Note: Descriptions are shown in the official language in which they were submitted.
1 324753
METHOD OF REMEDIAL CEMENT SQUEEZING
USING PHENOLIC-AlDEHYDE GELS
Field of the Invention
The pre6ent invention relates generally to an improved
method of remedial cement squeezing. More specifically, the
pregent invention concern6 a two-stage method wherein a
phenolic-aldehyde gelling solution is injected into an area
wherein a remedial cement squeeze is to be performed prior to
the cement squeeze operation.
Back~round of the Invention
For the successful production of a fluid from a
subterranean formation by way of a well, it has long been the
practice to cement the casing of the well in position. This
operation is termed primary cementing. It has also been the
practice to use cement as a means of 6ealing defects in the
primary cement sheath around the ca~ing. This treatment, termed
remedial cementing, is generally carried out through
perforations in the casing. Defects in the primary cement
integrity lead to undesired flow of fluids or gas from undesired
zones during production and to the 1066 of fluids to undesired
zone6 during injection. It has traditionally been difficult to
perform remedial cementing operations or to merely plug
perforations when the formation accessed by the perforations
contains vugs, large open fractures or is highly permeable.
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The cement employed in cementing operations concerned
with control of fluid production fxom subterranean formations
usually consists principally of a hydraulic cement and
sufficient water to provide a readily pumpable and flowable
composition which becomes, upon setting, a strong monolithic
solid. The term, hydraulic cement, as used herein~ refers to
portland cement, including aluminuous cements which contain a
relatively high proportion of tricalcium aluminate, cements
which contain a relatively high proportion of calcium
sulfoaluminate, and pozzolana cements which contain a relatively
high proportion of light weight mineral sources such a6 fly a6h
and volcanic rock.
Remedial fcfqueeze cementing is an operation wherein a
cement slurry i6 forced under pressure into a specific point in
a well for remedial purposes. The objective is to fill all the
perforations or defects in the primary cement sheath (channels)
behind casing with cement to obtain a 6eal be~ween the ca6ing
¦ and formation. However, because cement slurry is a suspension
1 20 having a high viscosity, the cement can only fill the larger
¦ channels, leaving the small channels near the wellbore
¦ unfilled. As a result, many conventional cement squeeze
¦ operations are not succe6sful in completely shutting off flow of
gas or fluids through the cement defects or channels.
I 25
¦ Formations containing natural fracture6, induced
¦ fractures or vugs present additional difficulty in cement
~quee~iDg. Io theDe ca~e~ the matris per=eability i~ Iov aod
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the high effective permeability is due to the interconnected
voidfi or fracture systems. In these formations, the cement
/ slurry may move into the large voids or fracture system under
S low pressure differentials. Consequently, cement filtercake i6
5 not formed on the vug or fracture faces to immobilize the cement
i~ in the near wellbore area. The problem then becomes one of
confining the remedial cement slurry to the near area of the
~ wellbore so that squeeze pressure can be developed. ~igher
;~ fluid 108s cements which form filtercake more easily or slurries
9 lO which set faster may be used in these cases, but they are
limited because they still require differential pressure to form
a cake. This differential pressure is difficult to develop in
the highly conductive channels, and as a result, all the cement
flows away from the near wellbore region resulting in an
15 unsuccessful cement squeeze.
It is known to inject solutions into highly permeable
formations, which at least partially plug the high permeability
zones. Phenol-aldehyde resins and gels have heretofore been
20 employed in permeability reducing and ~and consolidating
g operations. Solutions of pre-cstalyzed polyphenol-
paraformaldehyde resin have also been employed to permanently
plug ruptures or perforations in oil well ca6ings. However,
these prior art techniques are used as 6ub6titutes for cement
2~ 6queeze operations and do not offer a solution to the above
mentioned problems in cement 6queeze operations. Prior art
~ethodfi w ing resin6 or gels alone are ineffective to provide
depeDdoble, Bt~bl~, laDg-ter= pluggiDg of l~rg6 ch~DD~I8 or
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perforations. Prior art methods using cement alone are
ineffective to provide plugging or sealing of ~mall channels or
high permeability zones. Consequently, there exists a need in
the art for a method to make the frequently used remedial cement
squeeze technique effective in plugging channels and
perforations in vuggy, fractured and highly permeable formations.
