Self-heat-generation viscosity reduction system acting on deep part of thick oil as well as preparation method and application of self-heat-generation viscosity reduction system

Abstract

The invention discloses a self-heat-generation viscosity reduction system acting on the deep part of thick oil, the self-heat-generation viscosity reduction system is composed of a first agent and a second agent, the first agent is composed of urea, nitrite, a tackifier and a solvent, and the second agent is an acid catalyst wrapped by a temperature control microcapsule. The invention also discloses a preparation method of the self-heat-generation viscosity reduction system acting on the deep part of the thick oil, and the preparation method comprises the following steps: step (1), dissolving the formula amount of urea, the formula amount of nitrite and the formula amount of tackifier in a proper amount of water to form a first medicament with certain viscosity; and (2) adding the second medicament in the formula ratio into the first medicament obtained in the step (1), and uniformly stirring to obtain the self-heat-generation viscosity reduction system acting on the deep part of the thick oil. The invention further discloses application of the self-heat-generation viscosity reduction system acting on the deep part of the thick oil in viscosity reduction of the thick oil. The method has the advantages that the reaction delay time is adjustable, the reaction speed is controllable, the heat utilization rate is high, and the viscosity of the thick oil in the deep part of the reservoir is effectively reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of heavy oil development in oil fields, and particularly relates to a self-heating viscosity reduction system acting on the deep part of heavy oil, and a preparation method and application thereof. Background Art

[0002] In the process of heavy oil development, how to effectively reduce viscosity is the key to improving the development effect, among which thermal viscosity reduction is the most effective means. However, in the process of thermal viscosity reduction by steam injection in heavy oil reservoirs, due to factors such as heat loss in ground pipelines and wellbore, the heat of steam that plays a role in reducing viscosity in the reservoir is less than 40% of the heat of steam at the outlet of the ground boiler, resulting in a huge waste of energy. Due to the low heat and dryness of the steam entering the reservoir, the thermal wave range is small, and only the effective use of heavy oil within 30-40m around the wellbore can be achieved.

[0003] The self-generated heat viscosity reduction system is to inject different chemical substances into the heavy oil reservoir, so that they can directly react chemically in the formation to generate heat and various gases, and achieve viscosity reduction development of heavy oil reservoirs through the synergistic effect of multiple viscosity reduction mechanisms such as thermal viscosity reduction and gas dissolution viscosity reduction. This development method belongs to in-situ heat viscosity reduction, which avoids heat loss along the process of steam injection viscosity reduction and improves energy utilization.

[0004] Once the reactants in conventional self-generated thermal chemicals come into contact, a rapid and violent chemical reaction will occur, releasing a large amount of reaction heat in a short period of time, causing the heat and reaction products generated by the reaction to accumulate in a smaller area, causing the local temperature and pressure of the reservoir to rise sharply, and there are many problems in terms of safety and the scope of the viscosity reduction area.

[0005] At present, in order to avoid chemical reactions before entering the target reservoir, different reactants are generally prepared into solutions separately during the injection process. During the injection process, it is necessary to add isolation water plugs between the different reactant solutions. The reactants entering the reservoir diffuse freely in the reservoir. Chemical reactions can only occur when different reactants come into contact during the diffusion process. Although this method can ensure that chemical reactions occur in the reservoir, it is based on the free diffusion of reactants under the porous media conditions of the reservoir. Due to the poor dynamic conditions for the diffusion and migration of reactants in the reservoir, it is difficult to achieve sufficient contact between different reactants, and the reaction mainly occurs in the near-well area. Therefore, this method has always had a series of problems in the application of mines, such as low effective utilization of chemical agents, insufficient reaction, and difficulty in effectively achieving viscosity reduction in deep reservoirs. Summary of the invention

[0006] Purpose of the invention: In view of the shortcomings of the above-mentioned prior art, the present invention provides a self-generated thermal viscosity reduction system acting on the deep part of heavy oil and its preparation method and application. The present invention can solve the problems of large heat loss and small viscosity reduction range in the process of thermal viscosity reduction of heavy oil.

