ADDITIVE FOR THICK WELL CEMENT SUSPENSION AND METHOD FOR PRODUCING THIS ADDITIVE, THICK WELL CEMENT SUSPENSION AND WELL CEMENTING METHOD

Abstract

An additive for a thick cement slurry for a well is provided that is capable of suppressing the generation of free water and preventing the flotation / separation of low specific gravity aggregate while ensuring sufficient cement strength even at high temperatures. A method for producing this additive is also provided. This additive for a thick cement slurry for a well contains an aqueous dispersion of silica and a layered silicate.

Description

ADDITIVE FOR THICK WELL CEMENT SUSPENSION AND METHOD FOR PRODUCING THIS ADDITIVE, THICK WELL CEMENT SUSPENSION AND WELL CEMENTING METHOD FIELD OF INVENTION The present invention relates to an additive for thick cement slurry for a well and a method for producing the same, thick cement slurry for a well and a cementing method for a well. BACKGROUND OF THE INVENTION In drilling wells for oil fields, gas fields and the like, cementing work is carried out by injecting a thick slurry of cement into a space (also referred to as an annular space: annulus, etc.) between a casing pipe and the stratum (wall) in order to fix or reinforce the casing pipe inserted as an internal frame in the well, to prevent corrosion and to prevent the flow of groundwater into the well, in the completion of the well. The cement used in this work is defined as various types of oil well cement based on class or sulfate resistance according to the API specification (specification with respect to petroleum stipulated by the American Petroleum Institute) as a specification that takes into consideration the performance cncz Ln / Lznz / E / YiAi Ref. 325331 required. Among others, Class G cement is the most commonly used cement for drilling oil wells. The cement is designed according to the well conditions and is supplemented with water and a lightweight aggregate, as well as an additive such as a cement hardening accelerator, a cement hardening retarder, a cement dispersant, a cement dehydration regulator, a cement strength stabilizer, or a lost circulation material for its preparation. Cementing refers to the application of a thick cement slurry prepared from cement and water or dissolving water containing an additive in various locations within a well or inside or outside a casing. In drilling wells for oil, gas, and similar reservoirs, the drilling operations using a drill bit (drilling tool) and the cementing operations described above are performed repeatedly. Both the temperature and pressure at the work site increase with the depth of the oil well. In recent years, drilling techniques have improved, and drilling has been actively carried out in deep-layer oil and gas reservoirs with a depth of cncz Ln / Lznz / E / YiAi 500 to 1000 m or more. Therefore, there is a demand for the design of thick cement slurries that allow cementing even in a high temperature and high pressure environment. However, the diversion of the thick cement slurry to a high-permeability layer (hereafter also referred to as lost circulation) becomes critical with increasing pressure. Therefore, a casing cannot be set or reinforced even if the thick cement slurry is injected into the space between a casing pipe inserted into the well and the stratum. This causes significant adverse effects on the cementing operation. As a countermeasure, a method has been used that involves decreasing the specific gravity of the thick cement slurry, thereby reducing the pressure applied to the stratum and, consequently, suppressing lost circulation. Methods such as increasing the amount of water and adding a lightweight aggregate such as a hollow particle are known as methods for decreasing the specific gravity of the thick cement slurry. The increased water content causes visible segregation of the material (higher free water content) in the thick slurry. Adding a large amount of lightweight aggregate causes the lightweight aggregate to separate by flotation from the thick cement suspension (cncz Ln / Lznz / E / YiAi), resulting in a non-homogeneous aggregate and a marked reduction in flowability or non-homogeneous hardening (see Patent Document 2). As a solution to a non-homogeneous lightweight aggregate, Patent Document 1 describes that to avoid separation by flotation of a weight regulator such as perlite, fly ash, diatomaceous earth, or microsilica after preparation of the thick cement slurry, it is preferred to add 2% by weight or more (preferably 3 to 8% by weight; an amount with respect to a cement composition) of bentonite at the same time as the specific weight regulator. However, Patent Document 2 describes that if highly expandable bentonite is added in a large quantity, the addition of bentonite reduces the strength of the cement at a high temperature of 250 °C or more. Patent Document 3 describes the addition of 0.1 to 25% by weight of cement (BWOC) of a non-colloidal nanoclay mineral ranging from 1 to 400 nm for loss of circulation control. The thick cement slurry used herein has a specific gravity of 1.61 to 1.88 and is therefore a medium- to heavy-weight thick cement slurry. Patent Document 4 describes a lightweight cement composition containing 20% ​​to 100% BWOC of hollow spherical silica, 0.25% to 20% BWOC of bentonite component, 1% to 100% BWOC of a fine calcium carbonate powder, 1% to 100% BWOC of a medium calcium carbonate powder, 1% to 100% BWOC of a silica sand composition, and a silica flour composition. The density of the thick cement slurry described herein is 83 pcf (specific gravity: 1.33). As mentioned previously, suppressing lost circulation in thick cement slurries is required during the process of setting or reinforcing casing by injecting the thick cement slurry into the space between the casing inserted into the well and the stratum during the drilling of field wells for oil and gas reservoirs, particularly in high-temperature and high-pressure environments. Although lightweight thick cement slurry supplemented with lightweight aggregate is used for lost circulation suppression, the loss cannot be eliminated unless the flotation of the lightweight aggregate within the thick cement slurry is also suppressed. Therefore, there is a demand for an additive that suppresses the flotation of lightweight aggregate within a lightweight thick cement slurry. Prior Art Documents: Patent Document cncz Ln / Lznz / E / YiAi Patent Document 1: JP 62-38314 B Patent Document 2: Japanese Patent No. 3574536 Patent Document 3: US Patent No. 8603952 Patent Document 4: International Publication No. WO2015 / 034733 BRIEF DESCRIPTION OF THE INVENTION Technical Problem However, a method that relies on increasing the amount of water to obtain a light thick cement slurry causes material segregation (higher free water content) to be visible in the thick cement slurry. If highly expandable bentonite or a similar material is added to avoid this higher free water content, the addition of bentonite reduces the cement's strength at temperatures of 250°C or higher. Therefore, the additive is not preferred according to the description in Patent Document 2. The addition of a lightweight aggregate in a large quantity disadvantageously causes the separation by flotation of the lightweight aggregate in the thick cement slurry so that the aggregate becomes non-homogeneous, resulting in a marked reduction in fluidity or non-homogeneous hardening of the thick cement slurry. The present invention has been made in light of these circumstances. An objective of the present invention cncz Ln / Lznz / E / YiAi is to provide a thick cement slurry additive for a well that can suppress the appearance of free water even at elevated temperatures while ensuring sufficient cement strength, and suppress the flotation separation of a lightweight aggregate, and a method for producing the same. Another