CN114315256A - High-thermal-conductivity high-heat-insulation cement for geothermal well and preparation method thereof - Google Patents
High-thermal-conductivity high-heat-insulation cement for geothermal well and preparation method thereof Download PDFInfo
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Abstract
The invention belongs to the technical field of geothermal resource development, and particularly relates to high-thermal-conductivity high-heat-preservation cement for a geothermal well and a preparation method thereof, wherein the high-thermal-conductivity high-heat-preservation material comprises water, G-grade high-sulfate-resistance oil well cement, natural crystalline flake graphite, silicon carbide and alumina; the weight ratio of the water to the G-level high sulfate-resistant oil well cement is as follows: 0.42 to 0.60; the weight ratio of the natural crystalline flake graphite to the G-level high sulfate-resistant oil well cement is as follows: 0.05 to 0.10; the weight ratio of the silicon carbide and the alumina to the G-level high sulfate-resistant oil well cement is as follows: 0.02 to 0.04; the mass ratio of the silicon carbide to the aluminum oxide is 1: 1-2 nature. The formula of the invention is scientific and reasonable, the graphite, the silicon carbide and the alumina are all inorganic materials, the materials have excellent heat-conducting property, and the silicon carbide and the alumina can further fill pores and compact a matrix, so that the materials are mutually communicated, and the heat-conducting property of the well cementing material is improved.
Description
Technical Field
The invention belongs to the technical field of geothermal resource development, and particularly relates to high-thermal-conductivity high-heat-insulation cement for a geothermal well and a preparation method thereof.
Background
The well cementation of the geothermal well of the middle-deep layer which can not take water and heat is the construction process of injecting cement slurry into the annular space between the well wall and the outer pipe. The method aims to utilize the solidification of cement slurry to seal an annular space, support and protect an outer pipe, prevent corrosion of stratum fluid to a sleeve, prevent the stratum fluid from leaking mutually, protect a producing layer, require the whole underground system to be sealed and not to exchange fluid with the stratum, and directly relate to the service life of a geothermal well and the protection of geothermal resources on the condition of well cementation quality. The whole aspects, the whole process and all links of the well cementation operation must be fully considered.
For a long time, the well cementation of geothermal wells with middle-deep layers without taking heat or water basically adopts the well cementation technology of oil wells and cement paste, and after years of research, introduction and practice, the well cementation technology of the oil wells forms serialized cement, admixture and additive, and establishes a cement paste system and a process technology suitable for the well cementation of various types of oil wells. For the well cementation of a geothermal well with a middle-deep layer without taking water or heat, a cement sheath formed by the well cementation is a component of a geothermal well heat exchanger, the heat conductivity coefficient directly influences the heat exchange efficiency of a heat storage layer section, and the well cementation of the oil well cement paste generally does not consider the higher index requirement. The well cementation material with higher heat conductivity coefficient is selected for the middle-deep layer 'heat taking and water non-taking', so that the thermal resistance between the underground rock stratum and the sleeve can be effectively reduced, and the heat exchange efficiency of the heat exchanger is improved.
For the geothermal well constructed in the closing basin, the lost circulation well section exists due to low formation pressure, and the lost circulation is very easy to occur in casing running, circulation, slurry injection, displacement and coagulation waiting; the heat reservoir is mainly made of third-line loose sand mudstone, the mudstone cutting section is long, the friction resistance is large, and the difficulty of putting a large-size sleeve down to the designed well depth is large; according to the technical code DBJ61/T166-2020 applied to the middle-deep geothermal buried pipe heat supply system of the engineering construction standard, 4.5.5 the well cementation shall accord with the following contents: after the well cementation operation, the operation is finished within 24 hours after the drilling operation is finished, so that the phenomenon of underground water channeling in the construction process is prevented. On one hand, in the process of well cementation construction, a well cementation material can cover a well cementation layer of the whole middle-deep geothermal well only by overcoming flow resistance in a narrow and complex space, and the well cementation material is required to have good fluidity, so that grouting power is reduced, and well cementation accidents are avoided; on the other hand, the adoption of the well cementation material with poor heat conduction performance can obviously increase the thermal resistance in the underground heat exchange process of the geothermal well, and under the same heat production quantity, the drilling depth needs to be increased to make up the required heat, so that the initial cost investment and the operating cost are increased.
Therefore, the optimization and improvement of the geothermal well cementing material have important practical requirements and significance for ensuring the quality of the geothermal well.
Disclosure of Invention
The present invention is directed to overcoming at least one of the above problems of the conventional art and providing a high thermal conductivity and high thermal insulation cement for a geothermal well and a method for preparing the same.
In order to achieve the technical purpose and achieve the technical effect, the invention is realized by the following technical scheme:
a high heat conduction and high heat preservation cement for a geothermal well comprises water, G-level high sulfate-resistant oil well cement, natural crystalline flake graphite, silicon carbide and alumina;
the weight ratio of the water to the G-level high sulfate-resistant oil well cement is as follows: 0.42 to 0.60;
the weight ratio of the natural crystalline flake graphite to the G-level high sulfate-resistant oil well cement is as follows: 0.05 to 0.10;
the weight ratio of the silicon carbide and the alumina to the G-level high sulfate-resistant oil well cement is as follows: 0.02 to 0.04; the mass ratio of the silicon carbide to the aluminum oxide is 1: 1 to 2.
