GB2387613A

GB2387613A - High temperature well cementing using silica-rich minerals

Info

Publication number: GB2387613A
Application number: GB0208775A
Authority: GB
Inventors: Bruno Drochon; Michel Michaux
Original assignee: Schlumberger Holdings Ltd
Current assignee: Schlumberger Holdings Ltd
Priority date: 2002-04-17
Filing date: 2002-04-17
Publication date: 2003-10-22
Also published as: GB0208775D0

Abstract

A method of cementing high temperature wells (above 110{C [230{F]) by pumping into the well a cement mixture comprising Portland cement and a silica-rich mineral, the blend having a CaO/SiO2 molar ratio of less than about 2.0, preferably less than 1.6. This composition prevents the formation of "alpha dicalcium silicate hydrate" and the associated strength regression at high temperatures. Silica-rich minerals include silica flour, blast furnace slag, fly ash (preferably Class F), natural pozzolans and metakaolin. Preferably the silica-rich mineral is present in the mixture at 60 to 85%.

Description

23876-l 3 USE OF BLENDED PORTLAND CEMENTS CONTAINING HIGH-SILICA

MINERALS FOR HIGH-TEMPERATURE WELL CEMENTING

The present invention relates to the use of cement blends containing Portland cement and minerals having a high silica content for the cementing of wells at high temperature. Portland cement is essentially a calcium silicate material, the most abundant components being tricalcium silicate (C3S) and dicalcium silicate (C2S) which represent about 75-80% of cement (the following abbreviations are used unless otherwise indicated: C = CaO, A = Al2O3, S = sio2, H = H2O). Upon addition of water, both hydrate to form a quasi-amorphous calcium silicate hydrate called "C-S-H gel," which is responsible for the strength and dimensional stability of the set cement at ordinary temperatures. In addition to C-SH gel, a substantial amount of calcium hydroxide (CH) is liberated. C-S-H gel is an excellent binding material at well temperatures less than about 110 C (230 F). At higher temperature, C-S-H gel is subject to metamorphosis, which results in decreased compressive strength and increased permeability of the set cement. C-S-H gel converts to a phase called "alpha dicalcium silicate hydrate (oc-C2SH)." a-C2SH is highly crystalline and much more dense than C-S-H gel. As a result, a shrinkage occurs which is deleterious to the integrity of the set cement. This phenomenon is known as "strength retrogression."

The strength retrogression problem can be prevented by reducing the bulk lime-to-

silica molar ratio (C/S molar ratio) in the cement. The typical C/S molar ratio of Portland cement is around 3. The conversion of C-S-H gel into xC2SH at temperature above 110 C (230 F) can be prevented by the addition of 35% to 40% BWOC (By Weight Of Cement) of fine silica sand and/or silica flour, reducing the C/S molar ratio to about 1.0-1.2. At this level, a mineral known as 11 A tobermorite (CsS6Hs) is formed; fortunately, high compressive strength and low permeability are preserved. As the curing temperature increases to about 170 C (338 F), 11 A tobermorite normally converts to xonotlite (C6S6H) with minimal deterioration.

Toberrnorite sometimes persists to about 200 C (392 F) in Portland cement systems because of aluminium substitution in the lattice structure.

In some areas, oilwell cements are commercially available where 35-40% BWOC of quartz (silica) has been interground with Portland cement. However, in most cases the blend has to be made by cementing service companies. This operation is not very popular since it requires special equipments (at least two silos) and is time consuming.

Moreover, fine silica is dangerous to manipulate (silicosis).

The present invention seeks to avoid the problems involved in the use of silica in oilwell cements.

In accordance with the present invention, there is provided a method of cementing high-temperature wells (e.g. above 110 C [230 F]) by pumping into the well a cement blend comprising Portland cement and a silica-rich mineral, the blend having a C/S molar ratio of less than about 2.0, preferably less than 1.6.