object of the present invention is to provide a thick cement slurry for a well, comprising the additive described above. An additional object of the present invention is to provide a method of cementing a well using the thick well cement slurry described above. Solution to the Problem The present inventors have diligently studied the subject matter and, accordingly, have completed the present invention by discovering that: an additive containing an aqueous dispersion comprising layered silicate (for example, purified smectite or bentonite group layered silicate) and silica as a dispersoid, more preferably an aqueous dispersion produced under specific conditions and / or comprising the dispersoid under specific conditions, is suitable for use as a thick cement slurry additive for a well; and this additive contained in the thick cement slurry for a well can suppress the poor distribution of a lightweight aggregate in the thick cement slurry and produces sufficient cement strength. Specifically, the aspects of the present invention are as follows. < 1> A thick cement slurry additive for a well, comprising a layered aqueous silicate dispersion and silica. < 2> The additive for thick cement slurry for a well in accordance with <1> , where the well is an oil well or a geothermal well. < 3> The additive for thick cement slurry for a well according to <1> either <2> , wherein a solid content concentration of the layered silicate is from 0.01 to 5% by mass, a solid content concentration of the silica is from 0.3 to 30% by mass, a mass ratio of the layered silicate to the silica is from 0.01 to 0.1, and an average particle size of a dispersoid in aqueous dispersion by laser diffractometry is from 0.1 to 30.0 pm. < 4> The additive for thick cement slurry for a well in accordance with any of <1> to <3> , wherein the layered silicate is at least one layered silicate cncz Ln / Lznz / E / YiAi selected from the group consisting of montmorillonite, hectorite, saponite, stevensite, beidellite, volkonskoite, nontronite and sauconite. < 5> The additive for thick cement slurry for a well in accordance with any of <1> to <4> , wherein the layered silicate is purified bentonite containing 90% to 99.9% by mass of montmorillonite. < 6> The additive for thick cement slurry for a well in accordance with any of <1> to <5> , where the pH is from 2 to 11. < 7> A method for producing a thick cement slurry additive for a well in accordance with any of <1> to <6> , which comprises the step of adding layered silicate to an aqueous silica dispersion and mixing the resulting mixture with agitation to obtain a layered aqueous silicate and silica dispersion. < 8> A method for producing a thick cement slurry additive for a well in accordance with any of <1> to <6> which includes the following steps: Add layered silicate to water and mix the resulting mixture with stirring to obtain an aqueous layered silicate dispersion; and add the aqueous layered silicate dispersion to an aqueous silica dispersion and mix the resulting mixture with stirring to obtain an aqueous layered silicate dispersion (cncz Ln / Lznz / E / YiAi). <9> The method for producing a thick cement slurry additive for a well according to <7> or <8> , wherein the aqueous silica dispersion is formed using silica having an average particle size of 3 to 300 nm converted from a specific surface area obtained through measurement by a nitrogen adsorption method. <10> Thick cement suspension for a well, comprising an additive according to any of <1> to <6> , wherein the thick cement slurry for a well comprises 0.001 to 0.05% BWOC layered silicate, 0.01 to 0.8% BWOC of silica, to 80% BWOC of water, and 0 to 50% BWOC of a lightweight aggregate. < 11> The thick cement slurry for a well according to <10> which also includes 0.1 to 5% BWOC of a cement hardening retarder, and 0.001 to 10% BWOC of at least one auxiliary agent selected from the group consisting of a dehydration regulator, an antifoam, a hardening accelerator, a cement dispersant, a cement strength stabilizer, and a circulating material cncz Ln / Lznz / E / YiAi loss. < 12> The thick cement slurry for a well according to <10> either <11> , wherein the lightweight aggregate is at least one hollow particle selected from the group consisting of a hollow aluminosilicate particle, a hollow borosilicate glass particle, a hollow silica particle, a hollow pearl particle, a hollow fly ash particle, a hollow alumina particle, a hollow ceramic particle, a hollow polymer particle, and a hollow carbon particle. < 13> The thick cement slurry for a well according to any of <10> to <12> where the specific weight is 1.2 or more and less than 1.6. < 14> The thick cement slurry for a well according to any of <10> to <13> Wherein, when a hardened cylindrical cement 50 mm in diameter x 300 mm high obtained by hardening the thick cement suspension for a well is divided equally into 3 portions which are an upper portion, a middle portion and a lower portion, the difference in density of the hardened cement between the upper portion and the middle portion is 0.15 or more. < 15> A method of cementing a well, comprising injecting a thick slurry of cement into a well according to any of <10> to <14> to a space between a casing pipe inserted into the well and the stratum in the well drilling, followed by hardening. Advantageous Effects of the Invention The thick cement slurry additive for a well of the present invention can efficiently prevent the deviation of the thick cement slurry into the stratum (loss of circulation) because the separation by flotation of a light aggregate in the thick cement slurry for a well supplemented with this additive is suppressed in the step of fixing or reinforcing a casing by injecting the thick cement slurry into a space between a casing pipe inserted in the well and the stratum at the time of drilling field wells for oil or gas reservoirs in a high temperature and high pressure environment. The use of the thick cement slurry for a well of the present invention can suppress the appearance of free water while achieving high cement strength in a high temperature and high pressure oil layer, and suppress defective workmanship (e.g., insufficient fixing of a casing due to poor cement that cannot fill the space between the casing and the stratum). Therefore, the use of the cement slurry additive of the present invention in a cement slurry allows the well completion to be carried out stably with high productivity even in a high temperature and high pressure environment. BRIEF DESCRIPTION OF THE FIGURES Figures 1A-1G show photographs of the external appearance of hardened cement, and Figures 1D-1G show photographs of comparative examples. DETAILED DESCRIPTION OF THE INVENTION Preferred embodiments of the present invention will be described below. However, the embodiments described below are given to illustrate the present invention. The present invention is not limited by these embodiments in any way. In this description, a numerical interval represented by 'a' means an interval that includes numerical values ​​described before and after 'a' as the lower limit and upper limit values, respectively. Thick cement slurry additive for wells The thick cement slurry additive for a well of the present invention comprises an aqueous dispersion of layered silicate and silica, which are a dispersoid. As shown in the Examples mentioned below, the use of this aqueous dispersion comprising silica (aqueous silica sol) and layered silicate as essential constituents, as a thick cement slurry additive for a well, exerts a suppressive effect on poor distribution in a lightweight aggregate. In one embodiment of the present invention, in the additive for thick cement slurry for a well, the solid content concentration of the layered silicate is from 0.01 to 5% by mass, the solid content concentration of the silica is from 0.3 to 30% by