Further, in the cement with high thermal conductivity and high heat preservation for the geothermal well, the water is tap water in an experimental place.
Further, in the high-thermal-conductivity high-heat-insulation cement for the geothermal well, the natural crystalline flake graphite mainly comprises C, and the carbon content is 99.99%; the density of the natural crystalline flake graphite is 2.25g/cm3, the granularity is 35 mu m, the specific surface area is 115.9m2, and the thermal conductivity is 400W/(m.K).
Further, in the high-thermal-conductivity high-heat-insulation cement for the geothermal well, the silicon carbide is cubic silicon carbide, the main component of the silicon carbide is beta-SiC, and the purity is 99.99%; the density of the silicon carbide is 3.2g/cm3, the granularity is 2.5-3.5 mu m, and the thermal conductivity is 80W/(m.K).
Further, in the high-thermal-conductivity high-heat-preservation cement for the geothermal well, the main component of the alumina is alpha-Al 2O3, and the purity is 99.995%; the particle size of the alumina is 1 mu m, the density is 3.5g/m3, and the thermal conductivity is 33-36W/(m.K).
Further, in the above high thermal conductivity and high thermal insulation cement for geothermal wells, the weight ratio of the water to the G-level high sulfate-resistant oil well cement is: 0.45.
further, in the above high thermal conductivity and high thermal insulation cement for geothermal wells, the mass ratio of the silicon carbide and the alumina to the G-level high sulfate resistance oil well cement is: 0.03; the mass ratio of the silicon carbide to the aluminum oxide is 1: 2.
further, in the cement with high thermal conductivity and high thermal insulation for the geothermal well, the method comprises the following steps: accurately weighing the water quantity, the cement quantity and the admixture quantity required by each group of experiments by using an analytical balance, pouring the weighed materials into a slurry cup, and stirring the materials by using a stirrer for 3-5min to obtain the required high-heat-conductivity high-heat-insulation cement.
The invention has the beneficial effects that:
1. the invention has scientific and reasonable formula design, and the graphite, the silicon carbide and the aluminum oxide are all inorganic materials with excellent heat-conducting property. The graphite does not participate in the hydration reaction of cement, but can promote the hydration process of the cement, so that the cement is more fully hydrated, the hydration products are more, and unhydrated cement particles are effectively reduced, so that the overall structure of the high-heat-conductivity well cementing material is more compact, the pores are less, the pores are small, the thermal resistance in the heat transfer process is reduced, although the graphite has excellent heat conductivity, the addition is limited due to engineering feasibility, and the silicon carbide and the aluminum oxide can further fill the pores and compact the matrix, so that the materials are mutually communicated.
2. The cement matrix, graphite, silicon carbide and alumina constitute inorganic composite well cementing material, and the heat conducting principle of inorganic material is suitable for phonon heat transfer, i.e. energy is transferred from high temperature area to low temperature area through lattice vibration.
3. The research on the heat conduction mechanism of cement-based materials mostly refers to the heat conduction theory of inorganic high polymer materials, and the theory of heat conduction paths is mostly applied at present, graphite, silicon carbide and aluminum oxide are uniformly dispersed in a matrix and form heat conduction paths through mutual contact, and the heat conduction paths are further communicated with one another to form a heat conduction network, so that a heat conduction path mainly based on electron propagation is formed, the heat energy transfer rate is accelerated, and the heat conduction performance of a well cementation material is improved.
Of course, it is not necessary for any one product that embodies the invention to achieve all of the above advantages simultaneously.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to the drawings without creative efforts.
FIG. 1 is a graph of the variation of thermal conductivity with water-cement ratio in the example;
FIG. 2 is a graph showing the thermal conductivity change of the inorganic thermal conductivity single-doping test in the examples;
FIG. 3 is a schematic diagram of the hierarchical structure modeling according to the AHP method in the embodiment;
FIG. 4 is a schematic diagram illustrating the consistency detection of the judgment matrix according to the AHP method in the embodiment;
FIG. 5 is a diagram illustrating the calculation results according to the AHP method in the embodiment;
FIG. 6 is an SPSSAU online data input interface in an embodiment;
FIG. 7 is an interface of the results of the SPSSAU online calculation in the embodiment;
FIG. 8 is a graph of cement slurry rheology in the examples.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The experimental process adopts the following materials:
1. water for experiment: experimental tap water.
2. Cement: high-grade high-sulfate (HSR) type G oil well cement with a particle size of 300 meshes and an execution standard of API Spec 10A GB/T10238-2015, and the manufacturer is Ningxia bronze gorge cement.
3. Filler material
Natural flake graphite: the main component C, type T-C-04, has a density of about 2.25g/m3Particle size 35 μm and specific surface area 115.9m2The thermal conductivity coefficient is 400W/(m.K), and the carbon content is 99.99%. The manufacturer: shaanxi six-membered carbon Crystal Co., Ltd.
