Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR TREATMENT OF UNDERGROUND FORMATIONS
The process of the present invention is generally applicable to the production
of oil,
gas or water from wells drilled into underground reservoirs. It is also
applicable to
injection wells.
During drilling, workover and production operations there are numerous
situations
where the production rate of an oil, gas or water well following these
operations is
limited due to the presence of formation damage. Types of damage include, but
are
not limited to, the presence of polymer-containing filter cakes including
drilling mud
filter cakes, fluids (including hydraulic fracturing fluids) filtrates or
residues including
polysaccharide-containing filter cakes, fluids, filtrates or residues;
particulate
materials such as fluid loss control agents, bridging agents and rock fines,
biofilms,
scales and asphaltenes. Damage may arise as a result of drilling, production,
injection,
workover or other oilfield operations.
Damage can be near wellbore, for example the presence of drilling mud or
fracturing
fluid filter cake, or damage may be present deeper into the formation, for
example
mineral scale deposited in natural or induced fractures or in the rock matrix.
The
effective removal of damage, especially near wellbore damage such as filter
cake, can
significantly increase the production rate of hydrocarbon or water producing
wells
penetrating underground formations. The effective removal of damage can also
increase the injectivity of injection wells.
Treatment with acid (acidizing) has been used for many years to treat damage
in
underground formations and stimulate the rate of oil or gas production.
If acids can be delivered sufficiently far into the formation, acidizing may
also be
effective in stimulating undamaged formations, particularly carbonate
formations, by
increasing the permeability of the rock matrix around the wellbore. For
example,
increasing the permeability of a radial zone around a vertical or other
wellbore will
increase the rate of fluids production (or injection rate) in a situation
where there is no
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near wellbore damage. The efficient delivery of acids into fractures such as
induced
fractures or natural fractures or natural fracture networks can also increase
the
conductivity of the fractures allowing higher rates of fluid production or
injection.
Acids may also be used to break acid-sensitive gels such as crosslinked guar-
borate
gels used in hydraulic fracturing and other oilfield applications. Efficient
breaking of
gels is generally required to obtain maximum production after such treatments.
Acid
may also be used to break mixed metal hydroxides or complexes of mixed metal
hydroxides with materials such as bentonite.
However, conventional acids have several drawbacks. They react rapidly with
acid-
soluble materials which can prevent effective placement of reactive acid deep
into
carbonate formations or throughout long horizontal wellbores, resulting in
poor zonal
coverage. Conventional acids are also hazardous in use. To improve zonal
coverage
the use of high pressure, high rate injection is often attempted, which
increases the
hazards associated with the use of such acids.
One approach which can improve zonal coverage has been the use of solutions of
carboxylic acid esters which hydrolyse at high formation temperatures to
produce a
carboxylic acid downhole (US 3,630,285). Preferably, the formation temperature
for
this process is greater than about 150 C. Because the acid is produced
predominantly
after placement of the fluid improved zonal coverage can be achieved. The
preferred
esters used in US 3,630,285 were ethyl acetate and methyl formate. These
compounds
have the disadvantage of low flash points and have other health and safety
drawbacks
such as some degree of toxicity.
US 3,630,285 indicates that ester hydrolysis generally proceeds too slowly. It
considers the possible use of strong acids and alkalis to increase the rate of
hydrolysis
of the esters, but then discounts the use of such catalysts downhole, due to
the fact
that carbonate (limestone) formations will rapidly neutralise the catalytic
effects of
these materials.
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US 5,678,632 teaches the use of enzymes to increase the rate of hydrolysis of
esters,
providing a highly effective means of generating carboxylic acids in-situ for
acidizing
without using extremes of pH and extending the range of downhole temperatures
over
which useful rates of hydrolysis of esters can be obtained.
The use of certain types of non-enzyme catalysts to increase the rate of
hydrolysis of
esters and achieve effective acidizing has been taught in WO 01/02698. Non-
enzyme
catalysts taught included metal ions such as transition metal ions, organic
molecules
including amino acids, peptides, monosaccharides, oligosaccharides, nucleic
acids,
peptide nucleic acids and derivatives of organic molecules and combinations
thereof.
