Technical Field

The present invention relates to additives for oil well cement slurries and aqueous based drilling fluids comprising microsilica and a method for the production thereof. The additives are composite materials based on modified microsilica and can be used as fluid loss additives. The materials can also be used in other oilfield fluids such as a viscosifier in enhanced oil recovery (EOR) fluids and fracturing fluids. Further, the additives can be used in oil well cement in order to reduce the transition time from gel formation to hardening/curing.

Background Art

Microsilica is a co-product of silicon and ferrosilicon alloy production. Microsilica is a spherical, submicron material used in various applications among others oil well cementing as a gas-migration control additive and as an extender to lower the cement slurry density. Microsilica contributes in reducing the fluid loss of an oil well cement slurry on its own but desired filtration values cannot be achieved without adding further additives to the microsilica.

Additives such as fluid loss control additives are added into drilling fluids or oil-well cement to reduce the amount of fluid lost into the formation during drilling and completion operations. Most of the fluid loss additives used for aqueous systems are water-soluble polymers such as starch and starch derivatives, cellulose and cellulose derivatives, synthetic water-soluble polymers such as acrylate and vinyl copolymers. The downhole conditions such as the temperature of the well specify the type of chemical products that will fit for the purpose.

US 2013/0203951 Al discloses graft copolymers where silica is reacted with silane and then monomers are grafted onto the silane. The copolymers find use as additives in different chemical applications.

DE 102006061327 Al discloses graft copolymers comprising silica reacted with silane and a sulphonic acid containing polymer for use in construction chemical applications. CN 101818050 B discloses a nano-drilling fluid treating agent comprising nano-silica, silane-coupling agent and acrylamide.

The currently used additives for oil well cement slurries and aqueous based drilling fluids, particularly the class of additives used for high-pressure high-temperature (HPHT) conditions may have one or more of the following drawbacks:

1) Costly (expensive chemicals), due to the complex production processes.

2) Generate high viscosity at low temperature.

4) Retard the cement hydration and setting process.

5) Do not function well in salt containing cement.

6) Suffer from thermal degradation at extreme high temperatures.

It is thus an object of the present invention to provide an additive for oil well cement slurries and aqueous based drilling fluids for that overcomes the above-mentioned problems.

It is a further object of the present invention to provide a simplified method for the production of such additive.

Short Description of the Invention

In this invention, it is provided additives for oil well cement slurries and aqueous based drilling fluids that contain chemically modified microsilica and organic polymers.

In the present invention, it has been found, surprisingly, that by adding organic monomers to chemically modified microsilica, efficient additives for oil well cement slurries and aquesous based drilling fluids are provided.

The present invention provides an additive for oil well cement slurries and aqueous based drilling fluids comprising microsilica having a particle size in the range of 0.02 μπι to 45 μπι, the amount of microsilica being in the range of 1-80 weight % by the total weight of dry matter, the surface of the microsilica comprising polymerizable silane, the polymerizable silane being in an amount in the range of 0.01 - 20 weight % by the total weight of dry microsilica and the additive further comprising at least one acrylamide derivative monomer. In one aspect, the present invention provides a fluid loss additive for oil well cement slurries and aqueous based drilling fluids, wherein the fluid loss additive comprises microsilica, wherein microsilica is particulate, amorphous S1O2 obtained from a process in which silica is reduced to SiO-gas and the reduction product is oxidized in the vapour phase and condensed to form amorphous silica containing > 90 % by weight of silica (S1O2) and having a specific density of 2.1 - 2.3 g/cm ³ , a surface area of 12 - 40 m ² /g and a particle size in the range of 0.02 μπι to 45 μπι, in an amount in the range of 1-80 weight % by the total weight of dry matter, the surface of the microsilica comprises polymerizable silane in an amount in the range of 0.01 - 20 weight % by the total weight of dry microsilica, and the fluid loss additive further comprises at least one acrylamide derivative monomer selected from the group consisting of acrylamide (AAm), 2-acrylamido-2-methylpropane sulfonic acid (AMPS), N,N- dimethylacrylamide (NNDMA), vinylformamide, N-tert-butylacrylamide (NTBAAm), N-(Hydroxymethyl)acrylamide (NHMAAm), N,N'-Methylenebisaciylamide

(NNMBAAm), vinylbutyrolactam, acrylonitrile, and 2-(dimethylamino)ethyl methacrylate.

According to an embodiment of the invention, the additive further comprises at least one acidic unsaturated monomer.

According to an embodiment of the invention, the at least one acrylamide derivative monomer is selected from the group consisting of acrylamide (AAm), 2-acrylamido-2- methylpropane sulfonic acid (AMPS), N,N-dimethylacrylamide (NNDMA), vinylformamide, N-tert-butylacrylamide (NTBAAm), N-(Hydroxymethyl)acrylamide (NHMAAm), Ν,Ν'-Methylenebisaciylamide (NNMBAAm), vinylbutyrolactam, acrylonitrile, and 2-(dimethylamino)ethyl methacrylate.