[0007] The present invention forms a first agent by dissolving urea, nitrite and a thickener in a solvent, and then adds a second agent (a temperature-controlled microcapsule wrapped with an acidic catalyst) into the first agent to form a self-heating viscosity reduction system acting on the deep part of the heavy oil. After the self-heating viscosity reduction system is injected into the oil reservoir, as the self-heating viscosity reduction system migrates to the target depth in the deep part of the oil reservoir, the self-heating viscosity reduction system is gradually heated by the high temperature of the formation. When its temperature reaches or exceeds the rupture temperature of the temperature-controlled microcapsule, the temperature-controlled microcapsule ruptures in batches and releases the acidic catalyst therein. Under the action of the acidic catalyst, the urea in the solution reacts with the nitrite in an exothermic reaction to generate nitrogen, carbon dioxide, and high-temperature steam and release a large amount of heat, wherein:

[0008] The generated heat, high-temperature steam and carbon dioxide can reduce the viscosity of heavy oil. The expansion effect of nitrogen, carbon dioxide and other gases pushes the self-generated heat viscosity reduction system and reaction products to the deep part of the heavy oil reservoir, achieving expanded coverage and deep viscosity reduction.

[0009] Technical solution: The self-heating viscosity reduction system acting on the deep part of heavy oil is composed of a first agent and a second agent, wherein:

[0010] The mass ratio of the first agent to the second agent is (15-25):1;

[0011] The first agent is composed of urea, nitrite, thickener and solvent, wherein the mass ratio of urea, nitrite, thickener and solvent is (3-5): (5-7): (0.2-0.5): (88-92);

[0012] The second agent is an acidic catalyst encapsulated by temperature-controlled microcapsules.

[0013] Furthermore, the nitrite is sodium nitrite and / or potassium nitrite.

[0014] Furthermore, the viscosity enhancer is one of guar gum, hydroxyethyl tianpu powder and polyacrylamide.

[0015] Furthermore, the solvent is water.

[0016] Furthermore, the temperature-controlling microcapsules are prepared from shellac and low molecular weight polyethylene.

[0017] Furthermore, the mass ratio of shellac to low molecular weight polyethylene is (8-13):1.

[0018] Furthermore, the relative molecular weight of the low molecular weight polyethylene is less than 10,000.

[0019] Furthermore, the elastic modulus of the second agent is 0.78-0.82Pa.

[0020] Furthermore, the acidic catalyst is at least one of hydrochloric acid, acetic acid, citric acid and tartaric acid.

[0021] The preparation method of the above-mentioned self-heating viscosity reduction system acting on the deep part of heavy oil has the following specific steps:

[0022] Step (1), dissolving a formula amount of urea, a formula amount of nitrite, and a formula amount of a viscosity enhancer in an appropriate amount of water to form a first agent having a certain viscosity;

[0023] Step (2), adding the second agent in the formula amount to the first agent obtained in step (1), stirring evenly to obtain a self-heating viscosity reduction system acting on the deep part of the heavy oil.

[0024] Furthermore, in step 1, the viscosity of the first agent is 10 to 30 mPa.s.

[0025] The above-mentioned self-heating viscosity reduction system acting on the deep part of heavy oil is used in the viscosity reduction of heavy oil.

[0026] Furthermore, the specific steps of the above application are:

[0027] The self-generated heat viscosity reduction system acting on the deep part of the heavy oil is pumped into the target depth of the heavy oil reservoir. The self-generated heat viscosity reduction system will be heated by the high temperature of the oil reservoir in the process of flowing to the target depth of the deep part of the oil reservoir. When the temperature of the self-generated heat viscosity reduction system reaches or exceeds the rupture temperature of the temperature-controlled microcapsules, the temperature-controlled microcapsules rupture in batches, releasing the acidic catalyst wrapped therein, so that the reaction occurs in the deep part of the oil reservoir.

[0028] Furthermore, the rupture temperature range of the temperature-controlled microcapsule is 85% to 115% of the reservoir temperature at the target depth of the heavy oil reservoir.

[0029] Furthermore, the rupture temperature of the temperature-controlled microcapsule ranges from 90% to 110% of the reservoir temperature at the target depth of the heavy oil reservoir.

[0030] Furthermore, the pumping speed is 100-150 L / min.

