The invention concerns a cementitious aplite-based geopolymeric material.

Generally, two different types of cements may be distinguished : "hydraulic" cements such as Portland cements, and "geopolymer" cements.

Since the development of Portland cement, it has become the most common construction ingredient. Portland cement is also widespread material used in petroleum industry for sealing the annular space between casing and for zonal isolation. However, there are some drawbacks regarding chemo-physical properties of hardened Portland cement and emission of greenhouse gases in its manufacturing process.

Production of Portland cement contributes between 5 and 7 % of global carbon dioxide (C0 ₂ ) emission, while approximately 900 kg of C0 ₂ is emitted for manufacturing one ton of Portland cement. Emitted C0 ₂ arising from manufacture of Portland cement is attributed to:

(i) the formation and release of C0 ₂ due to decomposing of limestone (a key ingredient); and

(ii) high energy consumption during calcination of raw materials within a kiln. Further, chemical shrinkage and autogenous shrinkage, possible gas influx (permeability), long-term durability, and instability in corrosive environments and at high temperatures are also some of drawbacks, which persuade researchers to look for alternatives to Portland cement. Several alternative binders have been designated; calcium alumi- nate cement, calcium sulfoaluminate cement, supersulfated cements and alkali- activated binders.

Alkali-activated binders are receiving increasing attention as alternative to Portland cement due to their high enough strength, durability and low environmental impact. Unlike Portland cement, the source of alkali-activated binders can be waste-stream materials used with very limited further processing. Alkali-activated binders are developed by mixing an alkaline activator, which could be an alkaline solution or a mix of alkaline solution with alkaline silicate solution, with a source of aluminosilicate material such as fly ash, kaolin, metakaolin, blast furnace slag, etc. Concisely, the hydroxyl group (OH ^" ) penetrates to the original structure of aluminosilicate material and depol- ymerize the silicates. As a result of alkalination monomers of silicon tetra hedrons and aluminum tetrahedrons form covalently bonded oligomers. Oligomers are of rea rrangement of gel formation polycondensation networking solidification. Broadly, de- polymerization, transportation or orientation and polycondensation are three main mechanisms in development of alkali-activated binders. The product of reaction is an inorganic material, which has been coined "geopolymer".

Extensive research have been done to investigate the possibility of utilizing artificial pozzolans or aluminosilicate-type industrial waste materials such as fly ash, kaolin, etc. In contrast, few works have been done to study utilization of natural pozzolan or aluminosilicate-type rocks as a source material in geopolymerization. Kawano and Tomita (Kawano M, Tomita K. : "Experimental study on the formation of zeolites from obsidian by interaction with NaOH and KOH solutions at 150 and 200 °C. " Journal of Clay and clay minerals 45 (1997) 3, p. 365-377) studied the synthesis of the zeolites from obsidian in various concentrations of NaOH and KOH solutions at 150 and 200 °C. Their findings show that smectite, phillipsite and rhodesite formed in NaOH solution as pH increased and the smectite, merlinoite and sanidine yielded in KOH solution as pH increased . The pH value, Si/AI, and Na/K ratios of the reacting solution have been reported to be important factors determining the nature of the products formed from obsidian.

Allahverdi et al. (Allahverdi A, Mehrpour K, Najafi Kani E. : "Taftan pozzolan-based geopolymer cement." IUST International Journal of Engineering Science, Vol. 19, No. 3, 2008, p. 1-5) utilized a pumice-type natural pozzolan taken from Taftan Mountain located at the southeast of Iran to develop a Taftan-based geopolymer. The pumice- type natural pozzolan has a relatively high siliceous content. A mixture of NaOH and Na ₂ OSi0 ₂ was used as an activator in their study. Their X-ray diffraction study shows that Taftan pozzolan had four crystalline mineral phases; quartz, hornblende, anor- thite and biotite. However, biotite and amorphous part of Taftan pozzolan participated in the reaction while quartz, hornblende and anorthite were not reactive. Based on their result, the final setting times of all their systems were relatively long due to high liquid/solid ratio of 0.44.

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art. The object is achieved through features, which are specified in the description below and in the claims that follow.

Geopolymers are one of the materials which chemo-physical characteristics and abundance have attracted much attention, recently. Geopolymeric cements result from a mineral polycondensation reaction in an alkaline medium. If geopolymers (reactive alumino-silicate materials) are mixed with appropriate additives under suitable temperature and pressure, they set and the final product can withstand high pressures, temperatures and corrosive environments for long-term.