mass, the mass ratio of the layered silicate to the silica is from 0.01 to 0.1, and the average particle size of the dispersoid in aqueous dispersion by laser diffractometry is from 0.1 to 30.0 pm. In one embodiment of the present invention, the pH of the additive for thick cement slurry for a well is from 2 to 11. The lower limit of the solid content concentration range of the layered silicate is preferably 0.01% by mass, more preferably 0.5% by mass, and more preferably 0.8% by mass. The upper limit of the solid content concentration range of the layered silicate is preferably 5% by mass, more preferably 3% by mass, and more preferably 1.5% by mass. A layered silicate solids content concentration equal to or greater than the lower limit value is preferred because the thick cement slurry is easily prepared to suppress the flotation separation of a lightweight aggregate. A layered silicate solids content concentration equal to or less than the upper limit value is also preferred because it prevents the prepared thick cement slurry from having excessively high viscosity and from becoming pasty or waxy. The lower limit of the silica solids content concentration range is preferably 0.3% by mass, more preferably 1% by mass, more preferably 5% by mass. The upper limit of the silica solids content concentration range is preferably 30% by mass, more preferably 25% by mass, more preferably 20% by mass. A silica solids content concentration equal to or greater than the lower limit value is preferred because the thick cement slurry is easily prepared to suppress the flotation separation of a lightweight aggregate. A silica solids content concentration equal to or less than the upper limit value is also preferred because it prevents the prepared thick cement slurry from having an excessively high viscosity (cncz Ln / Lznz / E / YiAi) and from becoming pasty or waxy. The lower limit value of the layered silicate to silica mass ratio range is preferably 0.01, more preferably 0.02, more preferably 0.04. The upper limit value of the layered silicate to silica mass ratio range is preferably 0.1, more preferably 0.08, more preferably 0.06. A layered silicate-to-silica mass ratio equal to or greater than the lower limit value is preferred because the thick cement slurry is easily prepared to suppress the flotation separation of a lightweight aggregate. A layered silicate-to-silica mass ratio equal to or less than the upper limit value is preferred because the thick cement slurry is easily prepared to suppress the flotation separation of a lightweight aggregate. The lower limit of the average particle size range of the dispersoid in the aqueous dispersion is preferably 0.1 pm, more preferably 0.5 pm, more preferably 1 pm. The upper limit of the average particle size range of the dispersoid in the aqueous dispersion is preferably 30.0 pm, more preferably 20 pm, more preferably 10 pm. cncz Ln / Lznz / E / YiAi An average particle size of the dispersoid in the aqueous dispersion equal to or greater than the lower limit value is preferred from the standpoint of good shelf-life stability of the thick cement slurry additive for a well of the present invention. An average particle size of the dispersoid in the aqueous dispersion equal to or less than the upper limit value is preferred because collections readily form during the preparation of the aqueous dispersion of layered silicate and silica by mixing, and a suppression effect on the flotation separation of a lightweight aggregate is not inhibited. The pH of the additive for thick cement slurry for a well is generally from 2 to 7, preferably from 3 to 6, when the additive comprises a layered aqueous silicate dispersion and an acidic aqueous silica sol. The pH is usually from 9 to 11, preferably from 9.5 to 10.5, when the additive comprises layered silicate and an alkaline aqueous silica solution. The pH of layered silicate and a smectite aqueous dispersion is 2 to 11. The pH of a mixed dispersion of acidic aqueous silica sol and a layered silicate aqueous dispersion of the smectite group is 2 to 7. The pH of a mixed dispersion of alkaline aqueous silica sol and a layered silicate aqueous dispersion of the smectite group is 9 to 11. Layered silicate Examples of the layered silicate used in the present invention include kaolin group layered silicate, pyrophyllite group layered silicate, smectite group layered silicate, vermiculite group layered silicate, mica group layered silicate, mica-deficient intermediate layered silicate, brittle mica group layered silicate, chlorite group layered silicate, and interstratified mineral layered silicate. Examples of layered silicates in the kaolin group include lazardite, bertierine, amesite, cronstedtite, nepouite, kellyite, fraipontite, brindleyite, kaolinite, dickite, nacrite, halloysite, and odinite. Examples of layered silicates in the pyrophyllite group include talc, willemseite, kerolite, pimelite, pyrophyllite, and ferripyrophyllite. Examples of layered silicates in the smectite group include saponite, hectorite, sauconite, stevensite, swinefordite, montmorillonite, beidellite, nontronite, and volkonskoite. Examples of the layered silicate group of vermiculite include trioctahedral vermiculite and dioctahedral vermiculite. Examples of layered silicates from the cncz Ln / Lznz / E / YiA mica group include biotite, phlogopite, lepidomelane, eastonite, siderophyllite, tetraferriflogopite, lepidolite, polylithionite, muscovite, celadonite, ferroceladonite, ferroaluminoceladonite, aluminoceladonite, tobelite, and paragonite. Examples of silicate in mica-deficient mid-layers include illite, glauconite, and brammallite. Examples of layered silicates from the brittle mica group include clintonite, kinoshitalite, bityite, anandite, and margarita. Examples of layered silicates in the chlorite group include clinochlore, chamosite, pennantite, nimite, baileychlore, donbassite, cookeite, and sudoite. Examples of layered silicate interstratified minerals include corrensite, hydrobiotite, aliettite, kulkeite, rectorite, tosudite, dozilite, lunijianlite, and saliotite. In the present invention, layered silicate forming a 2:1 layer structure is preferred, where positively charged cations (potassium ions, sodium ions, or calcium ions) or other intercalated substances (water) are intercalated between layers. For example, smectite-group layered silicate or vermiculite-group layered silicate is preferred, and smectite-group layered silicate is particularly preferred. cncz Ln / Lznz / E / YiAi Saponite included in the group of layered silicate esmectite has a structure of (Ca / 2, Na) or,3 (Mg, Fe2+) 3 (Si, Al)40io (OH) 2 ·4Η2Ο, hectorite has a structure of Na0.3 (Mg, Li) has a structure of Nao.3Zn3(Si, Al) 4O10 (OH)2-4H2O, stevensite has a structure of (Ca / 2)0.3Mg3SÍ40io (OH)2-4H2O, swinefordite has a structure of (Ca / 2, Na) 0.3 (Li, M,O) (OH,O10) F) 2 ' 2H2O, montmorillonite has a structure of (Ca / 2, Na)0.3 (Al,Mg)2(Si) 4O10 (OH)2-nH20, beidellite has a structure of (Ca / 2, Na) 0.3AI2 (Si, Al) 4O10 (OHH20) nontronium structure Nao.3Fe3+(Si, Al) 4O10 (OH)2-nH20, and volkonskoite has a structure of Ca0.3 (Cr3+,Mg, Fe3+) 3 (Si, Al)4Oi0(OH)2-nH20. The layered silicate preferably comprises at least one layered silicate selected from the group consisting of montmorillonite, hectorite, saponite, stevensite, beidellite, volkonskoite, nontronite, and sauconite. Bentonite is a layered silicate containing montmorillonite. Natural bentonite contains impurities other than clay minerals, such as crystalline silica and feldspar components. These impurities do not effectively suppress the flotation separation of a lightweight aggregate and could impair the preservation stability of the cement slurry additive for a well. Therefore, purified bentonite with a montmorillonite content improved to 90% to 99.9% by mass through the removal of these