Silicon carbide, mainly beta-SiC, cubic silicon carbide, with a density of about 3.2g/m3The type is W3.5, the granularity is 2.5-3.5 mu m, the purity is 99.99 percent, and the thermal conductivity is 80W/(m.K). The manufacturer: sienbole new materials, llc.
Alumina, main component of which is alpha-Al2O3Particle size of 1 μm, purity of 99.995%, density of about 3.5g/m3The thermal conductivity is 33-36W/(mK). The manufacturer: ningxia Jingming science and technology Co.
Experimental procedure
The test for testing the heat conductivity coefficient of the well cementation composite material and the manufacturing process of the test block are as follows:
1. preparing cement paste, accurately weighing the water quantity, the cement quantity and the admixture quantity required by each group of experiments by using an analytical balance, pouring the weighed cement paste into a paste cup, and stirring the cement paste by using a stirrer (4000r/min) for 3-5min to prepare the required paste for later use;
2. testing fluidity, leveling a glass plate, horizontally placing a truncated cone round mold in the center of the glass plate, quickly pouring the slurry into the truncated cone round mold, leveling by using a scraper, quickly and vertically lifting the truncated cone round mold upwards, measuring the diameters of the slurry for 3 times in different radial directions by using a straight ruler when the slurry is stable and does not flow, and taking the average value of the diameters to test the fluidity;
3. measuring the density, namely introducing the slurry into a liquid densimeter to measure the density of the slurry;
4. performing rheological property test, namely introducing the slurry into a slurry cup special for a six-speed rotational viscometer, performing 6-speed test according to 3r/min, 6r/min, 100r/min, 200r/min, 300r/min and 600r/min, recording instrument readings at different rotating speeds, calculating the plastic viscosity PV value of the slurry to be 1.5 (reading of 300 r/min-100 r/min) mPa.s, and calculating the hydraulic shearing force YP value of the slurry to be 0.511 (reading of 300 r/min-PV value) Pa;
5. thickening test, namely guiding the slurry into a special slurry cup, and performing a pressurizing thickening test by using a pressurizing thickening instrument;
6. injecting the slurry into a copper model (5cm multiplied by 5cm) to prepare test blocks, and preparing 4 blocks in each group;
7. putting the test block into a cement quick curing box for curing, and setting curing conditions: keeping the temperature at 52 +/-2 ℃ and the relative humidity at over 95 percent, carrying out water bath, maintaining for 48 hours, then removing the mold and wiping the mold clean, and finishing the manufacturing of the cement stone.
8. Testing the compressive strength, wherein two cement stones are taken in each group, a microcomputer-controlled constant stress pressure testing machine is adopted to carry out 48-hour compressive strength testing, and the testing average value is taken to calculate the 48-hour compressive strength value;
9. and (3) testing the heat conductivity coefficient: two cement stones in each group are drilled and cored by a coring machine (the inner diameter of a drill bit is phi 25) to obtain a cylindrical sample, the two ends of the cutting machine are polished and leveled after being divided into two parts, and a standard test block (phi 25 multiplied by 20) for testing the heat conductivity coefficient is prepared;
10. and (3) placing the standard test block into a DRL-III type thermal conductivity tester to test the thermal conductivity, and taking the average value of the two tests.
Experimental protocol
1. Influence of water cement ratio on well cementation cement heat conductivity coefficient
As shown in Table 1, 5 groups of pure slurry are prepared by selecting different water-cement ratios, wherein the water-cement ratios are 0.42, 0.45, 0.5, 0.55 and 0.60 in sequence, test blocks with the specification of 50mm multiplied by 50mm are prepared, and relevant parameters are tested.
