A CONCRETE MIXTURE FOR USE IN OIL- AND GAS WELLS
This invention regards a concrete mixture for use in oil- and gas fields, more particularly a mixture comprising a significant proportion of glass balls with rough surfaces. The object of the invention is to provide a concrete of high compressive strength having low specific gravity.
In some circumstances when using concrete, it is necessary to reduce the specific gravity of the concrete mixture to reduce the stress on surrounding structures during the pouring phase. An obvious example of this is the cementing of an oil well. In here, concrete is used to fill the space between the borehole casing and the bedrock through which the hole extends. Some rock structures are unstable and may collapse if subjected to a hydrostatic pressure of liquid concrete that is filled around the casing. This may result in large, undesirable fractures in the formations surrounding the borehole.
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To reduce the specific gravity of the concrete mixture, flue ash, among other things, is commonly used as an additive aggregate, in the form of balls of a diameter down to 0.25 mm. The flue ash provides a desired option of adjusting the specific gravity of the concrete mixture, but the properties of the flue ash render it rather unsuitable when desiring to fulfil other purposes than a reduced specific gravity. The flue ash is a by-product of combustion or refining of silica, among other things. It often contains substances hazardous to health. Of greatest significance to the property of the concrete mixture, however, is the inability of the flue ash to withstand high pressures. Already at pressures of ca. 60 bars (800 psi), the flue ash collapses. This results in settling in the concrete prior to hardening, hence an increase in the specific gravity of the concrete.
The object of the invention is to .remedy the disadvantages of. prior art.
The object is achieved through features disclosed in the description below and in subsequent claims.
In a concrete mixture intended to be poured between a casing of an oil or gas well and the surrounding rock wall or between tubulars of different diameter, additive aggregates known per se are used, including cement, water, sand and possibly agents for adjusting density, viscosity and hardening.
To reduce the specific gravity of the concrete mixture, glass particles are mixed into the water, for example particles of the brand Liaver Super-K from Liapor GmbH & Co. KG,
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Hallendorf, Pautzfeld, Germany, essentially in the form-of glass balls with a „ diameter of 0.25-0.8 mm. Liaver Super-K is available at different specific gravities, typically from ca. 0.45 to 2.5 g/cm3. The most used type has a specific gravity from 0.45 to 1.0 g/cm3. The invention is not limited to use of spherical particles, inasmuch as other particle shapes also may be used to achieve the desired effect.
A concrete mixture ready for use advantageously contains ca. 30% glass balls, but the invention is not limited to this volume percentage.
Preferably, the most used types of glass particles have a specific gravity that allows them to stay afloat or, if a stirring motion is maintained in the water, to suspend them in the water. At the floating or suspended state of the glass particles in the water, a uniform distribution of the glass balls is maintained when the water-/glass ball mixture is added to the remainder of the additive aggregates during the concrete mixing process.
According to the invention, the specific gravity of the concrete mixture is adjusted by using a type of glass particle having a specific gravity that, at ca. 30% volume percentage admixture of glass particles, provides the desired concrete specific gravity required for the pouring task at hand. Given this, the concrete mixture may be used under conditions at which the pressure on surrounding structures must be minimized.
The glass particles define a compact cross section. The surface is rough.
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Concrete with glass balls admixed have great compressive strength. Testing of concrete mixtures show crushing of 0.3% of the glass balls at a pressure of nearly 1100 bars (15000 psi) and a temperature of 175 °C. Given this, a concrete mixture according to the invention exhibits insubstantial settling during hardening and great compressive strength when hardened.
In the preferred spherical form, the surface of the glass particles exhibits a great ability for adhering to cement. When pumping a concrete mixture containing glass balls, it has been found that the wear on pump- and pipe components is reduced due to the collective abrasiveness of the aggregate mixture being lowered. The cement-covered glass balls provide a reduced abrasive action as compared with the other main constituents of a concrete mixture. At the same time, the ability, of the. concrete, mixture- to- flow .through pipes and similar is improved, wherebythe power requirement when pumping is reduced.
In tests the concrete mixture according to the invention exhibits little shrinkage and a strong adhesion to steel.
This provides good contact between for example the casing in an oil/gas well and the surrounding concrete.
Glass has a relatively high thermal capacity. During the setting and hardening processes on the concrete, this high thermal capacity provides a moderating effect on the temperature increase naturally occurring in this phase. Said effect is of great significance particularly in wells wherein gas condensate exists in a frozen state. An temperature increase of the gas hydrate may bring the gas into a vapour
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state, resulting in a great volumetric expansion. The expansion may cause damage to technical installations within and around the well. Use of glass balls in the concrete mixture thus will reduce the risk of such damage during pouring operations in wellbores .
In the following, a non-limiting example of a preferred embodiment is described.
A concrete mixture according to the invention consisting of:
36.25 litres of seawater having a specific gravity of 1.025 g/cm3;
4.5 litres of CaCl having a specific gravity of 1.314 g/cm3; 30 litres of glass balls Liaver Super-K 1 having a specific gravity of 1.000 g/cm3; and 100 litres of G-cement having a' specific gravity of 3.220 g/cm3
exhibited a specific gravity of 1.71 g/cm3 in a soft state. After hardening at 69 bars (ca. 950 psi) pressure, the specific gravity was 1.71 g/cm3.
Laboratory tests were carried out, in which a concrete mixture I known per se and appropriate for the purpose was compared with a concrete mixture II according to the invention. A summary of several repeated tests shows the following measurement results:
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The measurement results show that the concrete mixture according to the invention has provided the expected effect concerning compressive strength and adhesion.
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