A method for preparing long core based on cementation

Abstract

This invention relates to a method for preparing long cores based on a cementation method, comprising: (1) obtaining cement formulations for different permeabilities; (2) obtaining a set of natural cores and testing their permeability respectively; (3) preparing a core splicing device, which includes a metal base plate and several external fasteners, the external fasteners being fixedly installed on the metal base plate; (4) splicing the natural cores within the external fasteners: placing the first core into the external fastener; taking the average permeability of the first and second cores to prepare a cement, spreading it evenly on the upper surface of the first core, placing the second core on the cement, and then fitting the second external fastener; repeating the above steps until all cores are spliced; removing all external fasteners along with the cores, slowly pushing out the long core, and placing it in a clamp. This invention splices short natural cores into long cores using quartz sand epoxy resin cement, giving the long cores a certain strength, and making the seepage situation in the cores closer to the actual strata.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas field development engineering, and specifically relates to an experimental method for splicing and preparing long rock cores. Background Technology

[0002] In conventional core displacement experiments, short cores are often used to evaluate oil displacement efficiency. However, this evaluation method has significant limitations when applied to actual reservoirs. For example, in gas drive experiments, directly using short cores results in shorter gas exposure times, smaller gas diffusion ranges, severe fingering, and fails to meet the effective interaction distance required for hydrocarbon miscibility. With the development of reservoir physical model similarity theory, short cores can no longer meet the similarity conditions of laboratory physical simulation experiments; using long cores is more effective in reflecting actual reservoir conditions.

[0003] Due to limitations in core sampling techniques, current conventional methods for long core displacement experiments employ the harmonic averaging method, sequentially splicing short cores to replace long cores. For example, the invention patent "A High-Temperature and High-Pressure X-CT Scanning Long Core Displacement Device for Analyzing Crude Oil Distribution in Oil and Gas Reservoirs" (CN112782205A) arranges cores and loads them sequentially into heat-shrink tubing, with filter paper applied between adjacent short cores to reduce end-effects. However, long cores produced by this method lack sufficient strength, and the pressure field distribution at the splicing end is complex. Furthermore, the filter paper may shift during the experiment, affecting fluid flow and creating "dead oil zones" at the core splicing locations, reducing the sweep efficiency of the injected fluid. Some researchers also use artificial cores instead of natural long cores. For instance, the invention patent "A Method and Device for Preparing Artificial Cores for Petroleum Geological Exploration" (CN115112458A) uses epoxy resin and quartz sand mechanically mixed with clay minerals to prepare artificial long cores. However, the wettability and pore structure uniformity of artificial long cores are difficult to match those of real formations. Furthermore, in chemical flooding experiments, the oil displacement efficiency of artificial long cores is significantly higher than that of natural cores. Therefore, it is crucial to develop a long core assembly method that more closely approximates the actual reservoir conditions. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing long cores based on cementation, which splices natural short cores into long cores using quartz sand and epoxy resin cement, giving the long cores a certain strength and making the seepage situation in the cores closer to the actual strata, thus providing a new physical model for core flow experiments.

[0005] To achieve the above technical objectives, the present invention adopts the following technical solution.

[0006] A method for preparing long cores based on cementation includes the following steps:

[0007] (1) Prepare a cementitious material, which is composed of quartz sand, epoxy resin and curing agent. That is, quartz sand of different mesh size is mixed with epoxy resin and curing agent in different proportions to prepare several small plunger cores, and their permeability is tested to obtain cementitious material formulations with different permeabilities.

[0008] (2) Obtain a set of natural rock cores and test their permeability respectively;

[0009] (3) Prepare a core splicing device, which includes a metal base plate and several external fasteners. The external fasteners are fixedly installed on the metal base plate. The metal base plate is provided with cylindrical pin holes in the 0°, 90°, 180° and 270° directions. The external fasteners are cylindrical structures made of plexiglass material. Four cylindrical pin holes are also provided on the upper and lower end faces of the external fasteners. Their positions and sizes are the same as the pin holes of the metal base plate. The external fasteners are connected to the metal base plate and to each other by pins.