The C/S molar ratio of the cement blend is decreased by adding silicarich minerals, such as blastfumace slag and/or Class F (low content in CaO) flyash (pulverized fuel ash), that take part in the cement hydration reactions and thereby make a substantial contribution to the hydration product. The silica-rich mineral addition may be ground together with the cement clinker and gypsum, or mixed with Portland cement when the latter is used. Flyash and slag are waste materials produced in large quantities, and concretes (construction cements, cement/aggregate mixtures) made with them can have properties similar to those of ones made with pure Portland cements at lower cost per unit volume. Flyash is ash separated from the flue gas of a power station burning pulverized coal. Blastfurnace slag is formed as a liquid at 1350-1550 C (2462-2822 F) in the manufacture of iron as a result of limestone reacting with materials rich in SiO2 and Al2O3 associated with the ore or present in ash from the coke. If cooled sufficiently rapidly to below 800 C (1472 F), it forms a glass which is a latent hydraulic cement.

Nowadays many blended cements are commercially available and can contain large proportions of flyash or slag, going up to over 80% by weight in the blend. Such cements are not normally used for oilwell cementing or the like. When large proportions of flyash or slag are present, the C/S molar ratio of blended cement can be less than 1.6, and such C/S molar ratios can be sufficiently low to prevent the formation of a-C2SH and calcium hydroxide when the cement is cured at temperature above 110 C (230 F). Thus, blended commercial cements can be suitable for high-

temperature oilwell cementing if high compressive strength and low permeability can be achieved. Thus, the need for a blend of Portland cement and silica might be eliminated. The present invention will now be described by way of examples and with reference to the accompanying drawings, in which: Figure I shows photomicrographs of a cement/slag blend (CLK cement) cured for four weeks at 150 C (302 F); Figure 2 shows photomicrographs of a Class G cement cured for four weeks at 1 50 C (302 F); and Figure 3 shows photomicrographs of a Class G cement/35% BWOC silica flour blend cured for four weeks at 1 50 C (302 F).

The hydrothermal behaviour of a commercial blended cement (supplied by Origny Company) is studied at 150 C (302 F), and compared to two other cement systems: 1) pure Class G Portland cement, 2) Class G Portland cement stabilized with 35% BWOC silica flour. This commercial cement, which is commonly used in construction industry, is composed of about 20% Portland cement and about 82% blastfurnace slag (of CLK type in French cement nomenclature). The oxide composition and C/S molar ratio of the three cement systems are given in Table 1.

The composition of cement slurries is given in Table 2.

Table 1: Oxide Composition and C/S Molar Ratio of Cement Systems CLK cement Class G* cement Class G cement + 35% BWOC

Silica Flour Oxide Composition: CaO 44.32 63.12 46.76 SiO2 31.41 22.55 42. 63 Al2O3 9.72 3.90 2.89 Fe2O3 1.37 4.71 3.49 MgO 6.39 0.75 0.56 C/S Molar Ratio 1.51 3.00 1.17 * API (American Pet' oleum Institute) Cla gasification for oilwe 11 cements. Class G cement is commonly used at high temperature.

Table 2: Composition of Cement Slurries CLK cement Class G cement Class G cement + 35% BWOC

Silica Flour l Anti-foam agent 2.7 1 2.7 2.7 (L/ tonne of cement) Retarder (% BWOC) 0.3 0.25 Retarder/Dispersant- solid 0.3 0.25 (% BWOC)

Retarder/Dispersant - liquid 6.1 (L/ tonne of cement) Water (L/ tonne of cement) 411 424 538 Slurry Density (kg/L) 1.87 1.92 1.92 Slurry Porosity (%) 55.4 56.9 54.6

600 mL of cement slurry is mixed according to the API procedure in a Waring blender mixer rotating at 12,000 RPM for 35 seconds. The slurry is then introduced in a UCA (Ultrasonic Cement Analyzer) cell to follow the development of compressive strength at 150 C (302 F) under a 3000 psi (20. 7 MPa) confining pressure. The cement is heated from ambient temperature to 150 C (302 F) at 2.78 C/min (5 F/min) to avoid a thermal shock.

After two or three days of curing at 150 C (302 F) the compressive strength of the three cement systems stabilizes at 4700 psi (32.4 MPa) for the CLK cement, 2500 psi (17.2 MPa) for the Class G cement without addition of silica flour, and at 5500 psi (37.9 MPa) for the Class G cement stabilized with 35% BWOC silica flour. No further evolution is noted after 4 weeks and the experiments stopped.

