Casing repair using a plastic resin

A method for replacing a damaged or corroded casing with a solid plastic is described herein. After removing the corroded or damaged casing from the wellbore, the borehole is underreamed to a desired diameter. Thereafter, a resin/curing agent mixture sufficient to form a hardened plastic or resin able to withstand downhole stresses, is placed into the wellbore so as to bind with the undamaged casing and close off any thief zone. Subsequently, the resin/curing agent mixture forms a hardened solid. The hardened solid is milled out so as to form a resinous casing the size of the original casing. Later, the hardened resinous casing is perforated to communicate with a targeted zone. Perforation is not required when isolating a zone.

FIELD OF THE INVENTION 
This invention is directed to a method for the replacement of a damaged 
section of wellbore casing located in a subterranean formation. 
BACKGROUND OF THE INVENTION 
During the course of well drilling operations, a wall of a wellbore being 
drilled is generally sealed and stabilized by means of a protective steel 
casing which is lowered through a wellbore. Afterwards, the casing is 
cemented in place after retrieval of the drilling assembly. Setting a 
steel casing in a well is a time consuming and expensive procedure. Since 
the wellbore is essential to removing desired fluids from a subterranean 
formation, it is necessary that the wellbore's casing remain intact to 
make for a more efficient operation and avoid the loss of wellbore fluids 
into the formation. 
Often during the production of hydrocarbonaceous fluids or other desired 
fluids from a formation via a wellbore, the wellbore becomes damaged or 
corroded. The damage may be caused by excessive pressure within the 
wellbore which will cause a section of wellbore casing to fail thereby 
interfering with its integrity. Also, wellbores which are located at 
levels in excess of about 5,000 ft. will often have an environment where 
high temperatures, high pressures, and corrosive chemicals are 
encountered. When these chemicals, pressures and temperatures combine, 
casing corrosion often occurs thereby necessitating the repair of a 
section of the casing so as to maintain its integrity thereby avoiding a 
loss of desired fluids into the formation. 
Depending upon the composition of the casing which is used in the wellbore, 
the expense of replacing the wellbore's casing can vary. When stainless 
steel casings are used for example, replacement costs can be substantial. 
For these reasons, it is desirable to have a method for repairing the 
casing in the wellbore so as to maintain the efficiency of operations for 
removing desired fluids from the formation while at the same time 
minimizing the downtime and repair costs. Heretofore, it has been 
necessary to remove the entire wellbore casing and replace it with new 
casing. This of course is a time consuming and expensive operation. 
Therefore, what is needed is a simple and inexpensive method of repairing a 
wellbore casing in situ so as to avoid loss of operational time, the 
production of desired fluids from the formation, or diversion of injection 
fluids to the oil-bearing formation. 
SUMMARY OF THE INVENTION 
In the practice of this invention, a section of corroded or damaged casing 
is removed from the wellbore. This can be achieved by various operations 
such as milling. Afterwards, if required, the section is further prepared 
with underreaming to a desired size. A drillable mechanical packer can be 
placed below the section of the wellbore from which the corroded or 
damaged casing was removed so as to allow a solidifiable mixture to fill 
the void created by removal of said section by containing the mixture 
thereabove. Subsequently, the solidifiable mixture is flowed into the 
voided section from which the damaged or corroded casing was removed. 
The solidifiable mixture is allowed to remain in the vicinity of the 
wellbore from which the damage or corroded section has been removed. It 
remains there for a time sufficient to form a solid plastic or resin. 
Where a drillable mechanical packer is used a solid will be formed 
thereabove. The solidifiable material forms a solid in the wellbore and 
the voided section which previously contained the damaged or corroded 
casing. The solid which forms is of a composition sufficient to withstand 
environmental conditions encountered at the wellbore depth from which the 
damage or corroded section of wellbore casing was removed. This solid also 
forms a bond with the undamaged or corroded casing so as to prevent liquid 
or gaseous fluids from flowing therethrough. 
Subsequently, solid material within the wellbore is removed, preferably by 
drilling so as to establish communication with the formation and the 
surface via the wellbore. Where a mechanical packer is utilized it too is 
drilled out. Once excess solid material has been removed from the 
wellbore, the remaining solid material reestablishes the integrity of the 
wellbore casing. The wellbore is now of a diameter substantially similar 
to the casing prior to its being repaired. 
The solidifiable material which is used herein is comprised of a 
resin/curing agent material which hardens in situ to form a solid plastic 
material. Any solid which is formed via said material should be of a 
composition sufficient to withstand environmental conditions, pressures, 
and temperatures located at the depth where the damage or corrosion 
occurs. 