The enzymes and non-enzyme catalysts of US 5,678,632 and WO 01/02698 are
typically used at relatively low concentrations of up to a few percent v/v
enzyme
concentration or 1 to 10 mM non-enzyme catalyst. The solution containing the
ester
and enzyme or non-enzyme catalyst is made up in a suitable aqueous fluid such
as
fresh water, produced water or seawater.
W004/007905A1 teaches that compounds with a carboxylate functionality can also
increase the rate of acid production from esters.
As well as esters, other organic acid precursors may be used for in-situ
acidizing
processes. The use of orthoesters, anhydrides, polyesters and polyorthoesters
as
alternatives to esters has been taught. For example, the use of polyesters in
oilfield
acidizing has been taught inW02005/095755.
The period required for sufficient acidizing to occur is determined by several
factors,
including the type and concentration of organic acid precursor used, solvent
composition, availability of water, pH, formation temperature and amount and
type of
any enzyme or other catalyst used to increase the rate of acid production.
There is a need for further methods by which the rate of production of acid
from
certain organic acid precursors can be increased in certain situations.
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For example, at formation temperatures in the range 60-100 C precursors of
formic
acid might hydrolyse too quickly, but precursors of acetic acid, even in the
presence
of suitable catalysts, might take a few days to produce acid. If a method was
available
to achieve controlled acidizing over a shorter period, for example 3 to 24
hours, this
would be attractive to operators wishing to minimise expensive rig time.
It is an object of the present invention to provide further simple and
effective
processes for increasing the rate of hydrolysis of organic acid precursors to
facilitate
acidizing of underground formations based on the in-situ production of organic
acids
within a reduced timescale.
It is a further object of the present invention to provide simple and
effective processes
for acidising underground formations in combination with one or more polymer
breakers, where desired. Additionally, the present invention provides
processes and
chemicals that are generally low hazard and environmentally acceptable. It is
yet
another objective of the present invention to provide processes that can
achieve
acidizing at a predictable rate with excellent zonal coverage.
Accordingly, the present invention provides a process for treating an
underground
formation, which process comprises:
(a) introducing into the underground formation a treatment fluid comprising an
organic acid precursor;
(b) heating a zone within the formation, which zone contains at least a
portion of
the organic acid precursor, to a temperature which is above the natural
formation
temperature and sufficient to increase the rate of hydrolysis of the organic
acid
precursor; and
(c) allowing the organic acid precursor to hydrolyse to produce an organic
acid in
an amount effective to acidise the underground formation.
The process of the present invention is intended to achieve controlled and
effective
acidizing for purposes such as increasing the rate of production or
injectivity of wells
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drilled into the formation, increasing the permeability of the formation, for
example
the permeability of the rock matrix or of induced or natural fractures or
fracture
networks, treating filter cake, including following a gravel packing
operation, and
treating biofilm.
The process of the present invention may produce acid in an amount effective
to
dissolve acid soluble material in the underground formation. Such material
includes
calcium carbonate and other carbonates. It may also be used to break acid-
sensitive
gels such as crosslinked guar-borate gels used in hydraulic fracturing and
other
oilfield applications or to disrupt acid sensitive materials such as
mixed metal hydroxides and complexes of mixed metal hydroxides with materials
such as bentonite. The process may also be used in any other downhole
application
where the in-situ production of acid may be useful.
Where the process of the present invention is used to treat filter cakes
following
gravel packing operations, the treatment fluid may be used as the base fluid
for a
gravel packing operation, or be introduced after gravel packing.
The formation may be a hydrocarbon reservoir, for instance a gas or oil
reservoir.
Alternatively the reservoir may be a water reservoir. When it is a hydrocarbon
reservoir the process of the invention may further include recovering a
hydrocarbon
from the treated reservoir. The hydrocarbon may be gas or oil. Likewise, when
it is a
water reservoir, the process of the present invention may further include
recovering
water from the treated reservoir.
Typically the reservoir is, or includes, carbonate or sandstone rock
structures.The
organic acid precursor may be any organic acid precursor that hydrolyses in
the
presence of water to produce an organic acid. One or more organic acid
precursors
may be used. When there is more than one organic acid precursor, these may be
different. Organic acid precursors useful in the process of the present
invention
include esters, orthoesters, anhydrides, polyesters and polyorthoesters. The
organic
acid precursor may be in the form of a liquid, a colloid, a gel, a semi-solid
or a solid.
