U.S. patent application number 13/642557 was filed with the patent office on 2013-03-07 for heterogeneous proppant placement.
This patent application is currently assigned to Schlumberger Technology Corporation. The applicant listed for this patent is Evgeny Borisovich Barmatov, Trevor Lloyd Hughes, Sergey Mikhailovich Makarychev-Mikhailov, Anatoly Vladimirovich Medvedev. Invention is credited to Evgeny Borisovich Barmatov, Trevor Lloyd Hughes, Sergey Mikhailovich Makarychev-Mikhailov, Anatoly Vladimirovich Medvedev.
Application Number | 20130056213 13/642557 |
Document ID | / |
Family ID | 44861747 |
Filed Date | 2013-03-07 |
United States Patent
Application |
20130056213 |
Kind Code |
A1 |
Medvedev; Anatoly Vladimirovich ;
et al. |
March 7, 2013 |
Heterogeneous Proppant Placement
Abstract
A method is given for inducing heterogeneous proppant placement
in a hydraulic fracture in a subterranean formation by causing
proppant aggregation through a gel phase transition or chemical
transformation in the proppant carrier fluid. Proppant aggregation
may be induced by causing or allowing syneresis of the polymer gel
that viscosifies the fluid; formation of a polyelectrolyte complex
from cationic and anionic polymers included in or created in, the
fluid; and by increasing the temperature of the fluid above the
critical solution temperature of a polymer in the fluid. The
proppant carrier fluid may be formulated such that these
transformations occur naturally during or after proppant injection,
and the transformations may be chemically triggered or delayed.
Inventors: |
Medvedev; Anatoly
Vladimirovich; (Moscow, RU) ; Makarychev-Mikhailov;
Sergey Mikhailovich; (St. Petersburg, RU) ; Barmatov;
Evgeny Borisovich; (Cambridge, GB) ; Hughes; Trevor
Lloyd; (Cambridge, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Medvedev; Anatoly Vladimirovich
Makarychev-Mikhailov; Sergey Mikhailovich
Barmatov; Evgeny Borisovich
Hughes; Trevor Lloyd |
Moscow
St. Petersburg
Cambridge
Cambridge |
|
RU
RU
GB
GB |
|
|
Assignee: |
Schlumberger Technology
Corporation
|
Family ID: |
44861747 |
Appl. No.: |
13/642557 |
Filed: |
April 27, 2010 |
PCT Filed: |
April 27, 2010 |
PCT NO: |
PCT/RU10/00207 |
371 Date: |
November 15, 2012 |
Current U.S.
Class: |
166/308.5 |
Current CPC
Class: |
C09K 8/685 20130101;
C09K 2208/08 20130101; E21B 43/267 20130101 |
Class at
Publication: |
166/308.5 |
International
Class: |
E21B 43/267 20060101
E21B043/267 |
Claims
1. A method of inducing proppant aggregation in a hydraulic
fracture comprising formulating a proppant carrier fluid
viscosified by a first polymer gel that can undergo syneresis;
injecting a slurry of the fluid and proppant; and triggering gel
syneresis.
2. The method of claim 1 wherein the fluid further comprises
fibers.
3. The method of claim 1 wherein at least a portion of the proppant
is resin coated.
4. The method of claim 1 wherein the polymer gel is
crosslinked.
5. The method of claim 1 wherein the gel is a borate crosslinked
polymer gel and the syneresis is triggered by incorporation of a
multivalent cation in the gel.
6. The method of claim 5 wherein the multivalent cation is a cation
of a metal selected from the group consisting of Ca, Zn, Al, Fe,
Cu, Co, Cr, Ni, Ti, Zr and mixtures thereof.
7. The method of claim 5 wherein the cation is incorporated by the
dissolution of a salt, oxide or hydroxide of the cation.
8. The method of claim 5 wherein the cation is in the form of a
hydroxide when it causes the syneresis.
9. The method of claim 1 wherein the syneresis is caused by
overcrosslinking
10. The method of claim 9 wherein the overcrosslinking is delayed
by a crosslink delay agent.