It is therefore an object of this invention to replace a damaged or 
corroded wellbore casing by use of materials in situ so as to avoid 
removing the steel casing from the wellbore. 
It is another object of this invention to provide for a simple, safe, 
economical, and effective means of repairing or replacing a damaged or 
corroded section of a wellbore in situ. 
It is yet another object of this invention to provide for a wellbore casing 
replacement material, which is equal to or better in composition than the 
original casing so as to withstand environmental conditions encountered 
within the wellbore, particularly deep wellbores.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As is shown in FIG. 1, a wellbore penetrates formation 10, producing zone 
12, and thief zone 14. The borehole contains cement 18 and casing 20. 
During the removal of hydrocarbonaceous or other desired fluids from the 
formation, conditions existing at the lower portion of the wellbore have 
caused casing 20 to be damaged by channels 22. These channels 22 allow 
fluids to move from the wellbore into a thief zone 14. Ordinarily, it 
would be necessary to remove the entire casing 20 and replace it with a 
new casing. 
In the practice of this invention, as is shown in FIG. 2, a portion of 
casing 20 that contains channels 22 which communicate with thief zone 14 
has been removed. After removing the damaged casing containing channels 
22, borehole 16 is underreamed to a desired size at a location just below 
perforations 24 above channels 22. Thereafter, a solidifiable mixture is 
flowed into casing 20 whereupon it also penetrates via channels 22 into 
thief zone 14. The solidifiable mixture is allowed to remain in the 
wellbore and thief zone 14 so as to form a thick solid wall which is able 
to withstand environmental conditions encountered at a preferred depth in 
formation 10. After the solidifiable mixture has formed a solid, a 
drilling operation is conducted within the wellbore to remove excess solid 
material from the wellbore. The underreaming provides the desired 
thickness of the plastic casing to withstand the downhole stress. 
Once the solid material is removed from the wellbore, the casing is 
comprised of the remaining solid plastic material that abuts the metal 
casing which was already in the wellbore. Any solidifiable material which 
flowed into thief zone 14 remains therein as a solid. The completed 
wellbore casing containing the repaired section comprised of the solid 
material is shown in FIG. 3. 
In those situations where it is desired to repair a damaged or corroded 
section of casing 20 at a level higher than the bottom of the wellbore as 
is shown in FIGS. 1, 2, and 3 a drillable mechanical packer can be placed 
below the portion of the casing which it is desired to repair or replace. 
Once the drillable packer has been positioned as desired in the wellbore, 
the solidifiable mixture is flowed into the wellbore and is placed on the 
drillable packer whereupon it flows into channels which communicate with a 
thief zone or other zone of the formation. Subsequently, the solidifiable 
mixture will form a solid plastic which is subsequently drilled out along 
with the drillable packer. Once this has been done, the well can be 
produced as desired. 
The solidifiable material can comprise a resinous material. 
One method for placing the resinous material into the formation is by use 
of a positive displacement dump bailer. This is a mechanical device, 
symmetrical in shape, which is filled with a mixture of resinous material 
and an acid or alkaline curing agent. It is lowered into the wellbore by a 
cable. The bailer is positioned at the desired depth above the damaged 
casing or packer and when activated, releases a metal bar in the top of 
the device. The bar falls downward inside the device and impacts the top 
of the fluid creating a downward-moving shock wave which travels through 
the fluid column contained in the bailer. The shock wave causes a shearing 
of metal pins in the bottom of the bailer and a subsequent downward 
movement of the small piston. This small piston uncovers ports to allow a 
release of the resinous material. The bar continues to fall through the 
bailer as fluid is released through the ports. The weight of the metal bar 
effectively adds to the weight of the fluid column being dumped. As the 
bar falls to the bottom of the bailer, the cylindrical bailer is wiped 
clean of the resinous material containing an acid or alkaline curing 
agent. 
Other types of positive displacement dump bailers, which operate in a 
similar manner, may also be used. It is possible to deliver the resinous 
viscous material with curing agent therein in an open gravity-feed dump 
bailer. This is a bailer which is open at the top and closed at the 
bottom. When activated, the bottom cover, which is held by metal pins, is 
sheared by an explosive or by other means so as to open the bottom. 
Opening the bottom allows the resinous viscous material with curing agent 
therein to flow by gravity from the bottom of the bailer and into the 
damaged casing area and thief zone 14. 