The compressive strength of CLK cement is quite comparable to that obtained with the Class G cement stabilized with silica, and is sufficiently high to protect the casing against mechanical stresses which can be encountered in the well. The lower compressive strength of Class G cement without addition of silica is due to "strength retrogression" phenomenon which occurs during the first day of curing.

After four weeks curing at 150 C (302 F), set cement samples are crushed and dried with acetone and ethyl ether to remove the water which is not chemically bound in cement hydrates. Hydration products are analysed by thermogravimetric analysis (TGA) and X-rays diffraction (XRD). XRD allows to detect crystalline compounds, whereas TGA enables to highlight the presence of a-C2SH and calcium hydroxide (CH) thanks to their characteristic water weight loss occurring in 450-500 C (842-

932 F) temperature range.

The only crystalline hydrate detected by XRD in hydrated CLK cement is 11 A aluminium-substituted tobermorite (up to 10% of silicon can be substituted by aluminium in its lattice structure), while oc-C2SH and calcium hydroxide is not detected by both techniques. The presence of poorly crystallized C-S-H gel is also likely. s

As expected the hydrated Class G cement is mainly composed of oc-C2SH and calcium hydroxide. Tobermorite and C-S-H gel is not detected. The presence of large amounts of a-C2SH and calcium hydroxide can explain the low compressive strength value.

The hydrated Class G cement stabilized with 35% BWOC silica flour reveals the presence of small quantities of a-C2SH and silica that has not reacted with the cement. Nevertheless, 11 A tobermorite is the major hydration product.

Set cements cured for four weeks at 150 C (302 F) are observed by Scanning Electron Microscopy (SEM).

Figure I shows that the matrix of CLK cement is very compact, probably resulting in low permeability value. In small holes hydration products have some space to develop, they are poorly crystallized and look like those obtained at temperature below 110 C (230 F) where the C-S-H gel is predominant. Crystals of a-C2SH and calcium hydroxide were not observed; this is in agreement with the results obtained by TGA and XRD.

Figure 2 shows that the matrix of Class G cement is very porous with the presence of big holes between crystallized hydrates. These crystals mainly appear as plates which are characteristic of oc-C2SH. Some smaller hexagonal crystals of calcium hydroxide could also be distinguished. It is most likely that the permeability of this cement matrix is quite high.

The effect of silica added to Class G cement can clearly be seen in Figure 3. The matrix is much more compact and, therefore, has probably a low permeability. The morphology of hydrates is representative of that described in the literature for well-

crystallized 11 A tobermorite.

The water permeability of CLK cement cured for five days at 150 C (302 F) is measured to be less than 6 uD which corresponds to the detection limit of the equipment used. Actually, the true permeability is likely much lower than this value.

It is generally recognized that the water permeability of oilwell cements should be no more than 100 ED to prevent interzonal communication.

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Claims

- CLAIMS

I A method of cementing an oil well having a temperature above 1 1 0 C (230 F), comprising pumping into the well an aqueous cement slurry including a mixture of Portland cement and a silica-rich mineral, the mixture having a C/S molar ratio of less than about 2.0.
2 A method as claimed in claim 1, wherein the cement blend is substantially free of finely divided silica, silica flour, silica sand or silica fume.
3 A method as claimed in claim 1 or 2, wherein the cementing composition develops high compressive strength and low permeability when cured at temperature above I 1 0 C (230 F).
4 A method as claimed in claim 1, 2 or 3, wherein the Portland cement comprises ordinary Portland cement (OPC); Class A, B. C, G or H of API classification, Type I to V of ASTM classification or pozzolan cement.
5 A method as claimed in any preceding claim, wherein the silica-rich mineral comprises blastfurnace slag, flyash, natural pozzolans or metakaolin.
6 A method as claimed in claim 5, wherein the flyash is of Class F (low content in lime, CaO < 10%).
7 A method as claimed in any preceding claims, wherein the silica-rich mineral is present in an amount in the range of from about 40% to 100% of the mixture.
8 A method as claimed in claim 7, wherein the silica-rich mineral is present in an amount in the range of from 60% to 85% of the mixture.
9 A method as claimed in any preceding claim, wherein the C/S molar ratio of À- _ =_ _ _