Geopolymers are a are used as alternative to cement and replacing binder in concrete. The term "geopolymers" coined by Davidovits to describe inorganic binders that have the empirical formula of _¾ {-ί5έ¾) _ζ - Αΐϋ~) - -ω ₂ 0, where M is a cation

(K÷,Ma.*,Li* _* or & * ), n is a degree of polycondensation, and z is the atomic ratio of

Si/AI which may be 1, 2, 3 or higher. In other words, geopolymers are alumino-silicate materials, which react in alkaline solution. The reaction shows a complex process but it could be said that in an alkaline medium the bonds of Si-O-Si are broken and Al atoms penetrate into the original Si-O-Si structure; alumino-silicate gels are mostly formed in the process. Cations must be present in the framework cavities to balance the negative charges of ions (J. Davidovits: "Geopolymer chemistry & applications", 3 ^rd edition, July 2011, p. 3-5, 228-230, 365-371, 375-386. F. Skvara : "Alkali activated materials or geopolymers?" Institute of Chemical Technology Prague, May 2007. H. Xu : "Geopolymerisation of alumino-silicate minerals." Ph.D. thesis, University of Melbourne, April 2002). The process is termed "geopolymerization" and the result is a cementitious phase with a high mechanical strength, high fire and acid and bacteria resistance. In addition, geopolymers are characterized by a number of physical characteristics including thermal stability, high surface smoothness, hard surface, long- term durability and high adhesive property to natural stone and steel (Davidovits, 2011. Xu, 2002). The geopolymerization development depends on many pa rameters including chemical and mineralogical composition, particle size and surface area, cu ring temperature and pressure, alkali cation type, Si/AI ratio of the used substances, activator/solid ratio, and types of additives (E. I. Diaz, E. N. Allouche, S. Eklund : "Factors affecting the suitability of fly ash as source material for geopolymers" Elsevier, Fuel, Vol. 89, 2010, p. 992-996. D. L. Y. Kong, J. G. Sanjayan, K. Sagoe-Crentsil; "Factors affecting the performance of metakaolin geopolymers exposed to elevated temperatures." Journal of Material Science, Vol. 43, 2008, p. 824-831. J. Nemecek, V. Smilauer, L. Kopecky; "Nanoindentation characteristics of alkali-activated aluminosili- cate materials." Elsevier, Cement & Concrete Composites, Vol. 33, 2100, p. 163-170. D. Ravikumar, S. Peethamparan, N. Neithalath; "Structure and strength of NaOH activated concretes containing fly ash or GGBFS as the sole binder." Elsevier, Cement & Concrete Composites, Vol. 32, 2010, p. 399-410. 1 Stark; "Recent advances in the field of cement hydration and microstructure analysis." Elsevier, Cement & Concrete research, Vol. 41, 2011, p. 666-678).

There are different types of geopolymers based on the used source e.g., kaolinite- based, metakaolin-based, fly ash-based, phosphate-based, etc.

Aplite is an intrusive rock in which quartz, alkali feldspar, microcline and albite are the dominant components. Oligoclase, muscovite, apatite and zircon are principally minerals of aplites. Biotite and all ferromagnesian minerals rarely appear in aplites. Aplite members are usually Na-rich. Aplites contain Si0 ₂ and Al ₂ 0 ₃ and whereby are similar to pozzolans, and they seem to have the potential to be utilized in development of an aplite-based geopolymer cement.

The present invention introduces a new geopolymeric material, which can be called as an aplite-based geopolymer and prepared for oilfield cementing applications like sealing annuli between casings, sealing an annulus between a liner and a formation, zonal isolation, temporary and permanent plugging, and squeeze operations. Aplite is mixed with additives and an alkali activator to prepare a geopolymeric slurry. The additives are Blast Furnace Slag (BFS) and Microsilica. The alkali activator is prepared by mixing various concentrations of alkali solution and alkali silicate solution. Several tests have been performed by utilizing an aplite to obtain aplite-based geopolymer binder and an aplite-based geopolymer cement. The aplite-based geopolymer sets at 25 - 200 °C under ambient pressure and high pressures.

