Low-permeability artificial rock core and manufacturing method thereof

Abstract

The invention relates to the field of oil-gas exploration and development displacement testing, and provides a low-permeability artificial rock core and a manufacturing method thereof.The manufacturing method comprises the steps of S1, quartz sand particle screening, S2, rock core framework particle combination, S3, rock core framework particle cementing agent preparation, S4, rock core raw material mixing, S5, high-pressure pressing forming, S6, constant-temperature and constant-pressure reaction, S7, cooling and depressurization, S8, temperature-controlled heating curing and S9, rock core cutting. The low-permeability artificial rock core with low porosity and low permeability is prepared under the combined action of double factors of temperature and pressure, the heat-resistant stability and strength reliability of the rock core are ensured by adopting a thermosetting polymer powder cementing agent, and meanwhile, the prepared rock core has point-point and point-surface contact and cementing forms; a micro-pore structure is similar to that of a natural core, so that the problem of insufficient natural core sources in a low-permeability core flow experiment is solved.

Description

Technical Field

[0001] The present invention relates to the field of displacement tests for oil and gas exploration and development, and more specifically, to a low-permeability artificial core and a method for manufacturing the same. Background Art

[0002] Cores are an important experimental component in indoor flow experiments such as enhanced oil recovery techniques and the study of seepage mechanisms in porous media. Due to limitations in sources, quantity, and price, natural cores also have the characteristic of strong heterogeneity, making it difficult to meet the requirements of controlling single variables and using a large amount in experiments. Therefore, preparing low-permeability artificial cores with physical properties similar to natural cores is an inevitable choice for simulation experiments. After years of development, artificial cores mainly have three preparation techniques: quartz sand filling, aluminum phosphate sintering, and epoxy resin cementing. Among them, the artificial cores prepared by the epoxy resin cementing method have a relatively high similarity in physical properties to natural cores, good repeatability, and simple manufacturing processes, and are widely used. However, with the development of unconventional oil and gas such as low permeability and tightness becoming the focus of development, there is an urgent need to develop artificial cores with low porosity and low permeability for indoor flow experiment research. When using the epoxy resin cementing method to prepare artificial cores with low porosity and low permeability characteristics, generally, the porosity and permeability of the cores are reduced by increasing the amount of epoxy resin cementing agent. However, the excessive cementing agent used is generally liquid epoxy resin, which is prone to blocking throats and pores during the manufacturing process, which is very different from the "point-to-surface, surface-to-surface contact cementing" method of natural cores. At the same time, due to the relatively weak heat resistance of epoxy resin itself, when the amount used is relatively large, it will have a certain impact on the strength and heat resistance of the cores, and there are certain drawbacks. Summary of the Invention

[0003] The purpose of the present invention is to provide a method for manufacturing a low-permeability artificial core, which prepares a low-permeability artificial core with low porosity and low permeability by the combined action of temperature and pressure factors. The manufacturing process is simple and highly repeatable, meeting the needs of simulation experiments.

[0004] Another purpose of the present invention is to provide a low-permeability artificial core. Using a thermosetting polymer powder cementing agent not only ensures the heat resistance stability and strength reliability of the core, but also the core manufactured has "point-to-point" and "point-to-surface" contact and cementing forms, and the microscopic pore structure is similar to that of natural cores, solving the problem of insufficient sources of natural cores in low-permeability core flow experiments.

[0005] To solve the above technical problems, the technical solution adopted in the present application is as follows:

[0006] On the one hand, an embodiment of the present application provides a method for fabricating a low-permeability artificial core, comprising the following steps: S1. Screening of quartz sand particles: Screening quartz sand coarse material using a standard sieve to obtain standard quartz sand base materials with different mesh numbers; S2. Combination of core skeleton particles: According to different core physical property parameters, mixing two or three different mesh numbers of standard quartz sand in different mass ratios to obtain core skeleton particles; S3. Preparation of core skeleton particle binder: Respectively putting high molecular epoxy resin, curing agent, and curing accelerator into a high-speed stirrer and fully mixing to obtain a binder; S4. Mixing of core raw materials: Putting the core skeleton particles and the binder into a rotary stirrer and mixing evenly to obtain mixture A; S5. High-pressure pressing and forming: Using an atomizer to atomize and spray an organic solution to wet mixture A into a wet state to obtain mixture B, and evenly spreading mixture B in the core forming mold cavity, then setting the target pressing pressure for pressing to obtain mixture C; S6. Isothermal and isobaric reaction: When the pressing pressure reaches the target set pressure, maintaining constant-pressure pressing, and simultaneously performing heat treatment on mixture C; S7. Cooling and pressure reduction: Keeping the heating temperature around the mold unchanged, waiting until the heating temperature at the bottom of the mold and the pressing pressure drop to room temperature and atmospheric pressure, then removing the mold and taking out the pressed and formed sample; S8. Temperature-controlled heating and curing: Putting the taken-out sample into a temperature-adjustable constant-temperature oven, setting a heating and curing temperature program for gradient heating and insulation to cure the sample into rock; S9. Core cutting: Taking cores from the heated and cured rock samples for physical property testing, and simultaneously cutting the heated and cured cores into different shapes according to experimental needs to obtain low-permeability artificial cores, preparing for subsequent flow experiments such as reservoirs and seepage.

[0007] In some embodiments of the present application, the above-mentioned organic solution includes, but is not limited to, one or more of ethyl acetate solution and alcohol solution.

[0008] In some embodiments of the present application, in the above step S1, the mesh number ranges of the quartz sand base materials are 30 - 70 mesh, 140 - 260 mesh, and 300 - 400 mesh respectively.

[0009] In some embodiments of the present application, in the above step S3, the binder includes 6 - 10 parts by mass of high molecular epoxy resin, 0.72 - 1.2 parts of curing agent, and 0.18 - 0.3 parts of curing accelerator.

[0010] In some embodiments of the present application, in the above step S3, the high molecular epoxy resin is high-purity linear thermosetting epoxy resin powder particles, the curing agent is heat-curing latent curing agent fine powder particles, and the curing accelerator is imidazole and its derivative fine powder particles.

[0011] In some embodiments of the present application, in the above step S4, a micro drum-type mixer is used for stirring and mixing, and the stirring and mixing duration is 20 - 60 min.

[0012] In some embodiments of the present application, in the above step S5, the pressing pressure is 10 - 21 MPa.

[0013] In some embodiments of the present application, in the above step S5, the pressing pressure is 21 MPa.

[0014] In some embodiments of the present application, in the above step S6, during heat treatment, a heating sheet is used to perform gradient heating and heat preservation on the peripheral surface and bottom of the core forming mold, the heating temperature is 40 - 80 °C, and the heat preservation duration is 5 - 8 h.

[0015] In some embodiments of the present application, in the above step S8, the heating and curing temperature is 60 - 200 °C, and the heating and curing duration is 8 - 12 h.

[0016] On the other hand, an embodiment of the present application provides a low-permeability artificial core prepared by the above manufacturing method.

[0017] In some embodiments of the present application, the porosity of the above low-permeability artificial core is 10 - 15%, and the gas permeability is 2 - 30 mD.

[0018] Compared with the prior art, the embodiments of the present invention have at least the following advantages or beneficial effects:

[0019] 1. Provide a new method for low-permeability artificial cores. All raw materials used in the manufacturing process are solid particles, without the participation of liquid materials, which is safe, environmentally friendly, has a simple preparation process, and high repeatability.

[0020] 2. Compared with the conventional liquid epoxy resin cementing preparation method, the present invention uses a solid particle cementing agent, avoiding the blockage and filling of pore throats by the epoxy resin cementing agent during the production process, and realizing the contact and cementing between particles in the form of "point-point" and "point-surface". The pore throat microstructure is similar to that of natural cores.

[0021] 3. Different from the conventional room temperature pressing process for preparing low-porosity and low-permeability artificial cores, the present invention uses the combined action of temperature and pressure to prepare low-permeability artificial cores, which can avoid achieving low porosity and low permeability by using excessive cementing agents, thereby overcoming the problems of insufficient heat resistance and strength of the cores caused by excessive use of organic cementing agents. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other relevant drawings can also be obtained based on these drawings.