[0031] Since the rupture temperature of the temperature-controlled microcapsules is within a certain temperature range suitable for the target reservoir, as the temperature gradually increases, the temperature-controlled microcapsules rupture in batches and release the acid catalyst wrapped therein, which plays a role in controlling and delaying the reaction rate;

[0032] Under the action of the released acid catalyst, the urea in the self-heating viscosity reduction system reacts with nitrite to generate nitrogen and carbon dioxide and release heat, while forming high-temperature steam. The reaction equation is as follows:

[0033]

[0034] Among them, CO(NH2)2 is urea, is nitrite, H + It is an acidic catalyst;

[0035] Heat, high-temperature steam, and carbon dioxide can play a role in reducing viscosity by heat and dissolution, and cooperate with the viscosity reducing effect of the viscosity enhancer in the first agent to reduce the viscosity of heavy oil;

[0036] At the same time, nitrogen and carbon dioxide have a good expansion effect in the reservoir, which can push the self-generated heat viscosity reduction system and the reaction products to the deep part of the reservoir, achieving the purpose of expanding the impact and deep viscosity reduction.

[0037] Beneficial effects: The present invention has the characteristics of adjustable reaction delay time (adjustable within the range of 0 to 25 minutes), controllable reaction speed (1°C / min to 6.5°C / min), high heat utilization rate and effective viscosity reduction of deep-seated heavy oil in the reservoir, which solves a series of problems in the process of using conventional self-generated thermal chemical agents to reduce the viscosity of heavy oil. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 The present invention discloses a flow chart of a method for preparing a self-heating viscosity reduction system for deep heavy oil. Specific implementation method:

[0039] The specific embodiments of the present invention are described in detail below.

[0040] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0041] In order to allow those skilled in the art to understand the technical solution of the present invention, the preparation process of the second agent is briefly described below:

[0042] Preparation steps of the second agent:

[0043] S1, the reaction container is composed of a cold water layer, a hot water layer, and a coating material layer from bottom to top, wherein:

[0044] The temperature of the cold water in the cold water layer is controlled at 10°C;

[0045] The temperature of hot water in the hot water layer is controlled at 95°C;

[0046] The coating material layer is composed of shellac and low molecular weight polyethylene, the mass ratio of the two is (8-13):1, and the relative molecular weight of the low molecular weight polyethylene is less than 10000.

[0047] S2. After heating until the coating material layer is in a molten state, an acidic catalyst droplet with a diameter of about 800 nm is dropped into the container through a nozzle at a height of 1.5 cm above the container. The acidic catalyst droplet first passes through the coating material layer on the top of the container, and its periphery is coated with a coating with a thickness of about 100 nm. As the droplet continues to fall, it is adjusted to a spherical shape in the hot water layer, and is condensed and hardened after entering the cold water layer to obtain a temperature-controlled microcapsule.

[0048] The rupture temperature of the microcapsules can be controlled by adjusting the ratio of shellac to low molecular weight polyethylene (relative molecular weight <10,000).

[0049] The temperature control performance of the second agent is verified according to the above method. The other parameters remain unchanged. The only change is the ratio of shellac and low molecular weight polyethylene in the coating material layer, which is as follows:

[0050] Mass ratio of shellac and low molecular weight polyethylene Coating material layer 1 8:1 Laminating material layer 2 10:1 Coating material layer 3 13:1

[0051] Under the experimental conditions of simulating reservoir temperature of 83°C and reservoir pressure of 19MPa, the temperature control performance of the temperature control microcapsules composed of the coating material layers 1-3 were tested respectively:

[0052] Temperature at which rupture begins (℃) Complete rupture temperature (℃) Coating material layer 1 89.9 112.2 Laminating material layer 2 81.1 98.9 Coating material layer 3 70.3 93.1

[0053] The temperature control performance of the temperature control microcapsules composed of the coating material layers 1-3 was tested respectively:

[0054] Elastic modulus(Pa) Coating material layer 1 0.82 Laminating material layer 2 0.78 Coating material layer 3 0.8

[0055] Example 1

[0056] The reservoir temperature of the target heavy oil reservoir is 70°C, and the crude oil viscosity under reservoir conditions is 1244 mPa.s.