impurities can be used appropriately. Specifically, in the present invention, the layered silicate is particularly and preferably purified bentonite containing 90% to 99.9% by mass of montmorillonite.The montmorillonite content can be determined as a delta by calculating a crystalline silica content (% by mass) from the peak diffraction intensity of crystalline silica in purified bentonite using powder X-ray diffractometry. Purified bentonite is commercially available primarily in powder form, and this commercially available product can be used. Examples of commercially available purified bentonite containing 90% to 99.9% montmorillonite (the montmorillonite content in bentonite is indicated by purity) include the trade name Kunipia F (purity: 99.3%, manufactured by Kunimine Industries Co., Ltd.), the trade name Kunipia G (purity: 99.3%, manufactured by Kunimine Industries Co., Ltd.), and the trade name Polargel HV (purity: 99.5%, manufactured by Volclay Japan Co., Ltd.). Examples of commercially available synthetic saponite include the trade name Sumecton-SA (purity: Examples of commercially available synthetic stevensite products include the trade name SumectonST (purity: 99.9%, manufactured by Kunimine Industries Co., Ltd.). Examples of commercially available synthetic hectorite products include the trade name SumectonSWN (purity: 99.9%, manufactured by Kunimine Industries Co., Ltd.). Silica In the present invention, the average particle size of aqueous silica solution (colloidal silica particle) refers to a specific surface area diameter obtained through measurement by a nitrogen adsorption method (BET method) or a Sears method particle size, unless otherwise specified. The specific surface area diameter obtained by measurement by a nitrogen adsorption method (BET method) [mean particle size (specific surface area diameter) D (nm) ] is given according to the expression D (nm) = 2720 / S of the specific surface area S (m2 / g) measured by the nitrogen adsorption method. The particle size of the Sears method refers to an average particle size measured on the basis of a rapid measurement method for particle sizes of colloidal silica cncz Ln / Lznz / E / YiAi described in the document: GW Sears, Anal. Chem. 28 (12), p. 1981, 1956. Specifically, the specific surface area of ​​colloidal silica is determined from an amount of 0.1 N NaOH required to titrate the colloidal silica corresponding to 1.5 g of SiO2 from pH 4 to pH 9, and an equivalent size (specific surface area diameter) is calculated from this. In the present invention, the average particle size of aqueous silica solution (colloidal silica particle) obtained by the nitrogen adsorption method (BET method) or the Sears method is 3 to 200 nm, 3 to 150 nm, 3 to 100 nm, or 3 to 30 nm. Aqueous silica solution with a silica concentration of 5 to 50% by mass is generally commercially available and readily obtainable. Either alkaline aqueous silica sol or acidic aqueous silica sol may be used as the aqueous silica sol. A particle size average value obtained by the Sears method can be used for a particle size average of less than 10 nm, while a particle size average value obtained by the BET method can be used for a particle size average of 10 nm or larger. Examples of commercially available alkaline aqueous silica sols include Snowtex (R) ST-XS, Snowtex ST-S, Snowtex ST-30, Snowtex ST-M30, Snowtex ST-20L, Snowtex cncz Ln / Lznz / E / YiAi ST-YL and Snowtex ST-ZL (all manufactured by Nissan Chemical Corp.). Examples of aqueous acid silica sol include Snowtex (R) ST-OXS, Snowtex ST-OS, Snowtex ST-O, Snowtex ST0-40, Snowtex ST-OL, Snowtex ST-OYL and Snowtex ST-OZL-35 (all manufactured by Nissan Chemical Corp.). At least a portion of a silane compound mentioned below can be attached to a partial surface of the silica particle in the aqueous silica sol of the present invention. Preferred examples of silane compounds may include silane coupling agents having at least one functional group selected from the group consisting of a vinyl group, an ether group, an epoxy group, a styryl group, a methacrylic group, an acrylic group, an amino group, and an isocyanurate group. Among these, a silane coupling agent having an epoxy group may be a particularly preferred silane compound. <Método para producir un aditivo para suspensión espesa de cemento para un pozo> In one embodiment of the present invention, the additive for thick cement slurry for a well comprises an aqueous dispersion in which layered silicate dispersoids and silica are uniformly dispersed in water. An example of the cncz Ln / Lznz / E / YiAi method for producing the cement thick slurry additive for a well of the present invention will now be shown (prescription 1 and prescription 2). Prescription-1 Prescription 1 is a method comprising adding a layered silicate powder to an aqueous silica dispersion (aqueous silica sol) and mixing the resulting mixture with agitation to obtain a uniform aqueous dispersion of layered silicate and silica. Prescription 2 Prescription 2 is a method comprising the steps of: adding a layered silicate powder to water and mixing the resulting mixture with agitation for dispersion to obtain an aqueous layered silicate dispersion; and adding an aqueous layered silicate dispersion to the aqueous silica dispersion and mixing the resulting mixture with agitation for uniform dispersion to obtain an aqueous layered silicate and silica dispersion. Any of the aforementioned materials can be used appropriately as the layered silicate and silica as a dispersoid. In particular, when using layered silicate such as bentonite, a purified form with a reduced amount of impurities is preferred, and ideally a form with a purity of 90% or higher. The aqueous silica dispersion described above is formed using silica that has an average particle size of 3 to 300 nm converted from a specific surface area obtained through measurement by a nitrogen adsorption method. The lower limit for the average particle size of silica is preferably 3 nm, more preferably 5 nm. The upper limit for the average particle size of silica is preferably 300 nm, more preferably 200 nm, more preferably 100 nm. An average silica particle size equal to or greater than the lower limit value is preferred for the sake of good shelf-life stability of the thick cement slurry additive for a well of the present invention. An average silica particle size equal to or less than the upper limit value is preferred for the sake of low cost of the aqueous silica dispersion (aqueous silica colloidal solution). A composition obtained by using the prescription can effectively function as the additive for thick cement slurry for a well of the invention of the present application. Thick cement slurry for well The thick cement slurry for a well of the present invention comprises, as an additive, a composition comprising an aqueous dispersion of layered silicate and silica, which are dispersoids. As shown in the Examples mentioned below, the use of this aqueous dispersion comprising both silica (aqueous silica sol) and layered silicate as essential constituents, contained as an additive for a thick cement slurry for a well, exerts a suppressive effect on poor distribution in a lightweight aggregate. Furthermore, the advantage of high cement strength is obtained because homogeneous hardened cement is achieved. In one embodiment of the present invention, the thick cement slurry for a well is a thick slurry comprising the thick cement slurry additive for a well in any form of the present invention, and comprises cement such as oil well cement and also comprises 0.001 to 0.05% BWOC of layered silicate, 0.01 to 0.8% BWOC of silica, 50 to 80% BWOC of water and 10 to 50% BWOC of a lightweight aggregate, with respect to the cement. In this context, % BWOC means % by mass based on the dry solid content of cement (by weight of cement) and is a