achieved by selecting intact core segments from the strata with stable lithological characteristics, avoiding fracture zones. Before any physical or chemical treatment, the selected massive shale samples are first subjected to density determination. Density determination is performed using a fully automated encapsulated density analyzer module, which can accurately determine the skeletal density of the sample using a gas expansion method (e.g., helium expansion method), i.e., the density of the solid material after deducting all pore volumes. This density value is denoted as . The unit is g / cm³ 3 This will be used to correct headspace volume in subsequent quantitative calculations of gaseous hydrocarbon content. Accurate density values ​​are fundamental to ensuring the accuracy of quantitative calculations.

[0027] Sample crushing and screening: After density determination, the blocky shale sample is mechanically crushed. Crushing can be achieved through manual hammering, a jaw crusher, or a hammer crusher module. Over-grinding should be avoided during crushing to preserve the closed pores in the sample to the greatest extent possible. The crushed sample is then sieved through a standard series of sieves to obtain particles within the target particle size range. The preferred particle size range for this invention is 3-8 mesh (corresponding to a particle size of approximately 2.36–7.93 mm). This particle size range is technically reasonable: smaller particle sizes (e.g., less than 8 mesh) facilitate effective gas escape from interconnected pores during subsequent thermal desorption, improving purification efficiency; simultaneously, particles of this size are large enough to ensure the integrity of most closed pores, preventing unintended rupture of closed pores during the initial crushing stage, thus creating conditions for subsequent controlled release. After sieving, the total mass of particles meeting the 3-8 mesh size requirement is weighed and recorded as _____. The unit is g. This mass This will serve as the basis for subsequent quantitative calculations. Thermal desorption of gases through interconnected pores: The obtained quality is Shale sample particles with a particle size of 3-8 mesh are placed in a heat treatment device, preferably a muffle furnace module in this embodiment.

[0028] The heat treatment equipment is started and its internal temperature is set and maintained at a preferred temperature of 100°C. This temperature parameter of 100°C is chosen because it is higher than the boiling point of light gaseous hydrocarbons (such as methane, ethane, etc.) that may be present in shale, and is sufficient to provide energy for the thermal desorption of hydrocarbon molecules adsorbed on the outer surface of the particles and the inner wall of the interconnected pore network.

[0029] Meanwhile, this temperature is below the initiation temperature of thermal decomposition of organic matter (such as kerogen) in the rock matrix and does not cause significant thermal expansion stress in the rock. This temperature control ensures that no new gaseous hydrocarbons are generated due to pyrolysis during the removal of gas from interconnected pores, nor does it cause unexpected rupture of closed pores due to thermal stress, thus guaranteeing the uniqueness of the source of the subsequent gas to be measured.

[0030] The sample is treated at a set time, preferably at a constant temperature of 100°C, and in this embodiment, it is preferably 2 hours. This treatment duration ensures that the quality is... The sample particles (i.e., 3-8 mesh size) can reach complete thermal equilibrium, providing the necessary time for the desorbed gas components to fully diffuse and escape from the interconnected pore network to the external environment.

[0031] After this step, the light hydrocarbon components present in the interconnected pores and particle surfaces are selectively removed, resulting in a pretreated sample for subsequent gas release from the closed pores.

[0032] Release of gas from closed pores: Experimental setup and environmental replacement: The shale sample (mass) after thermal desorption treatment The material is rapidly transferred to a sealed grinding jar module. This sealed grinding jar module has a known total internal volume. It is made of chemically stable materials (such as stainless steel) to ensure that it does not react with the sample or gas during ball milling.

[0033] The sealed ball mill jar module has a highly airtight structure and is equipped with two ball valves (referred to as the ball valve module and the ball valve module, respectively) that can communicate with the outside. In addition, the ball mill jar module is equipped with a sample injection septum device module, which (for example, mounted on the ball valve module) is used to perform gas sampling by puncture while maintaining the airtightness of the jar.

[0034] After the sample is loaded into and sealed in the ball mill jar module, an environmental replacement operation is performed. First, the ball valve module is closed, and then connected to the vacuum pump module. The vacuum pump module is then started to replace the internal volume of the ball mill jar module. A vacuum process is performed to remove residual air (especially oxygen) from the container, preventing it from interfering with subsequent gas chromatography detection.

