U.S. patent number RE35,891 [Application Number 08/538,664] was granted by the patent office on 1998-09-08 for process for increasing near-wellbore permeability of porous formations.
This patent grant is currently assigned to Noranda Inc.. Invention is credited to Abul K. M. Jamaluddin, Taras W. Nazarko.
United States Patent |
RE35,891 |
Jamaluddin , et al. |
September 8, 1998 |
Process for increasing near-wellbore permeability of porous
formations
Abstract
A method of increasing the near-wellbore permeability of porous
formation comprises exposing formation to an elevated temperature
of .[.400.degree..]. .Iadd.600.degree. .Iaddend.C. or greater to
cause dehydration of the clay lattices, vaporization of any blocked
water, mud filtrate or other fluids, and/or destruction of the clay
structure.
Inventors: |
Jamaluddin; Abul K. M.
(Calgary, CA), Nazarko; Taras W. (Calgary,
CA) |
Assignee: |
Noranda Inc. (Quebec,
CA)
|
Family
ID: |
4150889 |
Appl.
No.: |
08/538,664 |
Filed: |
October 3, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
070812 |
Jun 3, 1993 |
05361845 |
Nov 8, 1994 |
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Foreign Application Priority Data
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Dec 22, 1992 [CA] |
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2086040 |
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Current U.S.
Class: |
166/302;
166/272.1 |
Current CPC
Class: |
E21B
43/24 (20130101) |
Current International
Class: |
E21B
43/24 (20060101); E21B 43/16 (20060101); E21B
043/24 () |
Field of
Search: |
;166/302,303,272 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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915573 |
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Nov 1972 |
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CA |
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1282685 |
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Apr 1991 |
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CA |
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Primary Examiner: Neuder; William P.
Attorney, Agent or Firm: Keck, Mahin & Cate
Claims
We claim:
1. A method off increasing the near-wellbore permeability of porous
formations containing hydratable clays, shales, materials which
swell when contacted with water or mud filtrate, migratable clays
or a formation having a wettability which causes water or fluid
blockage, comprising exposing the formation to a temperature of at
least .[.400.degree..]. .Iadd.600.degree. .Iaddend.C. by using
downhole heaters with continuous inert gas injection to cause
dehydration of clay lattices, vaporization of any blocked water,
mud filtrate or other fluids, and destruction of the clay
structure.
2. A method of defined in claim 1, wherein the formation is exposed
to a temperature of at least .[.400.degree..]. .Iadd.600.degree.
.Iaddend.C. either prior to water and/or mud filtrate contact or
after damage with water and/or mud filtrate.
3. A method as defined in claim 1, wherein the heat treatment lasts
more than 4 hours after the desired temperature is reached.
4. A method as defined in claim 1, wherein the temperature of the
heat treatment is about .[.400.degree..]. .Iadd.600.degree. C.
.Iaddend.to 1000.degree. C.
5. A method as defined in claim 1, wherein inert gas is injected at
a pressure higher than the reservoir pressure.
6. A method as defined in claim 1, wherein the downhole heater is
an electrical resistance heater, a gas heater, or a high frequency
dipole heating device.
Description
This invention relates to a process for increasing near-wellbore
permeability of a porous subterranean formation.
BACKGROUND OF THE INVENTION
Most porous formations contain clay minerals, which are crystalline
in nature and have lattice-layer silicates and chain silicates. The
lattice-layer silicates are formed of combinations of two basic
building blocks, a silicone-oxygen tetrahedron, and an
aluminum-oxygen-hydroxyl octahedron. These units are polymerized
into sheets. Tetrahedral sheets are formed by sharing of corners,
while octahedral sheets are formed by sharing of edges. There are
two types of octahedral sheets: one in which every octahedral site
is filled by a divalent ion and one in which two out of three sites
are filled by trivalent ions. The first and second sheets are
referred to as trioctahedral and dioctahedral sheets, respectively.
The polymerization process can also be continued by hooking
together terahedral and octahedral sheets to form a 1:1 composite
layer. In the composite layer, the octahedral sheet could also be a
dioctahedral one. Similarly, a 2:1 composite layer can also be
formed by using two tetrahedral sheets to the central octahedral
layer. At 2:1 composite layer could be formed of dioctahedral or
trioctahedral sheets.