TABLE 1 Water cement ratio neat paste test block protocol
| Water cement ratio | 0.42 | 0.45 | 0.5 | 0.55 | 0.60 |
| Water/cement (g) | 0.42G/G | 0.45G/G | 0.5G/G | 0.55G/G | 0.6G/G |
2. Influence of different fillers on heat conductivity coefficient of well cementation cement
The thermally conductive fillers commonly used as thermally conductive particles are generally of three types: metal material, carbon material, and inorganic heat conductive particle. Due to the wide variety of materials, the availability of raw materials and the feasibility of the scheme are considered. Although the metallic material has a high thermal conductivity, the metallic powder has a density higher than that of cement (3.0-3.15 g/cm)3) For example, Fe powder and copper powder, on one hand, can be settled and separated in the uniform mixing process, and on the other hand, the pumping in is very difficult in the construction, which is not suitable for the admixture of well cementing cement. Therefore, the heat conducting filler in the experiment is made of carbon materials and inorganic heat conducting particles, the carbon materials are made of graphite, and the inorganic heat conducting particles are made of silicon carbide and aluminum oxide. The three properties are shown in table 2:
TABLE 2 Property parameter Table for different materials
In order to ensure that the cementing cement in the cementing composite material should account for the main part, the upper limits of the filler proportion are all 30% (the filler proportion is the ratio of the heat-conducting filler to the total material (cement to filler), and is the mass ratio and is expressed by M). The proportion of the filler was adjusted according to the specific conditions of the test mold and the filler, and the planned embodiment of the mix ratio of the components of the test block is shown in table 3:
TABLE 3 test piece ratio (GP, SiC, Al)2O3)
3. High heat-conducting cement experimental scheme setting
9 groups of heat conduction material test groups are set for research and test, and water-solid ratio, Graphite (GP), silicon carbide (SiC) and aluminum oxide (Al) are selected2O3) The mass fraction is 4 factors, and the codes are A, B, C and D respectively; each factor is respectively set at 3 levels, the mixing is carried out according to the mass percentage of the cement, and a four-factor three-level orthogonal test table L is set9(3)4The parameter settings are shown in table 4 below:
TABLE 4 orthogonal test Table
Experiment for analyzing influence factors of heat conductivity
Factors influencing the heat conductivity coefficient of the well cementing material include water cement ratio, filler type, filler particle size, filler proportion, curing time, curing environment, additives and the like. The well cementation is under the underground constant temperature and humidity condition, so the influence of factors such as maintenance time and maintenance environment has little significance on engineering application research; in engineering practice, the well cementation cement additive is mainly an antifoaming agent and is mainly used for reducing bubbles generated in the preparation process of cement slurry, so that formed cement stones are more compact, the heat conductivity coefficient is favorably improved, and the cement additives are required to be added according to a fixed proportion in experiments and engineering and can be regarded as constants, so that the cement additives have little significance in engineering application research. In view of the above, the experiment only performed the influence factor analysis experiment on the water-cement ratio, the filler type and the filler ratio.
Influence of water cement ratio on well cementation cement heat conductivity coefficient
1. Selecting different water-cement ratios to prepare 4 groups of neat pastes, manufacturing test blocks with the specification of 50mm multiplied by 50mm, and testing the heat conductivity coefficient of each test block for 48 hours, wherein the specific values are shown in Table 5:
TABLE 5 thermal conductivity coefficients of different water-cement ratios
| Water cement ratio | Amount of Water added (g) | Cement addition (g) | Thermal conductivity W/(m.K) | Sample number |
| 0.42 | 300 | 800 | 1.3527 | |
| 0.45 | 360 | 800 | 1.5809 | Number 2 |
| 0.5 | 400 | 800 | 1.2809 | No. 3 |
| 0.55 | 400 | 800 | 1.1138 | |
2. Influence Curve analysis
As can be seen from the curve (figure 1) of the thermal conductivity along with the change of the water-cement ratio, the thermal conductivity increases along with the increase of the water-cement ratio, the thermal conductivity is 1.5809W/(m.K) at the water-cement ratio of 0.45, the thermal conductivity reaches a peak value, and then the thermal conductivity decreases along with the increase of the water-cement ratio, which indicates that the thermal conductivity of the well cementing material is not in a linear relation with the water-cement ratio, the optimal value is 0.45, and the thermal conductivity is completely consistent with the water-cement ratio of 0.45 of the conventional well cementing material produced by the selected Ningxia Yi Jie special engineering materials Limited company. When the water-cement ratio is larger than 0.45 and is continuously increased, the thermal conductivity coefficient obviously shows linear reduction, because the water content of the slurry is increased, and the thermal conductivity coefficient of water is smaller than that of the cement material, the higher the water content is in the hardening process of the cement material, the looser the structure of the hardened material is, the pore structure of the material is increased, the heat energy transfer is not facilitated, and the thermal conductivity coefficient is reduced; when the water cement ratio is less than 0.45, the heat conductivity coefficient is also reduced, mainly because the water cement ratio is small, the hydration speed is slow, the residual water content is less or the cement is not hydrated sufficiently, materials forming colloid and crystal cannot be formed sufficiently, the unhydrated cement is hydrated again in the curing process after the set cement is fully hardened, and the formed structure of the set cement can be damaged by the expansive force caused by hydration products, so that the heat conductivity coefficient is reduced; description of the experiment: the water-cement ratio is 0.45, cement particles can be fully mixed with water for hydration, an ideal heat conduction effect is achieved, the water-cement ratio is basically consistent with the water-cement ratio of 0.44 in a test reported by the experimental G-level high-anti-sulfate (HSR) type oil well cement, and ideal and reliable water-cement ratio parameters are provided for subsequent experiments.