In an embodiment of the invention, the at least one acidic unsaturated monomer is selected from the group consisting of acrylic acid (AA), methacrylic acid, maleic anhydride (MA), itaconic acid, 4-vinylbenzenesulfonic acid, and vinylphosphonic acid.

In an embodiment of the invention, the additive comprises the acrylamide derivative monomers 2-acrylamido-2-methylpropane sulfonic acid (AMPS) and N,N- dimethylacrylamide (NNDMA). In an embodiment of the invention, the additive comprises the acrylamide derivative monomers 2-acrylamido-2-methylpropane sulfonic acid (AMPS), N,N- dimethylacrylamide (N DMA) and acrylamide (AAm).

In an embodiment of the invention, the additive comprises the acrylamide derivative monomers 2-acrylamido-2-methylpropane sulfonic acid (AMPS), N,N- dimethylacrylamide (NNDMA) and acrylamide (AAm) and the acidic unsaturated monomer acrylic acid (AA).

In another embodiment, the amount of microsilica comprising polymerizable silane on its surface is in the range 20 - 70 weight % by the total weight of dry matter, 20 - 60 weight % by the total weight of dry matter, 25 - 55 weight % by the total weight of dry matter, 25 - 50 weight % by the total weight of dry matter, 30 - 50 weight % by the total weight of dry matter, 30 - 40 weight % by the total weight of dry matter.

In another embodiment, the at least one acrylamide derivative monomer is present in an amount in the range of 20-90 weight % by the total weight of dry matter, 50-90 weight % by the total weight of dry matter, 60-80 weight % by the total weight of dry matter.

In another embodiment, the at least one acidic unsaturated monomer is present in an amount in the range of 0.1 -20 weight % by the total weight of dry matter, 1 -

15 weight % by the total weight of dry matter, 2-10 weight % by the total weight of dry matter.

In another embodiment, the particle size distribution of the additive measured by the light scattering method is in the range of 0.05 μιη to 500 μιη with an average size (D50) in the range of 5 μιη to 20 μιη.

In an embodiment, the polymerizable silane is selected from the group consisting of vinyltrimethoxysilane, triethoxy vinyl silane, vinyltris(2-methoxyethoxy)silane, dimethoxymethylvinylsilane, diethoxy (m ethyl )vinyl silane, trichlorovinylsilane, tris(trimethylsilyl)silyl vinyl ether, 3-(trimethoxysilyl)propyl acrylate, 3- (trimethoxysilyl)propyl methacrylate, 3-[tris(trimethylsiloxy)silyl]propyl methacrylate and 3 -(triethoxy silyl)propyl methacrylate. In an embodiment, the polymerizable silane is vinyltrimethoxysilane or 3- (trimethoxysilyl)propyl methacrylate.

The present invention further relates to a method for the production of an additive for oil well cement slurries and aqueous based drilling fluids comprising the steps of:

a) modifying the surface of microsilica with polymerizable silane by bringing the microsilica in an amount in the range of of 80 - 99.9 weight % by the total weight of dry matter in contact with polymerizable silane in an amount in the range of 0.1-20 weight % by the total weight of dry microsilica;

b) polymerizing the polymerizable silane modified microsilica in an amount of 1 - 80 weight % by the total weight of dry matter with at least one acrylamide derivative monomer in an amount in the range of 50 - 90 weight % by the total weight of dry matter.

In another aspect, the present invention provides a method for the production of a fluid loss additive for oil well cement slurries and aqueous based drilling fluids according to the invention, comprising the steps of:

a) modifying the surface of microsilica with polymerizable silane by bringing the microsilica in an amount in the range of 80 - 99,9 weight % by the total weight of dry matter in contact with polymerizable silane in an amount in the range of 0.1 - 20 weight % by the total weight of dry microsilica;

b) polymerizing the polymerizable silane modified microsilica in an amount of 1 - 80 weight % by the total weight of dry matter with at least one acrylamide derivative monomer selected from the group consisting of acrylamide (AAm), 2- acrylamido-2-methylpropane sulfonic acid (AMPS), N,N-dimethylacrylamide (N DMA), vinylformamide, N-(hydroxymethyl)acrylamide ( HMAAm), Ν,Ν'- methylenebisacrylamide (N MBAAm), vinylbutyrolactam, acrylonitrile, and 2- (dimethylamino)ethyl methacrylate, in an amount in the range of 50 - 90 weight % by the total weight of dry matter.

In an embodiment, the polymerizable silane modified microsilica is in addition polymerized with at least one acidic unsaturated monomer in an amount in the range of 0.1 - 20 weight % by the total weight of dry matter.

In an embodiment, the surface of microsilica is modified with polymerizable silane using a dry method. In an embodiment, the surface of microsilica is modified with polymerizable silane using a wet method.

In an embodiment, the polymerizable silane modified microsilica is polymerized using a free radical polymerization technique.

