WO2008146011A2

WO2008146011A2 - Well cementing methods

Info

Publication number: WO2008146011A2
Application number: PCT/GB2008/001851
Authority: WO
Inventors: Arild Saasen; Hallvar Eide; Rune Godoy
Original assignee: Statoilhydro Asa; Cockbain, Julian
Priority date: 2007-06-01
Filing date: 2008-06-02
Publication date: 2008-12-04
Also published as: GB0710519D0; GB2449847B; GB2449847A; WO2008146011A3

Abstract

The invention relates to a method for cementing- in a lining tube in a bore hole, said method comprising placing said lining tube at a distal end of said bore hole, and introducing an aqueous liquid hydraulic cement composition into said distal end of said bore hole, characterised in that at least part of the length of said lining tube is aluminium and in that said cement composition comprises pulverulent aplite.

Description

Well cementing methods

This invention relates to improvements in and relating to well cementing.

The term well or bore hole as used herein means a well for extraction of fluids from below the earth's surface or for the subsurface injection of fluids. Typically such wells will be for water or hydrocarbon (e.g. gas or oil) extraction or for injection of water, carbon dioxide or hydrocarbon gas, especially for introduction of carbon dioxide.

When drilling to retrieve fluids (e.g. water or more generally hydrocarbons) from subterranean reservoirs, drilling is generally done using a drill bit at the end of a drill string running from the drill rig which may be on land or water. The drill string is a pipe, generally of steel but also possibly of another metal, e.g. aluminium, titanium, titanium/steel, or a composite (generally carbon fibre reinforced plastics). Steel drill strings are cheaper than titanium, titanium/steel or composite but are heavier than those of these other materials.

To enable the desired fluid to be recovered without contamination by undesired fluids (e.g. water) from other strata through which the bore may pass and to prevent seepage of the desired fluid from the bore into other strata, once drilled the bore is lined with a tube, generally of steel for economic and other reasons, and the gap between this lining tube (referred to as a casing or liner) is sealed with hydraulic cement to ensure the desired fluid travels up to the surface through the lining tube rather than through gaps between the lining tube and the surrounding rock (also referred to as matrix or formation) . Otherwise there is a risk that the fluid escapes into the matrix or reaches the surface uncontained giving rise to the risk of fire or explosion. Once extraction is to begin, the lining tube is then pierced at the site at which extraction is to take place, e.g. using an explosive device. If drilling is to continue however, the end of the lining tube, if sealed, is drilled out.

The lining tube remains in place and thus there is a strong economic incentive not to use tubes of expensive materials such as composites, titanium or titanium/steel combinations. As a result, steel lining tubes are normally used.

Placement of the lining tube may be done in one operation or alternatively in stages, each covering a length of the bore successively further away from the drill rig. In the case of successive lining tube placement, a liner string is fed to the bore end through the existing cemented-in lining tube (the casing) and then expanded to roughly the same internal diameter as the casing. This is generally achieved through brute¹ force (internally applied mechanical pressure) and requires the liner string to be expandable.

Where the drill rig is an offshore platform, the storage capacity, in terms of both weight and volume, is restricted and this in turn places limits on the length of drill string and lining tube that can be used and hence on the overall length of the bore that is practicable. Further limiting factors result from the nature of the materials from which the drill string and lining tubes are made. Thus composite tubes cannot be used at great subsurface depths as the ambient temperature can be so high as to affect the tubes ' integrity, e.g. in the case of lining tubes causing them to become porous to hydrocarbons. Thus composites cannot normally be used at vertical depths beyond 3000 to 4000 m. For steel lining tubes, long and deep bores pose two weight-related problems. Firstly, the weight may reach the buoyancy limit for the platform and secondly, where the bore is not vertical, the friction between the lining tube and the underside of the bore may cause difficulties in inserting the tube and can cause drilling debris in the bore to be dragged down hole with the tube, again increasing friction possibly to the extent of jamming the tube in a position short of the distal end of the bore.

