CASING INTEGRITY PRESSURE TEST SYSTEM

SYSTEME DE TEST DE PRESSION D'INTEGRITE DE BOITIER

English Abstract

Oilfield wells lined with tubular steel casing are periodically filled with
water and
pressure tested to assess whether the casing is leaking. The present invention
involves
landing a charge of water-soluble, molten eutectic salt mixture at a
predetermined depth
where it will solidify due to temperature reduction. The mixture expands as it
cools and
seals against the casing. It forms a solid plug bridging across the casing
bore. The plug
may be used in connection with pressure testing the casing above it. The plug
will
eventually dissolve in water to allow regular production to resume.

Claims

I claim:
1. A temporary, removable plug, positioned downhole in a wellbore formed
by a string of casing in a well, comprising:
a column of solid eutectic salt mixture extending across the wellbore and
adhering
to the inner surface of the casing;
said salt column being sufficiently soluble in water to enable its removal by
dissolution in water and operative to seal against the casing to withstand a
pressure
differential, across the column, which is sufficient to allow a casing
integrity pressure test
to be performed.
2. A method for pressure testing a section of steel casing in a well
comprising:
delivering a slug of molten eutectic salt mixture downhole into the wellbore
of the
casing to a point, at the base of the section of casing to be tested, where
the slug cools
and solidifies to form a solid column extending across the wellbore, said
column adhering
to and sealing against the inner surface of the casing to form a temporary
removable plug
operative to withstand a pressure differential sufficient to allow a casing
integrity
pressure test to be performed;
said salt mixture being sufficiently soluble in water to enable its removal by
dissolution in water after pressure testing;
pressurizing the section of the wellbore above the plug with water for
sufficient
time to test the integrity of the casing section; and
dissolving the plug in water in the wellbore above the plug to remove the
plug.
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Description