11. The method of claim 9 wherein the overcrosslinking is induced
by an encapsulated crosslinker, a slowly dissolvable crosslinker,
or a temperature-activated crosslinker.
12. The method of claim 1 wherein the syneresis is caused by
including in the fluid, in addition to the polymer in the first
polymer gel, a second polymer and a delayed crosslinker for the
second polymer.
13. The method of claim 12 wherein the second polymer is at a
concentration below its overlap concentration.
14. The method of claim 1 wherein the syneresis is caused by a
superabsorbent polymer.
15. The method of claim 1 wherein the triggering is caused by a
second fluid that contacts the proppant carrier fluid downhole.
16. A method of inducing proppant aggregation in a hydraulic
fracture comprising (1) formulating a proppant carrier fluid
comprising (i) at least one anionic polyelectrolyte or the
precursor to at least one anionic polyelectrolyte, and (ii) at
least one cationic polyelectrolyte or the precursor to at least one
cationic polyelectrolyte; (2) injecting a slurry of the fluid and
proppant; and (3) triggering formation of a polyelectrolyte
complex.
17. The method of claim 16 wherein the fluid further comprises
fibers.
18. The method of claim 16 wherein at least a portion of the
proppant is resin coated.
19. The method of claim 16 wherein the formation of the
polyelectrolyte complex is induced by a pH change.
20. The method of claim 16 wherein the formation of the
polyelectrolyte complex is induced by conversion of at least one
polyelectrolyte precursor to a polyelectrolyte.
21. The method of claim 16 wherein the formation of the
polyelectrolyte complex is induced by formation of a cationic
polyelectrolyte downhole.
22. The method of claim 21 wherein the cationic polyelectrolyte is
formed downhole by a method selected from Mannich reaction, Hofmann
degradation of a polyacrylamide.
23. The method of claim 16 wherein the formation of the
polyelectrolyte complex is induced by formation of an anionic
polyelectrolyte downhole.
24. The method of claim 23 wherein the anionic polyelectrolyte is
formed downhole by hydrolysis.
25. The method of claim 16 wherein at least one polyelectrolyte or
polyelectrolyte precursor is initially present in the fluid in the
internal phase of an emulsion.
26. The method of claim 16 wherein at least one polyelectrolyte or
polyelectrolyte precursor is initially present in solid form.
27. The method of claim 16 wherein the formation of the
polyelectrolyte complex is delayed by incorporating at least one
polyelectrolyte in the fluid as a polyelectrolyte-surfactant
complex.
28. The method of claim 16 wherein the triggering is caused by a
second fluid that contacts the proppant carrier fluid downhole.
29. A method of inducing proppant aggregation in a hydraulic
fracture comprising (1) formulating a proppant carrier fluid
comprising a polymer below its lower critical solution temperature;
and (2) injecting a slurry of the fluid and proppant into a
subterranean formation that is above the lower polymer critical
solution temperature.
30. The method of claim 19 wherein the fluid further comprises
fibers.
31. The method of claim 29 wherein at least a portion of the
proppant is resin coated.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to reservoir stimulation by
hydraulic fracturing. More particularly it relates to methods of
heterogeneous proppant placement (HPP) in fractures, which
increases their conductivity and enhances fluid production. The HPP
is achieved by formation of proppant clusters in situ in the
fracture due to polymer gel phase transitions or polymer gel
chemical transformations.
[0002] There is a need for a method of inducing heterogeneous
proppant placement in subterranean formation hydraulic fractures
that does not require large changes in injected slurry proppant
concentration or viscosity.
SUMMARY OF THE INVENTION
[0003] One embodiment of the invention is a method of inducing
proppant aggregation in a hydraulic fracture including the steps of
(1) formulating a proppant carrier fluid viscosified by a first
polymer gel that can undergo syneresis; (2) injecting a slurry of
the fluid and proppant; and (3) triggering gel syneresis. The fluid
may contain fibers and at least a portion of the proppant may be
resin coated.