As a result of these various factors, bores are generally limited in length to a maximum of about 12 to 15 km although, using non-steel drill strings, e.g. aluminium drill strings, a maximum length in excess of 24 km could theoretically be reached.

Any limitation on the bore length is of course a limitation on the range within a subterranean reservoir that can be reached from a single drilling platform and thus on the efficiency of hydrocarbon recovery from the reservoir.

Aluminium lining tubes, which again theoretically could be used to line bores with lengths in excess of 24 km, is unfortunately not feasible since aluminium cannot be cemented in place sufficiently firmly due to the nature of the cement and its setting process which involves an exothermic reaction in which aluminium participates and which is normally terminated by the inclusion in the cement of gypsum. When an aluminium object is in contact with the unset cement, the gypsum in the cement is insufficient to terminate the exothermic reaction and the cement overheats . Heat of course causes the aluminium tube to expand and once cement setting is complete and the aluminium tube cools and shrinks an unacceptable gap is left between the tube and the cement. To this shrinkage of the aluminium tube is of course added the normal shrinkage of the cement on setting, generally about 3-4% by volume.

This problem cannot be addressed by plastics coating or surface treatment, e.g. anodization, of the aluminium tube since placement of the tube will abrade the tube's outer surface. Likewise the problem cannot be addressed by raising the gypsum content of the cement to a level sufficient to terminate the exothermic aluminium reaction since, conversely, the resulting cement tends to expand thereby raising the risk of fracturing the surrounding matrix. (High gypsum cement with an enormous capacity to expand is known as "Trollkraft" and is used for rock fragmentation) .

However, we have now surprisingly found that hydraulic cements containing aplite may satisfactorily be used for down hole cementing of aluminium lining tubes (i.e. liners or casings) .

Thus viewed from one aspect the invention provides a method for cementing-in a lining tube in a bore hole, said method comprising placing said lining tube at a distal end of said bore hole, and introducing an aqueous liquid hydraulic cement composition into said distal end of said bore hole, characterized in that at least part of the length of said lining tube is of aluminium and in that said cement composition comprises pulverulent aplite.

The lining tube may be placed at the distal end of the bore hole before or after the liquid cement composition is introduced. In the former case, the cement composition is typically introduced through the lining tube and into the annulus between the tube and the surrounding matrix. In the latter case, the distal end of the lining tube may be sealed (ie so as to prevent cement entering the tube) or cement which enters the tube may be driven into the annulus between tube and matrix by application of a drilling fluid which is denser than the cement.

The lining tube and cement need not of course be positioned all the way down to the very end of the borehole.

Aplite is a granitoid mineral found for example in Montpelier, Virginia, USA, Owens Valley, California, USA and in Finnvolldalen in Norway as well as in Japan, Russia and Tuscany, Italy. Aplite is currently used almost exclusively as a flux in single-fired ceramic tile production. Aplite may be obtained commercially, e.g. from Maffei Natural Resources, Italy and the US Silica Company, West Virginia, USA. Typically aplite contains silicon, magnesium, iron, sodium, aluminium, potassium, titanium and calcium with the major components (expressed as oxide content) being silicon and aluminium, these generally being present at 60-85% wt. and 10 to 25% wt . respectively.

The aplite used according to the present invention is preferably a high silicon content aplite, e.g. with a silicon content (expressed as oxide content) of at least 68% wt., more preferably at least 70% wt., especially at least 75% wt . The aplite from Finnvolldalen in Norway which has a silicon content (expressed as oxide content) of about 80% wt. is especially preferred.

The silicon content is expressed as an oxide content as it is standard geological practice to express elemental contents in this fashion. Thus for example the US Silica Company provides a typical chemical analysis for its aplite (from Montpelier) of SiO₂ 62.0%, Fe₂O₃ 0.18%, Al₂O₃ 21.7%, TiO₂ 0.30%, CaO 5.6%, MgO 0.034%, Na₂O 5.5%, K₂O 2.9%, P₂O₅ 0.22% and LOI (loss on ignition) 0.1%.