CA 02773339 2012-04-02
Casing Integrity Pressure Test System
Technical Field
The present application relates to the field of plugs which may be temporarily
and
removably provided downhole in a cased well to close off the annulus. It
further relates
to a method for running a casing integrity pressure test using the plug.
Background
The invention was specifically developed in connection with providing a system
which could be used to pressure test casing in cyclic steam stimulation wells.
It is
contemplated that it may find application in other contexts as well, where
there is a need
for and use of a temporary removable plug.
Cyclic steam stimulation is a known technique used to thermally treat
subterranean reservoirs containing viscous heavy oil or bitumen. The technique
typically
involves:
= injecting steam down the annulus of the cased well and into the reservoir
for a
period of time, perhaps sixty days, thereby heating the oil or bitumen to
reduce its
viscosity and improve its mobility;
= terminating injection and allowing the reservoir to "soak" for a period
of perhaps
15 days;
= producing heated oil or bitumen from the reservoir through the well
tubing string,
usually by a combination of initially flowing and subsequently pumping the
well,
until the rate of production diminishes significantly; and
= then repeating the cycle.
Due to the cyclic heating and cooling that the well's steel casing string
undergoes,
its integrity may eventually be compromised and leakage may begin, typically
at one or
more of the threaded couplings joining the joints of casing. These leaks
usually first
occur in the upper reaches of the casing string, where temperature changes are
most
pronounced.
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CA 02773339 2012-04-02
=
As a consequence, periodic pressure testing of the upper section of the casing
string is usually carried out. For example, in wells in the Lloydminster
region of Alberta,
such tests are carried out on a yearly basis.
A conventional casing pressure test typically involves:
= moving a service rig onto the well site and pulling the tubing and rod
strings;
= filling the wellbore with water;
= running an inflatable mechanical packer into the wellbore on the tubing
string;
= expanding the packer by pressurizing it internally, so that it seals
against the
casing;
= pressurizing the water in the annulus above the packer, perhaps to 2500 psi
for 10
minutes, and monitoring whether the pressure drops off, indicating a leak;
= collapsing the packer and removing it and the tubing string from the
wellbore;
= replacing the tubing string in the well; and
= moving the rig from the site.
The conventional test just described is expensive to carry out and can reduce
the
time available for normal production operation.
There has thus long existed a need for a less expensive and quicker technique.
Summary of the invention
In accordance with the invention, a water-soluble eutectic salt mixture, which
melts at an elevated temperature and solidifies at a lesser temperature, is
used. A slug of
the eutectic salt mixture is melted at ground surface and then delivered into
the wellbore
of a cased well at a depth where the temperature is sufficiently low to cause
"freezing" or
solidification of the molten salt mixture. Delivery may be effected by
displacing the slug
to the desired depth through a string of small diameter tubing. Sufficient
eutectic salt
mixture is supplied to form a column extending across the annulus. As it
solidifies, the
salt mixture expands, presses against the steel casing and adheres to it. The
solidified salt
column provides a temporary, removable plug, sealed against the casing, that
is operative
to withstand a pressure differential thereacross, sufficient for an integrity
pressure test of
the casing. A casing integrity pressure test can then be carried out by
pressurizing water
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CA 02773339 2012-04-02
in the wellbore annulus above the plug. Upon termination of the test, the
water in the
annulus will dissolve and thereby remove the plug.
Description of the Drawing
Figure 1 is a sectional view showing a cased well having a production tubing
string and a small diameter tubing string positioned in the wellbore, and a
eutectic salt
mixture plug temporarily closing off the annulus.
Description of a preferred embodiment
Having reference to the Figure, a well 1 is shown cased by a string 2 of steel
production casing 3. A string 4 of steel production tubing 5 extends down the
casing
wellbore 6 from a wellhead 7 to a heavy oil reservoir 8. The casing and tubing
strings 2,
4 define an annulus 9 between them (which is part of the wellbore 6). The
casing string 2
is perforated at the reservoir 8, to allow heavy oil to enter the annulus 9
and tubing string
bore 20 as well as to permit steam to be injected down the annulus and through
the
perforations 10 into the reservoir 8. A subsurface pump 21 is provided in the
lower end
of the tubing string bore 20, for pumping oil to ground surface.
A eutectic salt melter 11 is connected at ground surface with the small
diameter
tubing string 12, which extends down the annulus 9. The small diameter tubing
string 12
is landed above the bottom of the production tubing string 4. At its bottom
end, the small
diameter tubing string 12 is connected with a salt distribution nozzle 13.
The melter 11 is operative to heat the eutectic salt mixture sufficiently so
that it
melts. A pump (not shown) is provided to pump the molten salt mixture down the
string
12. The nozzle 13 is landed at a subsurface elevation which is at a
temperature lower
than the temperature at which the molten salt solidifies. The molten salt is
discharged
through the nozzle 13 into the annulus 9, where it contacts the casing string
2, cools,
solidifies and forms a plug 14 comprising a solid column extending across the
annulus 9
and sealing against the adjacent casing 3 and tubing 5.
The eutectic salt mixture used is selected so as to have the following
characteristics:
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CA 02773339 2012-04-02
= it needs to solidify if exposed to the temperature existing at the depth
where the
nozzle is landed and the plug is to form;
= it needs to expand on solidifying, so as to press against the casing (and
production
tubing, if present) and provide a pressure tight seal; and
= it needs
to be sufficiently water soluble so as to dissolve if contacted over time
(typically several hours) with water, for removal of the plug.
As stated, a slug or suitable volume of the molten salt mixture is pumped down
the bore of the small diameter tubing string 12 and is delivered into the
annulus 9 through
the nozzle 13. The molten salt mixture slug forms a column in contact with the
casing 3
and any other strings present in the wellbore. As the salt mixture cools upon
contacting
the casing, it solidifies. As the salt mixture solidifies, it expands and
seals against the
string(s) with which it is in contact. It thereby forms the plug 14, which
adheres to the
strings, so as to remain fixed in place, and seals to enable using it for a
casing integrity
pressure test.
Following the pressure test, the plug 14 will dissolve in the water present in
the
annulus 9 within a few hours.
Experimental
A test was conducted to assess the strength of a selected eutectic salt
mixture
plug.
A 3 inch diameter column was formed of a water-soluble eutectic salt mixture
of
sodium nitrate and potassium nitrate. The mixture was supplied by Houghton
International Inc., Valley Forge, PA 19482. The material was identified under
the
designation Draw Temp 430. It had a melt temperature of approximately 315 C
and a
solidification temperature of approximately 260 C.
The column was subjected to end loading in an MTS load frame. Load was
monitored with a load cell. The column was found to fail at a compressive load
of
3660 psi.
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CA 02773339 2012-04-02
The test indicated that the load rating of the eutectic salt mixture column
was
comparable to that of Glass G cement, which is commonly used to cement well
casing
into the geological formation.
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