Well cementation cement paste for middle-deep layer coaxial ground rock hot well and preparation method of well cementation cement paste

Abstract

The invention provides a well cementation cement slurry for a middle-deep layer coaxial ground rock hot well and a preparation method thereof, and relates to the technical field of well cementation cement slurrys.The well cementation cement slurry is prepared from, by weight, 60-70 parts of G-grade cement, 5-10 parts of superfine silicon dioxide, 5-10 parts of superfine aluminum oxide powder, 1-2 parts of graphite powder and 0.5-1 part of high-temperature strength enhancer, the composite material comprises the following components in parts by weight: 0.5-0.8 part of a toughening material, 0.5-1 part of a retarder and 127-160 parts of water. By using the well cementation cement slurry provided by the invention, well cementation cement with relatively high high-temperature strength, relatively high toughness and relatively high heat conductivity can be obtained at the same time, and the specific requirements of a middle-deep layer coaxial ground rock hot well cementation process and a service environment can be met.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of cementing slurry, in particular to a cementing slurry for a medium-deep coaxial geothermal well and a preparation method thereof. BACKGROUND

[0002] Geothermal resources are a clean, environmentally friendly, widely distributed and renewable resource. Geothermal energy is divided into two categories: hydrothermal type and rock heat type. Rock heat type geothermal energy includes shallow soil source heat energy, medium-deep geothermal energy and deep dry hot rock geothermal energy. Medium-deep geothermal energy and dry hot rock both belong to the form of rock heat type geothermal energy. Geothermal heat is mainly used for building heating, industrial drying and power generation. Through a deep well, a circulating working medium is injected into the underground rock layer. The circulating working medium absorbs geothermal energy from the formation during the downward process. The circulating working medium returns to the ground after descending to the bottom of the well. The outlet temperature of the working medium can reach 20-150℃. Through heat exchange and a ground heat pump circulation system, it is used for building heating or industrial drying. The cooled circulating working medium is circulated and used again by exchanging heat with the coaxial geothermal well formation through a circulating pump.

[0003] Medium-deep coaxial geothermal rock layers usually have the characteristics of small porosity and low permeability. There is a certain gap between the outer sleeve and the formation after drilling a coaxial geothermal well. Therefore, cementing is needed to make the formation and the outer sleeve fully contact, which is not only beneficial to the heat transfer between the formation and the wellbore, but also beneficial to improving the safety life of the wellbore. During production and operation, the outer sleeve of the wellbore faces complex mechanical / chemical environments such as high temperature, high stress, chemical scaling / corrosion. Therefore, the cementing cement is required to have good thermal conductivity to facilitate the rapid transfer of formation heat to wellbore medium. In addition, the cementing cement for medium-deep coaxial geothermal wells also needs to have high high-temperature strength and high toughness to meet the specific requirements of the medium-deep coaxial geothermal well cementing process and service environment. However, at present, it is difficult to obtain cementing cement with the above several properties at the same time by using existing cementing slurry. SUMMARY

[0004] The problem solved by the present application is that it is difficult to obtain cementing cement with high high-temperature strength, high toughness and high thermal conductivity at the same time by using existing cementing slurry, which cannot meet the specific requirements of the medium-deep coaxial geothermal well cementing process and service environment.

[0005] To address the aforementioned problems, this invention provides a cementing slurry for medium-deep coaxial geothermal wells. By weight, the cementing slurry for medium-deep coaxial geothermal wells comprises: 60 to 70 parts of Grade G cement, 5 to 10 parts of ultrafine silica, 5 to 10 parts of ultrafine alumina powder, 1 to 2 parts of graphite powder, 0.5 to 1 part of high-temperature strength enhancer, 0.5 to 0.8 parts of toughening material, 0.5 to 1 part of retarder, and 127 to 160 parts of water. The chemical composition of the Grade G cement contains a calcium oxide to silica molar ratio of 1.8 to 2.4, and the seven-day heat of hydration of the Grade G cement is no greater than 260 kJ / kg.

[0006] Optionally, the high-temperature strength enhancer is ultrafine silica powder.

[0007] Optionally, the particle size of the ultrafine silica powder is not greater than 15 μm.

[0008] Optionally, the toughening material includes at least one of carbon fiber and carbon nanotubes.

[0009] Optionally, the diameter of the toughening material is 5 μm to 7 μm.

[0010] Optionally, the particle size of the ultrafine silica is no greater than 15 μm.