In an embodiment, the at least one acrylamide derivative monomer is selected from the group consisting of acrylamide (AAm), 2-acrylamido-2-methylpropane sulfonic acid (AMPS), N,N-dimethylacrylamide (N DMA), vinylformamide, N- (Hydroxymethyl)acrylamide ( HMAAm), N,N'-Methylenebisaciylamide

(N MBAAm), vinylbutyrolactam, acrylonitrile, and 2-(dimethylamino)ethyl methacrylate.

In an embodiment, the at least one acidic unsaturated monomer is selected from the group consisting of acrylic acid (AA), methacrylic acid, maleic anhydride (MA), itaconic acid, 4-vinylbenzenesulfonic acid, and vinylphosphonic acid.

In an embodiment, the polymerizable silane is is selected from the group consisting of vinyltrimethoxysilane, triethoxyvinylsilane, vinyltris(2-methoxyethoxy)silane, dimethoxymethylvinylsilane, diethoxy (m ethyl )vinyl silane, trichlorovinylsilane, tris(trimethylsilyl)silyl vinyl ether, 3-(trimethoxysilyl)propyl acrylate, 3- (trimethoxysilyl)propyl methacrylate, 3-[tris(trimethylsiloxy)silyl]propyl methacrylate and 3-(triethoxysilyl)propyl methacrylate.

In an embodiment, the polymerizable silane is vinyltrimethoxysilane or 3- (trimethoxysilyl)propyl methacrylate.

These and other features, advantages and benefits and objects will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of representative embodiments of the invention below and the accompanying drawings.

Brief Description of the Drawings

Embodiments of the invention will now be described with reference to the following drawings. Fig. 1 shows scanning electron micrograph of an additive according to the invention prepared according to Example 4.

Fig. 2 shows scanning electron micrograph of an additive according to the invention prepared according to Example 4.

Fig. 3 shows scanning electron micrograph of an additive according to the invention prepared according to Example 4.

Fig. 4 shows the particle size distribution of an additive according to the invention prepared according to Example 4 measured by the light scattering method.

Fig. 5 shows the development of static gel strength (straight-line) and compressive strength (dots-line) for a conventional oil well cement slurry as a function of time.

Fig. 6 shows the development of static gel strength (straight-line) and compressive strength (dots-line) for an oil well cement slurry comprising an additive according to the invention as function of time.

Detailed Description of the Invention

The term "microsilica" as used in the specification and claims of this application refers to particulate, amorphous SiC obtained from a process in which silica (quartz) is reduced to SiO-gas and the reduction product is oxidized in the vapour phase and condensed to form amorphous silica. Microsilica may contain at least 70 % by weight silica (S1O2), and preferably > 90 % by weight and has a specific density of 2.1 - 2.3 g/cm ³ and a surface area of 12 - 40 m ² /g, typically 20 m ² /g. The particle size of microsilica is in the range of 0.02 μπι to 45 μπι and preferably is in the range of 0.05 μπι to 0.5 μπι. The primary particles are substantially spherical and have an average particle size (D50) below 1 μπι, the average particle size (D50) can be in the range of 0.12 μπι to 0.25 μπι, the average particle size (D50) can be around 0.15 μπι. Microsilica is preferably obtained as a co-product in the production of silicon alloys in electric reduction furnaces.

More specifically, the new additives are composite materials comprising microsilica, which are chemically modified with polymer(s). The bulk size of the silica particles within the polymer matrix improves the sealing properties of the additive. The use of inorganic functional fillers such as modified microsilica reduces significantly the overall production cost. It may offer other benefits for oil well cementing such as improving the bonding between the cement and the formation. In addition, the composite dispersion has a better viscosity profile compared to aqueous solutions of pure synthetic polymers. Modified microsilica additives are suitable for high temperature applications, as well as in salt containing cement. The presence of the inorganic part in the composite extends the thermal stability of the polymer matrix. The composite can be used in most of the water-based drilling fluids. In addition, composite comprising microsilica and hydrophobic copolymers such as styrene-butadiene, maleic anhydride or methyl methacrylate can be used for non-aqueous drilling fluids. The composite can be produced with high solid content without the need for dilution during the

polymerization, which is commonly done for conventional polymerization methods. The concentrated liquid dispersion can be used directly, thus reducing the cost for the drying process.

Chemically modified microsilica is produced by bonding / grafting water-soluble polymer into the surface of microsilica spheres through a chemical bond. There are different ways to bring microsilica in contact with said polymer in order to achieve such modification. One way is through the addition of polymers with functional groups, which react with the silica surface to form covalent or ionic bonds. Another way is to modify silica with a coupling agent such as silane and then graft the polymer into the surface through the coupling agent. The grafting / coupling of the silica with the polymers can take place in gas, liquid or solid states.