technical element well known to experts in the field. cncz Ln / Lznz / E / YiAi The lower limit of the layered silicate content ratio range is preferably 0.001% BWOC, more preferably 0.002% BWOC, and more preferably 0.003% BWOC. The upper limit of the layered silicate content ratio range is preferably 0.05% BWOC, more preferably 0.03% BWOC, and more preferably 0.02% BWOC. A layered silicate content ratio equal to or greater than the lower limit value is preferred because it ensures a sufficient amount of layered silicate and enhances the buoyancy-suppressing effect on the lightweight aggregate. A layered silicate content ratio equal to or less than the upper limit value is preferred because it prevents the thick cement slurry from having excessively high viscosity and allows a predetermined amount of cement to be loaded without difficulty. The lower limit of the silica content ratio (solid content) is preferably 0.01% BWOC, more preferably 0.02% BWOC, and more preferably 0.05% BWOC. The upper limit of the silica content ratio (solid content) is preferably 0.8% BWOC, more preferably 0.5% BWOC, and more preferably 0.3% BWOC. A silica content (solids content) ratio equal to or greater than the lower limit value is preferred because it prevents the thick cement slurry (cncz Ln / Lznz / E / YiAi) from having excessively low viscosity and improves the buoyancy-suppressing effect on the lightweight aggregate. A silica content (solids content) ratio equal to or less than the upper limit value is preferred because it prevents the thick cement slurry from having excessively high viscosity during preparation and allows for the easy loading of a predetermined amount of cement. The thick cement slurry for a well of the present invention may comprise 50 to 80% water (BWOC). Fresh water, tap water, industrial water, purified water, or seawater, etc., may be suitable as wastewater. The thick cement slurry for a well of the present invention may comprise a lightweight aggregate that is at least one hollow particle selected from the group consisting of a hollow aluminosilicate particle, a hollow borosilicate glass particle, a hollow silica particle, a hollow perlite particle, a hollow fly ash particle, a hollow alumina particle, a hollow ceramic particle, a hollow polymer particle, and a hollow carbon particle. The thick cement slurry for a well of the present invention may comprise from 10 to 50% BWOC of lightweight aggregate in terms of solid content. This can suitably decrease the specific gravity of the thick cement slurry (e.g., to a specific gravity of 1.2 or more and less than 1.6). Other included components The thick cement slurry for a well of the present invention may also contain a cement hardening retarder, a dehydration regulator, an antifoaming agent, a hardening accelerator, a cement dispersant, a cement strength stabilizer, and a lost circulation material, in addition to oil well cement, the thick cement slurry additive for a well, lightweight aggregate, and water. Any cement from Class A to Class H, as specified in APISPEC 10A Specification for Cements and Materials for Wells by the American Petroleum Institute (API), can be used as oil well cement. Among these, Class G and Class H cements are preferred because quality control is easily achieved with an additive or auxiliary agent, and they can be used at a wide range of depths and temperatures. The cement hardening retarder is used to maintain a moderate fluidity of the thick cement slurry until the cementing job is completed and a thickening time is set. The CNCZ Ln / Lznz / E / YiAi cement hardening retarder contains a main component such as ligninsulfonates, naphthalenesulfonates, and borates. The dehydration regulator can be used to, for example, protect the water-sensitive layer or prevent early dehydration of the thick suspension, and contains a main component such as high molecular weight organic polymers and vinylamide-vinylsulfonic acid copolymers. The antifoam contains a main component such as silicon compounds and higher alcohols. Cement hardening accelerator is used for the purpose of, for example, increasing initial strength or shortening a waiting time for hardening, and contains a main component such as calcium chloride, liquid glass, and gypsum. The cement dispersant can be used to, for example, reduce the viscosity of the thick cement slurry and improve the efficiency of mud water replacement, and contains a main component such as naphthalenesulfonic acid formalin condensates, polyacrylic acid condensates, and sulfonated melamine condensates. The cement strength stabilizer contains a main component such as fly ash and silica. The lost circulation material is used to prevent lost circulation, it is found in the form of, for example, inactive grains that do not influence the properties of the cement, and contains a main component such as walnut shells, vermiculite, gilsonite, mica and cellophane fragments. The thick cement slurry for a well of the present invention may contain a cement composition for an ordinary structure, various types of cement or aggregate for use in concrete compositions and other additives for use in these or similar cement compositions, in addition to cement such as oil well cement, the thick cement slurry additive for a well in any form of the present invention, the cement hardening retarder and other additives or auxiliary agents as described above. Examples of conventional common cement for an ordinary structure that can be used include Portland cement (e.g., ordinary Portland cement, high early strength Portland cement, ultra high early strength Portland cement, low heat / moderate heat Portland cement, and sulfate-resistant Portland cement), various types of blended cement (blast furnace cement, silica cement, fly ash cement, etc.).), white Portland cement, alumina cement, ultra-rapid hardening cement (one-clinker ultra-rapid hardening cement, two-clinker ultra-rapid hardening cement, and magnesium phosphate cement), grout cement, low-heat cement (low-heat blast furnace cement, mixed fly ash blast furnace cement, and belite-rich cement), ultra-high-strength cement, cement-based solidifying materials, and environmentally friendly cement (cement produced using one or more incinerated wastes and municipal wastes such as sewage sludge as raw materials). Additionally, a fine powder such as blast furnace slag, fly ash, volcanic ash, clinker ash, husk ash, fumed silica, silica powder, or limestone powder, or gypsum, may be added as an admixture. Gravel, crushed stone, granulated slag and recycled aggregate can be used, as well as refractory aggregate such as silica, clay, zirconium, high alumina, silicon carbide, graphite, chromium, chromemagnesia or magnesia as the aggregate. Known cement or concrete additives such as high-range water-reducing agents (HRAs), high-range water-reducing agents, water-reducing agents, water-reducing agents, air-entraining agents (AE agents), foaming agents, segregation control additives, thickeners, shrinkage-reducing agents, curing compounds, and water repellents may be blended like other additives for use in cement composition or similar. In the thick cement slurry for a well of the present invention, as shown in the Examples mentioned below, when a hardened cylindrical cement of 50 mm diameter x 300 mm height obtained by hardening the thick cement slurry for a well is divided equally into 3 portions, which are an upper portion, a middle portion, and a lower portion, the difference in the density of the hardened cement between the upper portion and the middle portion is 0.15 or more, and a maldistribution suppressing effect is exerted on the lightweight aggregate. Well cementing method In one embodiment of the present invention, the well cementing method is a method that uses the thick well cement slurry in any form of the present invention and is a method comprising injecting