Note: Descriptions are shown in the official language in which they were submitted.
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METHODS OF TERMINATING
UNDESIRABLE GAS MIGRATION IN ~1ELLS
Backcround of the Invention
1. Field of the Invention.
This invention relates generally to methods of
terminating undesirable gas migration in wells, and more
particularly, to such methods which are relatively simple and
inexpensive to carry out.
2. Description of the Prior Art.
After a well bore is drilled by rotary drilling wherein
a drilling fluid is circulated through the well bore, the
circulation of the drilling fluid is stopped and a production
casing string is typically run into the well bore. After the
casing has been run, primary cementing is performed. That is,
the string of casing disposed in the well bore is cemented
therein by placing a cement slurry in the annulus between the
casing and the walls of the well bore. The cement slurry is
permitted to set into a sheath of hard substantially
impermeable cement in the annulus which holds the casing in
the well bore and is intended to bond the casing to the walls
of the well bore whereby the annulus is sealed.
Undesirable gas migration in a well is the migration of
gas in the annulus from one or more pressurized gas formations
or zones penetrated by the well bore during and after primary
cementing. The gas migration can be between formations or
zones, e.g. , from a high pressure zone to a low pressure zone,
or the gas migration can be from one or more gas zones to the
surf ace .
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It is widely believed that gas migration is caused by the
behavior of the cement slurry during the transition phase in
which the cement slurry changes from a fluid to a highly
viscous mass having some solid characteristics. The
transition phase starts when the cement slurry develops enough
static gel strength to restrict the transmission of
hydrostatic pressure over its column height, and ends when the
cement slurry develops a gel strength which is sufficient by
itself to prevent the migration of gas through the cement
slurry. If the hydrostatic pressure exerted on one or more
pressurized gas formations or zones by the cement slurry falls
below the pressure of the gas in the zones, the gas enters the
annulus and migrates through the cement slurry. The initial
gas migration causes passages, e.g., flow channels or very
small annular spaces between the casing, the cement column and
the walls of the well bore known as "microannuli" to be
formed. Such flow channels and microannuli remain after the
cement slurry sets and undesirable gas migration continues.
While numerous techniques have been developed and used
heretofore for preventing the formation of passages in the
cement sheath in a well through which gas migration can occur,
such techniques are not always successful and gas migration
still results. Such gas migration often travels to the
surface in wells which penetrate shallow gas zones.
The elimination of surface gas migration has heretofore
been difficult, particularly in wells which penetrate shallow
gas zones up-hole from the completed producing formation or
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formations. The shallow gas zones usually have very low
permeability, and often include clays which swell upon contact
with water thereby making it difficult to introduce cement for
plugging the zones thereinto. Generally, the heretofore
utilized techniques for eliminating undesirable surface gas
migration have been unreliable, difficult to carry out and
very expensive. Thus, there is a need for an improved
relatively simple and inexpensive method of terminating
undesirable gas migration in wells, particularly in wells
where the gas migration originates from relatively shallow gas
zones.
Suamaar~r of the Invention
The present invention provides methods of terminating
undesirable gas migration in wells which meet the need
described above and overcome the shortcomings of the prior
art. The methods apply to the termination of undesirable gas
migration in a well comprised of a well bore having casing
held therein by a cement sheath wherein the gas migration
occurs at least in part through one or more passages such as
channels and microannuli in the cement sheath or near the well
bore in the formation.
The methods basically comprise the steps of locating a
substantially gas impermeable formation penetrated by the well
bore through which the gas migration occurs by way of passages
in the cement sheath. One or more lateral openings through
the casing and through the cement sheath into the
substantially gas impermeable formation are then formed, and
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one or more horizontal fractures are created in the formation
extending from the lateral openings. A fluid which sets into
a substantially gas impermeable solid is next deposited in the
openings and fractures, and the fluid is caused to set into a
substantially gas impermeable solid in the openings and
fractures whereby the passages in the cement sheath are
plugged and the gas migration is terminated.
It is, therefore, a general object of the present
invention to provide improved methods of terminating
undesirable gas migration in wells.
A further object of the present invention is the
provision of methods of terminating undesirable gas migration
in wells which are relatively simple and inexpensive to carry
out.
Other and further objects, features and advantages of the
present invention will be readily apparent to those skilled in
the art upon a reading of the description of preferred
embodiments which follows when taken in conjunction with the
accompanying drawings.
Brief Description of the Drawings
FIGURE 1 is a schematic illustration of a well which
penetrates a producing formation and also penetrates a shallow
gas zone from which gas migrates to the surface.
FIGURE 2 is a schematic illustration of the well after a
plurality of slots have been formed through the casing and
cement sheath thereof into a substantially gas impermeable
formation.