Microsilica on its own cannot reduce the filtration loss satisfactorily. With the present invention, the filtration was controlled to the desired value by using modified silica. The surface of microsilica is modified with polymerizable silane by bringing the microsilica in contact with said polymerizable silane. The surface of microsilica is modified with polymerizable silane through a covalent bond formation by the reaction of silanol groups on microsilica surface with the alkoxysilane. The end products are modified polymerizable microsilica and alcohols.

Both dry and wet methods can be used to bring microsilica in contact with

polymerizable silanes. In a dry method, polymerizable silane as it is or dissolved in alcohol such as methanol, ethanol can be sprayed over microsilica. Then the microsilica/polymerizable silane mixture is mixed or rotated for some time and then dryed to obtain the modified microsilica. The dry modification can also be conducted with a vapor phase deposition process, where polymerizable silane is gasified at elevated temperature and deposited over silica particles. In a wet method,

polymerizable silane can be added to a dispersion comprising microsilica and water or non-aquous solvent such as tolune or benzene.

Examples of suitable polymerizable silanes are vinyltrimethoxysilane,

triethoxyvinylsilane, vinyltris(2-methoxyethoxy)silane, dimethoxymethylvinylsilane, diethoxy(methyl)vinylsilane, trichlorovinylsilane, tris(trimethylsilyl)silyl vinyl ether, 3- (trimethoxysilyl)propyl acrylate, 3-(trimethoxysilyl)propyl methacrylate, 3- [tris(trimethylsiloxy)silyl]propyl methacrylate, or 3-(triethoxysilyl)propyl methacrylate.

Without polymerizable silane modification, microsilica adhere poorly to the polymer- matrix. The silica content can be up to 80 weight % of the composite material. The lowest limit is 0.1 weight % but then it will be almost pure polymeric fluid loss, which is expensive. The amount of microsilica in the additive can be in the range of 1 - 80 weight % by the total weight of dry matter, 20 - 70 weight % by the total weight of dry matter, 20 - 60 weight % by the total weight of dry matter, 25 - 55 weight % by the total weight of dry matter, 25 - 50 weight % by the total weight of dry matter, 30 - 50 weight % by the total weight of dry matter, 30 - 40 weight % by the total weight of dry matter. The amount of polymerizable silane is in the range of 0.01 - 20 weight % by the total weight of dry microsilica, in the range of 0.01 - 10 weight % by the total weight of dry microsilica, in the range of 0.5 - 5 weight % by the total weight of dry microsilica, or in the range of 1 - 2 weight % by the total weight of dry microsilica.

The additive according to the invention does not cause gelation at low temperature and is usable for most cementing applications such as high pressure high temperature (HPHT) cement, lightweight cement, high-density cementing, salt containing cement etc. The polymerizable silane modified microsilica can be used in other applications such as filtration control for drilling fluids and fracturing fluids. Further, it can be used as a viscosifier for enhanced oil recovery (EOR) fluid.

To produce the composite additive, the polymerizable silane modified microsilica is copolymerized with one or several of the monomers mentioned below. The polymer chain can be copolymer, terpolymer, tetrapolymer or pentapolymer, etc. For the sake of simplicity, it is referred to as copolymers in the application but the other polymer types are also within the scope of the invention.

Suitable acrylamide derivative monomers are selected from the group consisting of acrylamide (AAm), 2-acrylamido-2-methylpropane sulfonic acid (AMPS), N,N- dimethylacrylamide (NNDMA), vinylformamide, N-tert-butylacrylamide (NTBAAm), N-(Hydroxymethyl)acrylamide ( HMAAm), N,N'-Methylenebisaciylamide

(N MBAAm), vinylbutyrolactam, acrylonitrile, and 2-(dimethylamino)ethyl methacrylate.

Suitable acidic unsaturated monomers are selected from the group consisting of acrylic acid (AA), methacrylic acid, maleic anhydride (MA), itaconic acid, 4- vinylbenzenesulfonic acid, and vinylphosphonic acid.

Additives comprising polymerizable silane modified microsilica and comprising two monomers such as for example mixtures of two acrylamide derivative monomers, or mixtures of one acrylamide derivative monomer and one acidic unsaturated monomer, or mixtures of two acidic unsaturated monomers can be used. Such mixtures can be used in the temperature range 50-150 °C.

For aqueous drilling fluid applications, in particular fresh water systems, a composition comprising two acrylamide derivative monomers together with polymerizable silane modified microsilica has been shown to be sufficient. This is probably due to the low salinity in fresh water based fluids. For the use as a water based drilling fluid, a suitable additive according to the invention comprises polymerizable silane modified microsilica, and the acrylamide derivative monomers 2-acrylamido-2-methylpropane sulfonic acid (AMPS) and N,N-dimethylacrylamide (NNDMA).

A suitable additive according to the invention comprises polymerizable silane modified microsilica and the acrylamide derivative monomers 2-acrylamido-2-methylpropane sulfonic acid (AMPS) and acrylamide (AAm).