(II) influence of different filler types and mixing amounts on thermal conductivity
1. According to the experimental results, the water-solid ratio is set to be 0.45 in the experiment, and the filler is graphite, silicon carbide and alumina. Wherein the adding proportion of graphite is up to 10 percent, and the adding proportion of other materials is up to 30 percent. The results of the experiments are shown in tables 6 to 8:
TABLE 6 coefficient of thermal conductivity of graphite filler
TABLE 7 Heat conductivity of the filler silicon carbide
TABLE 8 thermal conductivity of filler alumina
2. Influence Curve analysis
From FIG. 2, it can be seen that the proportion of the filler varies with the materialThe thermal conductivity of the cementing composite material is improved to different degrees, when the single doping amount of the filler is 5%, the thermal conductivity of graphite, silicon carbide and alumina cement stone is 1.9601W/(m.K), 1.8695W/(m.K) and 1.8535W/(m.K), and the thermal conductivity of the cementing composite material is improved by 23.99%, 18.26% and 17.24% compared with that of pure cement stone 1.5809W/(m.K). When the graphite is singly doped by 10 percent, the heat conductivity coefficient reaches 2.0359W/(m.K), the heat conductivity coefficient is improved by 28.78 percent, which shows that the three fillers are all ideal fillers for strengthening the heat transfer of the well cementing material, and the graphite has more obvious effect, and when the fillers are silicon carbide and alumina, the silicon carbide has better effect of strengthening the heat conductivity coefficient of the well cementing material and the alumina is inferior. The density of the graphite, the silicon carbide and the alumina powder is 2.25g/cm in sequence3、3.2g/cm3、3.5g/cm3The filling materials with the same mass are doped, the filled volume sequentially comprises graphite, silicon carbide and aluminum oxide, and according to the theory of a heat conduction path, a good heat conduction path is formed to play a good reinforcing effect, so that the density (filling volume) of the filling materials is also the key influencing the filling effect in addition to the heat conduction performance of the filling materials. Different materials are mixed into the well cementation material, and the influence on the heat conductivity of the well cementation composite material is different due to the difference of physical properties of the materials.
The graphite doping amount reaches 10%, the uniform doping with cement is difficult in experiments, a stirrer is also difficult to stir at a constant speed (4000r/min), the calculated plastic viscosity YP value of the slurry is 96MPa & s, the dynamic shear force is 32.13Pa, and the plastic viscosity reflects that the strength of the internal friction among suspended solid-phase particles, between the solid-phase particles and a liquid phase and inside a continuous liquid phase is in dynamic balance when the damage and the recovery of a liquid-heavy grid structure are in the condition of laminar flow, which shows that the slurry plastic viscosity is increased along with the increase of the graphite addition amount, the pumping performance of the slurry is poor, the construction is expressed, and the preparation and the construction of well cementing materials are difficult to implement smoothly; from the cement compressive strength test results, the cement compressive strength is reduced along with the increase of the graphite amount, which indicates that the more the graphite is, the better the graphite is, and the experiment selects the addition of 10 percent at most reasonably. The same problem exists in the alumina in the experiment, especially when the 25 percent proportion is doped, the six-speed rotational viscometer can not read the corresponding data because the plastic viscosity of the slurry is too high.
Heat transfer performance coupling analysis
In order to comprehensively evaluate the influence of different admixtures on the heat-conducting property, an orthogonal test method is adopted in the test, 9 groups of well-cementing materials with different mixing ratios are designed, the tested heat-conducting coefficient, fluidity and 48h compressive strength of the set cement are used as evaluation indexes, the AHP-CRITIC mixed weighting method is adopted to determine the weight of each evaluation index, and the heat-conducting property influence factor analysis of the well-cementing materials and the optimal mixing ratio determination of the high-heat-conducting well-cementing materials are carried out by extreme difference analysis.
(one) orthogonal test
Through the tests, the water-solid ratio (A), the Graphite (GP) doping amount (B), the silicon carbide (SiC) doping amount (C) and the aluminum oxide (Al) are selected2O3) The doping amount (D) is 4 factors, each factor is divided into 3 levels, 9 test groups are totally arranged, a four-factor three-level orthogonal test method is adopted, and the basic performance of 9 groups of well cementing materials tested by the test is shown in a table 9:
TABLE 9 results of basic Performance test of orthogonal test
Note: the number preceding 0.44(1) in the table indicates the value of the parameter, and the number in parentheses indicates the level.
(II) thermal conductivity coupling analysis
In order to reasonably determine the comprehensive weight of multiple performance indexes of the well cementation material, not only can show the primary and secondary sequence of each index, but also can objectively and comprehensively show the data information of a sample and ensure the reliability and the effectiveness of an evaluation result, an AHP-CRITIC mixed weighting method is adopted for evaluation in a test, and the weight omega is omegaZiThe calculation formula is as follows:
ωZi＝ωAi·ωCi/∑ωAiωCiformula (I)
In the formula: omegaAiRepresents the weight calculated by the AHP method; omegaCiRepresenting the weight calculated by the CRITIC method; i represents an evaluation index.