[0010] (4) The natural rock cores are spliced ​​together within the external fasteners, as follows:

[0011] ① Sort and number the natural rock cores according to the harmonic averaging method to determine the order in which they are placed;

[0012] ② Insert the pin into the pin hole of the metal base plate, put on the first outer fastener, put the first rock core into the outer fastener, the length of the rock core must be less than the length of the outer fastener, confirm that there is no gap between the bottom end of the rock core and the metal base plate, and the outer wall of the rock core is tightly attached to the inner surface of the outer fastener.

[0013] ③ Take the average permeability of the first core and the second core to be placed, and obtain the average permeability. Based on the cement formulas obtained in step (1) for different permeabilities, select quartz sand, epoxy resin and curing agent according to the average permeability and mix them in the corresponding proportions to prepare the cement.

[0014] ④ Spread a layer of cement evenly on the upper surface of the first core sample;

[0015] ⑤ Place the second core on top of the cementitious material, insert a pin into the pin hole on the upper end face of the first outer fastener, put on the second outer fastener, and then compact the second core.

[0016] ⑥ After the cement has cured, repeat steps ③ to ⑤ until all core samples are joined together;

[0017] ⑦ Remove all external fasteners along with the core and place them in a constant temperature chamber to allow the cement to fully solidify. Use a rubber mallet to gently tap the external fasteners to separate the cement from the core at the joint. Then slowly push out the long core and place it in the clamp.

[0018] (5) The preparation of long core samples is complete.

[0019] Furthermore, coarse sand of 35-70 mesh and fine sand of 70-120 mesh are mixed in different proportions.

[0020] Furthermore, the mass ratio of epoxy resin to curing agent is 1:1.

[0021] Furthermore, when the formation temperature is below 60°C, a polyamide resin curing agent is selected; when the formation temperature is above 60°C, a methyltetrahydrophthalic anhydride curing agent is selected.

[0022] Furthermore, the inner surface of the outer fastener is in contact with the rock core, and the outer surface is marked with graduations.

[0023] Furthermore, the harmonic averaging method refers to arranging the cores according to the harmonic averaging method (invention patent "Water Injection Development Method for Abandoned Condensate Gas Reservoirs" CN102953717A). The process is as follows: calculate the harmonic average permeability of the core, then compare this value with the permeability of all cores, and place the core with the permeability closest to this value in the first position, and then deduce the core arrangement order in the same way.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) Because the present invention uses a cement with a permeability similar to that of short cores to splice long cores, compared with splicing long cores with filter paper, a more stable flow field is achieved in spliced ​​long cores;

[0026] (2) The cementitious material has high strength after solidification, and the spliced ​​long rock core has a certain strength, avoiding the flow field changes caused by the movement of filter paper due to internal pressure in the traditional filter paper splicing long rock core experiment;

[0027] (3) The use of strong spliced ​​long cores reduces the requirements for the use of core straighteners in long core holders. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the core splicing device.

[0029] In the diagram: 1-Metal base plate; 2-Natural rock core; 3, 4-External fasteners; 5, 6-Cylindrical pins; 7-Cylindrical pin holes. Detailed Implementation

[0030] The present invention will be further described below with reference to the accompanying drawings and examples to enable those skilled in the art to understand the invention. However, it should be understood that the present invention is not limited to the specific embodiments described herein. For those skilled in the art, any variations that fall within the spirit and scope of the invention as defined and determined by the appended claims are all within the scope of protection.

[0031] Figure 1The specific parameters of the core assembly device are as follows: The metal base plate 1 is made of aluminum alloy and is a cuboid with dimensions of 25cm × 25cm × 1cm. Four cylindrical pin holes 7, each φ0.5 × 1.0cm, are drilled at a distance of 1.775cm from the center point in the 0°, 90°, 180°, and 270° directions. The cylindrical outer fittings 3 and 4 are made of plexiglass with graduations on their outer surfaces for easy observation and measurement of the cementitious filler. They have an inner diameter of φ2.55cm, a wall thickness of 1cm, and a height of 8.5cm. Four cylindrical pin holes 7, each φ0.5 × 1.0cm, are also drilled on the upper and lower ends of the outer fittings, corresponding to the pin holes on the metal base plate 1. The remaining outer fittings are the same size as outer fitting 3. The outer fittings are connected to the metal base plate and to each other via pins 5 and 6. The natural core 2 is bonded within the outer fittings.