[0057] The self-generated heat viscosity reduction system acting on the deep part of heavy oil is composed of a first agent and a second agent, wherein:

[0058] The mass ratio of the first agent to the second agent is 25:1;

[0059] The first agent is composed of urea, nitrite, thickener and solvent, wherein the mass ratio of urea, nitrite, thickener and solvent is 5:7:0.5:92;

[0060] The second agent is an acidic catalyst encapsulated by temperature-controlled microcapsules.

[0061] Furthermore, the nitrite is sodium nitrite.

[0062] Furthermore, the viscosity increasing agent is guar gum.

[0063] Furthermore, the solvent is water.

[0064] Furthermore, the temperature-controlling microcapsules are prepared from shellac and low molecular weight polyethylene.

[0065] Furthermore, the mass ratio of shellac to low molecular weight polyethylene is 13: 1. This ratio is compatible with the reservoir temperature of the target heavy oil reservoir being 70°C.

[0066] Furthermore, the relative molecular weight of the low molecular weight polyethylene is less than 10,000.

[0067] Furthermore, the elastic modulus of the second agent is 0.8Pa.

[0068] Furthermore, the acidic catalyst is hydrochloric acid.

[0069] The preparation method of the above-mentioned self-heating viscosity reduction system acting on the deep part of heavy oil has the following specific steps:

[0070] Step (1), dissolving a formula amount of urea, a formula amount of nitrite, and a formula amount of a viscosity enhancer in an appropriate amount of water to form a first agent having a certain viscosity;

[0071] Step (2), adding the second agent in the formula amount to the first agent obtained in step (1), stirring evenly to obtain a self-heating viscosity reduction system acting on the deep part of the heavy oil.

[0072] Furthermore, the viscosity of the first agent in step 1 is 18 mPa.s.

[0073] The above-mentioned self-heating viscosity reduction system acting on the deep part of heavy oil is used in the viscosity reduction of heavy oil.

[0074] Furthermore, the specific steps of the above application are:

[0075] The self-heating viscosity reduction system acting on the deep part of the heavy oil is pumped into the target depth of the heavy oil reservoir. The self-heating viscosity reduction system will be heated by the high temperature of the reservoir in the process of flowing to the target depth of the deep part of the reservoir. When the temperature of the self-heating viscosity reduction system reaches or exceeds the rupture temperature of the temperature-controlled microcapsules, the temperature-controlled microcapsules rupture in batches, releasing the acidic catalyst wrapped therein, so that the reaction occurs in the deep part of the reservoir, wherein:

[0076] The pumping speed is 150L / min.

[0077] After later tests, the self-heating viscosity reduction system prepared in Example 1 for deep heavy oil had a heat enthalpy utilization rate of 84.2%, achieving effective viscosity reduction within 75 m of the oil well.

[0078] Example 2

[0079] The reservoir temperature of the target heavy oil reservoir is 80°C, and the crude oil viscosity under reservoir conditions is 2170 mPa.s.

[0080] The self-generated heat viscosity reduction system acting on the deep part of heavy oil is composed of a first agent and a second agent, wherein:

[0081] The mass ratio of the first agent to the second agent is 20:1;

[0082] The first agent is composed of urea, nitrite, thickener and solvent, wherein the mass ratio of urea, nitrite, thickener and solvent is 4:6:0.3:90;

[0083] The second agent is an acidic catalyst encapsulated by temperature-controlled microcapsules.

[0084] Furthermore, the nitrite is potassium nitrite.

[0085] Furthermore, the viscosity increasing agent is hydroxyethyl Tianpu powder.

[0086] Furthermore, the solvent is water.

[0087] Furthermore, the temperature-controlling microcapsules are prepared from shellac and low molecular weight polyethylene.

[0088] Furthermore, the mass ratio of shellac to low molecular weight polyethylene is 10: 1. This ratio is compatible with the reservoir temperature of the target heavy oil reservoir being 80°C.

[0089] Furthermore, the relative molecular weight of the low molecular weight polyethylene is less than 10,000.

[0090] Furthermore, the elastic modulus of the second agent is 0.78Pa.

[0091] Furthermore, the acidic catalyst is acetic acid.

[0092] In another embodiment, the acidic catalyst is citric acid.

[0093] In yet another embodiment, the acidic catalyst is tartaric acid.