[0011] Optionally, the particle size of the ultrafine alumina powder is not greater than 25 μm.

[0012] Optionally, the particle size of the graphite powder is not greater than 20 μm.

[0013] Optionally, the retarder is citric acid.

[0014] The present invention also provides a method for preparing cementing slurry for medium-deep coaxial geothermal wells as described above, comprising: Step S1: Mix G-grade cement, ultrafine silica, ultrafine alumina powder, graphite powder, high-temperature strength enhancer and toughening material evenly to obtain dry mix; Step S2: Mix the retarder and water evenly to obtain a mixture; Step S3: Add the dry mixture to the liquid mixture and stir evenly to obtain cement slurry for cementing medium-deep coaxial geothermal wells.

[0015] Compared with related technologies, the cement slurry for medium-deep coaxial geothermal wells provided by this invention is prepared from a specific ratio of Grade G cement, ultrafine silica, ultrafine alumina powder, graphite powder, high-temperature strength enhancer, toughening material, retarder, and water. Specifically, the molar ratio of calcium oxide to silica in the chemical composition of Grade G cement is 1.8 to 2.4, resulting in a low calcium-to-silica ratio in the cement slurry. This avoids the formation of high-calcium-to-silica hydration products (such as CSH, CSH(A), CSH(C)) in the cement, ultimately forming a hydration product system dominated by calcium silicate (CSH) and calcium siliceous silicate (CSH), which is beneficial for improving the high-temperature strength of the cement. Furthermore, because Grade G cement has a low heat of hydration, it avoids the problem of compressive strength degradation of the cement at high temperatures, which also helps to improve the high-temperature strength of the cement. Furthermore, the ultrafine silica, ultrafine alumina powder, and graphite powder in the cement slurry act as thermally conductive admixtures, which helps improve the thermal conductivity of the cement. Additionally, toughening materials (such as carbon fiber and carbon nanotubes) in the cement slurry ensure high toughness in the cement. In summary, using the cement slurry provided by this invention can yield cement with high high-temperature strength, high toughness, and high thermal conductivity, meeting the specific requirements of cementing processes and service environments for medium-deep coaxial geothermal wells. Attached Figure Description

[0016] Figure 1 This is a scanning electron microscope image of the cement stone obtained by curing the cementing slurry prepared in Example 1. Detailed Implementation

[0017] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Although some embodiments of the present invention are shown in the drawings, it should be understood that the present invention can be implemented in various forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the present invention. It should be understood that the accompanying drawings and embodiments of the present invention are for illustrative purposes only and are not intended to limit the scope of protection of the present invention.

[0018] Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0019] It should be noted that the 7-day hydration heat of G-grade cement refers to the total heat released after a unit mass of G-grade cement reacts with water for 7 days.

[0020] The term "comprising" and its variations as used herein are open-ended, meaning "including but not limited to"; the term "based on" means "at least partially based on"; the term "one embodiment" means "at least one embodiment"; the term "another embodiment" means "at least one additional embodiment"; the term "some embodiments" means "at least some embodiments"; and the term "optionally" means "optional embodiments". Definitions of other terms will be given in the following description. It should be noted that the concepts of "first," "second," etc., mentioned in this invention are used to distinguish different objects, not to describe a specific order or hierarchy. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0021] To address the problems existing in the aforementioned related technologies, this embodiment provides a cementing slurry for medium-deep coaxial geothermal wells. By weight, the cementing slurry for medium-deep coaxial geothermal wells comprises: 60 to 70 parts of Grade G cement, 5 to 10 parts of ultrafine silica, 5 to 10 parts of ultrafine alumina powder, 1 to 2 parts of graphite powder, 0.5 to 1 part of high-temperature strength enhancer, 0.5 to 0.8 parts of toughening material, 0.5 to 1 part of retarder, and 127 to 160 parts of water. The chemical composition of the Grade G cement has a molar ratio of calcium oxide to silica of 1.8 to 2.4, and the seven-day heat of hydration of the Grade G cement is no greater than 260 kJ / kg.