[0004] In one version of this embodiment, the polymer gel is
crosslinked, for example the gel is a borate crosslinked polymer
gel, and the syneresis is triggered by incorporation of a
multivalent cation in the gel. The multivalent cation is a cation
of a metal selected for example from Ca, Zn, Al, Fe, Cu, Co, Cr,
Ni, Ti, Zr and mixtures of these. The cation is incorporated for
example by dissolution or by slow dissolution, for example of a
salt, an oxide or a hydroxide of the cation. The cation may
optionally be in the form of a hydroxide or an in situ formed
hydroxide when it causes the syneresis.
[0005] In another version of this embodiment, the syneresis is
caused by overcrosslinking. The overcrosslinking may be delayed for
example by a crosslink delay agent, or may be induced for example
by an encapsulated crosslinker, a slowly dissolvable crosslinker,
or a temperature-activated crosslinker.
[0006] In yet another version of this embodiment, the syneresis is
caused by including in the fluid, in addition to the polymer in the
first polymer gel, a second polymer and a delayed crosslinker for
the second polymer. The second polymer is optionally at a
concentration below its overlap concentration.
[0007] In further versions of this embodiment, the syneresis is
caused by a superabsorbent polymer or is triggered by a second
fluid that contacts the proppant carrier fluid downhole.
[0008] Another embodiment of the method of inducing proppant
aggregation in a hydraulic fracture includes the steps of (1)
formulating a proppant carrier fluid containing (i) at least one
anionic polyelectrolyte or the precursor to at least one anionic
polyelectrolyte, and (ii) at least one cationic polyelectrolyte or
the precursor to at least one cationic polyelectrolyte; (2)
injecting a slurry of the fluid and proppant; and (3) triggering
formation of a polyelectrolyte complex. The fluid may optionally
contain fibers, and at least a portion of the proppant may be resin
coated.
[0009] In various versions of this embodiment, the formation of the
polyelectrolyte complex is induced by a pH change; the formation of
the polyelectrolyte complex is induced by conversion of at least
one polyelectrolyte precursor to a polyelectrolyte; the formation
of the polyelectrolyte complex is induced by formation of a
cationic polyelectrolyte downhole; the cationic polyelectrolyte is
formed downhole by a Mannich reaction or a Hofmann degradation of a
polyacrylamide; the formation of the polyelectrolyte complex is
induced by formation of an anionic polyelectrolyte downhole; the
anionic polyelectrolyte is formed downhole by hydrolysis; at least
one polyelectrolyte or polyelectrolyte precursor is initially
present in the fluid in the internal phase of an emulsion; at least
one polyelectrolyte or polyelectrolyte precursor is initially
present in solid form; the formation of the polyelectrolyte complex
is delayed by incorporating at least one polyelectrolyte in the
fluid as a polyelectrolyte-surfactant complex; and the triggering
is caused by a second fluid that contacts the proppant carrier
fluid down hole.
[0010] Yet another embodiment is a method of inducing proppant
aggregation in a hydraulic fracture including the steps of (1)
formulating a proppant carrier fluid containing a polymer below its
lower critical solution temperature; and (2) injecting a slurry of
the fluid and proppant into a subterranean formation that is above
the lower polymer critical solution temperature. The fluid may
optionally contain fibers, and at least a portion of the proppant
may be resin coated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 shows the dependency of syneresis vs. time for borate
crosslinked guar gel samples having different concentrations of
Ca(OH).sub.2 at room temperature.
[0012] FIG. 2 shows the syneresis of borate crosslinked guar gels
in samples having different concentrations of Mg(OH).sub.2 at room
temperature.
[0013] FIG. 3 shows the syneresis of the borate crosslinked gel
samples having varying concentrations of AlCl.sub.3.6H.sub.2O.