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The organic acid precursor may be dissolved or dispersed in water. Where
appropriate, it may be mixed, dissolved or dispersed in a non-aqueous solvent
such as
a mutual solvent, hydrocarbon or organic solvent. The organic acid precursor
may
also be incorporated into an emulsion such as a water-in-oil emulsion or oil-
in-water
emulsion or into a micellar dispersion (also referred to as a microeinulsion
or
transparent emulsion). In most applications the use of a water-soluble organic
acid
precursor such as an ester, or a solid organic acid precursor such as a
polyester, in an
aqueous treatment fluid will be preferred. Where a solid organic acid
precursor is
used it may be in any physical form that can be introduced into the formation.
The organic acid precursor will hydrolyse at a predictable rate in the
presence of even
a small amount of water to generate an organic acid. The organic acid
precursors will
preferably be low hazard and toxicity with a high flash point and high
environmental
acceptability. Generally they will also be biodegradable to an acceptable
extent.
The organic acid precursor is typically an ester, for instance an ester of a
carboxylic
acid or of a hydroxycarboxylic acid. Esters taught in US 5,678,632 US
5,813,466 US
6,702,023 and US 6,763,888 are suitable for use in the present invention. They
include esters of an aliphatic carboxylic acid of formula RCO2 H wherein R is
selected from the group consisting of hydrogen, an alkyl group having from 1
to 6
carbon atoms and -R'-C02 H where R' is a bond or an alkylene group having from
1
to 6 carbon atoms, the alkyl or alkylene group being unsubstituted or
substituted by
halogen or hydroxy.
Esters of short chain carboxylic acids including ethanoic and methanoic acid
(acetic
and formic acid) are particularly suitable. The calcium and magnesium salts of
these
acids have good solubility in water, formate brines, acetate brines and many
other
brines, which is advantageous when the acid produced by the process of the
present
invention is used to dissolve calcium carbonate, magnesium carbonate, calcium
magnesium carbonate (dolomite) or other acid soluble calcium or magnesium
salts.
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Esters of hydroxycarboxylic acids such as glycolic and lactic acid are also
particularly
suitable. For example hydroxyacetic acid can dissolve calcium sulphate.
Where the acid has a hydroxy substituent, the ester may be a cyclic ester such
as a
lactone. Esters of chelating compounds such as malonic acid, oxalic acid,
succinic
acid, ethylenediaminetetraacetic acid (EDTA) nitriloacetic (NTA) citric acid
or
hydroxyacetic acid as taught in US 6,702,023 and US 6,763,888 may also be
used. If
a base is present and the chelating acid is neutralised, it will be understood
that salts
of such chelating acids may also act as dissolving agents for acid soluble
materials as
taught in US 7,021,377. Where acid soluble materials are described in the
current
description, this also refers to material soluble in solutions of salts of
chelating acids
and agents.
The ester should be at least slightly water soluble. Preferably the ester
should be
soluble to at least 1% w/v in water and most preferably soluble to at least 5%
w/v in
water. Preferably 5% to 20% w/v ester will be used but concentrations of ester
higher
than 20% w/v may be used in some cases.
In general it has been found that 5% to 10% w/v ester is sufficient to give
good
increases in permeability or good removal of filter cake damage. The
solubility of
some esters may be reduced in high salt concentration fluids such as heavy
brines,
compared to their solubility in water.
In such cases an ester which is completely soluble in the base fluid to a
sufficient
concentration will normally be selected. In some situations, it may be
desirable or
necessary to use an emulsion of an ester in the treatment fluid.
The alcohol portion of the ester may be monohydric or polyhydric. The degree
of
esterification of polyhydric alcohols will affect the solubility of the ester
in water and
other solvents such as hydrocarbons. For example, partial esters of polyhydric
alcohols can be used in which case the unesterified hydroxyl groups serve to
increase
the water solubility of the ester, compared to fully esterified polyhydric
alcohols.