Additives comprising polymerizable silane modified microsilica and comprising three monomers such as for example mixtures of three acrylamide derivative mononers or mixtures of two acrylamide derivative monomers with one acidic unsaturated monomer can be used or mixtures of one acrylamide derivative monomer with two acidic unsaturated monomers can be used.

A suitable additive according to the invention comprises polymerizable silane modified microsilica and the acrylamide derivative monomers 2-acrylamido-2-methylpropane sulfonic acid (AMPS), N,N-dimethylacrylamide (N DMA) and acrylamide (AAm).

A suitable additive according to the invention comprises polymerizable silane modified microsilica and the acrylamide derivative monomers 2-acrylamido-2-methylpropane sulfonic acid (AMPS) and N,N-dimethylacrylamide (NNDMA) and the acidic unsaturated monomer acrylic acid (AA).

A suitable additive according to the invention comprises polymerizable silane modified microsilica and the acrylamide derivative monomers 2-acrylamido-2-methylpropane sulfonic acid (AMPS) and N,N-dimethylacrylamide (NNDMA) and the acidic unsaturated monomer maleic anhydride (MA).

Additives comprising polymerizable silane modified microsilica and comprising four monomers such as for example mixtures of four acrylamide derivative mononers or mixtures of three acrylamide derivative monomers with one acidic unsaturated monomer can be used, or mixtures of two acrylamide derivative monomers with two acidic unsaturated monomers or mixtures of one acrylamide derivative monomer with three acidic unsaturated monomers can be used.

For oil well cementing, additives comprising mixtures of acrylamide derivative monomers and acidic unsaturated monomers containing carboxylate group(s) are needed to achieve better adsorption onto the positively charged phases of the cement grains. For cementing applications, a suitable additive according to the invention comprises polymerizable silane modified silica and the acrylamide derivative monomers 2-acrylamido-2-methylpropane sulfonic acid (AMPS), acrylamide (AAm) and N,N- dimethylacrylamide (NNDMA) and the acidic unsaturated monomer acrylic acid (AA).

Additives comprising polymerizable silane modified microsilica and comprising five or more monomers wherein the monomers are combined in any possible manner such as described above, can also be used. The acrylamide derivative monomer(s) is(are) present in the additive in an amount in the range of 20-90 weight % by the total weight of dry matter, 50-90 weight % by the total weight of dry matter, 60-80 weight % by the total weight of dry matter.

The acidic unsaturated monomer(s) is (are) present in the the additive in an amount in the range of 0.1 - 20 weight % by the total weight of dry matter,

1 - 15 weight % by the total weight of dry matter, 2-10 weight % by the total weight of dry matter.

The particle size distribution of the fluid loss particles is in the range of 0.05 μιη to 500 μπι and preferably in the range 0.1 μιη to 15 μιη. The average size (D50) of the particles is in the range of 5 μιη to 20 μπι, preferably around 10 μιη.

The method for the production of an additive for oil well cement slurries and aqueous based drilling fluids comprises a first step of modifying the surface of microsilica with polymerizable silane, using a dry or a wet method, where microsilica is brought in contact with polymerizable silane. Microsilica is chemically modified to allow better integration into the polymer network. Microsilica is added in an amount in the range of 80 - 99.9 weight % by the total weight of dry matter and polymerizable silane is added in the range of 0.1-20 weight % by the total weight of dry microsilica. The

polymerizable silane is selected from the group specified above.

The silane modified microsilica is subsequently polymerized using a free radical polymerization technique. Free radical polymerization is a widely-used method to polymerize unsaturated monomers, wherein free radicals initiate the polymerization. The additives according to the invention can be produced by any free radical polymerization method such as solid, solution, suspension, or emulsion polymerization.

In the polymerization step, silane modified micosilica in an amount of 1 - 80 weight % by the total weight of dry matter is polymerized with at least one acrylamide derivative monomer; the at least one acrylamide derivative monomer being added in an amount in the range of 50 - 90 weight % by the total weight of dry matter and optionally at least one acidic unsaturated monomer. If present, the at least one acidic unsaturated monomer is added in an amount in the range of 0.5 - 20 weight % by the total weight of dry matter. The acrylamide derivative monomers and the acidic unsaturated monomers are selected from the groups specified above. Preparation of chemically modified silica fluid loss additivities

Unless stated otherwise, the microsilica slurry used in the examples was Microblock ® slurry, which is a slurry of ca. 50 weight % (wt %) water and ca. 50 wt % (amorphous silica powder with other additives, produced by Elkem AS.

The process of producing additives containing microsilica is composed of two consequent steps:

1) Silica modification

To incorporate silica into a polymer matrix, the surface of silica is modified with chemicals such as polymerizable silane. Common wet or dry modification methods are applicable.

Example 1: Microsilica modification with polymerizable silane using a dry method

1000 g of Microsilica was collected from an ARC silicon production furnace. The sample was dried at 105 °C overnight. After drying overnight, the sample was loaded in a Hubert mixer and 20 g of 3-(trimethoxysilyl)propyl methacrylate was sprayed over microsilica. The mixture was mixed for 30 min and subsequently vacuum dried at 40 °C overnight.