[0094] The preparation method of the above-mentioned self-heating viscosity reduction system acting on the deep part of heavy oil has the following specific steps:

[0095] Step (1), dissolving a formula amount of urea, a formula amount of nitrite, and a formula amount of a viscosity enhancer in an appropriate amount of water to form a first agent having a certain viscosity;

[0096] Step (2), adding the second agent in the formula amount to the first agent obtained in step (1), stirring evenly to obtain a self-heating viscosity reduction system acting on the deep part of the heavy oil.

[0097] Furthermore, the viscosity of the first agent in step 1 is 10.2 mPa.s.

[0098] The above-mentioned self-heating viscosity reduction system acting on the deep part of heavy oil is used in the viscosity reduction of heavy oil.

[0099] Furthermore, the specific steps of the above application are:

[0100] The self-generated heat viscosity reduction system acting on the deep part of the heavy oil is pumped into the target depth of the heavy oil reservoir. The self-generated heat viscosity reduction system will be heated by the high temperature of the oil reservoir in the process of flowing to the target depth of the deep part of the oil reservoir. When the temperature of the self-generated heat viscosity reduction system reaches or exceeds the rupture temperature of the temperature-controlled microcapsules, the temperature-controlled microcapsules rupture in batches, releasing the acidic catalyst wrapped therein, so that the reaction occurs in the deep part of the oil reservoir.

[0101] Furthermore, the pumping speed is 100 L / min.

[0102] After later tests, the self-heating viscosity reduction system prepared in Example 2 for deep heavy oil had a heat enthalpy utilization rate of 73.2%, achieving effective viscosity reduction within 68 m of the oil well.

[0103] Example 3

[0104] The reservoir temperature of the target heavy oil reservoir is 95°C, and the crude oil viscosity under reservoir conditions is 947 mPa.s.

[0105] The self-generated heat viscosity reduction system acting on the deep part of heavy oil is composed of a first agent and a second agent, wherein:

[0106] The mass ratio of the first agent to the second agent is 15:1;

[0107] The first agent is composed of urea, nitrite, thickener and solvent, wherein the mass ratio of urea, nitrite, thickener and solvent is 3:5:0.2:88;

[0108] The second agent is an acidic catalyst encapsulated by temperature-controlled microcapsules.

[0109] Furthermore, the nitrite is a mixture of sodium nitrite and potassium nitrite in an equal mass ratio.

[0110] Furthermore, the viscosity increasing agent is polyacrylamide.

[0111] Furthermore, the solvent is water.

[0112] Furthermore, the temperature-controlling microcapsules are prepared from shellac and low molecular weight polyethylene.

[0113] Furthermore, the mass ratio of shellac to low molecular weight polyethylene is 8: 1. This ratio is compatible with the reservoir temperature of the target heavy oil reservoir being 95°C.

[0114] Furthermore, the relative molecular weight of the low molecular weight polyethylene is less than 10,000.

[0115] Furthermore, the elastic modulus of the second agent is 0.82Pa.

[0116] Furthermore, the acidic catalyst is a mixture of hydrochloric acid, acetic acid, citric acid and tartaric acid in equal mass ratios.

[0117] The preparation method of the above-mentioned self-heating viscosity reduction system acting on the deep part of heavy oil has the following specific steps:

[0118] Step (1), dissolving a formula amount of urea, a formula amount of nitrite, and a formula amount of a viscosity enhancer in an appropriate amount of water to form a first agent having a certain viscosity;

[0119] Step (2), adding the second agent in the formula amount to the first agent obtained in step (1), stirring evenly to obtain a self-heating viscosity reduction system acting on the deep part of the heavy oil.

[0120] Furthermore, the viscosity of the first agent in step 1 is 29.8 mPa.s.

[0121] The above-mentioned self-heating viscosity reduction system acting on the deep part of heavy oil is used in the viscosity reduction of heavy oil.

[0122] Furthermore, the specific steps of the above application are:

[0123] The self-generated heat viscosity reduction system acting on the deep part of the heavy oil is pumped into the target depth of the heavy oil reservoir. The self-generated heat viscosity reduction system will be heated by the high temperature of the oil reservoir in the process of flowing to the target depth of the deep part of the oil reservoir. When the temperature of the self-generated heat viscosity reduction system reaches or exceeds the rupture temperature of the temperature-controlled microcapsules, the temperature-controlled microcapsules rupture in batches, releasing the acidic catalyst wrapped therein, so that the reaction occurs in the deep part of the oil reservoir.