the thick well cement slurry into a space between a casing pipe inserted into the well and the stratum, followed by hardening. In one embodiment of the present invention, the cementing method for a well can suppress unfavorable lost circulation by using the thick cement slurry for a well in any form of the present invention to fill the void space between the stratum and a casing pipe with oil well cement in drilling for oil or gas reservoirs. Examples The present invention will hereafter be described in more detail with reference to Examples of Preparation of a Thick (Light) Cement Slurry Additive for a Well, Examples, and Comparative Examples. However, the present invention is not limited by these examples in any way. Measuring apparatus and method The additive for thick suspension of lightweight cement was analyzed (silica concentration, pH value, laser diffractometric average particle size and viscosity) using the following apparatus. Silica solids concentration: The alkali was removed from aqueous silica solution using a hydrogen cation exchange resin, followed by drying. The silica solids concentration was determined from a residue after calcination at 1000 °C. pH: A pH meter was used (manufactured by DKK-TOA cncz Ln / Lznz / E / YiAi Corp.) . Viscosity: A type B viscometer (manufactured by Tokyo Keiki Inc.) was used. Average DLS (dynamic light scattering) particle size: A Zetasizer Nano dynamic light scattering particle size measuring device (manufactured by Malvern Panalytical, a part of Spectris Pie) was used. Laser diffractometric average particle size: A SALD-7500 laser diffractometric particle measuring apparatus (manufactured by Shimadzu Corp.) was used. Preparation conditions for additive for thick suspension of lightweight cement Additive A for thick suspension of lightweight cement According to prescription 1, a 500 ml styrol bottle was loaded with 300 g of the trade name Snowtex-XS aqueous silica solution (pH = 9.6, S1O2 concentration = 20.5% by mass, Sears particle size = 5.0 nm, manufactured by Nissan Chemical Corp.), then loaded with 3.5 g of the purified bentonite containing 99.3% montmorillonite component (trade name Kunipia F, manufactured by Kunimine Industries Co., Ltd.) with stirring at 500 rpm using a stirrer fitted with a 40 mm diameter propeller blade, and then stirred for 2 hours. In this way, additive A for a light thick cement suspension was prepared containing 1.2% by mass cncz Ln / Lznz / E / YiAi of montmorillonite and 20.3% by mass of silica and had a pH of 9.5, conductivity of 3980 pS / cm, viscosity of 17 mPa-s and an average laser diffractometric particle size of 19.4 pm. Additive B for thick suspension of lightweight cement According to prescription-1, a 500 ml styrene bottle was filled with 394 g of Snowtex-XS aqueous silica sol, then filled with 6.5 g of purified bentonite containing 99.3% montmorillonite component (Kunipia F, manufactured by Kunimine Industries Co., Ltd.) and stirred at 500 rpm using a stirrer equipped with a 40 mm diameter propeller blade, and then stirred for 2 hours. In this way, additive B for light cement slurry was prepared, containing 1.6% by mass of montmorillonite and 20.2% by mass of silica, and having a pH of 9.5, conductivity of 4860 pS / cm, viscosity of 19 mPa-s, and an average laser diffractometric particle size of 18.5 pm. Additive C for thick suspension of lightweight cement According to prescription 2, a 500 ml styrol bottle was loaded with 400 g of pure water, then loaded with 14.5 g of the purified bentonite containing 99.3% montmorillonite component (trade name Kunipia F, manufactured by Kunimine Industries Co., Ltd.) with stirring at 500 rpm using a stirrer equipped with a 40 mm diameter propeller blade, and then stirred for 2 hours to prepare the thick slurry with pH 9.9, conductivity 1190 pS / cm, a laser diffractometric average particle size of 1.8 pm and a montmorillonite concentration of 3.5% by mass. Next, a 500 ml styrol bottle was loaded with 300 g of the commercially available aqueous silica sol trade name Snowtex-XS (pH = 9.6, S1O2 concentration = 20.5% by mass, Sears particle size = 5.0 nm, manufactured by Nissan Chemical Corp.), then loaded with 48.6 g of the thick suspension having a montmorillonite concentration of 3.5% by mass with stirring at 500 rpm using a stirrer equipped with a 40 mm diameter propeller blade, and then stirred for 2 hours. In this way, additive C for a light thick cement slurry was prepared containing 0.89% by mass of montmorillonite and 15.5% by mass of silica and had a pH of 9.6, conductivity of 3750 pS / cm, viscosity of 21 mPa-s and an average laser diffractometric particle size of 1.7 pm. Preparation of thick cement slurry and measurement of the physical properties of the API specification For the preparation of the thick cement slurry in Example 1 to Example 3 and Comparative Example 1 to Comparative Example 4, 997 g of thick cement slurry were prepared in accordance with the API specification (specification with respect to petroleum stipulated by cncz Ln / Lznz / E / YiAi). American Petroleum Institute) 10B-2 using a dedicated apparatus and the materials and loading ratios (% BWOC) shown in Table 1. Specifically, a dedicated mixer was charged with pure water and charged with a commercially available dehydration regulator, lightweight cement thick slurry additive, a commercially available hardening retarder and antifoam, and Class G cement (manufactured by Ube-Mitsubishi Cement Corp.) in the mixing quantities shown in Table 1 for 90 seconds, while a stirring blade was rotated at 4000 rpm. The number of revolutions of the stirring blade was increased to 12,000 rpm, and stirring was carried out for 35 seconds to prepare the cement thick slurry. A specific gravity of thick slurry was calculated for each thick cement slurry prepared in this manner using the following procedures. In addition, the amount of free water, a thickening time test, cement strength, and fluid loss were evaluated using dedicated apparatus that met the API specification. 1) Measurement of the specific gravity of the thick suspension The specific gravity of 100 cc of the prepared thick cement slurry was measured using a stainless steel cup hydrometer having a capacity of 100 ml. cncz Ln / Lznz / E / YiAi 2) Measurement of the amount of free water The thick cement slurry was conditioned by the method described in section <Preparación de suspensión espesa de cemento y medición de las propiedades físicas de la especificación API> A resin measuring cylinder with a capacity of 250 cc was then filled with 250 cc of the thick cement slurry. The temperature was set to 88 °C for 30 minutes, and the measuring cylinder was tilted at a 45-degree angle and left upright for 2 hours. After the 2-hour standing time, the water released at the top of the thick slurry was collected with a dropper, and the amount of this water (as a percentage by volume of 250 cc of the thick slurry) was considered the amount of free water. The amount of free water is preferably 2% or less. 3) Thickening time test A 500 cc aliquot of the prepared thick cement slurry was separated. It was loaded into a Model 290 HPHT (High Pressure, High Temperature) Consistometer thickening time measuring apparatus (manufactured by Fann Instrument Company) as described in the API specification. The temperature and pressure were then raised to either 150 °C and 3700 psi or 180 °C and 5000 psi for 1 hour while the thick cement slurry was stirred with a stirring blade. The predetermined temperature and pressure were maintained. The consistency was measured over time using the thickening time measuring apparatus from the start of the test, and this temperature was maintained until the measured value (Bearden Consistency Unit (BC)) reached 70 BC. The time from the start of heating until reaching 70 BC was considered the thickening time (min). The thickening time is preferably from 2 hours to 6 hours. 