Example 2: Microsilica modification with polymerizable silane using a wet method

Microsilica was dried at a temperature of 105 °C overnight. Then, 300 g of the dried microsilica was dispersed in 1000 mL of toluene and flashed with nitrogen. The temperature was raised to 50 °C. After raising the temperature, 15 g of 3- (trimethoxysilyl)propyl methacrylate was added dropwise. The mixture was stirred under reflux overnight at 50 °C. The modified silica was filtered and washed with 200 ml ethanol twice. Finally, the mixture was dried under vacuum at 40 °C overnight. The dry modification method is faster and more cost efficient compared to the wet modification method.

2) Polymerization

The polymerizable silane modified microsilica is polymerized with one or several acrylamide derivative monomers and optionally one or several acidic unsaturated monomers using a free radical polymerization technique, which can be conducted through solid state, solution, emulsion, invert emulsion, mini-emulsion or suspension polymerization methods. Example of monomers which can be polymerized with polymerizable silane modified microsilica are acrylamide derivative monomers selected from the group consisting of acrylamide (AAm), 2-acrylamido-2-methylpropane sulfonic acid (AMPS), N,N-dimethylacrylamide (NNDMA), vinylformamide, N-tert- butylacrylamide (NTBAAm), N-(hydroxymethyl)acrylamide (NHMAAm), Ν,Ν'- methylenebisacrylamide (NNMBAAm), vinylbutyrolactam, acrylonitrile and 2- (dimethylamino)ethyl methacrylate and acidic unsaturated monomers selected from the group consisting of acrylic acid (AA), methacrylic acid, maleic anhydride (MA), itaconic acid, 4-vinylbenzenesulfonic acid, and vinylphosphonic acid.

Free radical initiators such as potassium persulfate, ammonium persulfate, benzoyl peroxide, 2,2'-azobisisobutyronitrile (AIBN), tert-butyl peroxide can be used. In addition, redox initiators can be used.

Example 3: Solution polymerization

1) 15 g of 3-(trimethoxysilyl)propyl methacrylate modified microsilica, prepared using the dry method with the use of 1.5 wt % 3-(trimethoxysilyl)propyl methacrylate by weight of dry microsilica, was dry blended with 25 g 2- acrylamido-2-methylpropane sulphonic acid (AMPS) and 7 g N,N- dimethylacrylamide (NNDMA) monomers and mixed into 200 mL of deionized water.

2) The pH of the mixture was adjusted to 7 using 50 wt % NaOH solution.

3) The mixture was then transferred into a 500-liter flask equipped with reflux condenser, magnetic stirrer, and thermometer.

4) The reaction vessel was heated to 60 °C.

5) 0.33 g ammonium persulfate dissolved in 10 ml water as a free-radical initiator was added to initiate the reaction.

6) The reaction was carried out for 2 hours, at the end providing a viscous

dispersion.

Example 4: Solution polymerization

1) 15 g of 3-(trimethoxysilyl)propyl methacrylate modified microsilica, prepared using the dry method with the use of 1.5 wt % 3-(trimethoxysilyl)propyl methacrylate by weight of dry microsilica was dry blended with 2.5 g acrylic acid (AA), 2 g acrylamide (AAm), 3 g N,N-dimethylacrylamide (NNDMA) and 25 g 2-acrylamido-2-methylpropane sulfonic acid (AMPS) monomers and mixed into 200 mL of deionized water. 2) The pH of the mixture was adjusted to 7 using 50 wt % NaOH solution.

3) The mixture was then transferred into a 500-liter flask equipped with reflux condenser, magnetic stirrer, nitrogen gas and thermometer.

4) The reaction vessel was heated to 60 °C.

5) 0.33 g ammonium persulfate dissolved in 10 ml water as free-radical initiator was added to initiate the reaction.

6) The reaction was carried out for 2 hours, at the end providing a viscous

dispersion.

Example 5: Semi-continuous solution polymerization process

1) Solution A consisting of 15 g of 3-(trimethoxysilyl)propyl methacrylate

modified microsilica dry blended with 25 g 2-acrylamido-2-methylpropane sulfonic acid (AMPS), 3 g acrylamide (AAm), and 7gm N, N- dimethylacrylamide (N DMA) monomers was mixed into 200 mL of deionized water.

2) The pH of the mixture was adjusted to 7 using 50 wt % NaOH solution.

3) Solution B consisting of 0.33 g ammonium persulfate was dissolved in 10 ml water as free-radical initiator.

4) 10 % of the total solutions A and B, in this case, 25.5 g of solution A and 1.03 g of solution B, were transferred into a 500-liter flask equipped with reflux condenser, magnetic stirrer, nitrogen gas, and thermometer.

5) The reaction vessel was heated to 60 °C.