[0022] The cementing slurry for medium-deep coaxial geothermal wells provided in this invention is prepared from a specific ratio of Grade G cement, ultrafine silica, ultrafine alumina powder, graphite powder, high-temperature strength enhancer, toughening material, retarder, and water. The Grade G cement has a calcium oxide to silica molar ratio of 1.8 to 2.4, resulting in a low calcium-to-silica ratio in the cementing slurry. This avoids the formation of high-calcium-to-silica hydration products (such as CSH, CSH(A), CSH(C)) in the cementing slurry. The cementing slurry ultimately forms a hydration product system dominated by calcium silicate (CSH) and calcium siliceous silicate (CSH), which is beneficial for improving the high-temperature strength of the cementing slurry. Furthermore, the low heat of hydration of Grade G cement avoids the degradation of compressive strength at high temperatures, further contributing to improved high-temperature strength. Furthermore, the ultrafine silica, ultrafine alumina powder, and graphite powder in the cement slurry act as thermally conductive admixtures, which helps improve the thermal conductivity of the cement. Additionally, toughening materials (such as carbon fiber and carbon nanotubes) in the cement slurry ensure high toughness in the cement. In summary, the cement slurry provided by the embodiments of this invention can yield cement with high high-temperature strength, high toughness, and high thermal conductivity, meeting the specific requirements of cementing processes and service environments for medium-deep coaxial geothermal wells.

[0023] In some embodiments of the present invention, the high-temperature strength enhancer is ultrafine silica powder, and the particle size of the ultrafine silica powder is no greater than 15 μm. Silica slag is a silica ore auxiliary material and has become a source of environmental pollution in silica mining areas. Grinding waste silica slag into ultrafine silica powder and using it as a high-temperature strength enhancer in cementing slurry is one of the effective ways to treat this type of slag waste.

[0024] In some embodiments of the present invention, the toughening material includes at least one of carbon fiber and carbon nanotubes, and the diameter of the toughening material is 5 μm to 7 μm. Through the fiber composite reinforcement effect, the flexural strength of the cement stone is improved, and under hydraulic pressure, it effectively resists the tensile failure of the first interface cement ring caused by the stress expansion of the casing, thereby improving the integrity of the cement ring.

[0025] In some embodiments of the present invention, the particle size of the ultrafine silica is no greater than 15 μm, the particle size of the ultrafine alumina powder is no greater than 25 μm, and the particle size of the graphite powder is no greater than 20 μm. In this embodiment, the combination of various ultrafine silica, ultrafine alumina powder, and graphite powder of different particle sizes is beneficial to further improving the thermal conductivity of cementing.

[0026] In some embodiments of the present invention, the retarder is citric acid. In this embodiment, using citric acid as a retarder enables the cementing slurry to thicken in a time of 3 to 9 hours.

[0027] This invention also provides a method for preparing cementing slurry for medium-deep coaxial geothermal wells as described above, comprising: Step S1: Mix G-grade cement, ultrafine silica, ultrafine alumina powder, graphite powder, high-temperature strength enhancer and toughening material evenly to obtain dry mix; Step S2: Mix the retarder and water evenly to obtain a mixture; Step S3: Add the dry mixture to the liquid mixture and stir evenly to obtain cement slurry for cementing medium-deep coaxial geothermal wells.

[0028] The present invention will be further described below with reference to specific embodiments.

[0029] Example 1 In this embodiment, the cement slurry comprises, by weight, 65 parts of Grade G cement, 7.5 parts of ultrafine silica, 7.5 parts of ultrafine alumina powder, 1.5 parts of graphite powder, 0.75 parts of high-temperature strength enhancer, 0.65 parts of toughening material, 0.75 parts of retarder, and 144 parts of water; wherein, the molar ratio of calcium oxide to silica in the chemical composition of the Grade G cement is 2.1, and the seven-day heat of hydration of the Grade G cement is 255 kJ / kg; the high-temperature strength enhancer is ultrafine silica powder with a particle size of 10 μm, the toughening material is carbon fiber with a diameter of 6 μm, the ultrafine silica has a particle size of 10 μm, the ultrafine alumina powder has a particle size of 20 μm, the graphite powder has a particle size of 15 μm, and the retarder is citric acid.

[0030] The preparation process of cement slurry for well cementing is as follows: A1. Mix G-grade cement, ultrafine silica, ultrafine alumina powder, graphite powder, high-temperature strength enhancer and toughening material evenly to obtain dry mix; A2. Mix the retarder with water until homogeneous to obtain a mixture; A3. Add the dry mixture to the liquid mixture and stir evenly to obtain cementing slurry.