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FIGURE 3 is a schematic illustration of the well after
cement has been placed in the slots formed therein as well as
in fractures formed in the substantially gas impermeable
formation.
FIGURE 4 is a schematic illustration of the well taken
along line 4-4 of FIG. 2.
FIGURE 5 is a schematic illustration of the well taken
along line 5-5 of FIG. 3 .
Description of Preferred Embodiments
In primary well cementing, a cement sheath is formed in
the annular space between casing disposed in a well bore and
the walls of the well bore. The cement sheath is generally
formed by displacing a pumpable hydraulic cement slurry
downwardly through the casing and upwardly into the annulus
between the casing and the well bore. After being placed, the
cement slurry is permitted to remain in a static state in the
annulus whereby it sets into a hard impermeable mass therein.
The resulting cement sheath provides physical support and
positioning to the casing in the well bore and is intended to
provide a bond between the casing and the walls of the well
bore whereby the annulus is sealed.
As mentioned above, primary cementing operations are
often unsuccessful in sealing the annulus and preventing gas
migration therethrough. Gas migration takes place in a well
bore which penetrates one or more pressurized gas zones as a
result of the hydrostatic pressure exerted in the well bore by
the cement slurry falling below the gas zone pressure. When
_._ ~~~JJ67
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the gas pressure is higher than the hydrostatic pressure, gas
enters and flows through the well bore before the cement
slurry develops sufficient gel strength to prevent such gas
entry and flow. Gas migration through the set cement slurry
continues through passages remaining therein whereby the gas
is free to flow to the surface and/or between zones penetrated
by the well bore. The passages formed in the cement sheath
usually take the form of flow channels or very thin annular
spaces between the casing, the cement sheath and the walls of
the well bore known as microannuli.
By the present invention, improved methods of terminating
undesirable gas migration in a well are provided. A well to
which the methods are applicable is comprised of a well bore
having casing or other pipe held therein by a cement sheath
and gas migration occurs at least in part through one or more
passages such as channels and microannuli in the cement sheath
or in the formation near the well bore as a result of
formation damage.
The improved methods of this invention for terminating
undesirable gas migration in a well of the type described
above basically comprise the steps of locating a substantially
gas impermeable formation penetrated by the well bore through
which the gas migration occurs by way of passages in the
cement sheath. One or more lateral openings are formed
through the casing and cement sheath into the substantially
gas impermeable formation, and one or more horizontal
fractures are created and extended in the formation from the
Z135~6'~
lateral openings. A fluid which sets into a substantially gas
impermeable solid is deposited in the openings and in the
fractures in the substantially gas impermeable formation, and
the fluid is caused to set into a solid therein. The gas
impermeable solid plugs the passages in the cement sheath and
provides a seal between the cement sheath and the
substantially gas impermeable formation whereby gas migration
is blocked and terminated.
The initial step of locating a substantially gas
impermeable formation penetrated by the well bore through
which the undesirable gas migration occurs by way of passages
in the cement sheath can usually be accomplished by a study of
the open hole log or logs run on the well when it was drilled.
If no previously run logs are available, a new log can be run.
A substantially gas impermeable formation for purposes of this
invention is a formation which has a relatively high
compressive strength and low permeability and porosity whereby
the pressurized gas which is the source of the gas migration
can not fracture or permeate through the formation.
Referring now to FIGS. 1-5 of the drawings, a well
generally designated by the numeral 10 is schematically
illustrated. The well 10 is comprised of a well bore 12 which
penetrates a subterranean producing formation 14. A string of
production casing 16 is disposed in the well bore 12 extending
from the surface through the producing formation 14 to a point
near the bottom of the well bore 12. The well l0 also
includes a relatively short string of surface casing 18. The
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production casing 16 and surface casing 18 are held in the
well bore by a cement sheath 20 disposed within the annulus
between the production casing 16 and surface casing 18 and the
walls of the well bore 12. A string of production tubing 22
is disposed within the production casing 16. The production
tubing 22 extends from the surface to the producing formation
14 .
The producing formation is communicated with the interior
of the production casing 16 by a plurality of perforations 24,
and hydrocarbons from the producing formation 14 flow through
the perforations 24 into the production casing string 16 and
upwardly by way of the open bottom of the tubing string 22
into and through the tubing string 22 to the surface. A
conduit 26 containing a valve 28 conducts the produced
hydrocarbons to storage or further processing.
The well bore 12 penetrates a pressurized gas zone 30
which is above the producing zone 14 or which may comprise a
part of the producing zone and which is the source of
undesirable gas migration (shown by arrows) through passages
in the cement sheath 20 into the surface casing 18. Some of
the gas which migrates upwardly in the annulus enters the soil
layer adjacent the surface and flows therethrough to the
surface. The gas collected within the surface casing can be
vented by way of a conduit 32 connected to the surface casing
and containing a valve 34 to a point of use or disposal. The
gas which migrates through the soil surrounding the well 10
constitutes a hazard to the environment and personnel.