The main objective is to create a cementitious material, which gets high enough compressive strength in order to withstand some degree of tectonic stresses. The product should be impermeable, non-shrinking, withstand corrosive environments and bonds with rocks and casings. Several tests were performed to find the effect of different additives on the rheological and physical properties of the slurry and final product after setting. Uniaxial Compressive Strength (UCS) measurements have been carried out to find the compressive strength development through time. Finally, sets of sensitivity analysis tests have been performed to find the influence of alkali concentration, liquid/solid ratio, curing temperature and pressure on the chemo-physical property of the developed aplite-based geopolymers. Little studies have been done on the reactivity of natural rocks to make geopolymeric binder and geopolymeric cement in alkaline medium in contrast to acid medium. It could be as a result of lower solubility of natural rocks in alkaline medium than in acid medium (J. A. Chermak, "Low temperature experimental investigation of the effect of high pH NaOH solutions on the opalinus shale, Switzerland". Clay and clay minerals, Vol. 40, No. 6, 1992, p. 650-658. Davidovits, 2011). M. Kawano, K. Tomita (see above) reported; smectite, phillipsite and rhodesite formed in NaOH solution as pH increased and the smectite, meriinoite and sanidine were produced in KOH solution as pH increased. They also mentioned that the pH value, Si/AI, and Na/K ratios of the reacting solution are important factors determining the nature of the products formed from obsidian. Gougazeh, M., Buhl, J.-C. "Synthesis and characterization of zeolite A by hydrothermal transformation of natural Jordanian kaolin". Journal of the Association of Arab Universities for Basic and Applied Sciences (2013),

http://dx.doi.Org/10.1016/i .iaubas.2013.03.007 synthesized zeolite A by treating the activated metakaolin from natural kaolin with various concentrations of NaOH at 100 °C. Their obtained results show that the zeolite A is the major constituent phase and quartz and hydroxysodalite were the minor constituents.

The invention is defined by the independent patent claims. The dependent claims define advantageous embodiments of the invention.

In a first aspect the invention relates more particularly to a cementitious aplite-based geopolymeric material, characterised in that a mixture of fine-grained aplite and an alkaline medium concentration including an alkali solution and an alkali silicate solution is forming a curable slurry.

The aplite particle size might be maximum 75 μηη.

The alkali solution may comprise NaOH in the range of 6M-10M. Alternatively, the alkali solution may comprise NaOH in the range of 7M-9M.

The alkali solution may comprise KOH in the range of 4M-8M. Alternatively, the alkali solution may comprise KOH in the range of 5M-7M.

The liquid/solid ratio of the mixture may be in the range of 0.40-0.50 by weight. Alternatively, the liquid/solid ratio of the mixture may be in the range of 0.42-0.47 by weight.

In a second aspect the invention relates more particularly to a method of providing a curable slurry of a cementitious aplite-based geopolymeric material, characterised in that the method comprises the steps of:

providing a fine-grained aplite;

adding a concentration of an alkali solution and an alkali silicate solution to a liquid-solid ratio in the range of 0.40-0.50 by weight.

The method may comprise the further steps of:

grinding the aplite to a particle size of maximum 75 μηη prior to adding said solution to the mixture.

The invention offers several advantages over prior art:

• High pressure/high temperature conditions (up to 8.000 psi (approx. 55 MPa) and 700 °C)

• Low bulk shrinkage factor (less than 4%)

• Withstand corrosive environments

• Low permeability and more suitable for gas reservoirs (less than 20 micron Darcy)

• Retarders, accelerators and viscosity-control additives exist

• Same equipment for placing the slurry at desired depths as used in well cementing operations

Experimental section

Materials

Table 1 tabulates the chemical composition of grinded aplite which was supplied by HELI Utvikling AS, Namskogan, Norway. Fig. 1 depicts the particle size distribution analysis of the used aplite. Elkem microsilica grade 955 was supplied by Elkem AS, Oslo, Norway. Blast Furnace Slag (BFS) was made in Sweden and supplied by SSAB Merox AB, Oxelosund, Sweden under the brand name of Merit 5000. Sodium hydroxide (NaOH) and potassium hydroxide (KOH) came as pellets with 99% purity delivered by Merck KGaA, Darmstadt, Germany. Sodium silicate solution (Na ₂ Si0 ₃ ) was supplied by Merck. The chemical composition of the Na ₂ Si0 ₃ was: 28.5% Si0 ₂ , 8.5% Na ₂ 0 and 63% H ₂ 0. Potassium silicate solution (K ₂ Si0 ₃ ) used was supplied by Univar AS, Oslo, Norway. Potassium silicate solution reported to have 38% of K ₂ Si0 ₃ and 62% H ₂ 0. Distilled water was used throughout the experiments. Table 1

Chemical composition of the original aplite

*) LOI = loss on ignition.