In the practice of the present invention, an improved
method of remedial cement squeezing is provided to overcome the
difficulties of cement squeezing in the formations discussed
above. This improvement comprises a two-stage method of
remedial cementing.
SUMMARY OF T~E INVENTION
The present invention i A directed to a two-stage method
for remedial cementing of an area in a wellbore. The method
comprises the steps of: a) injecting a phenolic-aldehyde gelling
solution into an area wherein remedial cementing i~ to be
20 performed such that a gel form~ near the wellbore, and b)
sequentially squeezing cement into the area. This method is
useful to effectively seal perforation~ or areas in fractured,
vuggy or highly permeable zones having large pore sizes as well
as to block small channels in cement behind the casing resulting
25 in an effective remedial cement squeezing operation.
In a preferred embodiment, the cement is squeezed into
the area a6 the gel forms.
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DESCRIPTION OF T~E PREFERRED EMBODIMENTS
The present invention provides a method for remedial or
~queeze cementing of a specific point in a well 80 that all
perforations or channels behind the casing are filled to obtain
a ~eal between the casing and the formation. This method
overcomes the difficulty of cement squeez~ng in fractured, vuggy
or highly permeable rock wherein the cement flows into the voids
or highly permeable regions under low pressure differentials.
In such formations, cement filtercake i6 prevented from being
built up on the vug or fracture faces to seal the vug or
fracture and the cement is therefore not confined to the near
wellbore area. This method al60 provides an effective means for
¦ blocking small, hairline channels in cement behind the casing
that cannot be reached with a conventional cement squeeze.
¦~ According to the present invention, a two-stage process -~
l~ is provided wherein a phenolic-aldehyde gelling solution is
first injected into the area to be shut off in a wellbore. The
20 gelling solution is prepared as is known in the art by adding
l; any commercially available mixture of a phenolic resin and an
¦ aldehyte to water to which caustic has been added. The caustic
' acts as the catalist for the polymerization reaction which forms
i a stiff, i.e. highly viscous, impermeable gel. Rate of
! 25 polymerization is controlled, as i8 ~nown in the art, ~o that
the co1ution vill not ~tiffen in the wellbor-.
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The phenolic component may be one or more of any
phenolic compounds such as phenol, resorcinol, catachol, and the
like, as well as selected oxidized phenolic compound6 ~uch as 1,
4-benzoquinone and natural or modified tannins. The aldehyde
may be either a monoaldehyde, such as formaldehyde and
acetaldehyde, or a dialdehyde, such as glyoxal. Formaldehyde is
the most preferred aldehyde. The aldehyde may algo be generated
in-6itu by an aldehyde precusor, guch as paraformaldehyde. The
phenolic compound to aldehyde ratio may be any ratio suitable to
form a ~tiff gel upon polymerization.
For purposes of this invention, the concentration of
the phenolic resin and aldehyde mixture is from about 1 to about
50 weight percent of the gelling solution. Preferably, the
concentration is from about 5 to about 30 weight percent, and
most preferably from about 10 to about 20 weight percent.
The amount of gelling solution injected depends on the
application and may be determined based on previous plugging
treatments, formation permeability, and desired extent of
plugging.
The gelling golution as injected preferably has a low
ViAC06ity (2-3 Cp) and will invade gmall channels in the
25 formation rock. The ~olution forms a time-controlled stiff gell
thae hat a low deo~ity aDd teDds to stay where it i~ placed.
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The re~in ~cts to seal the small channel6, w gs, fractures or
high permeability zones in the near wellbore region. The resin
also forms a cushion for the second stage conventional cement
squeeze. The phenolic-aldehyde gel is superior to other known
resins because of its tolerance of high temperature and pH
variations.