A coiled tubing may also be used to place the viscous resinous material at 
the site from which the damaged casing has been removed. The coiled tubing 
consists of a one-inch or other small pipe which is wound on a spool at 
the surface of borehole 16. The viscous resinous material and curing agent 
therein are placed in the end of the tubing and held in place by wiper 
balls at the top and at the bottom of the resinous material. The tubing is 
then uncoiled and lowered into the wellbore above the site where it is 
desired to replace the casing. Thereafter, the viscous resinous material 
with curing agent therein is pressured through the tubing and released 
into the wellbore where it flows into the thief zone via channels 22 and 
contacts casing 20. Here it forms a solid in the wellbore 16 and thief 
zone 14. As is shown in FIG. 2, the resinous material enters thief zone 14 
via channels 22. Because the resinous material with curing agent therein 
is fast acting, a solid 22 is formed in the wellbore and thief zone 14. 
This material, of course, can be held in place by a drillable packer if 
required. The material is allowed to harden in thief zone 14 and the 
wellbore. 
The preferred resin for use herein comprises an epoxy resin, a curing 
agent, a reactive diluent, and a filler. An example of epoxy resin is 
Shell's EPON-828.sup..RTM., a bisphenol-A epichlorohydrin epoxy resin with 
an epoxide equivalent weight of 185-192. Another epoxy resin is Shell's 
EPON DPL-862.sup..RTM., a bisphenol-F epichlorohydrin epoxy resin with an 
epoxide equivalent weight of 166-177. The epoxy resin is blended with a 
reactive diluent and a filler. An example of the reactive diluent is 
Scherling Berlin's Diluent 7, a monofunctional glycidyl ether based on 
alkyl groups of C.sub.8 -C.sub.10. The diluent is used to increase pot 
life or gel time of the epoxy resin and to increase load capacity for the 
filler. In some cases, a large amount of filler (up to 50% by weight 
relative to the epoxy resin) is added to the resin mixture. It serves to 
increase the specific gravity of the resin mixture for gravity 
dump-bailing applications and for application in deep wells. The filler is 
also used as a heat sink to allow more working time. An example of the 
filler is a fine powder of calcium carbonate or silica flour. A 
crosslinking or curing agent is then added to the resin mixture. This 
makes a fast-reacting gel which hardens in a short period of time. 
An example of a curing agent is Schering Berlin's Euredur.sup.200 3123, a 
polyamide epoxy curing agent. For fast curing at or below room 
temperature, Schering Berlin's Euredur 3254 can be used. Euredur 3254 is a 
Mannich base aliphatic polyamine attached to a phenol group. A catalytic 
tertiary amine can also be blended with the aforementioned curing agents 
to promote even faster curing. For high temperature applications, an 
anhydride such as Ashland Chemicals' phthalic anhydride or a liquid 
anhydride of methyl tetrahydrophthalic anhydride can be used. The 
concentration and volume of curing agent utilized must be customized 
according to the temperature of the well right before the dump-bailing 
operation. This allows the resin to have about twenty minutes of flow time 
and to gel in about 60 minutes. The amount of various batches of the 
resinous material to be utilized depends on the hole size to be filled. 
The resin plugs the bottom of thief zone 14 and binds with the undamaged 
portion of casing 20. 
The resinous or solid plastic which forms should have a fracture toughness 
able to withstand perforations being placed therein so as to remove fluids 
from a producing zone. In addition to forming a solid liner, the resinous 
material or plastic should be able to preclude formation gases and liquids 
from flowing therethrough. 
As mentioned above, in order to thin the epoxy resin thus increasing the 
pot life, a five to fifteen weight percent solution of a reactive diluent 
is utilized. Use of this concentration of diluent allows for efficient 
draining of the dump-bailer or for speeding up resin penetration into any 
cracks or channels behind the casing. It has also been determined that it 
is best to use a resin which is substantially fresh. Freshness can be 
determined by a measurement of the resinous material's viscosity. If the 
viscosity is over a recommended tolerance limit, it should be rejected. 
The preferred viscosity range is about 4,000 to 11,000 centipoise 
@75.degree. F. A simple, rugged capillary viscometer is available to 
measure the viscosity obtained. This viscometer can be obtained from 
Baxter Scientific Products. 
Where desired, a steam-flooding, CO.sub.2 -flooding, or water-flooding 
process can be initiated in either producing zone 20 or thief zone 14 of 
the formation. Steam-flooding processes which can be used when employing 
the procedure described herein are detailed in U.S. Pat. Nos. 4,489,783 
and 3,918,521 which issued to Shu and Snavely, respectively. These patents 
are hereby incorporated by reference herein. Of course, the viscous 
material which is utilized, must be capable of withstanding steam 
temperatures. 
Although the present invention has been described with preferred 
embodiments, it is to be understood that modifications and variations may 
be resorted to without departing from the spirit and scope of this 
invention as those skilled in the art will readily understand. Such 
modifications and variations are considered to be within the purview and 
scope of the appended claims.