[0023] Figure 1 Schematic flow chart of a method for manufacturing a low-permeability artificial core provided for an embodiment;

[0024] Figure 2 Physical diagram of a low-permeability artificial core provided for an embodiment. Specific embodiments

[0025] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. For those not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. Those reagents or instruments not specified by the manufacturer can all be obtained as conventional products through commercial purchase.

[0026] It should be noted that, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to specific embodiments.

[0027] The features and properties of the present invention will be further described in detail below in conjunction with embodiments.

[0028] Please refer to Figure 1 , Figure 1 as shown in the schematic flow chart of the manufacturing method of the present invention.

[0029] The first aspect of the present invention provides a method for manufacturing a low-permeability artificial core, including the following steps:

[0030] S1. Screening of quartz sand particles: The coarse quartz sand is screened using a standard sieve to obtain standard quartz sand base materials with different mesh numbers. The mesh number ranges of the quartz sand base materials are 30 - 70 mesh, 140 - 260 mesh, and 300 - 400 mesh respectively;

[0031] S2. Combination of core skeleton particles: According to different core physical property parameters, two or three different mesh number standard quartz sands are mixed in different mass ratios to obtain core skeleton particles;

[0032] S3. Preparation of core skeleton particle binder: 6 - 10 parts by mass of high-purity linear thermosetting epoxy resin powder particles, 0.72 - 1.2 parts by mass of heat-curing latent curing agent micropowder particles, and 0.18 - 0.3 parts by mass of imidazole and its derivative micropowder particle curing accelerators are respectively put into a high-speed stirrer and fully mixed to obtain a binder;

[0033] S4. Mixing of core raw materials: The core skeleton particles and the binder are put into a micro drum-type mixer and mixed and stirred for 20 - 60 minutes to obtain mixture A;

[0034] S5. High-pressure Compression Molding: Use an atomizer to atomize and spray the organic solution to moisten the mixture A into a wet state, obtaining mixture B. Then evenly spread mixture B in the core forming mold cavity. Subsequently, set the pressing pressure to 21 MPa for pressing to obtain mixture C;

[0035] S6. Isothermal and Isobaric Reaction: When the pressing pressure reaches the target set pressure, maintain constant-pressure pressing. At the same time, perform heat treatment on mixture C. Use heating sheets to perform gradient heating and insulation on the four sides and the bottom of the core forming mold. Set the heating temperature to 40 - 80 °C, and the insulation duration to 5 - 8 h;

[0036] S7. Cooling and Depressurization: Keep the heating temperature around the mold unchanged. After the heating temperature at the bottom of the mold and the pressing pressure drop to room temperature and atmospheric pressure, remove the mold and take out the compression-molded sample;

[0037] S8. Temperature-controlled Heating and Curing: Put the taken-out sample into a temperature-adjustable constant-temperature oven, set the heating and curing temperature program for gradient heating and insulation. The temperature setting gradient range is 60 - 200 °C, and the total heating and insulation duration is set to 12 h to cure the sample into rock;

[0038] S9. Core Cutting: Take cores from the heated and cured rock samples for physical property testing. At the same time, according to experimental needs, use a cutting machine to cut the heated and cured cores into different shapes to obtain low-permeability artificial cores, preparing for subsequent reservoir, seepage, and other flow experiments.

[0039] Please refer to Figure 2 , Figure 2 The physical diagram of the low-permeability artificial core prepared by the present invention is shown as follows.

[0040] The second aspect of the present invention provides a low-permeability artificial core prepared by the method described above.

[0041] According to the present invention, the low-permeability artificial core includes quartz sand base materials of different mesh numbers and a binder composed of high-purity linear thermosetting epoxy resin powder particles, heat-curing type latent curing agent fine powder particles, and imidazole-based and its derivative fine powder particle curing accelerators. The ratio is determined according to the physical property parameters of the target core. After high-pressure pressing and isothermal treatment, the required low-permeability artificial core is formed after curing. The porosity of the prepared low-permeability artificial core is 10 - 15%, and the gas permeability is 2 - 30 mD.