Clay surfaces of the most common clays have many negatively charged
sites, which make them fresh-water sensitive. Previous studies have
established that clay occur naturally as either pore-lining or
pore-filing minerals. These clay minerals usually are surrounded by
saline connate water layer. The cations (e.g., Na.sup.+, Ca.sup.++
etc.) from the saline water neutralizes the negative charges in
clay minerals. The introduction of fresh water or less saline water
into the formation, dilute the connate water and reduce its saline
content. Because of this cation-charge deficiency around clay
minerals, water molecules can easily invade in between clay
platelets and results in swelling or dispersion. Therefore, charge
deficiency in the minerals is an important quantity. It determines
the forces holding the layers together. With mica group, these
forces are relatively large. For smectites the forces are
relatively small. The results is that interlayer cations can leave
and enter the structure readily. In addition, water and organic
molecules can enter and leave readily. Water tends to hydrate
interlayer cations and result in a swelling of structure
perpendicular to these layers. Organic materials also cause
swelling.
Clay materials either were originally deposited during
sedimentation, were formed later by the action of heat, pressure,
and time on minerals already present, or were precipitated from
fluids flowing through the matrix. The major components of clay are
smectite, kaolinite, illite and mixed layer (i.e.,
illite-smectite). The two major mechanisms by which these minerals
cause permeability damage are swelling and migration. In swelling,
clay imbibes fresh water into its crystalline structure and
subsequently increases in volume, plugging the pores in which it
resides. Mixed layer and smectite are examples of swelling clays.
In migration, clay minerals can be dispersed by contact with a
foreign fluid or can be entrained by produced fluids and
transported until a restriction is encountered (usually a pore
throat), where the entrained particles bridge and restrict flow in
the capillary. Kaolinite, illite, chlorite and mixed-layer are
examples of migrating clays.
During drilling, if water-based drilling mud is used, mud filtrate
will invade and damage the near-wellbore formation to some degree.
During completion, the completion fluid can also invade and damage
the near-wellbore formation. The cause of the formation damage can
be explained by several possible factors including:
1. the invasion of drilling fluid causes clay minerals to swell and
to constrict pore throats; this constriction causes a decrease in
formation porosity and permeability, and an increase of the
capillary effects.
2. the invasion of water-based fluid also causes water blockage due
relative-permeability effects (two-phase flow).
The above-mentioned factors and other possible factors subsequently
cause permeability reduction. Hydraulic-fracture treatments are
often effective in by-passing the clay-related formation damage.
However, these treatment techniques of clay-related formation
damage, especially in horizontal wells, are difficult to perform
and could be uneconomic. Therefore, there is a need in the
petroleum industry for a new and improved method of treating
clay-related formation damage.
In addition to the conventional acid and hydraulic-fracture
treatments, several unconventional methods are disclosed in the
literature. The following is a brief description of some of these
disclosures.
U.S. Pat. No 4,844,169 presents a method of injecting non-reactive
gas (i.e., nitrogen) into the formation at atmospheric temperature
to fluidize the clays, including migratable fines, for their
removal. Subsequently, an aqueous solution of soft water containing
potassium chloride is proposed to be injected into the formation to
cause a potassium-sodium cationic exchange within the swellable
clays to reduce their swelling. In this method, temperature is kept
low and clay structures are not altered. The fluidized clay
particles can also block pore throat and subsequently, the treating
fluid will be unable to contact the swelling clays. After
low-temperature injections, chemical treatments may cause
reswelling of the clays.
Canadian patent No. 915,573 discloses a method of treating the
near-wellbore formation damage by contacting the formation with
heated air or gas at a 121.degree. C. (250.degree. F.) temperature
to cause partial dehydration of clays. Thereafter, the
near-wellbore formation is treated with non-ionic vinyl pyrrolidone
polymer to prevent reswelling of clays. In this 2-step method, the
partial dehydration remedies the formation damage temporarily.
However, subsequent chemical treatment may not be very effective
because of the lack of good contact between the polymer solution
and the formation.
