FIELD OF THE INVENTION
The present invention relates to an improved method of cementing the annulus of a hydrocarbon well.
BACKGROUND OF THE INVENTION
Conventionally the process of cementing an oil well (or other bored holes in earth or rock) requires the on-site preparation of the spacer (or scavenger) and cement slurry, which are then pumped under pressure to the tubing or casing at the well head. The casing or tubing typically runs the full length of the well, top to bottom, except in special cases which are not described herein.
Cement slurry design is not a simple task. It is essential to isolate the drilling mud from the cementitious materials for incompatibility reasons. For this purpose, either a mechanical device or spacer is used to prevent contamination of these fluids. For purely economic reasons, typically a spacer or scavenger fluid is used.
In this case, cement slurries or water-based separating fluids or “spacers” are pumped to the tubing or casing. Ideally a water-based spacer fluid is used, although in many instances no spacer will be used but a dilute, first portion of the cement slurry, called the “scavenger” is pumped ahead of the regular portion of cement slurry.
These fluids perform two main functions, the first being to “displace” the drilling mud efficiently, and the second being to prevent any contact between the cement and the mud. Here rheological, density and velocity profile characteristics are of vital importance, as are fluid loss control and stability or settling control.
As the spacer and the cement slurry or scavenger and cement slurry are pumped down the casing to the bottom of the hole, they are displacing the drilling mud which rises via the annulus (i.e. the gap between the exterior of the casing and the inner face of the bored hole), and are discharged to a retaining vessel at the surface (mud tank) for the disposal.
Once all of the slurry has been pumped, it is propelled or displaced by an inert fluid, i.e. water, or drilling mud. Pumped volumes are typically calculated in such a manner that pumping is halted when the cement slurry has replaced all of drilling mud in the annulus, and the interior of the casing is filled with the inert fluid. The volume of spacer and cement slurry are based on a number of different calculations, well-known to those skilled in the art. This annular volume is often very difficult to calculate. Thus, in many cases, the oilwell operator will mix cement slurry and pump it down the casing until the annular volume between the casing and the wellbore is entirely full, at which time the cement slurry in the casing will be displaced by the inert fluid.
In order to accomplish this, without a volume calculation relating the annular volume to the volume of cement slurry pumped into the casing, a method of knowing when the annulus is full is needed for the oilwell operator. In this context, it should be understood that cement and mud are typically difficult to discern visually.
SUMMARY OF INVENTION
Thus, in accordance with the present invention, a method is provided of cementing the annulus of a hydrocarbon well having a casing centering within a hole bored from a surface. A method comprises steps of introducing an effective amount of colouring agent contained in a water-soluble bag into a water-based separating fluid, pumping the separating fluid ahead of a portion of cementing slurry down the casing to the bottom of the hole to displace drilling mud upwardly through the annulus to the surface; until the colouring agent becomes visible at the surface displacing the remaining cement inside the casing with an inert material such as drilling mud or water, and then stopping the pumping and allowing the pumped cement slurry to harden.
The bags are made of sheets of polyvinyl alcohol or any other water soluble material. These are of preferably very thin, for example 2 mil thickness, since they require and receive very little agitation to dissolve in the spacer or scavenger fluid.
It will be understood that when the colouring agent reaches the upper surface of the annulus, the oilwell operator then displaces the casing with the certainty that the entire annular space between casing and bore hole is full of cement, thus ensuring the maximum hydraulic seal between various hydrocarbon layers in the ground around the casing.
Although the colouring agent technology used in the present case is not unique, the delivery of colouring agents to the spacers or scavengers by way of a water soluble bag is. The bag containing the colouring agent is made of fast dissolving polyvinyl alcohol and is sealed in a “zip-lock” type bag, or otherwise sealed, to keep it dry until use. Upon contact with any of the fluids described above, the bag deteriorates rapidly, releasing a powder material to the water phrase, to be quickly dispersed with minimal mechanical agitation and providing good colour difference from the surrounding fluids.
The colouring agent is preferably in powder form, is almost inert and has no effect on varying components contained within a cement slurry or spacer. Its sole purpose is to provide colour to the fluid being pumped, so that as it is returning from the wellbore, it can be discerned from the drilling fluid, to alert the operator that the certain slurry, returning to the surface via the annulus is close to the surface. The powder is usually an admixture composed of dyes or pigments conventionally used to colour concrete such as for example carbon black, iron oxide, phthalocyanine, umber, chromium oxide, titanium oxide, and cobalt blue.
It should be noted that colouring agents in water soluble bags have been added to aqueous cementitious mixtures in the past, as described and illustrated for example in Smith et al U.S. Pat. No. 5 120367, issued Jun. 9, 1992. When that has been done in the past however, the purpose has been to completely colour a load of concrete in a concrete wet mixer so that the resultant concrete product will have a uniform colour. Such prior art usage has not suggested Applicant's use of water soluble bags to colour spacer or scavenger fluid in a well environment. Moreover, because of the use to which the colouring agent/water soluble bag is applied, in accordance with the present invention, much smaller amounts of colouring agent are required in the bags, than would be the case for application in colouring cement in a wet mixer. As well, the significant agitation required in a wet mixer environment, to ensure that the dye permeates the concrete being mixed, is not needed in the context of the present invention since, in accordance with the present invention, the bag containing the colouring agent deteriorates to enable the spacer or scavenger to show colour just through normal passage of the bag containing spacer or scavenger down the casing or up the annulus.