6) The rest of the solutions A and B were fed into the reaction vessel using 2 metric pumps within 75 min.

7) The reaction was carried out for 1 hour after the addition of the solutions was completed.

8) The reaction was terminated by cooling down to room temperature and finally a viscous dispersion was obtained.

The morphology of the modified microsilica additive

Figures 1, 2 and 3 show scanning electron micrographs for an additive according to the invention produced by the procedure described in Example 4. The composite material is a large agglomerate filled with microsilica, which is surrounded by polymeric materials. At high shear, such particles deform and fill the voids between the cement particles and reduce the fluid loss. The size of said composite material accoding to the invention was also measured by light scattering method as shown in Figure 4. Oil Well Cementing

To illustrate the application of the new invention, oil well cementing tests using the chemically modified microsilica with polymers according to the invention were conducted according to the API 10 standard. Microblock® slurry produced by Elkem AS was used as a starting material to prepare the modified microsilica. The other chemicals such as conventional fluid loss additive, dispersant, retarder and defoamer are common chemicals to formulate an oil well cement. Silica flour, Milisil M10 (23 μπι & 2.65 sg (specific gravity)) supplied by Sibelco was used to prevent strength

retrogression at temperatures above 110 °C. G-cement was supplied by Dyckerhoff (fineness (Blaine) was 326 m ² /kg).

The following equipment was used to prepare and characterize the cement slurry: Fann 35 rheometer with thermo-cup, atmospheric and HPHT (high-pressure high- temperature) consistometers, equipment for measuring fluid loss (HPHT), ultrasonic compressive strength analyzer (UCA), constant-speed warring mixer, 200 - 250 ml measuring cylinder and precision balance.

Use of chemically modified microsilica in standard 1.9 sg (specific gravity) oil well cement slurry

Example 6

The cement formulation shown in Table 1 was used to prepare cement slurry with a density of 1.9 g/cm ³ . In this example, the performance of the developed chemical was tested in a typical oil well cement design with a density (specific gravity) of 1.9 g/cm ³ . Slurries 1 and 2 were prepared without fluid loss additives. In slurries 1 and 2,

Microblock® slurry with an amount of 25 % and 33 % by the weight of cement, respectively, were added. Slurry 3 was prepared using a conventional polymeric HPHT fluid loss additive containing 2-acrylamido-2-methylpropane sulfonic acid, acrylamide and N-vinyl-N-alkylalkanamide with an amount of 1 % by the weight of cement (BWOC). Slurry 4 was prepared with a fluid loss additive according to the invention prepared according to Example 4 with an amount of 1 weight % dry equivalent by the weight of cement.

The plastic viscosity was measured in centipoise (cP), which is equal to one millipascal- second (mPa s) in SI units (1 cP = 10 ^"3 Pa s = 1 mPa s). The yield point was measured in pound /100 square feet (lbs/lOOft ² ), which is equal to 0.049 kg/m ² in SI units.

The density or the specific gravity (sg) of slurries is measured in gram per cubic centimeter (g/cm ³ ).

Table 1 : HPHT oil well cement formulations with and without fluid loss additives.

The results shown in Table 1 indicate that microsilica cannot control the filtration on its own. Slurries 1 and 2 show high fluid loss. The increase of the microsilica amount reduces the fluid floss but the value of 267 ml after 30 min is still quite high and it is not possible to actually use such cement. In addition, the increase of the microsilica content increases the viscosity to an undesirable value. Cement slurry prepared with a conventional polymeric UPHT fluid loss additive containing 2-acrylamido-2- methylpropane sulfonic acid, acrylamide and N-vinyl-N-alkylalkanamide (slurry 3) provides a fluid loss of 59 ml. Slurry 4 prepared with a fluid loss additive according to the invention prepared according to Example 4 with 1 weight % dry equivalent by the weight of cement gives a fluid loss of 48 ml only and a thinner filter cake. The viscosity of slurry 4 was stable and quite similar at temperatures of 20 °C and 85 °C.

Table 2 shows the formulation of UPHT oil well cement without the use of

Microblock® slurry or any other pozzolanic materials. Just silica flour was added to prevent strength retrogression. Conventional polymeric HPHT fluid loss additive containing 2-acrylamido-2-methylpropane sulfonic acid, acrylamide and N-vinyl-N- alkylalkanamide was used in slurry 5. Slurry 6 was prepared using a fluid loss additive according the invention, prepared according to Example 4. Again, HPHT oil well cement comprising an additive according to the invention shows good results at 150 °C. Table 2: HPHT oil well cement formulations and performance.

Slurries 7 and 8 were prepared with the addition of sodium chloride salt 18 vol % by the volume of fresh water. Salt cement is commonly used to cement salt-containing formations to avoid salt hydration with regular oil-well cement. Control of the filtration for this type of cement requires a fluid loss additive which is compatible with NaCl salt. Slurry 7 was prepared with a conventional polymeric HPHT fluid loss additive containing 2-acrylamido-2-methylpropane sulfonic acid, acrylamide and N-vinyl-N- alkylalkanamide, while slurry 8 was prepared with a fluid loss additive according to the invention, prepared according to Example 4.