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In one embodiment the ester is a carboxylic acid ester of 1,2,3-propanetriol,
ethylene
glycol, diethylene glycol or triethylene glycol. Suitable esters include but
are not
limited to ethylene glycol monoformate, diethylene glycol diformate, glycerol
monoformate, glycerol triacetate, glycerol diacetate, butyl lactate, propyl
lactate and
ethyl lactate.
The organic acid precursor is alternatively a polyester, for instance an
aliphatic
polyester. Any polyester that hydrolyses in the presence of water to produce
an
organic acid may be used. Suitable polyesters include those which comprise one
or
more of lactic acid, lactide, glycolic acid, glycolide, caprolactone and
(optionally)
other compounds which may condense with lactic acid, lactide, glycolic acid,
glycolide or caprolactone.
Suitable monomers include but are not limited to hydroxy, carboxylic acid or
hydroxy-carboxylic acid compounds tribasic acids such as citric acid, dibasic
acids
such as adipic acid, and diols such as ethylene glycol and polyols. They also
include
difunctional molecules such as 2,2-(bishydroxymethyl) propanoic acid.
Organic acids produced from the hydrolysis of the organic acid precursor which
are
useful in the process of the present invention include any organic acid which
reacts
with acid soluble materials downhole to produce salts of sufficient solubility
to ensure
substantive dissolution of the acid soluble materials takes place, for example
calcium
formate or calcium lactate. It is important that the dissolution of acid
soluble material
does not result in the deposition of another solid or other chemical form that
produces
a different type of damage. Sufficient water needs to be present to dissolve
the salts
produced by the reaction between the acid and acid soluble materials such as
carbonates.
Sufficient organic acid precursor (or precursors) are provided to produce
sufficient
acid, when the organic acid precursor is hydrolysed, for the acid produced to
have a
substantive acidizing effect on acid soluble material present in the
underground
formation. By substantive effect it is meant that sufficient acid is produced
on the
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hydrolysis of the organic acid precursor to give sufficient dissolution of
acid soluble
material, present in or adjacent to the filter cake, scale or other damage, to
assist in the
removal of damage or to increase the permeability of carbonate rock
formations.
Preferably the organic acid produced from hydrolysis of the organic acid
precursor
should be water soluble to at least 1% w/v and preferably higher.
The process of the present invention may be used to treat formation damage.
The
process of the present invention may also be used to increase the permeability
of
undamaged regions of an acid soluble rock matrix adjacent to a wellbore,
natural or
induced fracture.
Examples of situations where the dissolution of acid soluble material is
desirable
include the dissolution of carbonate present in a filter cake produced from a
water-
based or oil-based drill-in fluid, the dissolution of carbonate rock adjacent
to a filter
cake, the dissolution of carbonate rock adjacent to induced or natural
fractures and the
dissolution of carbonate scales in the wellbore, formation or tubulars.
The concentration of organic acid precursor used in the process of the present
invention will typically be at least 1% w/v but may be up to 20% w/v or
higher. For
applications such as treatment of filter cake a minimum of several percent w/v
of acid
is normally produced. The use of formulations capable of producing between 5
and
10% w/v organic acid precursor have been found to be particularly suitable for
treatment of filter cakes.
Where using particulate, solid organic acid precursors, such as polyesters,
the extent
of any penetration into the formation will be determined, amongst other
factors, by the
particle size of the organic acid precursor and the size of the pore throats
in the
formation and this will be taken into account when designing the treatment.
Solid
organic acid precursors may be used in any solid configuration, including, but
not
being limited to spheres, cylinders, cuboids, fibres, powders, beads or any
other
configuration which can be introduced into the formation. They will preferably
be
used in the form of particles in the size range 1 micron to 4 mm, most
preferably 10
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microns to 1 mm. Solid organic acid precursors may also be in the form of a
moulded
tool or article or as a film, coating or as a fill in the annular spaces on
downhole
equipment such as a screen or expandable screen.
Where deep penetration is needed, solutions or other liquid forms of the
organic acid
precursor will normally be used.
The organic acid precursor is normally placed in the wellbore or adjacent
formation
following drilling of the well, for example as part of a completion or
workover
operation. In some cases, such as where a polyester is incorporated into a
drill-in
fluid, as taught by PCT/GB05/01193, it may be introduced during the drilling
process.