Injection of aqueous solution of nitrogen at an elevated
temperature of 260.degree. C. to 310.degree. C. (500.degree. F. to
590.degree. F.) to transform montmorillonite clays to more stable
illitic-type clays was disclosed in U.S. Pat No. 4,227,575. These
illitic-type clays are less sensitive to fresh water. The
transformation of montmorillonite clays to illitic-type clays are
possible by this method, but the aqueous solution of nitrogen can
also trigger swelling of other minerals (e.g. glauconite
paloids).
The use of saturated and superheated steam at temperatures of
104.degree. C. to 871.degree. C. (220.degree. F. to 1600.degree.
F.) and at pressures of 14.7 to 8000 psia was proposed in U.S. Pat.
No. 3,847,222 to treat the near-wellbore formation damage.
Subsequent to steam treatment, the injection of guanidine
hydrochloride in methanol was shown to achieve better results. In
this two-step process, the condensed steam will act as a source of
fresh water and cause formation damage in the untreated
regions.
The simultaneous injection of steam and vaporized hydrogen chloride
to rectify clay-related formation damage is presented in U.S. Pat
No. 4,454,917. The purpose of steam is to clean the formation and
the purpose of hydrogen chloride is to react with calcium and
magnesium salts in the near-wellbore formation to form
water-soluble chloride salts. In this process, the condensed steam
is also a source of fresh water and could cause formation damage
due to reswelling of clay minerals.
Another preventive technique disclosed in Canadian patent No.
1,282,685 is the removal of precursor ions from the injection water
using reverse osmosis before injection into the formation. The
removal of precursor ions will reduce precipitation in the
formation and subsequently reduce the chances of formation damage.
In this technique, the removal of precursor ions may not
necessarily prevent the swelling and/or migration of clay materials
in the formation.
SUMMARY OF THE INVENTION
It is the object of the present invention to provide a new method
for increasing the permeability of near-wellbore formation
containing hydratable clays, shales and other materials which tend
to swell when contacted with fresh water and/or mud filtrate.
It is also the object of the invention to provide a new method for
increasing the permeability of near-wellbore formation containing
migratable clays, which are fluid velocity sensitive.
It is a further object of the present invention to provide a new
method for increasing the permeability of near-wellbore formation
whose wettability tends to cause water and/or fluid blockage.
It is thus generally the object of the invention to provide a new
method for increasing near-wellbore permeability of any porous
formation either containing hydratable clays, shales or other
minerals which tend to swell when contacted with fresh water and/or
mud filtrate, or a formation which has migratable clays, or a
formation whose wettability tends to cause water and/or fluid
blockage.
The method in accordance with the present invention consists of
exposing the formation to an elevated temperature of
.[.400.degree..]. .Iadd.600.degree. .Iaddend.C. or greater to cause
dehydration or the clay lattices, vaporization of any blocked
water, mud filtrate, or other fluids, and/or destruction of the
clay structure.
The porous formation can be effectively treated to improve
hydrocarbon permeability either prior to water and/or mud filtrate
contact or after damage by water and/or mud filtrate. The heat
treatment typically lasts for several hours, preferably more than 4
hours after the desired temperature is reached.
The temperature of this heat treatment is desirably about
.[.400.degree..]. .Iadd.600.degree. C. .Iaddend.to 1000.degree. C.,
preferably 600.degree. to 800.degree. C.
The above heat treatment may be carried out using downhole heaters
including electrical resistance or gas heaters with air and/or
inert gas injection. The desirable injection pressure must be
higher than the reservoir pressure. Use of high frequency dipole
heating with or without gas injection is also envisaged.
The advantages of the preset invention is that the high temperature
destroys clay structure so that there is no possibility of
rehydration and reswelling of clay minerals. Therefore, chemical
post treatments are not required. In addition, laboratory tests
have shown that the destruction of clay structure not only improves
the damaged permeability but also improves the original
permeability of the virgin formation.
DETAILED DESCRIPTION OF THE INVENTION
In this invention, a subterranean formation containing one or more
hydratable clays, one or more migratable clays, one or more
hydratable shales, and/or one or more combinations thereof, where
the clays and shales both tend to swell when contacted with fresh
water and/or mud filtrate, and/or formation whose wettability tends
to cause water and/or fluid blockage, is exposed to an elevated
temperature of .[.400.degree..]. .Iadd.600.degree. .Iaddend.C. or
higher, either prior to water contact or after the formation has
been contacted with water from underground or other sources and
therefore has become hydrated and expanded and/or water blocked so
as to substantially reduce the permeability of that formation
relative to the original permeability of the virgin reservoir.