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In carrying out the present invention for terminating the
gas migration from the pressurized gas zone 30 associated with
the well 10, as mentioned above the first step is to locate a
substantially gas impermeable formation 36 through which the
migrating gas from the pressurized gas zone 30 flows. Since
the formation 36 is a substantially gas impermeable formation,
the migrating gas must pass through the formation by way of
passages either in or very near the cement sheath 20.
The next step involves the formation of one or more
lateral openings through the production casing 16 and through
the cement sheath 20 into the gas impermeable formation 36.
Preferably, the lateral openings are comprised of a plurality
of horizontal slots which are coextensive with a horizontal
annular area extending from the inside surfaces of the
production casing 16 into the formation 36. In the drawings,
the use of four overlapping horizontal slots 40, 42, 44 and 46
which extend through the casing 16, through the cement sheath
20 and into the gas impermeable formation 36 is illustrated.
As best shown in FIG. 4, the horizontal slots 40 and 42 are
opposite each other and are on the same level while the
horizontal slots 44 and 46 are opposite each other on a level
a short distance above the slots 40 and 42. Also, the slots
are angular whereby they are coextensive with a horizontal
annular area around the casing 16. Each of the slots 40, 42,
44 and 46 has a shape corresponding to an annular segment the
angle of which is generally in the range of from about 91o to
about 1200, preferably about 1100.
__ X13556?
As will be understood by those skilled in the art, a
number of slots greater or less than four can be utilized.
Also, a single circular slot can be formed through the casing
16 and cement sheath 20, but it is preferred that more than
one disconnected slot be used to avoid completely severing the
casing and cement sheath whereby shifting between the upper
and lower parts could occur.
The slots 40, 42, 44 and 46 can be formed in the casing
16, cement sheath 20 and formation 36 in any convenient
manner. A preferred technique for forming the slots is to
utilize an abrasive hydro-jetting process. Abrasive hydro-
jetting processes are well known to those skilled in the art
and direct a high velocity jet of a fluid containing abrasive
particles against a surface to be cut. In the application of
the present invention, a tool can be utilized which
simultaneously directs high velocity jets of the abrasive
fluid against opposite portions of the casing 16, the cement
sheath 20 and the formation 36 while rotating the tool over
the desired angle to form two opposing horizontal slots, e.g. ,
40 and 42, simultaneously. As is generally understood, the
term "horizontal" is used herein to mean about 90o from
vertical and deviations therefrom in the range of from about
60o to about 1200 from vertical.
After the horizontal overlapping slots 40, 42, 44 and 46
are formed, one or more horizontal fractures extending from
the slots covering a continuous or near continuous annular
area are created in the formation 36. The fractures are
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created by applying fluid pressure to the formation 36 by way
of the slots 40, 42, 44 and 46. That is, a fracturing fluid
is pumped into the formation 36 by way of the slots at a rate
and pressure to create and extend one or more horizontal
fractures in the formation 36. Because the slots overlap, the
fractures formed also overlap and are usually communicated
whereby a single annular fracture 50 extending into the
formation 36 is formed as illustrated in FIGS. 3 and 5. As is
understood by those skilled in the art, the horizontal slots
formed in the formation 36 through which fluid pressure is
applied promote the creation of fractures in the horizontal
plane.
A fluid which sets into a substantially gas impermeable
solid 52 is deposited in the slots 40, 42, 44 and 46 and in
the fractures 50. After deposit, the fluid is caused to set
in the slots 40, 42, 44 and 46 and in the fractures 50 whereby
the gas migration passages in the cement sheath 20 are
plugged, the annulus is sealed and the gas migration is
terminated.
As will be understood, the fracturing fluid utilized for
forming the fractures 50 can be the fluid which sets into a
substantially gas impermeable solid, and once the fractures
are formed the fluid can be caused to set by permitting it to
remain in the fractures 50 under sufficient pressure to
maintain the fractures 50 in the open position.
Any of a variety of fluids which set into a substantially
gas impermeable solid can be utilized in accordance with this
' 2135567
12
invention. Examples of such fluids are aqueous slurries of
Portland cement, high alumina cement, slag, fly ash, gypsum
cement and other similar cementitious materials as well as
mixtures of the materials. A variety of single component and
multi component hardenable resinous materials can also be
utilized including acrylic, epoxy and phenolic resinous
materials. Combinations of such resinous materials with the
above described cementitious and other materials can also be
utilized. Of the various fluids which can be used, an aqueous
cement slurry is preferred. The most preferred cement is a
fine particle size Portland cement or mixture of Portland
cement and slag. Such fine particle size cements are
described in U.S. Patent No. 5,086,850 entitled "Squeeze
Cementing" issued on June 16, 1992 and assigned to the
assignee of this present invention.