[0042] The inventors of the present invention have discovered a solid particle binder that can avoid the clogging of epoxy resin binders and filling of pore throats during the manufacturing process, enabling the particles to contact and bond in the forms of "point-point" and "point-plane", solving the problem that the conventional liquid epoxy resin bonding preparation method cannot achieve "point-plane and plane-plane contact bonding" of natural cores. At the same time, the pore throat microstructure is similar to that of natural cores, and the present invention uses the combined action of temperature and pressure to prepare low-permeability artificial cores, which can avoid achieving low porosity and low permeability by using excessive binders, solving the problems of high porosity, high permeability, insufficient heat resistance and strength in the conventional room-temperature pressing process for preparing low-porosity and low-permeability artificial cores.

[0043] Example 1

[0044] This example provides a method for manufacturing a low-permeability artificial core, including the following steps:

[0045] S1. Screening of quartz sand particles: Using a standard sieve to screen the coarse quartz sand to obtain standard quartz sand base materials with mesh numbers of 80, 160, and 300 for standby;

[0046] S2. Combination of core skeleton particles: Weigh 40 parts by mass of the 80-mesh quartz sand base material, 20 parts by mass of the 160-mesh quartz sand base material, and 40 parts by mass of the 300-mesh quartz sand base material respectively and mix them to obtain core skeleton particles;

[0047] S3. Preparation of core skeleton particle binder: Weigh 10 parts by mass of bisphenol A thermosetting epoxy resin powder particles, 1.2 parts by mass of dicyandiamide heat-curing latent curing agent micropowder particles, and 2 parts by mass of 1-methylimidazole curing accelerator, put them into a high-speed stirring crusher and mix well for 5 s to obtain the binder;

[0048] S4. Mixing of core raw materials: Put the core skeleton particles and the binder into a micro drum mixer and mix and stir for 30 min to obtain mixture A;

[0049] S5. High-pressure pressing and forming: Use an atomizer to atomize and spray out the alcohol solution to moisten mixture A into a wet state to obtain mixture B, evenly spread mixture B in the core forming mold cavity, level the upper surface of the sand material with a scraper and then horizontally place the pressing male mold, and then set the pressing pressure to 21 MPa for pressing to obtain mixture C;

[0050] S6. Isothermal and isobaric reaction: When the pressing pressure reaches the target set pressure, maintain constant-pressure pressing, and at the same time perform heat treatment on mixture C. Use heating sheets to perform gradient heating and insulation on the four sides and bottom of the core forming mold, set the heating temperature to 60 °C, the heating duration to 1 h, after heating at 60 °C for 1 h, set the temperature to rise by 10 °C per minute, and maintain a constant temperature of 80 °C for 5 h after the temperature reaches 80 °C;

[0051] S7. Cooling and pressure reduction: Keep the heating temperature around the mold unchanged. After the heating temperature at the bottom of the mold and the pressing pressure drop to room temperature and atmospheric pressure, remove the mold and take out the pressed and formed sample.

[0052] S8. Temperature-controlled heating and curing: Put the taken-out sample into a temperature-adjustable constant-temperature oven and set the heating and curing temperature program. The temperature gradient program is divided into 4 stages: Stage 1, the temperature rises from room temperature to 60 °C at a rate of 12 °C per minute and remains at 60 °C for 60 minutes; Stage 2, the temperature rises from 60 °C to 90 °C at a rate of 12 °C per minute and remains at 90 °C for 90 minutes; Stage 3, the temperature rises from 90 °C to 120 °C at a rate of 12 °C per minute and remains at 120 °C for 30 minutes; Stage 4, the temperature rises from 120 °C to 150 °C at a rate of 12 °C per minute and remains at 150 °C for 30 minutes to cure the sample into rock.

[0053] S9. Core cutting: After the heating and curing temperature program ends, wait for the core temperature to drop to room temperature. Use a water drill to obtain a Φ2.5×10 cm cylindrical core. Dry the cylindrical core and conduct permeability and porosity physical property tests. After the physical property tests are completed, use a cutting machine to cut the heated and cured core into different shapes for standby.