In order to eliminate the influence of unit dimensions of 3 evaluation indexes and facilitate the comparison among the evaluation indexes, the test results of the tests shown in the tables 2 to 10 are subjected to linear standardization treatment, namely: the evaluation index standard value (measured value/maximum value) × 100, and the calculation results of each index are shown in table 10:
TABLE 10 comprehensive scoring results table
1. AHP method
According to the determination method of index weight of AHP (analytic hierarchy process), 3 performance indexes of heat conductivity coefficient, 48h compressive strength and fluidity of the well cementation material are divided into 3 levels, and the priority of each index is determined according to the relative importance degree of the indexes as follows: the heat conductivity coefficient is more than 48h, and the compressive strength is slightly important relative to the fluidity, so the heat conductivity coefficient is compared with the compressive strength and then is assigned 4; the heat conductivity coefficient is compared with the fluidity and then is assigned with a value of 5; the 48h compressive strength and the fluidity are assigned 1, and a judgment matrix table 12 of the model is constructed by referring to each level of the scale meaning table 11 of the judgment matrix according to the scale meaning and the assignment result.
TABLE 11 judgment matrix level Scale of meanings Table
TABLE 12 index decision matrix
| Coefficient of thermal conductivity | Compressive strength | Degree of fluidity | |
| Coefficient of |
1 | 4 | 5 |
| |
1/4 | 1 | 1 |
| Degree of |
1/5 | 1 | 1 |
Using yaahp12.6 software, the calculation procedure is shown in fig. 3 to 5.
The AHP weights of the thermal conductivity coefficient, the 48h compressive strength and the fluidity are calculated to be 0.6908, 0.1603 and 0.1488 respectively; and (4) judging that the priority matrix has consistency by comparing the indexes in pairs and judging that the priority matrix has consistency and the weight coefficient is effective when the consistency scale factor CR is 0.0053 < 0.10.
2. CRITIC method
According to the idea of obtaining the index weight by the CRITIC method, SPSSAU software is used, as shown in fig. 6 and 7, the standard values of the thermal conductivity, the 48h compressive strength and the fluidity are substituted for people, and the CRITIC weights of the thermal conductivity, the 48h compressive strength and the fluidity are obtained by calculation and are 0.3675, 0.3639 and 0.2686 in sequence.
Calculating AHP-CRITIC weights of the heat conductivity coefficient, the 48h compressive strength and the fluidity of each group of well cementing materials according to the formula (I) to be 0.7209, 0.1656 and 0.1135 in sequence, thereby obtaining the final comprehensive scores of each group, namely: the composite score (standard value of thermal conductivity × 0.7209+48h standard value of compressive strength × 0.1656+ standard value of fluidity × 0.1135) × 100. The overall score results for each test group are shown in table 10.
(III) analysis of range differences and determination of optimal mix proportion
The range analysis was performed in tables 9 and 10, and the procedure was as follows:
1. summing the values of the factors at each level to obtain KijnWherein i represents index thermal conductivity L, 48h compressive strength M, fluidity N and comprehensive score O; level j is 1, 2, 3; n represents factors A, B, C, D.
2. For the same level K of each indexijnAveraging to obtain km，km＝Kijn/3；
3. Calculating the range R of each factor column of each indexn，Rn＝km,max-km,min。RnThe larger the value is, the more important the factor has an influence on a certain evaluation index. Thus obtaining the level value K of each factorijnAnd RnValues, results are shown in table 13:
TABLE 13 results of the orthogonal test are very poor
According to the results in Table 13, the primary and secondary influence orders of all factors on the heat conductivity coefficient of the well cementing material are B > D > C > A; the primary and secondary sequence of the influence on the 48h compressive strength of the well cementing material is D > B > C > A, and the primary and secondary sequence of the influence on the fluidity is B > C > D > A; the primary and secondary sequence of the comprehensive influence on three indexes of the well cementation material is B > D > C > A; according to the range analysis result of each group of comprehensive scores, according to the principle that the larger the evaluation index value is, the better the evaluation index value is, the optimal factor level combination (namely the optimal mixing ratio) of the well cementation material is A2B3C1D2, and the optimal mixing ratio of the test is as follows: the water-solid ratio is 0.45%, 8% of graphite, 1% of silicon carbide and 2% of aluminum oxide.