[0031] Example 2 In this embodiment, the cement slurry comprises, by weight, 60 parts of Grade G cement, 5 parts of ultrafine silica, 5 parts of ultrafine alumina powder, 1 part of graphite powder, 0.5 parts of high-temperature strength enhancer, 0.5 parts of toughening material, 0.5 parts of retarder, and 127 parts of water; wherein, the molar ratio of calcium oxide to silica in the chemical composition of the Grade G cement is 1.8, and the seven-day heat of hydration of the Grade G cement is 250 kJ / kg; the high-temperature strength enhancer is ultrafine silica powder with a particle size of 15 μm, the toughening material is carbon fiber with a diameter of 5 μm, the ultrafine silica has a particle size of 15 μm, the ultrafine alumina powder has a particle size of 25 μm, the graphite powder has a particle size of 20 μm, and the retarder is citric acid.

[0032] The preparation process of cement slurry for well cementing is as follows: A1. Mix G-grade cement, ultrafine silica, ultrafine alumina powder, graphite powder, high-temperature strength enhancer and toughening material evenly to obtain dry mix; A2. Mix the retarder with water until homogeneous to obtain a mixture; A3. Add the dry mixture to the liquid mixture and stir evenly to obtain cementing slurry.

[0033] Example 3 In this embodiment, the cement slurry comprises, by weight, 70 parts of Grade G cement, 10 parts of ultrafine silica, 10 parts of ultrafine alumina powder, 2 parts of graphite powder, 1 part of high-temperature strength enhancer, 0.8 parts of toughening material, 1 part of retarder, and 160 parts of water; wherein, the molar ratio of calcium oxide to silica in the chemical composition of the Grade G cement is 2.4, and the seven-day heat of hydration of the Grade G cement is 260 kJ / kg; the high-temperature strength enhancer is ultrafine silica powder with a particle size of 8 μm, the toughening material is carbon fiber with a diameter of 7 μm, the ultrafine silica has a particle size of 8 μm, the ultrafine alumina powder has a particle size of 15 μm, the graphite powder has a particle size of 10 μm, and the retarder is citric acid.

[0034] The preparation process of cement slurry for well cementing is as follows: A1. Mix G-grade cement, ultrafine silica, ultrafine alumina powder, graphite powder, high-temperature strength enhancer and toughening material evenly to obtain dry mix; A2. Mix the retarder with water until homogeneous to obtain a mixture; A3. Add the dry mixture to the liquid mixture and stir evenly to obtain cementing slurry.

[0035] Comparative Example 1 The difference from Example 1 is that the molar ratio of calcium oxide to silicon oxide in the chemical composition of the G-grade cement is 2.8, and the seven-day heat of hydration of the G-grade cement is 270 kJ / kg.

[0036] Comparative Example 2 In this comparative example, the cement slurry, by weight, comprises: Grade G cement: 65 parts, ultrafine silica: 9 parts, ultrafine alumina powder: 7.5 parts, high-temperature strength enhancer: 0.75 parts, toughening material: 0.65 parts, retarder: 0.75 parts, and water: 144 parts. The Grade G cement has a molar ratio of calcium oxide to silica of 2.1, and a seven-day heat of hydration of 255 kJ / kg. The high-temperature strength enhancer is ultrafine silica powder with a particle size of 10 μm. The toughening material is carbon fiber with a diameter of 6 μm. The ultrafine silica has a particle size of 10 μm, the ultrafine alumina powder has a particle size of 20 μm, and the retarder is citric acid.

[0037] The preparation process of cement slurry for well cementing is as follows: A1. Mix G-grade cement, ultrafine silica, ultrafine alumina powder, high-temperature strength enhancer and toughening material evenly to obtain dry mix; A2. Mix the retarder with water until homogeneous to obtain a mixture; A3. Add the dry mixture to the liquid mixture and stir evenly to obtain cementing slurry.

[0038] Comparative Example 3 In this comparative example, the cement slurry, by weight, comprises: Grade G cement: 65 parts, ultrafine silica: 15 parts, graphite powder: 1.5 parts, high-temperature strength enhancer: 0.75 parts, toughening material: 0.65 parts, retarder: 0.75 parts, and water: 144 parts. The Grade G cement has a molar ratio of calcium oxide to silica of 2.1, and a seven-day heat of hydration of 255 kJ / kg. The high-temperature strength enhancer is ultrafine silica powder with a particle size of 10 μm. The toughening material is carbon fiber with a diameter of 6 μm. The ultrafine silica has a particle size of 10 μm, the ultrafine alumina powder has a particle size of 20 μm, the graphite powder has a particle size of 15 μm, and the retarder is citric acid.