[0032] The specific steps for core assembly are as follows:

[0033] (1) Sort and number the natural short core groups according to the harmonic averaging method to determine the placement order;

[0034] (2) Insert a φ0.5×2.0cm pin into the pin hole of the metal base plate and put on the outer fastener. Place the first core into the outer fastener and make sure that there is no gap between the bottom end of the core and the base plate, and that the outer wall of the core is as close as possible to the inner surface of the outer fastener;

[0035] (3) Take the arithmetic average of the permeability of the first core and the second core to be placed, and prepare the cementitious material according to this average permeability: take the corresponding proportion of coarse sand and fine sand to mix into base sand, and place the base sand, epoxy resin, polyamide resin curing agent and stirring vessel in a 35℃ constant temperature box for 10 minutes to ensure the fluidity of epoxy resin.

[0036] (4) Place the base sand and the mixing vessel on the electronic balance and zero it. Slowly pour in the epoxy resin and the same mass of curing agent to the required mass and make them fully contact and stir. Take a certain amount (0.5 ml) of cement and pile it evenly on the surface of the first core to a thickness of about 1 mm. Use a thin glass rod to stir the cement to make it spread evenly.

[0037] (5) Slowly place the second core on top of the cementitious material, put a pin on the top of the outer fastener, put on the new outer fastener, and then use a wooden stick to gently compact the second core.

[0038] (6) After waiting 10 minutes for the cement to initially solidify, repeat steps (3) to (5) to continue splicing subsequent cores. The softening temperature of this cement after solidification is about 60°C. If the formation conditions are higher than 60°C, the polyamide resin curing agent in the formula can be replaced with methyltetrahydrophthalic anhydride curing agent, and after step (5), the device can be placed in an 80°C constant temperature oven to wait for solidification.

[0039] (7) Remove all the outer fasteners along with the core, tap the outer surface of the outer fasteners lightly to separate the cement at the connection between the inner surface and the core, and then slowly push the long core out and place it in the clamp.

[0040] Example (Description of the core preparation stage for a set of long core displacement experiments)

[0041] A method for preparing long cores based on cementation splicing is as follows:

[0042] (1) The five core samples were sorted according to the harmonic average method, and their parameters were as follows: Core #1 had a permeability of 569.52 mD and a length of 8.89 cm; Core #2 had a permeability of 453.8 mD and a length of 8.2 cm; Core #3 had a permeability of 398.29 mD and a length of 8.9 cm; Core #4 had a permeability of 1100.26 mD and a length of 8.77 cm; Core #5 had a permeability of 1204.57 mD and a length of 7.7 cm.

[0043] (2) Insert the pin into the pin hole of the metal base plate and put on the outer fastener. Put the No. 1 core into the outer fastener, and the core should fit against the inner surface of the outer fastener;

[0044] (3) The average permeability of core #1 and core #2 was calculated to be 511.66 mD. The cementitious material was prepared according to the formula in Table 1. 14g of coarse sand and 7g of fine sand were mixed to form base sand. The base sand, epoxy resin, polyamide resin curing agent and stirring vessel were placed in a 35℃ constant temperature oven for 10 min to ensure the flowability of epoxy resin.

[0045] Table 1. Permeability of small plunger cores prepared with different cementitious formulations

[0046]

[0047] (4) Place the base sand and mixing vessel on the electronic balance and zero it. Slowly pour in 3g of epoxy resin and 3g of polyamide resin curing agent and stir them thoroughly. Use a syringe to draw a certain amount of cementitious material and pile it evenly on the surface of the No. 1 core.