[0054] For the low-permeability artificial core prepared in Example 1, the permeability was tested by the gas measurement method and the porosity was tested by the dry and wet weight method. The average gas permeability of the core was measured to be 12.3 mD, and the average porosity was 14.2%. The average values of the permeability and porosity of the target reservoir were 13.4 mD and 15.3% respectively. The low-permeability artificial core prepared in Example 1 was close to the values of the target natural core, indicating that the pore-throat microstructure of the low-permeability artificial core prepared in Example 1 was similar to that of the natural core.

[0055] Comparative Example 1

[0056] This Comparative Example 1 is basically the same as Example 1, except that the heat treatment in step S6 was not carried out. The prepared core was dried and the permeability was tested by the gas measurement method and the porosity was tested by the dry and wet weight method. The average gas permeability of the core was measured to be 143.2 mD, and the average porosity was 21.3%. From the data results, it can be seen that the porosity and permeability of the core prepared without heat treatment deviated greatly from those of the target natural core. At the same time, due to the lack of heat treatment during the pressing process, the compressibility of the core matrix decreased, the core density was small, and the diagenetic strength was weaker than that of the heat-treated core, and there were significant differences in mechanical properties from the natural core.

[0057] Comparative Example 2

[0058] This Comparative Example 2 is basically the same as Example 1, except that the binder in Step S3 is replaced with a liquid epoxy resin binder. The prepared core is dried and the permeability is tested by gas measurement method and the porosity is tested by dry and wet weight method. The average gas permeability of the core is measured to be 11.6 mD and the average porosity is 10.1%. From the data results, it can be seen that for the core prepared by replacing the solid particle binder with a liquid epoxy resin binder, although the permeability is similar to that of the target reservoir, the porosity value is on the small side, and there is a large deviation from the target reservoir parameters. At the same time, since the core is pressed with a liquid epoxy resin binder by increasing the amount of epoxy to reduce the permeability and porosity, an increase in the amount of epoxy resin will lead to a decrease in the heat resistance of the core and a change in the surface wettability, resulting in a large difference from the properties of natural reservoirs.

[0059] Comparative Example 3

[0060] In this Comparative Example 3, a low-permeability artificial core is prepared by a different preparation method from that of Example 1. This Comparative Example 3 adopts a conventional core preparation method, including the following steps:

[0061] S1. Assemble a forming mold. The forming mold is made of carbon steel and consists of 2 long side plates, 2 end side plates and a bottom plate. The long side plates are fixed by four screws and nuts;

[0062] S2. Composition of core skeleton particles. The core skeleton particles are composed of quartz sands of different mesh numbers. The mesh number combination and mass ratio of quartz sands are determined according to the target reservoir parameters, and quartz sand particles of different particle sizes are mixed and evenly mixed;

[0063] S3. Preparation of liquid binder. Determine the target reservoir parameters. The amount of binder is 10% of the mass of quartz sand. The binder consists of liquid epoxy resin and three additives, and is prepared according to the mass ratio: the mass ratio of epoxy resin is 1, the mass ratio of dibutyl phthalate is 0.2, the mass ratio of ethylenediamine is 0.08, and the mass ratio of acetone is 0.4. The three additives with different mass ratios are mixed with epoxy resin evenly to obtain the binder;

[0064] S4. Preparation of core matrix by coating quartz sand: Mix the quartz sand mixture of different particle sizes with the binder and stir evenly to make the mixture free of agglomeration, and disperse the coated quartz sand with a 20-mesh sieve to obtain the core matrix;

[0065] S5. Loading the core matrix into the mold: Fill the core matrix into the mold groove in multiple times, and use a vertical scraper to evenly fill the core matrix in the mold groove after each filling;

[0066] S6. Compression molding: After uniformly filling the core matrix into the mold groove, slowly place the compression block and move the mold to the working area of the pressure testing machine on the central axis. Set the compression pressure parameters for compression. When the compression pressure is reached, hold the pressure for 15 minutes, and then remove the mold after the pressure of the pressure testing machine drops to normal pressure;

[0067] S7. Heating and curing into rock: Place the compression-molded core into the oven, set the curing temperature at 60 °C, and the curing duration at 6 hours to accelerate the cross-linking and curing of the binder, causing the core to turn into rock;

[0068] S8. After the core is cured into rock, use a water core drill to obtain a cylindrical core with a size of Φ2.5×10 cm. Dry the cylindrical core and conduct physical property tests on permeability and porosity.