The graphite is a main factor influencing the comprehensive performance of the high-heat-conductivity well cementing material, and as the graphite doping amount is increased, the heat conductivity coefficient of the well cementing material is increased, the 48-hour compressive strength is reduced, and the fluidity of slurry is reduced; the water-solid ratio is a secondary factor influencing the comprehensive performance of the well cementing material, and as the water-solid ratio increases, the heat conductivity coefficient of the well cementing material decreases, the compressive strength decreases within 48h, and the fluidity of slurry increases; the silicon carbide and the aluminum oxide have small influence on the comprehensive performance of the well cementing material and belong to common factors. The reason is mainly that: the graphite has high self heat conductivity coefficient which is 4-5 times that of silicon carbide and 11-12 times that of aluminum oxide, and contributes to the improvement of the heat conductivity coefficient of the well cementing material to the maximum extent. The graphite can adsorb water with stronger polarity, and after the graphite coats a part of mixing water, the mixing water participating in cement hydration is reduced, so that the consistency of the whole slurry system is increased, and the fluidity is reduced. Along with the increase of the graphite doping amount, the contact degree between graphite particles is increased, the contact area is increased, so that slippage occurs between the graphite particles, and the compression strength of the set cement is reduced; and along with the increase of the graphite doping amount, in order to keep the better flowing property and the lower consistency of the well cementation slurry, the mixing water amount is increased, the mass percentage of cement in the total mixing material is reduced, the water-cement ratio of the slurry is increased, so that the defects of internal pores of cement stones formed by the hardening of the slurry and the like are increased, and the compressive strength of the cement stones is finally reduced. The silicon carbide has good hydrophilicity, stable chemical property, oxidation resistance and acid and alkali corrosion resistance. The density of the beta-SiC is close to that of cement, the mixing effect with the cement slurry is good in the experimental process, the fluidity of the cement slurry is not changed basically, and the mixing effect is closely related to the crystal structure of the silicon carbide, wherein the crystal structure is hexagonal or rhombohedral. The main chemical components of the portland cement are as follows: CaO accounts for 64-67%; SiO 22＝20～23％；Al2O3＝4～8％；Fe2O33-6% of the total; the main mineral components of the cement clinker are: tricalcium silicate (3cao. sio3 abbreviated as C3S), dicalcium silicate, tetracalcium aluminoferrite, tricalcium aluminate; the addition of alumina powder does not change the properties of the cement slurry, and the chemical composition of the cement itself contains Al2O3The same physical properties, but aluminaThe addition of (2) has obvious change on the fluidity of cement paste, and the mixing is relatively easy. Therefore, in order to ensure that all indexes of the well cementing material meet the well cementing requirements of the geothermal well, the graphite mixing amount needs to be controlled.
Verification experiment of optimized cement cementing material
To further confirm the effectiveness of the optimal mix ratio A2B3C1D2, 3 sets of parallel validation tests were designed to validate the preferred results.
(first) test conditions
According to the mixing ratio: the water cement ratio is 0.45+ 8% of graphite, 1% of silicon carbide and 2% of alumina, the actual addition amount of each group is that water amount is 360g + cement 704g + graphite 64g + silicon carbide 8g + alumina 16g to prepare 800ml of slurry, various performance indexes are actually measured to prepare 3 groups of cementing materials, and the performance index results obtained by measurement are shown in tables 14-16:
table 14 verification test results table
Table 15 verification test results table
TABLE 16 evaluation index standard value calculation table for verification test
(II) evaluation of comprehensive analysis
For a well cementation material for well cementation construction of a geothermal well with a middle-deep layer without taking water and heat, the heat conductivity of the well cementation material needs to meet engineering requirements, and each index of the well cementation material needs to meet the stipulation of SY/T6544-2017 oil well cement performance requirements at the same time to ensure that the engineering can be used, the test is carried out according to GB/T19139-2012 oil well cement test method, a straight well cementation material produced by Ningxia Yi special engineering material company is used as a comparison group, and the test mixing ratio is verified from four aspects of heat conductivity, rheological property, 24-hour compressive strength and thickening property.
1. National standard cementing slurry performance requirement
SY/T6544-2017 oil well cement slurry performance requirements are given in Table 17 below:
TABLE 17 Cement paste Performance requirements
(1) The thickening time is the time elapsed from the start of the increase in temperature and pressure until the consistency reaches 100Bc, and the consistency of the cement paste at the end of the thickening time test is recorded. And the minimum time for thickening the cement of the G-grade oil well is not less than 90 min. (in accordance with GB/T10238-2015 acceptance requirements for cement thickening time for oil wells).
(2) And (4) measuring the fluidity of the cement slurry according to a standard testing method of the fluidity of the cement clean slurry. And injecting the stirred neat paste into the truncated cone circular mold, lifting the truncated cone circular mold, and measuring the maximum diameter of the cement neat paste which freely flows on the glass plane. The time from the start of stirring the cement slurry to the measurement of the diameter of the cement slurry less than 14cm is the pumpable period of the cement slurry.
2. Basic parameters of control group well cementation cement slurry performance
A vertical well cementing material produced by Ningxia hundred and eleven special engineering material company is adopted as a control group.
The formula is as follows: cement, fluid loss additive, reinforcing agent, stabilizing agent, rheological agent and retarder.
The control parameters are shown in table 18:
TABLE 18 control group parameters
(1) Coefficient of thermal conductivity: from Table 13, the thermal conductivity of three groups of test set cements is 2.1638 w/(m.K), 2.1838 w/(m.K) and 2.1797 w/(m.K), the average thermal conductivity is 2.176 w/(m.K), and the thermal conductivity is improved by 27.4% compared with the thermal conductivity of 1.58 w/(m.K) of the control group, thus achieving the target of expected thermal conductivity.
(2) Compressive strength: from table 13, the 48-hour compressive strength of the three sets of test set cements were: 18.2MPa, 14.9MPa and 15.2MPa, all of which are more than 14MPa, and meet the performance requirements of oil well cement paste.