The pulverulent aplite used according to the invention preferably has a particle size of less than 200 μm, more preferably less than 100 μm, e.g. 1 to 100 μm, more typically 10 to 100 μm, e.g. 50 to 100 μm, especially less than 75 μm. Preferably, the aplite used contains fines, ie particles below 15 micrometers, eg below 10 micrometers . This can readily be achieved by grinding the aplite and only screening to remove oversized particles. Particle size in this regard may be measured by screening or using particle size measuring apparatus. Where it is stated that the particle size is less than a certain value, then normally at least 50% volume will be that size or smaller, preferably at least 80% volume. Alternatively particle size may be taken to be mode particle size as measured by a particle size analyser, e.g. a Coulter particle size analyser. Coarse aplite may be transformed into finer grained aplite by conventional rock pulverizing techniques, optionally followed by screening to separate out oversized and/or undersized grains.

On a dry solids basis, the pulverulent aplite additive preferably constitutes at least 10% bwoc (i.e. "by weight of cement", i.e. by weight relative to the basic composition which is capable of forming a cement), more preferably at least 30% bwoc, especially at least 35% bwoc, more especially at least 50% bwoc, for example up to 400% bwoc and even higher concentrations by weight of cement, more typically up to 200% bwoc, e.g. at least 100% bwoc. Typically the aplite will constitute no more than 85% wt . , for example no more than 65% wt . , preferably no more than 60% wt., more preferably no more than 55% wt . , of the settable cement composition on a dry solids basis.

Compositions containing at least 100% bwoc aplite, e.g. 125 to 200% bwoc, are especially interesting as they are suitable for both low and high temperature usage. Currently different cements have to be used for different depths and temperatures.

The basic cement composition, i.e. the cement base in the compositions used according to the invention, may be any cement capable of use in down-hole conditions, for example Portland cement or other conventional cements. Such cement compositions are widely available and have been written about extensively.

Hydraulic cements are usually set by exposing the cement mixture to a base (i.e. to a pH above 7) . Exposure to an acid environment, before or after setting, can lead to failure to set properly or to cement corrosion. Thus for example exposure of set Portland cements to carbon dioxide is known to lead to cement corrosion and porosification. The more porous the set cement is, the higher will be the corrosion rate and loss of zonal isolation.

However in some down-hole uses, cements are exposed to acid environments, e.g. to oxide gases such as carbon dioxide, or acidic fluids leaching from the matrix.

Hydraulic cements based on a high aplite content however can be set by the use of acids rather than bases and thus offer the prospect of acid resistant down-hole cements. Such acid-setting aplite cements are particularly useful where the bore-hole is to be used for carbon dioxide injection.

These high aplite content cement compositions may contain aplite as the sole cement base or alternatively they may additionally contain at least one further cement base, preferably an inorganic hydraulic cement such as Portland cement. Typically the aplite will constitute at least 82% wt . of the total cement content, preferably at least 84% wt . , more preferably at least 85% wt., especially at least 90% wt . , e.g. at least 95% wt . The acid used in setting these high aplite content cements may be any strong or weak acid, e.g. a mineral acid such as hydrochloric acid or an organic acid such as a carboxylic acid, e.g. citric, malic, acetic, etc. acids .

In one preferred embodiment of the invention, the cement is formulated as a solid mix using a solid or encapsulated water-soluble acid, e.g. an acid encapsulated in a soluble polymer, for example a biopolymer such as gelatin. Alternatively an acid may be applied in fluid form, e.g. as a pure liquid acid or an aqueous solution. The acid may even be applied in gaseous form, e.g. by bubbling it through the cement composition.

Generally the acid will be used at a concentration or in an amount such that the pH of the aqueous phase of the cement composition is in the range 2 to 6.9, preferably 3 to 6, more preferably 4 to 5.