The porous formation is preferably treated to improve hydrocarbon
permeability prior to water and/or fluid contact or after damage by
water and/or fluid. The heat treatment typically last for several
hours, preferably more than 4 hours after the desired temperature
is reached. The three basic principles of formation heat treatment
are given below:
1. dehydration of clay lattices,
2. vaporization of any blocked water, mud filtrate, or other
fluids, and/or
3. destruction of clay structure.
The temperature ranges for this heated gas treatment are desirably
about .[.400.degree..]. .Iadd.600.degree. .Iaddend.C. to
1000.degree. C., preferably 600.degree. C. to 800.degree. C.
The above-mentioned three steps can be carried out using a tubing
or wireline-conveyed-downhole heating device placed in the
wellbore. Air and/or inert gas (e.g., nitrogen) is preferably
injected into the wellbore at atmospheric temperature and at a
pressure higher than the reservoir pressure. Air and/or inert gas
will be heated as it passes through and/or around the heating
device and hot gas will be forced into the formation. The heating
device can be made of an electrical-resistance heating element or a
gas heater or any device that can generate heat downhole. The
near-wellbore formation will be heated by the air and/or inert gas
being heated by the downhole heater. This heating process is
designed for cased or openhole vertical or horizontal wells. In
order to reduce wellbore heat losses in the vertical direction, air
and/or inert gas injection through the annular space, for the case
of tubing-conveyed heaters, may be provided. For the case of
wireline-conveyed heaters, injection of air and/or inert gas into
the formation will reduce the heat losses.
The injection of hot air or inert gas can also be carried out by
heating air and/or inert gas at the surface.
High-frequency dipole heating is another procedure which can be
used in the field. In this case, the formation is heated by high
frequency energy transmitted through an antenna located in the
wellbore. This heating procedure is suitable only for openhole
vertical or horizontal wells. It can also be applied to a newly
drilled well before casing is placed into the formation of
interest. In this case, it is not required to inject air and/or
inert gas into the wellbore to carry the heat into the formation.
However, the injection of air and/or inert gas into the formation
during heating will prevent heat front propagation towards the
antenna and also can mobilize the clay minerals and be beneficial.
The high-frequency dipole heating is rapid and propagates into a
large area.
By the application of either of the above-mentioned procedures for
several hours, depending on the injectivity of the formation and
the desired degree of treatment, the permeability of the
near-wellbore formation can be increased significantly. The
injected heat completely or partially dehydrates the clay-bound
water, evaporates the blocked water and/or fluid and destroys the
clay structures, thus leaving no possibility of rehydration when
the formation is resaturated with formation water.
The invention will now be disclosed, by way of example, with
reference to the following two examples:
EXAMPLE 1
Small core plugs, measuring 3.98 centimeters in length and 3.75
centimeters in diameter, were obtained from full-diameter cores,
taken from the gas-bearing formation, in a conventional manner. The
average porosity was estimated to be 12% and the initial absolute
permeability (i.e., at zero connate-water saturation) to air was
17.85 millidarcies (md). This permeability was considered to be the
base permeability.
The petrographic studies indicated that the sandstone formation
under consideration was of poor quality due to the presence of
swelling clays and glauconitic peloids. The formation contained 78%
quartz, 9% clays, and 13% glauconite materials. The major
components of clay are 58% illite, 38% mixed layer (i.e.,
illite-smectite), and 4% kaolinite.
The core sample was saturated with produced formation water. The
post-brine-desaturation permeability of 5.19 md reflects a 70%
decrease in air permeability when a residual-brine phase remains in
the core. The core was then saturated with KCd/Polymer mud
filtrate. A nitrogen flood was performed to reduce the mud-filtrate
saturation, thereby establishing an irreducible mud-filtrate
saturation level. At this point a post-mud-filtrate permeability of
2.86 md was measured which indicated an 84% reduction from the
initial air permeability.
The core under consideration was subjected to a sequential heat
treatment at temperatures ranging from 200.degree. C. to
800.degree. C. During heating, the core was placed into a reactor
and heated in a high-temperature oven. A constant pressure of 2,413
kPa was maintained inside the reactor using a regulated nitrogen
source and a back-pressure regulator. The heating was maintained
for 4 to 6 hours after the desired temperature was reached in the
core sample. The permeability of the treated core was measured
after cooling the core sample to atmospheric temperature.