(3) Thickening performance: and pouring the stirred cement paste into a slurry cup of a thickening instrument, measuring the consistency change of the cement paste according to the heating and pressure increasing rates of 52 ℃/30min and 35.6MPa/30min, recording the maximum consistency of the cement paste within 15-30 min as the initial consistency, and taking the time when the consistency of the cement paste reaches 100Bc as the thickening time. In the test, the initial temperature value is 24.1 ℃, the initial pressure is 0.8MPa, the final temperature value is 52.1 ℃, the final pressure value is 35.9MPa, the test duration is 168min, the final consistency value is 99.4Bc, the initial consistency of the slurry for verifying the proportioning is 15.9Bc, the slurry meets the specification requirement and is less than 30Bc, and the thickening time can be adjusted according to the actual construction condition of the engineering.
(4) Rheological properties: in the experiment, a vertical well cementing material produced by Ningxia hundred thousand special engineering material company is used as a control group, a six-speed rotary viscometer is used for measuring and recording data of a preferred group and the control group, and GB/T19139-2012 oil well cement test method is adopted to calculate rated shear rate and shear stress:
γ＝1.7045×nr
τ(Pa)＝0.511×F×θ
in the formula: gamma-nominal shear rate in units of one second(s)-1)；
nrViscometer rotation speed in revolutions per minute (r/min);
τ -shear stress in pascals (Pa);
θ — viscometer reading;
f is the torque spring coefficient, and is 1;
the results are shown in Table 19, and the rheological curves are shown in FIG. 8.
TABLE 19 shear Rate and shear stress calculation Table
As can be seen from FIG. 8, as the shear rate of the two slurries increased, the shear stress of the two slurries also increased correspondingly, the curve change rules were substantially consistent, the performance was stable, no sudden change occurred, indicating that the preferred group had excellent rheological properties as the control group.
(5) Relative standard deviation, the relative standard deviation was calculated using the following formula:
in the formula: RSD — relative standard deviation; s-standard deviation; x is the number ofi-a composite score; x-mean composite score; n-number of calculations
The relative standard deviation is calculated to be 0.50 percent, which shows that the test is stable and the repeatability is good.
In conclusion, the optimal mixing ratio thermal conductivity is 2.176 w/(m.K) in the test, the expected thermal conductivity target is achieved, and all indexes meet the requirements of geothermal well cementation construction.
The preferred embodiments of the invention disclosed above are intended to be illustrative only. The preferred embodiments are not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Obviously, many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and the practical application, to thereby enable others skilled in the art to best utilize the invention. The invention is limited only by the claims and their full scope and equivalents.
Claims (8)
1. The cement with high thermal conductivity and high thermal insulation for the geothermal well is characterized in that the material with high thermal conductivity and high thermal insulation comprises water, G-level high-sulfate-resistance oil well cement, natural crystalline flake graphite, silicon carbide and alumina;
the weight ratio of the water to the G-level high sulfate-resistant oil well cement is as follows: 0.42 to 0.60;
the weight ratio of the natural crystalline flake graphite to the G-level high sulfate-resistant oil well cement is as follows: 0.05 to 0.10;
the weight ratio of the silicon carbide and the alumina to the G-level high sulfate-resistant oil well cement is as follows: 0.02 to 0.04; the mass ratio of the silicon carbide to the aluminum oxide is 1: 1 to 2.
2. The high thermal conductivity and high thermal insulation cement for geothermal wells according to claim 1, wherein: the water was experimental tap water.
3. The high thermal conductivity and high thermal insulation cement for geothermal wells according to claim 1, wherein: the natural crystalline flake graphite mainly comprises C, and the carbon content is 99.99%; the density of the natural crystalline flake graphite is 2.25g/cm3Particle size of 35 μm and specific surface area of 115.9m2The thermal conductivity is 400W/(mK).
4. The high thermal conductivity and high thermal insulation cement for geothermal wells according to claim 1, wherein: the silicon carbide is cubic silicon carbide, the main component of the silicon carbide is beta-SiC, and the purity is 99.99 percent; the density of the silicon carbide is 3.2g/cm3The particle size is 2.5-3.5 μm, and the thermal conductivity is 80W/(m.K).
5. The high thermal conductivity and high thermal insulation cement for geothermal wells according to claim 1, wherein: the main component of the alumina is alpha-Al2O3Purity of 99.995%; the particle size of the alumina is 1 mu m, and the density is 3.5g/m3The thermal conductivity is 33-36W/(mK).
6. The high thermal conductivity and high thermal insulation cement for geothermal wells according to claim 1, wherein: the weight ratio of the water to the G-level high sulfate-resistant oil well cement is as follows: 0.45.
7. the high thermal conductivity and high thermal insulation cement for geothermal wells according to claim 5, wherein: the weight ratio of the silicon carbide and the alumina to the G-level high sulfate-resistant oil well cement is as follows: 0.03; the mass ratio of the silicon carbide to the aluminum oxide is 1: 2.
8. the method for preparing the cement with high thermal conductivity and high thermal insulation according to any one of claims 1 to 7, characterized in that the method comprises the following steps: accurately weighing the water quantity, the cement quantity and the admixture quantity required by each group of experiments by using an analytical balance, pouring the weighed materials into a slurry cup, and stirring the materials by using a stirrer for 3-5min to obtain the required high-heat-conductivity high-heat-insulation cement.
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