DETAILED DESCRIPTION OF THE INVENTION
[0014] Although the following discussion emphasizes fracturing, the
polymer gel phase transitions and polymer gel chemical
transformations of the invention may be used in fracturing, gravel
packing, and combined fracturing and gravel packing in a single
operation. The invention will be described in terms of treatment of
vertical wells, but is equally applicable to wells of any
orientation. The invention will be described for hydrocarbon
production wells, but it is to be understood that the invention may
be used for wells for production of other fluids, such as water or
carbon dioxide, or, for example, for injection or storage wells. It
should also be understood that throughout this specification, when
a concentration or amount range is described as being useful, or
suitable, or the like, it is intended that any and every
concentration or amount within the range, including the end points,
is to be considered as having been stated. Furthermore, each
numerical value should be read once as modified by the term "about"
(unless already expressly so modified) and then read again as not
to be so modified unless otherwise stated in context. For example,
"a range of from 1 to 10" is to be read as indicating each and
every possible number along the continuum between about 1 and about
10. In other words, when a certain range is expressed, even if only
a few specific data points are explicitly identified or referred to
within the range, or even when no data points are referred to
within the range, it is to be understood that the inventors
appreciate and understand that any and all data points within the
range are to be considered to have been specified, and that the
inventors have possession of the entire range and all points within
the range.
[0015] While hydraulic fracturing is currently one of the most
important and widely used methods of reservoir stimulation, it is
still not free from serious fundamental limitations, which can
restrict hydrocarbon production. Conventional stimulation jobs
involve pumping a viscosified fluid downhole at a rate and pressure
sufficient to fracture the formation; the resulting fracture is
filled with a proppant material usually delivered into the fracture
with the same fluid. The proppant is intended to prevent fracture
closure and usually is sand or a ceramic. The packed proppant bed
provides hydraulic conductivity orders of magnitude higher than
that of the formation, thus, allowing enhanced fluid flow towards a
wellbore. However, despite significant efforts focused on the
development of new proppant materials with optimized properties
(high crush resistance, low density and cost), the achievable
permeability (conductivity) of conventionally propped fractures can
still be a limiting factor for fluid production.
[0016] Heterogeneous proppant placement (HPP), for example
placement of proppant in a fracture as consolidated clusters (for
example pillars) thus creating open channels in the fracture, can
drastically improve fracture conductivity above the limits of
conventional proppant packs. In contrast to an approach in which
proppant placement mainly relies on a special pumping schedule, the
present invention encompasses a family of HPP methods in which
proppant clusters, i.e. agglomerates or aggregates, are generated
in situ in the fracture, and cluster formation timing and location
are controlled by chemical means through polymer gel phase
transitions or chemical transformations.
[0017] In one embodiment of the invention a polymer gel used as the
viscosifier of a fracturing fluid is deliberately subjected to
syneresis. This process is usually considered highly undesirable,
as it drastically affects rheological properties of the fracturing
fluid, and special efforts are often undertaken to avoid or
diminish it. However, if properly controlled, syneresis with
expulsion of water from the gel can lead to proppant particle
aggregation. The resulting polymer clots entrap and retain proppant
inside them; the distance between particulates in the clots is
significantly smaller than in the original homogeneous slurry. The
proppant aggregates (clusters) keeping the fracture from closure
provide channels in between them and, thus, significantly enhanced
fracture conductivity.
[0018] In another embodiment of the invention the polymer clots are
formed due to interactions between two different polymers,
triggered chemically. An example is the formation of complexes
between two oppositely charged polyelectrolytes. The complex
formation is accompanied by aggregation of proppant particulates
and consequently HPP. In yet another embodiment of the invention,
the proppant aggregation into clusters takes place due to a phase
transition in a polymer solution. A polymer solution with a low
critical solution temperature (LCST) undergoes phase separation at
bottomhole temperature and the resulting polymer precipitate
consolidates proppant particles.
[0019] The proppant aggregates formed by the method of the
invention can further be reinforced by resin curing, with fibers,
or by other means known in the art.
[0020] The present invention discloses a method of heterogeneous
proppant cluster formation by utilizing gel phase transitions and
chemical transformations that lead to proppant aggregation.
[0021] Formation of heterogeneous proppant structures by the method
of the invention may be controlled by syneresis of the fracturing
gel. Syneresis is defined herein as a process of water expulsion
from a gel. Syneresis leads to a phase separation in the gel and to
formation of a water phase caused by the collapse of the gel. When
the gel contains proppant particles, the syneresis leads to
proppant aggregation, which generally depends upon the degree of
gel shrinkage. In the present invention the syneresis can be
controlled by various means.