The additive according to the invention provides an efficient control of the filtration of salt cement.

Table 3 : HPHT oil well salt cement formulations and performance.

Table 4 shows the design and performance of a fluid loss additive according to the invention (slurry 10), which was produced according to Example 4, vs a conventional polymeric HPHT fluid loss additive containing 2-acrylamido-2-methylpropane sulfonic acid, acrylamide and N-vinyl-N-alkylalkanamide (slurry 9) in a lightweight formulation with density of 1.35 sg. At an amount of 1.5 % BWOC, the additive according to the invention provides good cement slurry design.

Table 4: Formulations and performance of lightweight FIPHT cement, density 1.35

Similar to oil well cement, synthetic polymeric materials are used as fluid loss control for HPHT (high pressure high temperature) aqueous drilling fluid, which is used to drill wells with bottomhole temperatures >150 °C. Such materials should stand the high temperature, which can be in the range of 150-250 °C. Modified microsilica composite has deformable large-size particles, which can form an elastic filter cake on the formation surface and reduce the fluid loss.

Tables 5 and 6 shows the formulation and the performance of a 2.1 specific gravity (sg) HPHT aqueous drilling fluid containing the additive according to the invention as a fluid loss additive. The fluid loss additive in Tables 5 and 6 are produced according the procedure described in Example 3.

The results show that the developed material is stable at a temperature of 200 °C. The fluid loss was in the range 20-22 ml after heat aging at 175 °C and 200 °C, respectively. The viscosity was stable after heat aging.

Table 5: Formulation of a 2.1 sg HPHT aqueous drilling fluid comprising a fluid loss additive according to the invention prepared according to the procedure described in Example 3.

Table 6: Performance of a 2.1 sg HPHT aqueous drilling fluid comprising fluid loss additive according to the invention prepared according to the procedure described in Example 3.

Composites of modified microsilica and acrylamide derivatives have a high capacity of water retention and can control the filtration of aqueous oilfield fluids and are thus suitable as fluid loss additives in aqueous drilling fluids as shown in Table 6 above. Table 7 shows examples of formulations of additives according to the present invention. The amount of water in the formulations was 200 ml. All the mass amounts in Table 7 are in grams. In addition to the monomer amount, 0.2 g of potassium persulfate and 5 g sodium hydroxide were added.

Table 7 -additive formulations according to the invention

Example 7

The additive according to the invention was tested in cement slurry. During the curing process, cement slurry is converted first into a gel state and finally is converted into solid state. To mitigate the risk of gas migration through the cement column during the hardening / curing process, the transition time should be as short as possible and the development of the compressive strength should be as fast as possible. The transition time is the time it takes for static gel strength to increase from 50 to 500 psi (3.45 - 34.5 bar). The transition time should be less than 40 min and preferably less than 30 min to prevent gas migration.

Static Gel Strength Analyzers (SGSA) provided by Ametek Oil & Gas company use acoustic /ultrasonic attenuation to monitor the hardening process of an oil well cement under downhole temperature and pressure conditions. The equipment measures the static gel strength development and the compressive strength development of a cement slurry as a function of time.

A shortening of the transition time was observed for the cement slurry containing the additive according to the invention. Figures 5 and 6 show the curing process for the oil well cementing prepared slurries 5 and 6 as shown in Table 2. Slurry 6 comprises an additive according the invention, prepared according to Example 4. Slurry 5 comprises a conventional polymeric HPHT fluid loss additive containing 2-acrylamido-2- methylpropane sulfonic acid, acrylamide and N-vinyl-N-alkylalkanamide. The tests were done at a temperature of 150 °C and a pressure of 3000 psi (206.8 bar). The transition time was 36 min for slurry 5 and 9 min for slurry 6. In addition, the compressive strength was developed much faster for slurry 6 formulated with the additive according to the present invention compared to slurry 5 formulated with a conventional polymeric HPHT fluid loss additive. Within 12 h curing time, slurry 5 reached a compressive strength of 1802 psi (124.24 bar) while slurry 6 reached a compressive strength of 2427 psi (167.33 bar).

This indicates that cement prepared with an additive according to the present invention provides additional benefit to the properties of a cement slurry, namely preventing gas migration by shortening the transition time and increasing the early compressive strength to a great extent.

The additives according to the invention can be used in formulations for oil well cementing or water-based drilling fluids. The additives according to the invention are for example used as fluid loss additives and also as viscosity modifiers (viscosifiers) for fracturing or enhanced oil recovery (EOR) applications.

Having described preferred embodiments of the invention it will be apparent to those skilled in the art that other embodiments incorporating the concepts may be used. These and other examples of the invention illustrated above are intended by way of example only and the actual scope of the invention is to be determined from the following claims.