[0035] After achieving the predetermined vacuum level, disconnect the vacuum pump module and connect the ball valve module to a high-purity helium (He) gas source module. Fill the tank with helium until the pressure inside is slightly higher than atmospheric pressure. Then, briefly open the ball valve module to allow the pressure inside the tank to equalize with the external atmospheric pressure, and then close both the ball valve module and the gas source module. This step ensures that subsequent ball milling is carried out in a controlled, inert (helium) atmospheric pressure atmosphere.

[0036] High-energy ball milling: The sealed ball mill jar module, after undergoing the aforementioned environmental replacement, along with its internal grinding ball module (e.g., stainless steel grinding balls) and the sample, are symmetrically mounted on a planetary or high-energy ball mill module. To ensure stable equipment operation, a balancing jar is used to adjust the balance.

[0037] Start the ball mill module and set the operating parameters. In this embodiment, the preferred rotation speed is 600 r / min. This rotation speed is selected as a high-energy ball milling parameter to apply high-frequency impact and shear forces to the shale sample particles through the grinding ball module.

[0038] The ball milling process is preferably set to a duration of 2 hours. This processing time ensures that the 3-8 mesh sample particles are sufficiently ground to a finer particle size (e.g., refined to 200 mesh, <75 μm).

[0039] Under this high-energy mechanical action, the rock matrix structure of the shale sample is destroyed, and the crystals and cement are crushed, causing the originally physically sealed pores inside the rock matrix to rupture. After the pores rupture, the original gaseous hydrocarbon components sealed in them are released and diffuse into the headspace gas (i.e., helium background gas) of the sealed ball mill jar module, forming the mixed gas to be tested.

[0040] Capture and analysis of gas components: After the ball milling process is completed, allow the sealed ball milling jar module to stand. At the sample inlet septum device module of the ball milling jar module, first open the ball valve (e.g., ball valve module) connected to the septum device module.

[0041] This invention employs a gas-tight syringe module for gas sampling. The needle of this gas-tight syringe module pierces the sample inlet septum module and extracts a predetermined volume of gas sample from the headspace of the grinding jar module; in this embodiment, 1 mL is preferably extracted. After extraction, the gas-tight syringe module is removed, and the sample inlet septum module automatically seals itself due to its elasticity, maintaining a sealed state for the remaining gas inside the jar. This sampling process ensures that the sample gas does not come into contact with the external atmosphere from release to capture, avoiding component loss or contamination.

[0042] Then, the needle of the sealed gas sampling needle module, containing 1 mL of gas sample, was inserted into the injection port of a thermal evaporative gas chromatograph module. The gas sampling needle module was then pushed to inject the gas sample into the chromatograph module.

[0043] The gaseous sample is vaporized in the heated injection port of the chromatography module and carried into the chromatographic column by a carrier gas (e.g., helium). Each gaseous hydrocarbon component (e.g., (e.g., flame ionization detectors) are separated in a chromatographic column based on their physicochemical properties and sequentially enter a detector (e.g., flame ionization detector FID) in chronological order.

[0044] The data processing system of the chromatograph module receives the detector signal and converts it into a chromatogram. This chromatogram displays the chromatographic peaks corresponding to different gaseous hydrocarbon components. The data processing system automatically or manually integrates and calculates the peak area for each target compound; this peak area value is denoted as... Its unit is pA·s. The value will serve as the raw data for the next step of quantitative calculation.

[0045] For the quantitative calculation of gaseous hydrocarbon content, this invention employs the external standard method to quantitatively calculate the content of each detected gaseous hydrocarbon component. In this embodiment, n-butane is preferably used as the standard gas. First, the content of the standard gas (n-butane) injected into the chromatograph under standard conditions (STP) is calculated. The unit is The calculation was performed using a separate gas chromatographic analysis (not shown), in which a known volume was used. Quantitative circulatory injection of known concentration The standard gas. The calculation formula is: ; In the formula, This indicates the volume of the quantitative loop in a gas chromatograph, in units of... ; The concentration of the standard gas is expressed in ppm; 58 represents the relative molecular mass of n-butane; 22.4 represents the molar volume of the gas under standard conditions. .