[0022] One preferred method of causing and controlling syneresis is
the use of borate-crosslinked polymer gels and multivalent cations.
It is believed that this works with Ti and Zr-crosslinked gels as
well. For example, the addition of calcium hydroxide to a
borate-crosslinked gel causes syneresis. For example, calcium
chloride, borate, and polymer are mixed at the surface. A
hydroxide, or a delayed source of hydroxide such as magnesium oxide
to generate the calcium hydroxide in situ, is added. Syneresis
occurs after sufficient multivalent cation hydroxide is present.
Generating the multivalent cation hydroxide in situ is preferred.
The more calcium ion present, the greater and faster the syneresis.
Other multivalent cations may be used, for example Zn, Al, Mg, Fe,
Cu, Cr, Co, Ti, Zr, and/or Ni. The level of syneresis depends not
only on the multivalent ion concentration but also on the borate
crosslinking density. It should be noted that an inexpensive and/or
unmodified guar may be used because the critical function may not
be to provide viscosity and because impurities are not a problem if
they end up in the agglomerated proppant.
[0023] Yet one more method of causing and controlling syneresis is
to use gel overcrosslinking. It is well known in the industry that
high crosslinker concentration can lead to an increased density of
crosslinked sites and finally to gel collapse, which is why special
precautions are often undertaken to avoid overcrosslinking.
However, in the present invention, controlled syneresis is promoted
by the use of at least one crosslinker and/or at least one
crosslinking delaying mechanism. Having more than one crosslinker
and/or delaying mechanism allows initial pumping of a slurry having
a conventional viscosity with the required degree of crosslinking,
ensuring good proppant transport deep into a fracture. The gel
overcrosslinking takes place in the fracture and is induced by
either a crosslinking system different from the system active
during initial pumping or by additional delaying mechanism, or by
both. Crosslink delay agents, which are known to those skilled in
the art; examples include polyols, encapsulated crosslinkers,
slowly dissolvable crosslinkers, and pH controlled and/or
temperature-activated crosslinkers. Slowly dissolvable crosslinkers
can be used in a pure form or can be deposited/impregnated
onto/into proppant particles. Various crosslinking systems can be
used according to the present invention, based on boron, any
metal-based crosslinker systems known from the art (such as
zirconium, chromium, iron, boron, aluminum, and titanium), and also
based on organic compounds (such as aldehydes, dialdehydes,
phenolic-aldehyde compositions, multifunctional amines and imines).
In all cases, a slow crosslinker concentration increase in the gel
leads to controlled gel overcrosslinking and syneresis. The size of
the resulting gel aggregates (clots) is controlled by shear
history, gel composition and environment conditions.
[0024] Another method of syneresis control is the use of selective
crosslinking. In a mixture of crosslinkable and non-crosslinkable
polymers, in which the non-crosslinkable polymer is a viscosifier,
and in which crosslinkable polymer is present at a concentration
below its overlap concentration, when the crosslinkable polymer
crosslinks it forms what are commonly known as "microgels" (i.e.
gel pieces which cannot overlap to fill space). These would form
inside a viscous matrix of the non-crosslinkable polymer.
Optionally, a mixture of two polymers and two crosslinkers may be
used, in which each crosslinker crosslinks only one of the
polymers. One polymer/crosslinker combination viscosifies the fluid
and the other polymer/crosslinker combination forms a microgel.
Optionally the viscosifying polymer may be crosslinked for normal
fracturing purposes and later the microgels could be formed to
induce proppant heterogeneity. Examples include (1) matrix
polymer=guar/borate+microgel polymer=xanthan/Cr.sup.3+ (delayed) or
(2) matrix polymer=guar/borate+microgel
polymer=polyacrylamide/Cr.sup.3+ (delayed) or polyacrylamide
formaldehyde (delayed for example using
hexamethylenetetramine).
[0025] Another method of syneresis control is the use of polymer
mixtures. Such a mixture may include similar-type polymers (for
example different polysaccharides such as non-derivatized guar and
carboxymethyl hydroxyethyl guar) or different-type polymers (for
example polysaccharides and polyacrylamides). The crosslinking
systems may be, as examples, any of those mentioned above.