[0039] The preparation process of cement slurry for well cementing is as follows: A1. Mix G-grade cement, ultrafine silica, graphite powder, high-temperature strength enhancer and toughening material evenly to obtain dry mix; A2. Mix the retarder with water until homogeneous to obtain a mixture; A3. Add the dry mixture to the liquid mixture and stir evenly to obtain cementing slurry.

[0040] Experimental Example The cement slurry prepared in Example 1 was cured (at 180℃ for 7 days) to form cement stone. The cement stone was characterized by scanning electron microscopy. The results are shown in the figure. Figure 1 The cement slurry prepared in Examples 1 to 3 and Comparative Examples 1 to 3 was cured (at 180℃ for 7 days) to form cement stone. The high-temperature compressive strength (tensile strength at 180℃), elastic modulus, and thermal conductivity of the cement stone were tested. The test results are shown in Table 2. Table 2 shows that, compared with Comparative Example 1, the high-temperature compressive strength of the cement stone corresponding to Examples 1 to 3 is higher. Compared with Comparative Examples 2 to 3, the thermal conductivity of the cement stone corresponding to Examples 1 to 3 is higher. Compared with Comparative Examples 1 to 3, the elastic modulus of the cement stone corresponding to Examples 1 to 3 is not significantly different, indicating that their toughness is not significantly different.

[0041] Table 2

[0042] The testing method involved in this invention: (1) The compressive strength test shall be conducted in accordance with GB / T 19139-2012 Oil Well Cement Test Method.

[0043] (2) The elastic modulus test was first cured using a standard mold of φ25.4mm×50.8mm, and then tested using a triaxial compression testing machine.

[0044] (3) Thermal conductivity shall be determined in accordance with GB / T 10295-2008 standard.

[0045] While the present invention has been disclosed above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention, and all such changes and modifications will fall within the scope of protection of the present invention.

Claims

1. A cementing slurry for medium-deep coaxial geothermal wells, characterized in that, The cement slurry for cementing medium-deep coaxial geothermal wells comprises, by weight, 60 to 70 parts of Grade G cement, 5 to 10 parts of ultrafine silica, 5 to 10 parts of ultrafine alumina powder, 1 to 2 parts of graphite powder, 0.5 to 1 part of high-temperature strength enhancer, 0.5 to 0.8 parts of toughening material, 0.5 to 1 part of retarder, and 127 to 160 parts of water; wherein the molar ratio of calcium oxide to silica in the chemical composition of the Grade G cement is 1.8 to 2.4, and the seven-day heat of hydration of the Grade G cement is not greater than 260 kJ / kg.

2. The cement slurry for cementing medium-deep coaxial geothermal wells according to claim 1, characterized in that, The high-temperature strength enhancer is ultrafine silica powder.

3. The cement slurry for cementing medium-deep coaxial geothermal wells according to claim 2, characterized in that, The particle size of the ultrafine silica powder is no greater than 15 μm.

4. The cementing slurry for medium-deep coaxial geothermal wells according to claim 1, characterized in that, The toughening material includes at least one of carbon fiber and carbon nanotubes.

5. The cementing slurry for medium-deep coaxial geothermal wells according to claim 4, characterized in that, The diameter of the toughening material is 5 μm to 7 μm.

6. The cementing slurry for medium-deep coaxial geothermal wells according to claim 1, characterized in that, The particle size of the ultrafine silica is no greater than 15 μm.

7. The cement slurry for cementing medium-deep coaxial geothermal wells according to claim 1, characterized in that, The particle size of the ultrafine alumina powder is no greater than 25 μm.

8. The cementing slurry for medium-deep coaxial geothermal wells according to claim 1, characterized in that, The particle size of the graphite powder is no greater than 20 μm.

9. The cement slurry for cementing medium-deep coaxial geothermal wells according to claim 1, characterized in that, The retarder is citric acid.

10. The method for preparing cement slurry for medium-deep coaxial geothermal wells as described in any one of claims 1 to 9, characterized in that, include: Step S1: Mix G-grade cement, ultrafine silica, ultrafine alumina powder, graphite powder, high-temperature strength enhancer and toughening material evenly to obtain dry mix; Step S2: Mix the retarder and water evenly to obtain a mixture; Step S3: Add the dry mixture to the liquid mixture and stir evenly to obtain cement slurry for cementing medium-deep coaxial geothermal wells.

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