In certain instances, a neutral pH may be used to set high aplite-content cements and concretes and such use is also deemed to fall within the scope of the invention.

One particular advantage of the high aplite content elements of the invention is that by selection of the aplite particle size and the aplite content, the temperature reached within the cement during setting may be regulated, e.g. to keep it below 60⁰C in temperature sensitive environments or end-uses.

While aplite is a well understood geological term, it should be emphasized herein that other granitoid rocks having the same or similar cement-shrinkage reducing effect, relative to silica, may be used according to the invention in place of materials formally recognised as aplites and that such usage is considered to be according to the invention, although less preferred than the use of materials recognised as aplites.

In addition to aplite, other pulverulent silicates, e.g. silica, in particular silica flour, may also be used in the cement compositions according to the invention. Typically the weight ratio of non-aplite silicate to aplite will be in the range of 0:100 to 90:10, more particularly 2:98 to 70:30, especially 10:90 to 30:70. The use of a non-aplite silicate in addition to aplite is especially preferred when the aplite content is relatively low.

One benefit of the inclusion of aplite is to reduce cement shrinkage on setting. In the absence of aplite, shrinkage may be as high as 4% vol. With 40% bwoc aplite this has been shown to be reduced to 1.2% vol. and at 50% bwoc aplite to 0.7% vol. (tested after 68 hours of curing at 150⁰C) . The use of such low- shrinkage cements form a preferred embodiment of the invention and in this embodiment the aplite-containing cements of the invention may have a shrinkage on setting of less than 3% by volume. This shrinkage will preferably be less than 2.5%, more preferably less than 2.0% and most preferably less than 2%.

A further considerable advantage of the use of aplite-containing cements according to the present invention is the very low porosity and/or very low permeability of the resulting set cement compositions. Reduced permeability will reduce the invasion of any fluid or gas (e.g. CO₂) and will thus reduce the corrosion of the cement and the transfer of gas or fluid across the cement plug or wall. The water permeability of set Portland cement with slurry density of 1.90 specific gravity (SG) (similar to the aplite free cement composition in Example 3) is around 0.0010 itiD (millidarcies) , and increases as density is reduced. If reduced to 1.44 SG the water permeability increases to approximately 0.1380 mD. API Spec.10, section 11.4 describes how these permeability tests are performed and will be familiar to one of skill in the art.

The aplite-containing cements used according to the present invention have reduced permeability in comparison with non-aplite containing equivalents . For example, aplite in a Portland cement reduces the permeability over a Portland cement composition of equivalent density. This decreased permeability thereby reduces the invasion of any fluid or gas which will cause cement corrosion and/ or loss of zonal isolation. In a preferred embodiment, the aplite-containing cements used according to the present invention thus have a lower permeability once set, according to API Spec.10, section 11.4, than the equivalent set cement prepared in the absence of aplite, and/or the equivalent set cement containing an equivalent quantity of silica flour in place of the aplite component. In this embodiment, the porosity of a cement of density 1.9 SG is typically no more than 0.0005 mD, preferably no more than 0.0003 mD and more preferably no more than 0.0002 mD. Such an absolute or comparative test will easily be carried out according to the known standard.

Aplite (and pulverulent silicate) content in the cement compositions of the invention is defined, as is normal in the industry, as a percentage by dry weight relative to the basic cement composition, e.g. a Portland cement composition, i.e. excluding other additives such as colorants, antimicrobials, organic polymers, fibres, (e.g. inorganic fibres such as glass or "rock wool" fibres) , etc. Such other additives, with the exception of additives significantly contributing to the structural (e.g. load-bearing) properties of the set cement, such as silica, will generally contribute no more than 10% wt . dsb (dry solids basis) to the total cement composition, typically less than 5% wt. Besides such additives, the cement composition comprises a cement base, i.e. a material capable of setting to form a cement, more particularly an inorganic cement base. Cement bases, such as Portland cement, are well known in the technological field and require no further description here. Cements are discussed for example in Lea, "The Chemistry of Cement and Concrete", 3rd Edition, Edward Arnold, Old Woking, UK, 1970, and Taylor, "Cement Chemistry", Academic Press, London, UK, 1990.