The heat treatment of the core under consideration at 200.degree.
C. yielded an increased air permeability to 56% below the base
permeability. The increase in permeability is most likely
attributable to the partial evaporation of the residual
mud-filtrate phase. Total evaporation of the mud filtrate during
the 200.degree. C. heat treatment did not occur because the
internal reactor pressure was maintained at 2,413 kPa, which is
above the saturation pressure at this temperature. It was also
observed from mass measurements that the total fluid in the core
was not evaporated. From the gas analyses conducted at 200.degree.
C., hydrocarbon evolution from the core is evident as well as
possible degradation of carbon-based minerals.
The second heat treatment at 400.degree. C. revealed a further
permeability increase to 11.9% below the base permeability. The
mass measurements indicated that the residual fluid was completely
evaporated. The reduction of residual hydrocarbons, a more
extensive decrease in hydration water and a partial degradation of
carbonaceous minerals increased the permeability significantly.
The third heat treatment at 600.degree. C. yielded a 51% increase
in air permeability above the base permeability. Further decrease
in sample mass indicated that the heating at 600.degree. C. has had
a significant effect on the mineral structures. The petrographic
studies revealed that the permeability reducing minerals have
broken down, resulting in a significant permeability increase. The
petrographic studies also revealed that the heating at 600.degree.
C. improved the core porosity from 12% to 15%.
A dramatic permeability increase of 764% occurred during the fourth
heat treatment at 800.degree. C. An additional decrease in sample
mass was also observed. The petrographic studies suggested that the
swelling-clay and shale structures were completely destroyed during
this heating phase. Even after the rehydration of the test core
with formation water (after heating at 800.degree. C.), the
permeability was maintained at 622% above the base
permeability.
EXAMPLE 2
Heating tests were also carried out on cores taken from the
oil-bearing formation. The average porosity of the formation was
estimated to be 15% and the air permeability is on the order of 25
md and the oil phase permeability was 0.9 md at 100% oil
saturation. The petrographic studies on cores indicated that the
formation was a moderately sorted, fine-grained, quartzose
sublitharenite with good porosity and moderate permeability. The
XRD analysis indicated that quartz material dominated the
mineralogy (85%). The total clay content was about 15%. Kaolinite
dominated the clay mineralogoy (86%) and illite constituted the
remaining 14%. Smectite and mixed-layer illite-smectite clays were
not found in the XRD analysis. The reservoir had modified
intergranular porosity of about 8% and a supplemental grain moldic
porosity of about 3%. The sandstone formation appeared to be
sensitive to water and to conditions that could induce fines
migration.
The core sample was sequentially exposed to brine, mud filtrate,
heat and brine. In these tests, one temperature (800.degree. C.)
was used to evaluate the effect of heat on oil-saturated core
permeability. It was anticipated that the exposure of an
oil-saturated core to heat would result in coking of the oil and
eventual reduction in permeability. The experimental setup was
modified to flush nitrogen through the core. This way the oil is
pushed out of the core as the core is exposed to heat. During the
experiment, no liquid phase was seen at the outlet end of the core.
In this experiment, the reactor was maintained under 16,500 kPa
confining pressure (reservoir pressure). In a field situation, the
injection of hot nitrogen would push the near-wellbore fluid far
into the reservoir and expedite the heating around the
well-bore.
The results indicated that mud filtrate caused substantial (38%)
reduction in oil-phase permeability, likely due to a combination of
phase trapping and clay deflocculation. However, the
high-temperature (800.degree. C.) exposure for four hours increased
the oil permeability by about 1000% over the original permeability.
Even after rehydration with conate water, permeability was still
748% greater than the initial "undamaged" baseline
permeability.
The results of the petrographic studies indicated that most of the
kaolinite was destroyed with only a few kaolinite pseudomorphs
remaining. SEM studies suggest that the hydrocarbon was not been
coked to insoluble carbon. The increase in permeability is mostly
due to the destruction of the kaolinite minerals and to the
subsequent transport of the degraded clay with hydrocarbon through
the pore system.
* * * * *