4) Measurement of cement strength (compressive strength test) A 130 cc aliquot was separated from the prepared thick cement slurry. A Model 304 Ultrasonic Cement Analyzer, as described in the API specification, was charged with this sample. The temperature and pressure were then raised to either 120°C and 3700 psi or 150°C and 5000 psi for 1 hour. This temperature and pressure were maintained for 3 hours. The temperature was then raised to either 150°C or 180°C for 20 hours. The compressive strength was considered the cement strength. A higher cement strength of 1000 psi was preferred. 5) Fluid loss measurement The thick cement slurry was conditioned by the method. A 130 cc aliquot of the thick cement slurry, temperature-adjusted to 88°C for 30 minutes, was then separated and loaded with a cncz Ln / Lznz / E / YiAi Fluid Loss Test Instrument (manufactured by Fann Instrument Company) described in the API specification. The water generated (dehydration) from the thick cement slurry when a pressure of 1000 psi was continuously applied to it for 30 minutes at 88°C was recovered in a 100 cc resin measuring cylinder. The amount of dehydration (Vt) for the measuring time (30 min) was applied to Equation 1 to calculate the fluid loss. Expression 1 Fluid loss ™ 21^ (Expression 1) V The API specification does not specifically define the numerical range for fluid loss, but states that 100 mi or less is adequate. Table 2 shows the results obtained from the evaluation of specific weight, amount of free water, thickening time, cement strength, and fluid loss. Measurement of the density distribution of hardened cement The thick cement slurry was prepared twice under the same preparation conditions as in Example 1, Example 3, or Comparative Example 3. Each thick cement slurry prepared in this way was evaluated for the cncz Ln / Lznz / E / YiAi hardened cement density distribution using the following procedures. A polyethylene bleed bag (dimensions: 50 mm diameter * 500 mm) was filled with 1200 g of each prepared thick cement slurry. The top portion of the bleed bag was tied with a vinyl string, and the resulting slurry was hung on a wire mesh rack, followed by curing for 1 week in a constant temperature room of 20 °C to produce hardened cement. Next, the prepared hardened cement was cut into three equal portions (top, middle, and bottom) using a concrete cutter (manufactured by Nittoku KK) to prepare the core donors. The dimensions and weight of each core donor were then measured, and a density was calculated to determine the density distribution of the hardened cement. The density difference between the top and middle portions was also calculated. If the poor distribution of a lightweight aggregate is not corrected, the cementitious components accumulate in the lower portion of the hardened product, while the lightweight aggregate floats in the middle and upper portions. Therefore, the density difference between the upper and middle portions is small. On the other hand, if the poor distribution of a lightweight aggregate is corrected, the cementitious components also spread throughout the middle portion of the hardened product (cncz Ln / Lznz / E / YiAi), so that the weight of the middle portion is greater than that of the upper portion. As a result, the density difference between the upper and middle portions is large. Here, the density difference is preferably 0.15 or more, and more preferably 0.20 or more. Table 2 shows the results obtained on the densities of the upper portion, the middle portion and the lower portion of the hardened cement and the difference in density between the middle portion and the upper portion. Figures 1A-1G show a photograph of the external appearance of the hardened cement obtained. Table 1 Table 1 Thick cement slurry component Example 1 Example 1 Example 2 I 3 Example Comp 1 Example j Example Comp2 j Comp3 Example Comp 4 Additive content for cement suspension Additive A Additive C Additive Not added Silica solution alone Purified bentonite alone Purified bentonite and silica solution separated; additionally during the preparation of cement suspension Additive G k NO. 00 aqrt!i|adu hqtro 33.34 31 34 tdlice 0. Í i 0. d SX ti 0 a 11 F jrti.uL of layered alise 3. íW> s. o 0 0 3. itoSS Pure water 62. Yes « A0 0X 10 re 0& GS. 23 AA: «2. 80 Dehydration regulator i 2.3» 2.63 Hardening retarder 0. 80 Antifoam 0. 1.2 ¢, ti Table 2 Table 2 Evaluation results cncz Ln / Lznz / E / YiAi Example 1 Example 2 s Example | 3 Example Comp 1 Example 1 Comp2 Example j Example | CompS Comp 4 Specific gravity h 41 i. 48 143 , 48 i L 47 I 1,48 L 48 Quantity of free water [% in ''o irruí ] ñ ΓΞ 0 0 4 Fluid loss 40 42 § j 4« 28 Thickening time Cement strength («só Poor distribution of lightweight aggregate Density of upper portion of hardened cement (d / tm 3j 6 hr y θθ ” Small 6 hr y tus Small 1 3 hr yj 53 min . § Small Very large | 6 hr y 07 mh | n »7 i Large | 7 hr y | 54 minñi | Very í large I 133 5 hr y 51 rain mi Slightly large Density of middle portion of hardened cement (g / cm3) Density of lower portion of hardened cement (q / cm sj Difference in density between middle portion and portion hardened cement top L 8S ..................... 17 p„.------ 1 L 41 |______________________________ 0, §8 | 2.11 1 0, QS J_______ Notes: The hyphen (-) means there is no data Analysis In the case of the thick cement slurry in Examples 1 to 3 supplemented with the additive for thick lightweight cement slurry prepared as an aqueous dispersion containing a silicate particle and a silica particle, it is evident that the poor distribution of the lightweight aggregate was clearly small compared to Comparative Examples 1 to 4 (see Table 2 and the photograph of the external appearance of the hardened cement in Figures 1A). 1G) As shown in Table 2, in Examples 1 through 3, the cement strength was 1300 psi or higher in all cases. Therefore, it is evident that high-strength hardened cement was obtained. These results demonstrated that, in Examples 1 through 3, the poor distribution of the lightweight aggregate was minimal, and high-strength hardened cement was obtained. Conversely, when the additive for thick lightweight cement slurry was not added (Comparative Example 1), when only aqueous silica sol was added (Comparative Example 2), and when only purified bentonite was added (Comparative Example 3) during the preparation of the thick cement slurry, the poor distribution of the lightweight aggregate was significant. In Comparative Example 2, the cement strength was 1157 psi, demonstrating that the strength of the hardened cement was also insufficient compared to Examples 1 through 3 (see Table 2 and the photograph of the external appearance of the hardened cement in Figures 1A-1G). In the case of adding the layered silicate particle (purified bentonite) and the aqueous silica sol separately (Comparative Example 4) in the preparation of the thick cement slurry, the poor distribution of the lightweight aggregate was slightly large and the cement strength was 1181 psi, demonstrating that the strength of the hardened cement was also insufficient compared to Examples 1 to 3 (see Table 2 and the photograph of the external appearance of the hardened cement in Figures 1A-1G). As is evident from the result described above, the use of the lightweight cement thick slurry additive consisting of an aqueous dispersion comprising a layered silicate particle such as a smectite group layered silicate particle and a silica particle in the preparation of the lightweight cement thick slurry suppresses the poor distribution of a lightweight aggregate and improves the strength of the cement. It is hereby stated that, as of this date, the best method known to the applicant for putting the present invention into practice is the one that is clear from the present description of the invention.