Carbon fibre may be added to the aplite-containing cement to affect several important properties thereof. The most essential of these properties are those related to the set cement, but also in the fluid state, carbon fibres in the cement may increase the ability of the cement to reduce fluid losses to the rock formation. Fluid loss is often a problem during well cementing operations, since the cement often has a higher density than the drilling fluid it displaces. The carbon fibres might in some cases bridge the small fractures causing the losses, and thus lessen the losses during the pumping operation.

More important are the properties of the set cement, since the carbon fibres will effect properties such as compressive strength, tensile strength and bond to casing/ formation. Compressive strength is important, but even more important is the increased tensile strength the carbon fibres will give the set cement. Temperature- and pressure-cycling in a well is especially critical for the set cement, since it causes the casing/tube to expand/contract . This movement of the casing is known to cause the set cement to fail, causing poor zonal isolation along the wellbore. By using carbon fibres together with aplite in a well cement, the most critical mechanical properties can be controlled for optimum zonal isolation.

Suitable carbon fibres for use in the invention include those from Devoid AMT AS, N-6030 Langevag, Norway. Preferably the carbon fibres are between 0.1 cm and 10.0 cm in length, more preferably between 0.3 cm and 2.5 cm especially preferably between 0.5 cm and 1.0 cm. Preferred fibres have a diameter of between 1 urn and 15 μm, preferably between 3 urn and 10 μm, more especially between 6 μm and 8 μm, particularly 7 μm. The amount of fibre added per m³ of cement mix (i.e. cement plus aplite, or cement plus aplite plus blast furnace slag) is preferably 0.1 kg to 10 kg, more preferably 0.3 kg to 7 kg, especially preferably 0.5 kg/m³ to 5 kg/m³.

In a particular embodiment of the invention, blast furnace slag (BFS) may be used as all or as part (e.g. from close to 0 (e.g. 2%) to nearly 100% wt (e.g. 90 wt%) ) of the cement base. The use of BFS in down-hole cementing applications is discussed for example by Saasen et al in SPE28821, a paper presented at the European Petroleum Conference, London, UK, 25-27 October 1994. BFS is useful in particular as the base for non- self-curing cement compositions, i.e. compositions which can be placed in situ before a further action is taken to initiate setting, e.g. addition of a pH modifier, more specifically an alkaline agent. For down-hole applications, the non-self curing cement composition is applied separately from the curing initiator. For example, the cement composition may be pumped into place before the curing initiator is added or released (e.g. the activator may be placed in the desired location prior to addition of the cement, such as by release from the surface of the metal pipe etc.), e.g. to bring the pH to above about 9.0. If the cement base is only partly BFS, e.g. with the balance provided by Portland cement, the use of an activator may be unnecessary as the material forming the balance may function as the activator .

Where the cement base is wholly or largely BFS (e.g. greater than 80%, especially greater than 90%, and particularly essentially 100%) , the concentration of aplite may be any non-zero concentration but will typically be in the proportions described supra. In particular, the amount of aplite used in this embodiment of the invention may be above 30% by weight of cement base (BFS) .

BFS-based cement compositions are currently of particular interest for use in down-hole locations where high temperatures may be encountered; however the conventional BFS-based cement compositions still suffer from undesired shrinkage problems that are addressed by the use of aplite according to the invention.