Differing affinities to the crosslinker of different polymers lead
to formation of gel volumes having different viscosities. The size
and distribution of the volumes can be controlled by solution
composition, mixing efficiency, and polymer properties.
[0026] Yet another method of gel syneresis control is utilization
of superabsorbent polymers (SAPs) to cause water extraction from a
crosslinked gel. The molecular weight and chemical properties of
SAPs can be adjusted in such a way as to cause osmotic pressure to
move water from the gel phase into the SAP phase. Loss of water by
the gel leads to proppant particle aggregation. Superabsorbent
polymers may be added to a slurry in a dry state or in partially
swollen state. The degree of swelling and the choice of the solvent
used with the SAP can be used for control of competitive swelling
of the gel and the SAP. Furthermore, the absorbance of water by a
superabsorbent can be triggered by pH, solution/gel ionic strength,
temperature and by other factors. SAP molecules can be either
crosslinked or not.
[0027] Yet another method of controlling syneresis is the addition
of fibers, for example polylactic acid fibers, to any of the
systems mentioned above. Fibers do not affect the degree of
syneresis but they do control the volume occupied by the shrunken
gel. The more fibers used, the greater the final volume of the gel
phase at the same degree of syneresis. In addition, the presence of
fibers greatly changes the mechanical properties of the gel
phase.
[0028] Polyelectrolyte solutions are widely used in various
oilfield technologies, usually providing a unique combination of
properties. Carboxymethylated guars and celluloses (such as
carboxymethyl guar, (CMG), carboxymethyl hydroxypropyl guar
(CMHPG), carboxymethyl cellulose (CMC), polyanionic cellulose
(PAC), carboxymethyl hydroxyethyl cellulose (CMHEC), etc.) are the
most common such polymers used in drilling fluids and fracturing
gels. These derivatized polysaccharides have polar carboxylic
groups, making the polymers more water soluble, chemically
resistant and crosslinkable with metals. Many natural and
semi-synthetic polymers are also polyanions, such as xanthan,
carrageenan, lignosulfonate, etc. Among purely synthetic
polyanions, the polymers based on polyacrylic acid (PA) and
polyacrylamide (PAM) are very important. They are utilized as
flocculants, dewatering agents, and friction reducers and have many
other applications. The PAMs contain anionic groups due either to
intrinsic hydrolysis of acrylamide to acrylic acid, or due to
deliberately incorporated sulfonic groups (e.g.
acrylamido-2-methyl-1-propane sulfonic acid, (AMPS). Complexes of
guar with borate ion show polyanionic properties in basic
environments.
[0029] Polycations are used less often in oilfield technologies, as
they are usually more expensive than their anionic counterparts.
Examples of the most common polycations include different
polyacrylamide copolymers with diallyldimethylammonium chloride
(DADMAC), acryloyloxyethyltrimethylammonium chloride (AETAC) and
other quaternary ammonium monomers, polyvinyl pyrrolidone (PVP),
polyethyleneimine (PEI) and natural polymers, such as chitosan,
gelatin (and other polypeptides), and poly-L-lysine.
[0030] The interaction of polyelectrolytes of opposite charge in
solution results in aggregation and formation of a polyelectrolyte
complex (PEC). Upon PEC formation, small counterions localized near
charged groups of free polyelectrolytes are released, resulting in
a gain in entropy, which is considered to be the main driving force
of the interaction as shown below. Long chain polyanions and
polycations, each with their small organic or inorganic
counterions, form complexes of the polymers in which they serve as
one another's' counterions and the original small counterions are
no longer included in the complexes. Other effects may also
contribute, including formation of interpolymer hydrogen bonds,
hydrophobic interactions, etc.