[0124] Furthermore, the pumping speed is 135 L / min.

[0125] After later tests, the self-heating viscosity reduction system prepared in Example 3 for deep heavy oil had a heat enthalpy utilization rate of 78.2%, achieving effective viscosity reduction within 72 m of the oil well.

[0126] The above describes the embodiments of the present invention in detail. However, the present invention is not limited to the above embodiments, and various changes can be made within the knowledge of ordinary technicians in the relevant technical field without departing from the purpose of the present invention.

Claims

1. A self-heating viscosity reduction system acting on the deep part of heavy oil, characterized in that: It consists of a first agent and a second agent, wherein: The mass ratio of the first agent to the second agent is (15-25):1; The first agent is composed of urea, nitrite, thickener and solvent, wherein the mass ratio of urea, nitrite, thickener and solvent is (3-5): (5-7): (0.2-0.5): (88-92); The second agent is an acidic catalyst encapsulated by temperature-controlled microcapsules.

2. The self-heating viscosity reduction system for heavy oil deep in the oil as claimed in claim 1, characterized in that: The nitrite is sodium nitrite and / or potassium nitrite.

3. The self-heating viscosity reduction system acting on the deep part of heavy oil as claimed in claim 1, characterized in that: The viscosity enhancer is one of guar gum, hydroxyethyl tianpu powder and polyacrylamide.

4. The self-heating viscosity reduction system for deep-seated heavy oil as claimed in claim 1, characterized in that: The solvent is water.

5. The self-heating viscosity reduction system acting on the deep part of heavy oil as claimed in claim 1, characterized in that: The temperature-controlling microcapsule is prepared from shellac and low-molecular-weight polyethylene.

6. The self-heating viscosity reduction system acting on the deep part of heavy oil as claimed in claim 5, characterized in that: The mass ratio of shellac to low molecular weight polyethylene is (8-13):

1.

7. The self-heating viscosity reduction system for deep-seated heavy oil as claimed in claim 5, characterized in that: The relative molecular weight of the low molecular weight polyethylene is less than 10000.

8. The self-heating viscosity reduction system for deep-seated heavy oil as claimed in claim 5, characterized in that: The elastic modulus of the second agent is 0.78-0.82Pa.

9. The self-heating viscosity reduction system for deep-seated heavy oil as claimed in claim 1, characterized in that: The acidic catalyst is at least one of hydrochloric acid, acetic acid, citric acid and tartaric acid.

10. The method for preparing the self-heating viscosity reduction system acting on the deep part of heavy oil according to any one of claims 1 to 9, characterized in that: The specific steps are as follows: Step (1), dissolving a formula amount of urea, a formula amount of nitrite, and a formula amount of a viscosity enhancer in an appropriate amount of water to form a first agent having a certain viscosity; Step (2), adding the second agent in the formula amount to the first agent obtained in step (1), stirring evenly to obtain a self-heating viscosity reduction system acting on the deep part of the heavy oil.

11. The method for preparing the self-heating viscosity reduction system acting on the deep part of heavy oil as claimed in claim 10, characterized in that: The viscosity of the first agent in step (1) is 10 to 30 mPa.s.

12. Use of the self-heating viscosity reduction system acting on the deep part of heavy oil as claimed in any one of claims 1 to 9 in reducing the viscosity of heavy oil.

13. The use according to claim 12, characterized in that The specific steps of the above application are: The self-heating viscosity reduction system acting on the deep part of heavy oil is pumped into the target depth of the heavy oil reservoir.

14. The use according to claim 13, characterized in that The rupture temperature range of the temperature-controlled microcapsule is 85% to 115% of the reservoir temperature at the target depth of the heavy oil reservoir.

15. The use according to claim 14, characterized in that The rupture temperature range of the temperature-controlled microcapsule is 90% to 110% of the reservoir temperature at the target depth of the heavy oil reservoir.

16. The use according to claim 12, characterized in that The pumping speed is 100-150L / min.

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