The cement composition used in the present invention is a hydraulic cement, i.e. an inorganic cement rather than a settable organic resin. Such cements are well-known and set and develop strength as a result of hydration. The best known such cement is Portland cement which is a combination of tricalcium silicate, dicalcium silicate, tricalcium aluminate, tetracalcium aluminoferrite, and gypsum. Other components may of course be present, for example the chemical setting retarders or setting accelerators . Examples of retarders (often also referred to as dispersants) include: lignosulphonic acid salts (e.g. the sodium and calcium salts) ; hydroxycarboxylic acids and their salts, e.g. gluconates and glucoheptonates ; citric acid; saccharides and other polyols (e.g. glycerol, sucrose and raffinose) ; saccharinic acids; cellulosic polymers (e.g. carboxymethylhydroxyethylcellulose) ; alkylene phosphonic acids and their salts; inorganic acids and their salts (e.g. boric, phosphoric, hydrofluoric and chromic acids and their salts) ; sodium chloride; and metal oxides (e.g. zinc and lead oxides) . For the present invention, saccharide and polyol retarders are preferred. If desired, the cement compositions used according to the invention may contain a delayed release coated setting accelerator so that, after an initial period within which setting is retarded, release of the accelerator, e.g. due to dissolution of a release delaying coating, will then serve to counteract the effects of the chemical retarders. Many inorganic salts, e.g. chlorides (e.g. calcium chloride) , carbonates, silicates (for example sodium silicate) , aluminates, nitrates, nitrites, sulphates, thiosulphates and hydroxides, serve as accelerators (see for example Nelson et a.1, "Cement additives and mechanisms of action", Chapter 3, pages 3- 1 to 3-37 in "Well cementing" Ed. Nelson and Guillot, 2nd Edition, Schlumberger, 2006, the contents of which book are hereby incorporated by reference) .

The lining tube may be placed in a single operation or may be placed to line a bore hole between the distal end of the hole and the distal end of a casing that has earlier been cemented in place. In the latter case, the lining tube (a liner) will have an external diameter small enough to allow it to pass through the casing. Once the liner is positioned, a tool is used to expand it to substantially the same internal diameter as the casing. This may be- done before the cement is applied but more preferably it is done after the cement has been applied and before it has set. Expansion of the liner may be done in either direction, i/e. distal to proximal or proximal to distal . Systems for liner expansion in this way are available from Enventure and from Baker Oil Tools .

The lining tube may comprise lengths (sections) of different materials, e.g. steel, titanium, titanium/steel, or composite, as well as at least one length of aluminium. Preferably however the lining tube has a length of aluminium at or near its distal end, e.g. constituting from 10 to 100% of the final 100 m of the distal end or at least 10% of the length of the liner string if that is less than 100 m. In certain circumstances, largely for reasons of economy, and generally when placing a casing, it may be desirable to use a lining tube string of which the proximal end is steel and the distal end is aluminium. The distal tip of the lining tube, known as the "shoe", which for the purposes of this paragraph is not part of the lining tube, may of course be of steel or composite. This shoe may be up to about 100m in length.

By "aluminium" when used in relation to material from which a lining tube is constructed is meant herein aluminium and alloys and laminates thereof containing at least 30% wt aluminium, preferably at least 90% wt aluminium, more preferably at least 95% wt aluminium.

Aluminium lining tubes used according to the invention will generally have external diameters in the range 5 to 100 cm, especially 15 to 55^" cm, and wall thicknesses in the range 10 to 50 mm, especially 20 to 30 mm. Lining tube strings will generally be assembled by coupling together individual tubes having lengths in the range 7 to 12 m, especially 8 to 10 m.

Using the method of the present invention it is now possible to drill and line bore holes in excess of 12 km in length; such lined holes and their preparation thus form further aspects of the present invention.

Viewed from another aspect the invention thus provides a bore hole having a lining tube cemented in place therein, said bore hole having a length of at least 12 km (preferably at least 15 km, e.g. at least 20 km, for example up to 30 km) and preferably having a maximum depth of at least 2000 m (more preferably at least 4000 m) from sea level, and wherein at least part of said lining tube is aluminium, preferably at least 10% of the 100 m length to the distal end thereof.