The process of the present invention is used in the situations where the rate
of
hydrolysis of the organic acid precursor is not sufficient at the bottom hole
static
temperature (BHST) to achieve the required degree of acidizing within an
acceptable
or desired period of time. The process of the present invention is used to
shorten the
time required for the acidizing treatment. For example, at BHST, hydrolysis of
the
organic acid precursor (in the absence or presence of an optional catalyst)
may take a
longer period than the operator may desire in order for acidizing to proceed
to a
sufficient extent. For example it might take several days. By employing the
process
of the present invention and supplying heat to the zone containing the organic
acid
precursor, the time required to generate acid in-situ may be reduced or even
substantially reduced, so that acid production takes place over a shorter
period (i.e.
fewer days or even over a few hours) depending on the temperature employed.
An essential feature of the present invention is that heat is supplied to the
zone
containing the organic acid precursor to increase the temperature to above the
normal
formation temperature. Heat will normally be supplied after placement of the
organic
acid precursor, but alternatively may be supplied before the organic acid
precursor is
placed in the formation or at the same time that the organic acid precursor is
placed in
the formation.
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Heat may be supplied by any method of delivering heat to a wellbore or
formation
that is known to one skilled in the art of providing heat to underground
formations.
Such methods may include but not be limited to hot watering, hot oiling, steam
injection, close circuit heating, such as described in US4641710, use of
exothermic
chemical reactions, microwave heating, electric heating, controlled combustion
or
oxidation, or use of a heat exchange compressor such as described in US
2005/0034852. For convenience, heating may be achieved using a heat source
which
is readily moveable. When treating a short section, a single source of heat
used in a
single location may be sufficient. When treating long sections, for example
when
treating filter cake in long horizontal wells, heat will generally applied to
one section
of the well at a time, unless multiple heat sources can be used. If using a
single heat
source to treat a long interval, the heat source will generally be placed in
the wellbore
and moved by methods that will be known to those skilled in the art, so that
heat is
supplied sequentially to different treatment zones along the interval.
When using exothermic chemical reactions as a heat source the chemicals used
to
generate heat may be introduced to the wellbore using any convenient means,
for
example the drill string, coiled tubing, production tubing or by bullheading.
At least a portion of the zone containing organic acid precursor is heated to
above the
normal formation temperature. Heating will generally be to a temperature at
least 10
C above the formation temperature and preferably to 20 to 120 C above the
formation temperature. The temperature to which the organic acid precursor
needs to
be heated to achieve acid production within a desired time period will be well
understood by those skilled in the art. For example, formic acid precursors
will
generally need to be heated less than precursors of other organic acids. Also,
esters
will generally need to be heated less than polyesters, and orthoesters less
than
polyorthoesters. It will be understood that a temperature gradient will be set
up in the
formation around the heat source. Organic acid precursor at different
distances from
the heat source will therefore by heated to different extents and produce acid
at
different rates and this will be taken into account when designing treatments.
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In one embodiment, the process of the present invention further comprises
introducing
one or more polymer breakers into the formation to achieve polymer breaking in
combination with acidizing. Polymer breakers may be selected where polymers or
gels are present in the formation, for example in filter cakes including those
formed
from drilling muds or drill-in fluids fluids, fracturing, completion or
workover fluids.
In one embodiment the polymer breaker is an oxidant (oxidative breaker). In
another
embodiment the polymer breaker is an enzyme. A polymer breaker component may
thus be introduced into the formation in an amount effective to degrade
polymers
present within the formation.
Oxidative breakers used in the present invention may be any one of those
oxidative
breakers known in the art to be useful to react with polymers to reduce the
viscosity of
polymer thickened compositions or to disrupt filter cakes. The oxidative
breaker is
typically introduced in a treatment fluid containing the organic acid
precursor
component. The oxidative breaker may be present in solution or as a
dispersion.
Suitable compounds include persulphates, peroxides, perborates, percarbonates,
perphosphates, hypochlorites, persilicates, metal cations and hydrogen
peroxide
adducts such as urea hydrogen peroxide and magnesium peroxide.