[0069] The low-permeability artificial core prepared by the conventional core preparation method in Comparative Example 3 was measured to have an average permeability of 10.8 mD by the gas measurement method and an average porosity of 9.7% by the dry-wet weight method.

[0070] In Comparative Example 3, the conventional core preparation method was used, and liquid epoxy resin was used as the binder. Since the liquid binder has greater fluidity, during the process of mixing with quartz sand particles to prepare coated quartz sand, large-mesh quartz sand particles are easily coated by the liquid binder, losing the surface chemical properties of the quartz sand. At the same time, during the pressing process, under the action of the vertical downward pressure, the liquid epoxy resin on the outer surface of the coated quartz sand spreads around under the action of the extrusion pressure, blocking the micro-pores, resulting in a lower porosity compared to the target reservoir and lacking a micro-pore throat structure.

[0071] The embodiments described above are some, but not all, of the embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the present invention claimed, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

Claims

1. A method for making a low-permeability artificial core, characterized in that: The following steps are involved: S1. Use a sieve to sieve the quartz sand coarse material to obtain quartz sand base materials of different mesh sizes; S2. According to the physical parameters of the core, two or three different mesh sizes of the quartz sand base materials are mixed in different mass ratios to obtain core skeleton particles; S3. The polymer epoxy resin, curing agent and curing accelerator are mixed to obtain a binder; S4. stirring and mixing the core skeleton particles and the binder to obtain a mixture A; S5. Atomizing and spraying the organic solution to make the mixture A wet to obtain a mixture B, spreading the mixture B evenly in the core forming mold cavity, setting a target pressing pressure, and obtaining a pressed mixture C, wherein the organic solution is one or more of an ethyl acetate solution and an alcohol solution; S6. Maintaining constant pressure and heat treating the mixture C; S7. After the heat treatment is completed, the mold is cooled to room temperature and normal pressure and then the pressed sample is taken out; S8. Place the pressed sample into a constant temperature box, set a heating and curing temperature program to perform a gradient heating and heat preservation to solidify the sample into rock, and obtain a solidified rock sample; S9. coring the solidified diagenetic sample to obtain a low-permeability artificial core.

2. The method for making a low-permeability artificial core according to claim 1, characterized in that: In the step S1, the mesh size ranges of the quartz sand base material are 30-70 mesh, 140-260 mesh and 300-400 mesh respectively.

3. The method for making a low-permeability artificial core according to claim 1, characterized in that: In step S3, the binder includes 6-10 parts by mass of a high molecular weight epoxy resin, 0.72-1.2 parts by mass of a curing agent, and 0.18-0.3 parts by mass of a curing accelerator.

4. The method for making a low-permeability artificial core according to claim 1, characterized in that: In step S3, the polymer epoxy resin is high-purity linear heat-curing epoxy resin powder particles, the curing agent is heat-curing latent curing agent powder particles, and the curing accelerator is imidazole and its derivative powder particles.

5. The method for making a low-permeability artificial core according to claim 1, characterized in that: In step S4, a micro drum mixer is used for stirring and mixing, and the stirring and mixing time is 20-60 minutes.

6. The method for making a low-permeability artificial core according to claim 1, characterized in that: In the step S5, the pressing pressure is 10-21 MPa.

7. The method for making a low-permeability artificial core according to claim 1, characterized in that: In the step S6, the heat treatment uses a heating plate to perform gradient heating and heat preservation on the surrounding surfaces and the bottom of the core forming mold, the heating temperature is 40-80° C., and the heat preservation time is 5-8 hours.

8. The method for making a low-permeability artificial core according to claim 1, characterized in that: In the step S8, the gradient heating curing temperature is 60-200° C., and the heating curing time is 8-12 hours.

9. A low-permeability artificial core prepared by the preparation method according to any one of claims 1 to 8.

10. The low permeability artificial core according to claim 9, characterized in that: The porosity of the low-permeability artificial core is 10-15%, and the gas permeability is 2-30 mD.

Images