Preparation of setting slurries

Abstract

When preparing a settable slurry such as a cement slurry for cementing an oil well, cement or other solid powder is mixed with water and a set retarder. In order to be able to run a convenient and rapid check on the concentration of retarder after dilution at the site of use, a tracer material is mixed with the retarder in known amount during manufacture. The tracer is chosen to enable its concentration to be determined analytically after dilution at the site of use, thereby providing a way to determine the concentration of set retarder after such dilution. The tracer may be a redox-active material and its concentration may be determined by voltammetry. Tracer may likewise be mixed with additives other than set retarder.

Description

FIELD OF THE INVENTION[0001]This invention relates to the preparation of slurries which set to solid form after they have been prepared. These may in particular, but not exclusively, be cement slurries used in wellbore cementing.BACKGROUND OF THE INVENTION[0002]Cement slurry is typically made by mixing cement powder, water (sometimes referred to as the mix-water) and various additives that may include retarders, dispersants, fluid-loss additives and anti-foam additives.[0003]When the cement slurry is going to be used to fill the space between a drilled bore hole and a casing inserted into that borehole, the slurry must flow for a considerable distance before it reaches its final position where it is required to set. It is therefore normal to include a retarder to delay setting. Typically the retarder and other additives are supplied to the rig-site as manufactured products, which may be stock solutions, and these are added to the mix-water before adding the cement powder. So, when m...

AI Technical Summary

Benefits of technology

[0006]The invention provides a way to check the concentration of an additive in a cement slurry and / or in the mix-water before cement powder is added. The invention can be embodied as a facile, inexpensive, and accurate measurement technique which can be carried out rapidly at the site of use without requiring a fully equipped laboratory. Although the invention has been conceived in the specific context of checking retarder concentration when cementing a wellbore, it could also be applied to other additives and / or in other contexts.

[0018]Linear or cyclic voltammetry may be entirely adequate when used in this invention, but the present inventors have appreciated, as a further feature of some forms of this invention, that square wave voltammetry may be advantageous for this invention because it can observe the redox reaction of a tracer while excluding inteference by other chemical species which may be present. It also provides good sensitivity and can be carried out quickly.

[0023]An advantage of voltammetry as an analytical technique is the small size and portability of the apparatus required. Analytical determination of tracer in accordance with this invention may be carried out with a beaker or similar container to hold a sample of slurry or mix-water, a disposable three-electrode array, a potentiostat to supply the electrical potential and observe current and a computer to control operation and store and display results.

Problems solved by technology

Using too much retarder will delay set of the cement unnecessarily and hence increase the “waiting on cement” delay during which the well site stands idle because further work cannot be done until the cement has set.

On the other hand premature setting of the cement can be hugely expensive to rectify.

Yet, there is currently no satisfactory technology for checking that the concentration of retarder as diluted in the mix-water used to make the cement slurry is correct.

This process is time-consuming, potentially taking up to 8 or 9 hours (depending on the thickening time) and so results in job delays and (depending on the outside temperature) in the possible degradation of the quality of the mix water.

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