Preferred oxidative breakers are peroxides which can decompose to generate
hydrogen peroxide. Of the oxidative breakers most preferred are percarbonates
and
perborates, most especially sodium percarbonate and sodium perborate.
Preferred polymer breaking enzymes used in the present invention include
hydrolases
and lysases, such as any one of those polysaccharide degrading enzymes known
in the
art to be useful to degrade (normally hydrolyse) polysaccharides and to reduce
the
viscosity of polysaccharide thickened compositions or to disrupt filter cakes.
The
polymer breaking enzymes will be selected on the basis of their known ability
to
hydrolyse the polysaccharide components known or believed to be contributing
to the
damage. Examples of suitable enzymes which may be used to break polymers
include
enzymes which can hydrolyse starch, xanthan, cellulose, guar, scleroglucan,
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succinoglycan or derivatives of these polymers. If suitable enzyme activities
are
available, enzymes could also be used to hydrolyse any other polymers suitable
for
use in drilling, workover or production operations. Appropriate enzymes for
this
purpose are well documented in the literature and would be well known to a
person of
skill in the art.
Oxidative or enzyme breakers or catalysts capable of hydrolysing other, non-
polysaccharide polymers may also be incorporated into treatment fluids used in
the
present invention. Where a breaker is incorporated into a treatment fluid to
be used in
the process of the present invention, sufficient polymer breaker or gel
breaker is
normally included to have a substantive effect on the polymer component. The
concentration of polymer breaker incorporated into the formulation will vary
according to the type of breaker employed, the nature of the polymer and its
concentration in the base fluid but will be of the order of 0.005 to 60 kg /
m3,
preferably 0.2 to 10 kg / m3.
In another embodiment, the process of the invention further comprises
introducing
one or more of an enzyme catalyst, non-enzyme catalyst and carboxylate
compound
into the reservoir to further increase the rate of production of acid from the
organic
acid precursor. For example, enzyme catalysts, non-enzyme catalysts and
carboxylate
containing compounds that increase the rate of ester hydrolysis are taught in
US
5,678.632, WO 01/02698 and WO 04/007905.
Where an enzyme is additionally used in conjunction with the process of the
present
invention, to hydrolyse an organic acid precursor or as a polymer breaker, it
is
necessary to select an enzyme which remains active in the treatment fluid
under
reservoir conditions for at least as long as the catalytic activity is needed.
It will be
understood by those skilled in the art of using enzymes in underground
formations
that the enzymes may only be active in a particular temperature range. Heating
of the
formation to a temperature above the maximum for a given enzyme may result in
its
inactivation. Nonetheless, if the enzyme is active during the heating process,
useful
hydrolysis of an organic acid precursor or polymer breaking may be achieved.
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It is generally advantageous for the enzymes to be readily water soluble
although the
enzymes may also be active and be used in low water activity environments or
two-
phase systems such as emulsions or dispersions or in microemulsions.
Typically,
isolated enzymes are used. Enzymes may be isolated from plant, animal,
bacterial or
fungal sources. The enzymes may be produced from wild-type, conventionally
bred,
mutated or genetically engineered organisms. The enzymes may, optionally, be
chemically modified, as long as they retain or possess the desired catalytic
ability.
Preferably, the enzymes will be industrial enzymes available in bulk from
commercial
sources. Typical enzyme concentrations will be 0.05% to 5% v/v of commercial
liquid enzyme preparations or about 0.005 to 0.5% v/v of dried enzyme
preparation.
Preferably liquid preparations of enzymes will be used for ease of handling.
The treatment fluid containing one or more organic acid precursor is generally
prepared at the surface before being introduced downhole. Suitable processes
for
preparing the selected treatment fluids and placing them in the formation will
be well
known to those skilled in the art. For example, when using an ester, a
treatment fluid
is normally prepared by blending the ester batchwise with suitable water, for
example
city (drinking) water, fresh surface or well water, produced water, sea water
or a base
brine. In other embodiments of the present invention, the organic acid
precursor may
be incorporated into mutual solvent, mutual solvent blended with water, mutual
solvent blended with water and hydrocarbon, hydrocarbons or other organic
solvents.