[0031] Many PEC structures are available. One is based on the
formation of nearly stoichiometric complexes between
polyelectrolytes of similar molecular weights; this is usually
called a "ladder"-type complex, in which oppositely charged
polymeric chains are aligned and linked ionically (as shown in
route A, FIG. 4). Water-soluble, ordered, non-stoichiometric
complexes with the ladder-type structure are also known. In more
disordered PECs, the structure of which has been referred to as
"scrambled egg"-type, the polymer chains coil, forming a structure
with statistical charge compensation (as shown in route B, FIG. 4).
Such complexes often have highly non-stoichiometric ratios of
polyelectrolytes and are usually characterized by very low
solubility. Utilization of these complexes is one of the
embodiments of the present invention.
[0032] Formation of PECs with the scrambled egg structure allows
entrapment of proppant particulates in the clots. It should be
mentioned that the aggregation forces holding the particles in
clusters are much stronger than in the case of flocculation.
Flocculation is widely used in water treatment; particles subjected
to flocculation have sizes generally not exceeding about 150
microns (100 US mesh). Organic flocculants, which usually include
water-soluble polymers, provide molecular interlinks between the
particles, so the resulting flocs are held by coiled, yet linear,
polymer chains. In contrast, PEC clots represent highly crosslinked
3D networks of polymer chains, which may, additionally, as in the
case of flocculants, have an affinity to the surfaces of entrapped
particles due to electrostatic, van der Waals, hydrogen bonding and
other forces.
[0033] The formation of PECs can be controlled in a variety of
ways. pH delaying agents, known to those skilled in art, can be
used to adjust the pH of a fracturing fluid and initiate PEC
formation in a fracture. In a non limiting example, the fracturing
slurry, in addition to proppant and other additives, is made from
two polyacrylamide copolymers, the first of which is made with
acrylic acid as one monomer and the second of which is made with
DADMAC as one monomer. When the slurry pH is kept below about 4.0,
most of carboxylic groups of PAM-PA (polymer of acrylamide and
acrylic acid) exist in a non-dissociated (protonated) form and the
PAM-PA polymer does not exhibit any polyelectrolyte properties.
Once the slurry pH is raised above about 5.0, carboxylic acid
groups start dissociating and the resulting PAM-PA polyelectrolyte
undergoes complexation with the PAM-DADMAC, forming low soluble PEC
clots with entrapped proppant particles.
[0034] Another method of controlling PEC formation is in situ
synthesis of one polyelectrolyte downhole. In a non-limiting
example, the Mannich aminomethylation or Hofmann degradation
reactions of polyacrylamide polymer are used to produce
polycationic species from initially neutral PAM. Both reactions
proceed in aqueous solutions at temperatures above about 50.degree.
C. In the Mannich reaction, a PAM is treated with formaldehyde and
an amine which results in formation of Mannich base groups
(--NH--CH.sub.2--NR.sub.2), which are positively charged even in
solutions with relatively high pH values; the product is a
polycation. Secondary amines, for example diethyl and dipropylamine
are preferred, but ammonia and primary amines may also be used.
Formaldehyde can be obtained downhole from a precursor (for example
urotropin (hexamethylenetetramine)), so no toxic substances are
needed at a wellsite. Another method of generating a
polyelectrolyte downhole is the Hofmann degradation reaction, in
which a PAM is treated with hypohalogenites in alkaline solution,
which leads to polyvinylamine, a cationic polyelectrolyte. Details
of chemical transformations of PAMs under downhole conditions can
be found in co-filed Patent Application "Subterranean Reservoir
Treatment Method" invented by Makarychev-Mikhailov, and
Khlestkin.
[0035] Yet another method of controlling (delaying) PEC formation
is the utilization of any type of emulsion (oil-in-water,
water-in-oil, water-in-water) to transport at least one
polyelectrolyte downhole. In a non-limiting example, a fracturing
slurry, in addition to proppant and other additives, contains
emulsion droplets, stable at ambient conditions, which confine a
polyelectrolyte, which therefore is non-reactive towards its
oppositely charged counterpart, also present in the slurry. The
emulsion breaks either under downhole conditions (at elevated
temperature) or by means of a delayed emulsion breaker, releasing
the polyelectrolyte, which immediately participates in a PEC
formation reaction.
[0036] Yet another method of utilizing PEC's is to add one of the
polymers or polymer precursors in solid form.