Viewed from a still further aspect the invention provides the use of aluminium tubes to line bore holes.

Viewed from a yet further aspect the invention provides a method of producing a bore hole comprising drilling a hole using a drill bit attached to a drill string, placing a lining tube at the distal end of said hole, and cementing said lining tube in place, characterized in that as at least part of said lining tube is used an aluminium tube. In this method, it is preferred to use a drill string which also is of aluminium at least in part, preferably one which is of aluminium for at least 20%, more preferably at least 80% of that portion of its length which is over 12 km from the end remote from the drill bit. Once again drill strings of aluminium at the distal end and of steel at the proximal end, optionally with a mid-portion of titanium, titanium/steel or composite may be used.

Besides the use of aluminium string components and of aplite based cements, drilling, lining, finishing and operation of such bore holes may be performed conventionally. The cement compositions used according to the invention may be applied down-hole by procedures and equipment conventional in the art for the down-hole application of settable cement compositions. Thus, for example, placing the cement in the method of the invention, will generally involve pumping a reselected volume of cement down hole and into the annulus . The lining tube may then be sealed at its distal end to prevent re-entry of the cement into the tube, or alternatively a quantity of a denser liquid, e.g. densified drilling fluid, may then be pumped down hole to prevent such re-entry.

In the method of the invention, where aluminium lining tubes are used and drilling is to continue thereafter, it is preferred to use an oil-based drilling fluid rather than a water-based one.

The method of the invention will now be illustrated further with reference to the following non-limiting Examples .

Example 1

Aplite-containinq cement composition

A dry cement composition was prepared by mixing 100 parts by weight Class G Portland cement (from Norcem) with 50 parts by weight pulverulent aplite (sieved to a particle size of 75 μm or less) from Finnvolldalen,

Norway (content SiO₂ 79.20%; MgO 0.11%; Fe₂O₃ 0.20%;

Na₂O 3.0%; Al₂O₃ 11.10%; K₂O 3.90%; TiO₂ 0.02%; CaO

1.29%; P₂O₅ 0.1%) .

To this was added 62.01 L/100kg fresh water.

The mixture was cured in a high pressure / high temperature consistometer at 5000 psi and 150⁰C. The volumetric shrinkage observed was 0.7%.

Compressive strength, measured in an ultrasonic cement analyser, at 3000 psi and 150⁰C (according to the API Recommended Practice for Testing Well Cements, 22nd Edition, 1997) was as set out in Table 1 below: Table 1

Example 2

Aplite-free cement composition (comparative)

A cement composition was prepared by adding 45.55

L/100kg fresh water to Class G Portland cement from

Norcem. The mixture was cured and tested as in Example

1 showing a volumetric shrinkage of 3.4% and compressive strength as in Table 2 below:

Table 2

Example 3

Aplite-containinq cement composition

A dry cement composition was prepared as in Example 1 but using 40 parts by weight of the aplite. This was mixed with 58.72 L/kg fresh water and cured and tested as in Example 1. The composition showed a volumetric shrinkage of 1.2% and compressive strength as in Table 3 below:

Table 3

The cement compositions of Examples 1 and 3 may be formulated and applied down-hole using conventional well-cement application equipment.

Example 4

Crushing and Shrinkage Tests

Cement compositions as set out in Table 4 were prepared and tested using an ultrasonic cement analyser as in

Example 1.

Table 4

% for silica is bwoc, i.e. relative to the Norcem G

The aplite used in these tests was inhomogeneous drilling dust. The aplite used in the remaining tests had a particle size of less than 75 μm.

These results show that relatively high quantities of aplite, especially that with a particle size below 75 μm, may be used with advantage, even at low temperatures such as cause problems when using traditional cements in deep water.