It may also be incorporated in emulsions incuding oil-in-water and water-in-
oil
emulsions or into a micellar dispersion (also referred to as a microemulsion
or
transparent emulsion). However, it will be understood that sufficient water
needs to
come into contact with the organic acid precursor to allow hydrolysis to occur
during
heating or in the presence of catalysts. This water may be provided in the
treatment
fluid at the outset, or may come from fluids introduced into the wellbore
before or
after the treatment fluid of from the formation. Sufficient water needs to be
present to
dissolve the products of the reaction of the produced organic acid with acid
reactive
material, for example calcium carbonate.
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Optionally, other components such as catalysts to increase the rate of organic
acid
precursor hydrolysis and/or polymer breakers may be incorporated into the
treatment
fluid by suitable means. In some cases, they may be placed in proximity to the
organic acid precursor using a separate fluid treatment stage. For example,
when a
well is drilled using a polyester incorporated into a mud or drill-in fluid,
as taught by
PCT/GB05/01193, the organic acid precursor is incorporated into a filter cake.
Displacing the liquid mud or drill in fluid to a clear brine containing a
catalyst or
polymer breaker will allow acidizing in conjunction with polymer breaking.
The organic acid precursor and any other components selected for incorporation
into
the treatment fluid will be chosen to reflect the requirements of the
treatment, such as
the amount of acid and rate of production of acid required. The chemical
compatibility with the formation will also be considered.
The treatment fluid may also contain further chemical additives such as are
commonly
used in the treatment of underground formations, such as surfactants, mutual
solvents,
foaming and chelating agents if their inclusion is deemed to be beneficial and
if they
are sufficiently compatible with the other components of the treatment fluid
and the
formation fluids.
Polymer breakers, enzymes and non-enzyme catalysts may, if desired, also be
used in
the form of delayed release preparations, such as will be well known by those
skilled
in the art.
The organic acid precursor and other chemicals required for the process of the
present
invention will normally be technical grade to reduce the cost of the process.
If it is preferred the treatment fluid may be prepared by adding the organic
acid
precursor to the water on a continuous, preferably carefully controlled and
monitored
basis ("on the fly") while the treatment fluid is being injected into the
underground
reservoir.
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The treatment fluids are conveniently introduced into the underground
formation via a
wellbore that extends into the reservoir, for instance via injection or
production wells.
The wellbore may be vertical, deviated, inclined or horizontal. If being
introduced
into a newly drilled well, particularly if being used to remove damage caused
during
drilling, such as filter cakes, a treatment fluid may conveniently be
introduced via the
drill string. This may be achieved using the mud pumps. The treatment fluid
may also
be introduced via coiled tubing or bullheading.
The corrosivity of a treatment fluid will be taken into account in deciding if
it may be
introduced into wells or the drill string without the need to add corrosion
inhibitors. If
required, suitable corrosion inhibitors or buffers (to raise the pH) may be
added to
treatment fluids. The treatment fluid will normally be introduced at below
fracture
pressure but may if desired be injected at above fracture pressure.
For near wellbore treatments, the volume of treatment fluid introduced into
the
reservoir will typically be at least equal to the wellbore volume plus an
allowance for
some leak off into the formation. A fluid volume of between 100% and 200% of
the
wellbore volume will normally be used although if a high rate of fluid loss is
expected
a volume up to 300% or higher of the well bore volume may be selected.
For treatments where the target is stimulation deeper into the formation such
as within
induced fractures or natural fractures or natural fracture networks a volume
of
treatment fluid will be selected appropriate to the requirements of the
treatment.
In one embodiment of the present invention, a volume of the treatment fluid
which is
sufficient to allow the fluid to penetrate up to several metres into a
formation around a
wellbore or behind a fracture face may be used. The production of acid in-situ
can
result in an increase in the matrix permeability of a carbonate formation to a
depth up
to several metres. Such treatments may be referred to as deep matrix
stimulation.
Using conventional acidizing it is extremely difficult to obtain a uniform
effect. A
much more uniform effect can be obtained using the process of the present
invention
if adequate heat penetration can be achieved.
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The well will normally be shut in during the period of heating and optionally
for some
time after the application of heat ceases. Typically a period of between 0.2
hours and
48 hours will be used to heat each treated zone, depending on the formation
temperature and the temperature to which the formation is heated although in
some
cases the period might be shorter or longer. The well is then put on or
returned to
production, or in the case of injection wells, put on injection.