Fine particle size Portland cement or mixtures thereof
with slag useful in accordance with this invention are
preferably made up of particles having diameters no larger
than about 30 microns, most preferably no larger than about 11
microns. The distribution of the various sizes of particles
within the cement is preferably such that 90% of the particles
have a diameter no greater than about 25 microns, most
preferably about 7 microns, 50% of the particles have a
diameter no greater than about 10 microns, most preferably
about 4 microns and 20% of the particles have a diameter no
greater than about 5 microns, most preferably about 2 microns.
'~.~,.E
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13
The Blaine Fineness of the particles is preferably no less
than about 6,000 square centimeters per gram. Most preferably
the Blaine Fineness is no less than about 13,000 square
centimeters per gram. An aqueous slurry of fine particle size
Portland cement or Portland cement and slag quickly develops
gel strength after placement. Further, because of the fine
particle size, the cement slurry enters the very small
passages in the cement sheath through which gas migrates and
readily bonds thereto whereby such openings are plugged.
A particularly preferred method of the present invention
for terminating undesirable gas migration in a well comprised
of a well bore having casing held therein by a cement sheath,
and the gas migration occurs at least in part through one or
more passages in the cement sheath is as follows . One or more
lateral openings, preferably one or more slots, are formed
through the casing and the cement sheath into a substantially
gas impermeable formation, such as, for example, a reservoir
cap rock, through which the gas migration occurs by way of the
passages in the cement sheath or near the well bore in the
formation. A fluid which sets into a substantially gas
impermeable solid is next pumped into the substantially gas
impermeable formation by way of the lateral openings at a rate
and pressure to create and extend one or more horizontal
fractures in the formation. The pumping of the fluid is
terminated while maintaining the fluid in the openings and
fractures, and the fluid is permitted to set into a
substantially gas impermeable solid in the openings and
2135~6~'
14
fractures whereby the passages in the cement sheath are
plugged and the gas migration is terminated. As mentioned
above, the fluid that sets into a substantially gas
impermeable solid is preferably an aqueous fine particle size
Portland cement composition. The fluid is maintained in the
openings and fractures and permitted to set therein by
shutting in the well under a pressure which maintains the
fractures in the open position for a time period sufficient
for the fluid to set.
As will now be understood by those skilled in the art,
the presence of the set cement or other gas impermeable fluid
in the slots formed in the casing and cement sheath plugs the
passages in the cement sheath whereby gas is prevented from
flowing through the passages. Since the set cement or other
fluid extends from the slots into fractures in a substantially
gas impermeable formation and the set cement or other fluid is
bonded to the formation, the gas below the set cement or fluid
is prevented from migrating around the plugged cement sheath.
In order to further illustrate the present invention, the
following Example is given.
Example
A well drilled in 1982 to a total depth of about 3,000
feet was completed in the Cutbank and Livingstone formations
in southern Alberta, Canada. The well included surface casing
to about 469 feet and production casing to total depth.
Immediately after completion, a surface casing gas vent flow
was discovered at an average flow rate of about 12,350 cubic
213567
feet per day. The vent flow was the result of gas migration
at least in part through the primary cement sheath in the
well.
Subsequently, a gas zone which was thought to be the
source of the undesirable gas migration was identified at
about 1,640 feet. An attempt was made to plug the gas zone by
perforating and squeezing cement into the zone without
success.
Subsequently, the method of the present invention was
performed on the well as follows. An abrasive hydro-jetting
tool with 1800 opposed nozzles was positioned adjacent a
substantially gas impermeable formation located at about 649
feet. Utilizing 20/40 mesh sand and clay swelling inhibited
water, two opposed 1100 horizontal slots were abrasively cut
through the production casing and the cement sheath into the
formation. Two additional opposed 1100 horizontal slots were
cut on a center line 90o from the center line of the initial
slots at a level about one foot above the initial slots . Four
overlapping 1100 horizontal slots were thus formed through the
production casing and the cement sheath into the gas
impermeable formation. Using clay swelling inhibited water,
the formation was fractured. A fine particle size cement
slurry having an average cement particle size of 5 microns was
then squeezed into the slots through the casing and cement
sheath and into the fractures in the formation. The well was
shut-in after the cement thickening time was exceeded under
high pressure (about 800 psi - 1000 psi) overnight during
2I~~S~'
16
which time the cement set. Thereafter, no measurable gas flow
from the production casing vent occurred indicating that the
undesirable gas migration was terminated.
Thus, the present invention is well adapted to carry out
the objects and attain the ends and advantages mentioned as
well as those which are inherent therein. While numerous
changes may be made to the compositions and methods by those
skilled in the art, such changes are encompassed within the
spirit of this invention as defined by the appended claims.