After a predetermined amount of gelling solution is
injected, a conventional cement squeeze operation is performet.
The variou~ methods of remedial cementing are old and known by
those skilled in the art. Although the gel may be allowed to
stiffen before cement is injected, a water or other suitable
6pacer must be injected to displace the gelling 601ution from
the perforations or other treated area prior to cement
injection. It i6 therefore preferred that the cement squeeze
take place as the gel 6tiffen6 60 that the cement di6place6 the
~- gelling 601ution. The gel forms a cushion ~o that pre6sure may
- be applied to the cement 61urry for cement filtercake buildup
and consequent sealing of the w gs, f ractures and high
20 permeability zone~ without 1068 of cement Erom the near wellbore ~-
region.
In another embodiment, a second treatment of the
gelling solution may be injected prior to cement injection.
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In order to illustrate the benefits of this invention,
a field test was conducted to shut off water which was
5 channeling through suspected fractures or channels behind the
casing to the perforated interval in a well. The well was
completed in a carbonate reef which is dolomitized and w gular
in nature. The poro6ity is le6s than 9~ and the permeability is
in the Darcy range due to fractures and w gs. The oil
10 production mechanism is bottom water drive. The well when
, initially completed at a first set of perforations produced
i 40-50 m3 oil per day, but gradually started to produce large
amounts of water.
Initially, the first set of perforations were cement
squeezed and the well was completed 4 meters higher up with a
second set of perforations. The production of oil went up to
35 m3 per day and water production dropped to about 5 m3 per
day temporarily for a period of about two months. However, the
20 oil protuction started to decline again and the well started to
produce water at about 15 m3 per day. At this time it was
suspected that the water was coming to perforations either
through vertical channel6 near the wellbore or through very fine
channels in cement behind the casing.
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The well was then treated in accordance with the
present invention. The treatment consi6ted of pumping 10 m
of a 10% phenolformaldehyde gelling solution tagged with
radioactive Indium 111, followed by 0.7 m3 of class G cement
through the retainer in the second set of perforation6. Three
days after the treatment, the cement was drilled out and the
cemented interval held the pressure well. A gamma-ray log was
run to trace the phenolformaldehyde gel di~tribution behind the
ca6ing. The gamma-ray log suggested that the gelling solution
traveled 15m vertically. It is interesting to note that while
the injected gelling solution only went 2n above the injected
perforation6, a sub6tantial portion of it went into the
formation at the treatment perforation depth as well as downward
from the treatment perforations, past the lower previously
cemented perforations, evidently through channels or voids in
the near wellbore area.
The well was completed 2m higher at the ga6 oil contact
at a third set of perforations. The well started production
20 with a hi8her ga6 cut a6 expected, but five months after the
treatment it produced largely oil and no water. The initial
result6 suggest that gel blocked the water channels not blocked
by the cement. Therefore, a two-stage treatment involving
phenolformaldehyde gel followed by cement ha6 been shown to work
25 better than a cement squeeze alone.
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Example II
A well completed near the top of a carbonate reef in a
vugular, low pressure and fractured formation was treated in
accordance with the present invention. The well was producing
22m3 oil and 295m3 water per day. To shut off the excessive
water production, the existing perforations were treated with
24m3 of radioactively tagged phenolformaldehyde gelling
solution, followed by 2.4m3 cement. Three days after the
treatment, the cement was drilled out and pressure tested to
7MPa. The test indicated that the treated perforations were
successfully shut off. The gamma ray log indicated 9m of gel
placement vertically behind the casing. The well was then
perforated just above the plugged perforations. Initial data
indicates production of 55m3 oil and 27m3 water per day.
This corresponds to a reduction of the water-oil-ratio from 14
to 0.5.
The preferred embodiments of the present invention have
20 been described above. It should be understood that the
foregoing description is intended only to illustrate certain
l~ preferred embodiments of the invention and is not intended to
¦ define the invention in any way. Other embodiments of the
invention can be employed without departing from the full scope
25 of the lnveotloo as ~et forth io the appeoded cl~
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