Sample preparation

Sodium hydroxide solutions were prepared with three different concentrations of 6, 8, and 10 M of NaOH. Potassium hydroxide solutions were prepared with concentrations of 4 and 6 M. Activators were prepared with different proportions as Table 2 summarizes alkali solution/alkali silicate ratios. It is recommended to prepare the activator one day before to get the components uniformly mixed. Liquid-solid mixing should be respected in order to get the most efficient product. Prime, micro silica should be added to the activator and mixed for 2 minutes. Then aplite was added and mixed for 2 minutes. Latter BFS was added and mixed . Liquid and solid phases were mixed by using a Hamilton Beach blender. Slurries were cast into cylindrical plastic moulds of dimensions of 5.2 cm in diameter and 10 cm in length. Table 3 outlines the different recipes, which have shown the prime results. Specimens were cured at ambient pres- sure and temperature for 7 and 28 days within a plastic box, which was filled with tap water. Note that specimens could be cured out of the plastic box.

Table 2

Alkali solution/alkali silicate solution ratio

Table 3

Recipes used in the development of aplite-based aeopolvmers

Sample Activator Ingredient

ID ID

Solid/total solid ratio Liquid/total Water/solid rasolid ratio tio (distilled

Aplite Microsilica BFS

(activator) water)

316 a 0.61 0.09 0.30 0.37 0.05

319 b 0.64 0.05 0.31 0.47 0

320 c 0.61 0.09 0.30 0.37 0.05

323 d 0.64 0.05 0.31 0.47 0

324 e 0.61 0.09 0.30 0.37 0.05

327 f 0.64 0.05 0.31 0.47 0

328 g 0.61 0.09 0.30 0.37 0.05 331 h 0.64 0.05 0.31 0.47 0

Analytical methods

In order to investigate the mechanical properties of the developed aplite-based geo- polymer cement, compressive strength development of geopolymers has been estimated. The compressive strengths of the specimens were measured by deploying a Toni Technik-H mechanical tester. The apparatus applies a TestXpert v7.11 testing software to assess the uniaxial compressive strength.

A Zeiss Supra 35VP model scanning electron microscope (SEM) analyzer was used to reveal the microstructure of the aplite-based geopolymers.

Particle Size Distributions (PSD) of aplite was estimated by Sympatec HELOS laser diffraction particle size analyzer and experiment was performed at Tel-Tek national research institute in Norway. Sauter Mean Diameter (SMD) and Volume Mean Diameter (VMD) were reported 3.20 and 19.68 μιτι, respectively. Aplite's density estimated to be 1.18 (g/cc).

Results and discussion

Effect of curing time

Different specimens were prepared and cured for 7, 28 and 56 days. Measured compressive strength of the specimens shows strength development with time after 56 days.

X-ray diffraction (XRD) analysis

X-ray powder diffraction (XRD) analysis of the aplite and the produced geopolymers were performed using synchrotron radiation with wavelength of 0.6888A. The XRD measurements were performed with a PILATUS2M -based diffracto meter at European Synchrotron Radiation Facility (ESRF). The angular range was 0-46, 2theta.

The obtained result shows the aplite and aplite-based geopolymers are crystalline.

Permeability measurement

A QX series of Quizix pump manufactured by AMETEK Chandler Engineering was utilized for permeability measurement. Injection pressure of 3045 psi (210 bar) was selected based on the experiences for permeability measurement. Overburden pressure selected to be 3480 psi (240 bar) and pump pressure limit was set on 3045 psi (210 bar). Distilled water used for injection. Outlet pressure was selected to be ambient pressure. The experiment carried out at ambient temperature. The measured permeability after 30 days of running test is k=0.07xl0 ^"7 mD, which can be accounted as zero in comparison to the Portland cement permeability.

Conclusion

Some specimens were cured at 87 °C for 28 days and estimated compressive strength was 7500 psi and in some cases 8660 psi.

Aplite-based geopolymers were cured at 500 °C for 12 hours and no change was observed in their mechanical property.

Due to low CaO content (20%) the aplite-based geopolymers can withstand carbon dioxide attacks.

Bulk shrinkage has been measured and a value of less than 4% was measured and is some cases it was 0.0 %.

During production the aplite-based geopolymer does not produce greenhouse gases, it can be called an environmental friendly cementitious material.

Sodium hydroxide concentration can vary between 6 and 10 M and even higher.

Potassium hydroxide concentration can vary between 4 and 8 M.

Alkali solution to alkali silicate solution ratio can vary between 0.3 and 1.

List of illustrations:

Fig. 1 Particle Size Distribution (PSD) analysis of the used aplite;

Fig. 2a Estimated compressive strength of the aplite-based geopolymer with an alkali solution comprising NaOH after 7 and 28 days; Fig. 2b Estimated compressive strength of the aplite-based geopolymer with an alkali solution comprising KOH after 7 and 28 days.