[0048] (5) Slowly place the No. 2 core vertically above the cementitious material, put the pin on the upper end of the outer fastener, put on the new outer fastener, and then use a wooden stick to gently compact the core.

[0049] (6) After waiting 10 minutes for the cement to initially solidify, continue splicing the subsequent cores according to steps (3) to (5);

[0050] (7) After all the cores are assembled, wait 30 minutes, then remove all the outer fasteners along with the cores and place them in a constant temperature chamber at 35°C for 72 hours to allow the cement to fully solidify. Gently tap the outer surface of the outer fasteners with a rubber mallet to separate the cement from the inner surface of the core. Then slowly push out the long core. The long core fabrication is now complete.

Claims

1. A method for preparing long cores based on cementation, comprising the following steps: (1) Prepare cementitious material, which is composed of quartz sand, epoxy resin and curing agent. That is, quartz sand of different mesh size is mixed with epoxy resin and curing agent in different proportions to prepare several small plunger cores, and their permeability is tested to obtain cementitious material formulations with different permeability. (2) Obtain a set of natural rock cores and test their permeability respectively; (3) Prepare a core splicing device, which includes a metal base plate and several external fasteners. The external fasteners are fixedly installed on the metal base plate. The metal base plate is provided with cylindrical pin holes in the 0°, 90°, 180° and 270° directions. The external fasteners are cylindrical structures made of plexiglass material. Four cylindrical pin holes are also provided on the upper and lower end faces of the external fasteners. Their positions and sizes are the same as the pin holes of the metal base plate. The external fasteners are connected to the metal base plate and to each other by pins. (4) The natural rock core is spliced ​​together within the external fastener, as follows: ① Sort and number the natural rock cores according to the harmonic averaging method to determine the order in which they are placed; ② Insert the pin into the pin hole of the metal base plate, put on the first outer fastener, put the first rock core into the outer fastener, the length of the rock core must be less than the length of the outer fastener, confirm that there is no gap between the bottom end of the rock core and the metal base plate, and the outer wall of the rock core is in close contact with the inner surface of the outer fastener. ③ Take the average permeability of the first core and the second core to be placed, and obtain the average permeability. Based on the cement formulas obtained in step (1) for different permeabilities, select quartz sand, epoxy resin and curing agent according to the average permeability and mix them in the corresponding proportions to prepare the cement. ④ Spread a layer of cement evenly on the upper surface of the first core sample; ⑤ Place the second core on top of the cementitious material, insert a pin into the pin hole on the upper end face of the first outer fastener, put on the second outer fastener, and then compact the second core. ⑥ After the cement has cured, repeat steps ③ to ⑤ until all core samples are joined together; ⑦ Remove all external fasteners along with the core and place them in a constant temperature chamber to allow the cement to fully solidify. Use a rubber mallet to gently tap the external fasteners to separate the cement from the core at the joint. Then slowly push out the long core and place it in the clamp. (5) The preparation of long core samples is complete.

2. The method for preparing long cores based on cementation as described in claim 1, characterized in that, Quartz sand is screened, and coarse sand of 35-70 mesh and fine sand of 70-120 mesh are mixed in different proportions.

3. The method for preparing long cores based on cementation as described in claim 1, characterized in that, The mass ratio of epoxy resin to curing agent is 1:

1.

4. The method for preparing long cores based on cementation as described in claim 1, characterized in that, When the formation temperature is below 60℃, polyamide resin curing agent should be selected; when the formation temperature is above 60℃, methyltetrahydrophthalic anhydride curing agent should be selected.

5. The method for preparing long cores based on cementation as described in claim 1, characterized in that, The inner surface of the outer fastener is in contact with the rock core, and the outer surface is marked with graduations.

6. The method for preparing long cores based on cementation as described in claim 1, characterized in that, The harmonic average method refers to numbering and arranging the cores according to the harmonic average method. The process is as follows: calculate the harmonic average permeability of the core, then compare this value with the permeability of all cores, and place the core with the permeability closest to this value in the first position, and then deduce the core arrangement order in the same way.

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