[0046] Secondly, based on the standard gas content and the corresponding chromatographic peak area measured in chromatographic analysis. Calculate the absolute content of gaseous hydrocarbons in each component of the sample gas. The unit is mg / g. The calculation formula is: ; In the formula, This indicates the absolute content of a certain component of gaseous hydrocarbons in a sample, expressed in mg / g. This indicates the chromatographic peak area of ​​the component compound in the sample, expressed in pA·s; This indicates the content of injected standard gas (n-butane) calculated according to the above formula, in μg; This represents the chromatographic peak area obtained when measuring standard gas, expressed in pA·s; This represents the total internal volume of the sealed ball mill jar module d, in cm³. 3 ; This indicates the total mass of the ball-milled shale sample, expressed in grams. This indicates the density of the shale sample, expressed in g / cm³. 3 .

[0047] In this calculation formula, the expression The response factor used to determine a standard gas is the gas mass per unit peak area. .

[0048] expression The mass (in μg) of the target component contained in 1 mL of extracted gas sample was calculated.

[0049] expression Used to calculate the gas headspace volume inside the sealed ball mill jar module d, i.e., after deducting the shale sample (mass). ,density After considering the volume occupied by the skeleton, the actual volume of the gas inside the tank (in cm³) 3 .

[0050] By multiplying the mass of the component in 1 mL of sample by the headspace volume (the sampling volume is assumed to be 1 mL, i.e., 1 cm³),... 3， Therefore, in the calculation ( / 1cm 3 Simplified to ( The total mass of the component released into the entire headspace (in μg) is obtained.

[0051] Finally, divide the total mass by the total mass of the shale sample. The absolute content of the gaseous hydrocarbon component in a unit mass sample was obtained. .

Claims

1. A method for testing the composition of gas in closed pores of shale under in-situ conditions, characterized in that, Includes the following steps: (1) Select blocky shale samples and determine their shale sample density; (2) Crush the blocky shale sample to the target particle size range and weigh the total mass of the sample that conforms to the target particle size range; (3) Perform thermal desorption treatment on the sample weighed in step (2); (4) Place the sample processed in step (3) into a sealed ball mill jar with an internal total volume, evacuate the sealed ball mill jar, and then fill it with inert gas to atmospheric pressure. (5) Fix the sealed ball mill jar onto the ball mill for ball milling; (6) Extract the gas from the sealed ball mill jar after the treatment in step (5) and perform gas chromatography analysis to obtain the chromatographic peak area of ​​the target component compound. (7) Based on the chromatographic peak area of ​​the target component compound, the content of standard gas, the standard chromatographic peak area, the total internal volume of the sealed ball mill jar, the total mass of the sample, and the density of the shale sample, calculate the content of gaseous hydrocarbons in the sealed pores of the sample.

2. The method for testing the composition of gas in closed pores of shale under in-situ conditions according to claim 1, characterized in that, The target particle size range in step (2) is 3-8 mesh.

3. The method for testing the composition of gas in closed pores of shale under in-situ conditions according to claim 1, characterized in that, The temperature of the thermal desorption treatment in step (3) is 90℃-110℃, and the treatment time is 1-3 hours.

4. The method for testing the composition of gas in closed pores of shale under in-situ conditions according to claim 1, characterized in that, The inert gas introduced in step (4) is helium.

5. The method for testing the composition of gas in closed pores of shale under in-situ conditions according to claim 1, characterized in that, In step (5), the ball milling speed is 500 r / min-700 r / min, and the processing time is 1-3 hours.

6. The method for testing the composition of gas in closed pores of shale under in-situ conditions according to claim 1, characterized in that, The sealed ball mill jar in step (4) is equipped with two ball valves that can communicate with the outside world and a sample injection septum device.

7. The method for testing the composition of gas in closed pores of shale under in-situ conditions according to claim 6, characterized in that, In step (6), the gas is extracted using a sealed gas extraction needle, which punctures the sample injection septum device to extract the sample.

8. The method for testing the composition of gas in closed pores of shale under in-situ conditions according to claim 1, characterized in that, The gas chromatography analysis in step (6) is thermal evaporative gas chromatography analysis.

9. The method for testing the composition of gas in closed pores of shale under in-situ conditions according to claim 1, characterized in that, In step (1), the density of the shale sample was determined using a fully automated encapsulation density analyzer.

10. The method for testing the composition of gas in closed pores of shale under in-situ conditions according to claim 1, characterized in that, The standard gas in step (7) is n-butane.

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