Example 5

Carbon fibre reinforced cement

A cement composition may be prepared using Norcem G cement mixed with 150% bwoc aplite (particle size below

75 μm) , 0.1 - 0.3% bwoc (e.g. 0.2% bwoc) carbon fibre and 94 L/100kg fresh water. The carbon fibre will typically have a fibre length of 5 to 50 mm, e.g. 10 to

40 mm. Example 6

Aplite-containinq cement composition

A dry cement composition is prepared by mixing 23.5 parts by weight Class G Portland cement (from Norcem) with 127.5 parts by weight pulverulent aplite (drilling dusts of particle size 50 - 150 μm) from Finnvolldalen,

Norway (content SiO₂ 79.20%; MgO 0.11%; Fe₂O₃ 0.20%;

Na₂O 3.0%; Al₂O₃ 11.10%; K₂O 3.90%; TiO₂ 0.02%; CaO

1.29%; P₂O₅ 0.1%) .

To this is added 62.01 L/100kg fresh water, and optionally hydrochloric acid to bring the aqueous phase pH to below 6.

The mixture is cured under ambient conditions for 2 to 3 hours and then at 40⁰C for 8 hours.

Example 7

Aplite-onlv cement composition

A cement composition was prepared according to Example 6 by adding fresh water to aplite of particle size 10-75μm (achieved by crushing and sieving) . pH was adjusted to 4-5 using hydrochloric acid and the composition was allowed to set for 24 hours at 150⁰C.

Compressive strength, measured in an ultrasonic cement analyser, at 3000 psi and 150⁰C (according to the API Recommended Practice for Testing Well Cements, 22nd Edition, 1997) was as set out in Table 5 below: Table 5

Example 8

Testing of cement compositions

The following cement compositions were tested:

Sample I - pure building cement Sample II - Class G Portland cement + aplite* (50:50 cement :aplite by volume)

Sample III Class G Portland cement Sample IV - building cement + aplite* (50:50 cement: aplite by volume)

* The aplite was crushed and then ground in a ball mill (type: Tecon 400 VL) . Particles passing through a 75μm sieve were used.

Each sample was mixed with water (94 lbs cement mix: 41.36 lbs water) and set in a cylindrical plastic flask having an internal diameter of 70 mm. An aluminium pipe (isolated inside with isopore) having an outer diameter of 45 mm was placed in the middle of the flask. The cemented length of the pipe was 110 mm.

Both samples I and III showed cracks (longitudinal creep crack) . After 5 days, the force needed to push the aluminium pipes out of the flask was measured using a "Materials Testing Machine" (from ZWICK, type Z020/TH2S) . The results are set out in Table 6 below: Table 6

Claims

Claims :

1. A method for cementing-in a lining tube in a bore hole, said method comprising placing said lining tube at a distal end of said bore hole, and introducing an aqueous liquid hydraulic cement composition into said distal end of said bore hole, characterised in that at least part of the length of said lining tube is aluminium and in that said cement composition comprises pulverulent aplite.

2. A method as claimed in claim 1 wherein said composition contains at least 30% bwoc aplite.

3. A method as claimed in either of claims 1 and 2 wherein said composition further contains carbon fibre.

4. A method as claimed in any one of claims 1 to 3 wherein said composition further contains silica flour.

5. A method as claimed in any one of claims 1 to 4 wherein said composition contains blast furnace slag.

6. A bore hole having a lining tube cemented in place therein, said bore hole having a length of at least

12 km, and wherein at least part of said lining tube is aluminium.

7. A bore hole as claimed in claim 6 wherein at least 10% of the distal end of the lining tube is aluminium.

8. The use of aluminium tubes to line bore holes .

9. Use as claimed in claim 8 of aluminium tubes to line at least 10% of the distal end of a bore hole having a length of at least 12 km.

10. A method of producing a bore hole comprising drilling a hole using a drill bit attached to a drill string, placing a lining tube at the distal end of said hole, and cementing said lining tube in place, characterized in that as at least part of said lining tube is used an aluminium tube.