Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02424800 2003-04-07
IMPROVED METHOD AND PRODUCT FOR
CEMENTING HYDROCARBON WELLS
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.
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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
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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 "ZIPLOC" (Trade Mark) 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
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for example in Smith et al U.S. Patent No. 5 120367, issued June 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.
Example
A polyvinyl alcohol pouch was made up of sheets of 2 mil thickness that
were formed into pouches to contain about 2.5 Ibs of iron oxide powder as
colouring agent. The colouring agent was sealed in the bag. At a test site
well
head, one of the pouches was placed into a water-based spacer and pumped into
the well casing, ahead of cement slurry. The pumping of cement slurry into the
casing was continued, displacing drilling mud from the bottom of the hole, at
the
bottom end of the casing, upwardly through the annulus to the surface. When
the
distinctive reddish colour of the iron oxide was noted in the displaced fluid
at the
surface (meaning that the drilling mud had been completely displaced from the
casing and annulus about the casing), the remaining cement slurry within the
casing was then displaced with water. The pumping was then stopped and the
pumped cement slurry remaining in the annulus was allowed to harden.
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It was found that there was no trace of the sheet material of the pouches in
the spacer and colouring agent which became displaced back to the surface
during
the pumping process.
Also it was found that, by using the pouch of colouring agent and the
process in the manner described, an immediate and accurate indication of when
the annulus was filled with cement slurry was achieved, resulting in a more
efficient
use of cement slurry and a faster completion of the annulus cementing process.
Thus, there has been provided in accordance with the invention of an
improved method and product for cementing hydrocarbon wells that fully
satisfies
the objects, aims and advantages set forth above. While the invention has been
described in conjunction with specific embodiments thereof, it is evident that
many
alternatives, modifications and variations will be apparent to those skilled
in the art
in light of the foregoing description. Accordingly, it is intended to embrace
all such
alternatives, modifications and variations as fall within the spirit and broad
scope of
the invention.