Claims

CLAIMS Having described the invention as above, the following claims are claimed as property:

1. An additive for thick cement suspension for a well, characterized in that it comprises an aqueous dispersion of layered silicate and silica.

2. The additive for thick cement slurry for a well according to claim 1, characterized in that the well is an oil well or a geothermal well.

3. The cement thick slurry additive for a well according to claim 1 or 2, characterized in that: a solid content concentration of the layered silicate is from 0.01 to 5% by mass, a solid content concentration of the silica is from 0.3 to 30% by mass, a mass ratio of the layered silicate to the silica is from 0.01 to 0.1, and an average particle size of a dispersoid in aqueous dispersion by laser diffractometry is from 0.1 to 30.0 pm.

4. The cement thick slurry additive for a well according to any of claims cncz Ln / Lznz / E / YiAi 1 to 3, characterized in that the layered silicate is at least one layered silicate selected from the group consisting of montmorillonite, hectorite, saponite, stevensite, beidellite, volkonskoite, nontronite and sauconite.

5. The cement thick slurry additive for a well according to any of claims 1 to 4, characterized in that the layered silicate is purified bentonite containing 90% to 99.9% by mass of montmorillonite.

6. The additive for thick cement slurry for a well in accordance with any of claims 1 to 5, characterized in that the pH is from 2 to 11.

7. A method for producing a thick cement slurry additive for a well according to any of claims 1 to 6, characterized in that it comprises the step of: adding layered silicate to an aqueous silica dispersion and mixing the resulting mixture with agitation to obtain an aqueous layered silicate and silica dispersion.

8. A method for producing a thick cement slurry additive for a well according to any of claims 1 to 6, characterized in that it comprises the steps of: adding layered silicate to water and mixing the resulting mixture with stirring to obtain an aqueous dispersion cncz Ln / Lznz / E / YiAi of layered silicate; and adding the aqueous dispersion of layered silicate to an aqueous dispersion of silica and mixing the resulting mixture with stirring to obtain an aqueous dispersion of layered silicate and silica.

9. The method for producing a thick cement slurry additive for a well according to claim 7 or 8, characterized in that the aqueous silica dispersion is formed using silica having an average particle size of 3 to 300 nm converted from a specific surface area obtained through measurement by a nitrogen adsorption method.

10. The thick cement slurry for a well, characterized in that it comprises an additive according to any of claims 1 to 6, wherein the thick cement slurry for a well comprises: 0.001 to 0.05% BWOC of layered silicate, 0.01 to 0.8% BWOC of silica, 50 to 80% BWOC of water, and 10 to 50% BWOC of a lightweight aggregate.

11. The thick cement slurry for a well according to claim 10, characterized in that it further comprises: 0.1 to 5% BWOC of a cement hardening retarder, and 0.001 to 10% BWOC of at least one auxiliary agent selected from the group consisting of a dehydration regulator, an antifoaming agent, a hardening accelerator, a cement dispersant, a cement strength stabilizer, and a lost circulation material.

12. The thick cement slurry for a well according to claim 10 or 11, characterized in that the lightweight aggregate is at least one hollow particle selected from the group consisting of a hollow aluminosilicate particle, a hollow borosilicate glass particle, a hollow silica particle, a hollow perlite particle, a hollow fly ash particle, a hollow alumina particle, a hollow ceramic particle, a hollow polymer particle, and a hollow carbon particle.

13. The thick cement slurry for a well according to any of claims 10 to 12, characterized in that the specific gravity is 1.2 or more and less than 1.

6.

14. The thick cement slurry for a well according to any of claims 10 to 13, characterized in that when a hardened cylindrical cement 50 mm in diameter x 300 mm high obtained by hardening the thick cement slurry for a well is divided equally into 3 parts which are an upper portion, a middle portion and a lower portion, the difference in the density of the hardened cement between the upper portion and the middle portion is 0.15 or more.

15. A method of cementing for a well, characterized in that it comprises injecting a thick slurry of well cement according to any of claims 10 to 14 into a space between a casing pipe inserted into the well and the stratum in the wellbore, followed by hardening.