The process of the present invention may be used in a number of different
acidizing
applications, such as to dissolve acid soluble material in the reservoir, to
increase the
rate of production or injection of wells drilled into the formation, to treat
filter cake, to
treat filter cake following a gravel packing operation, to treat a biofilm, to
break a pH
sensitive cross-linked gel or to disrupt mixed metal hydroxides or complexes
of mixed
metal hydroxides.
It will be understood that removal of damage or dissolution of acid-soluble
material
using the process of the present invention may not be complete. The treatment
may
however be judged a success if damage is substantially remediated, resulting
in higher
rates of production or injection than would be the case with no treatment.
The process of the present invention has a number of advantages.
It is a feature of the process of the present invention that the use of an
organic acid
precursor rather than a reactive acid avoids wormholing and improves the
placement
of the treatment fluid. Use of an organic acid precursor rather than a
reactive acid
gives the advantages described in US 3,630,285 and US 5,678,632 with respect
to
effective placement of the fluid and results in acidizing with much better
zonal
coverage.
In order to maintain this advantage in certain applications such as the
treatment of
long horizontal intervals, too rapid a breakthrough of filter cakes should in
general be
avoided. Ideally, breakthrough of filter cakes will be achieved after a period
longer
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than that amount of time needed to place a treatment fluid throughout the zone
requiring treatment. A delay in producing a substantive amount of acid and in
breaking the filter cake allows even treatment of the target zone and
excellent zonal
coverage. In the case of the present invention, an organic acid precursor
which
hydrolyses at a rather slow rate at formation temperature may be selected.
Increased
rates of hydrolysis and acid production occur on heating to the required
temperature.
It will be understood that localised heating may result in localised leak-off
when a
filter cake or other damage is removed and this will be taken into account
when
designing treatments. The use of suitable fluid loss or diverting agents may
be
required in some circumstances. One useful combination is the use of an ester
in
combination with a polyester, where heating results in relatively fast acid
production
from the ester to dissolve acid-soluble components of the filter cake, but the
polyester
hydrolyses relatively slowly and can act as a fluid loss agent,
Where suitable organic acid precursors are selected, in particular where low
toxicity,
high flash point organic acid precursors such as esters and polyesters are
used, the
process of the present invention has health, safety, operational and
environmental
advantages.
The treatment fluids used in the process of the present invention typically
start at near
neutral pH. The process therefore permits the incorporation of acid
incompatible gel
breakers such as enzymes and oxidative breakers into fluids capable of
delivering a
high concentration of acid. This avoids the compatibility problems which
typically
arise when attempting to incorporate such breakers into highly acidic
formulations
based on mineral or organic acids.
The process provides a simple, effective and convenient way to acidise
underground
formations. In its simplest embodiment, a single fluid and a single heat
source are
used at one location. The process increases the rate of organic acid precursor
hydrolysis to achieve acidizing within a shorter and more acceptable period of
time in
situations where the rate of ester hydrolysis might not otherwise be fast
enough. For
example, a treatment that may otherwise take 2 days might be completed in
several
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hours. Shortening the time needed to carry out a treatment can save
significant
amounts of downtime and lost production. In more complex embodiments, more
than
one fluid may be used, more than one heat source may be used, or the heat
source may
be moved along long intervals.
Compared to other treatment processes where a separate conventional mineral or
organic acid ("live acid") stage may be required it is a very low hazard
process for the
controlled rate dissolution of acid soluble materials. Handling of live acid
by
operators is avoided and there is generally no need for high pressure, high
rate
injection which is often used in conventional acidizing processes to counter
the high
reaction rate of live acids.
Due to the good zonal coverage that can be obtained, the process of the
present
invention is particularly effective for the removal of filter cakes over long
horizontal
intervals and in sand control completions including gravel packs, stand alone
and
expandable screens. Uniform cleanup of filter cakes in such situations is
critical in
minimising the risk of premature failure.
The process of the present invention may also provide deep matrix acidizing of
the
formation around a wellbore or fracture at the same time as removing near
wellbore
damage.
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