[0037] Any other methods of controlled (delayed) PEC formation may
be used, for example based on temporary protection of the charged
groups of at least one of the polyelectrolytes by means of chemical
protection groups or surfactants (by using
polyelectrolyte-surfactant complexes).
[0038] Other non-limiting examples of pH triggering that may be
used to initiate PEC formation include: [0039] 1. Use of a mixture
containing polyethylene imine (which is non-ionic at alkaline pH)
plus sulphonated polymer (in which the anionic charge persists at
high, neutral and low pH); no PEC will be formed until the pH is
changed from alkaline conditions to acidic conditions (whereupon
the PEI becomes positively charged). Such a pH change could be
triggered by controlled hydrolysis of polylactic acid and/or
polyglycolic acid (PLA/PGA) particles. [0040] 2. Use of a mixture
of chitosan (which is insoluble at alkaline pH) plus sulphonated
polymer (in which the anionic charge persists at high, neutral and
low pH); no PEC will be formed until the pH is changed from
alkaline conditions to acidic conditions (wherein chitosan is
dissolved as a cationic polymer). Again, such a pH change could be
triggered by controlled hydrolysis of polylactic acid and/or
polyglycolic acid (PLA/PGA) particles. [0041] 3. Use of a mixture
of polyDADMAC (in which the cationic charge persists at high,
neutral and low pH) plus a carboxylate polymer (which is anionic at
high pH, but non-ionic at pH near and below the pKa). No PEC is
formed under acidic conditions; the pH is raised to induce PEC
formation.
[0042] Triggers other than PEC polymer complexes will lead to
similar results. In addition to electrostatic interactions, other
forces may be used as a driving force for polymer complex
formation. As a non limiting example, complexes based on hydrogen
bonding provide a function similar to that of PECs described above.
In a wider sense, in the discussion above, instead of PECs any
complex may be used which involves at least one polyelectrolyte.
Such a polyelectrolyte can be complexed with a variety of
compounds, such as non-ionic polymers, surfactants, and inorganic
species (for example, metal ions).
Polymers with Low Critical Solution Temperature (LCST)
[0043] Stimulus-responsive polymers are a wide class of modern
functionalized materials. They are able to perceive small changes
in external signals, such as pH, temperature,
electric/magnetic/mechanical field, or light, and produce
corresponding changes or transformation of the physical structure
and chemical properties of a polymer solution or gel. Much
attention has been paid to chemical design and investigation of
thermally sensitive or thermo-responsive polymers. In particular,
they exhibit sensitive responses in their structure, properties,
and configuration to changes in temperature. Aqueous solutions of
certain polymers undergo fast, reversible changes around their
lower critical solution temperature (LCST). Below the LCST, the
free polymer chains are soluble in water and exist in an extended
conformation that is fully hydrated. On the contrary, above the
LCST, the polymer chains become more hydrophobic, resulting in the
assembly of a phase-separated state. Thermo-responsive polymers
have a variety of applications, such as temperature or pH-sensitive
materials for drug delivery applications, biosensors, thermally
responsive coatings, catalysis, soluble polymeric ligands for heavy
metal scavenging, size selective separation and as
water-dispersible hydrophobic thickening agents in the oilfield
industry.
[0044] The solubility of most polymers increases with increasing
temperature, but certain LCST polymers have inverse temperature
dependent solubilities. Polymers bearing amide groups form the
largest group of thermo-sensitive polymers. Among them,
poly(N-isopropylacrylamide) (PNIPAM) and
poly(N,N'-diethylacrylamide) (PDEAAM) are most well known. They
have similar LCSTs of 32-33.degree. C. Poly(ethylene oxide) (PEO)
is one of the most-studied biocompatible polymers that exhibit LCST
behavior. The LCST transition of PEO aqueous solutions occurs at
temperatures ranging from about 100.degree. C. to about 150.degree.
C., depending upon the molecular weight. This temperature range
extends PEO applications for temperature-sensitive purposes. The
properties of a polymer solution, such as the phase transition
temperature, depend on the chemical composition and the molecular
weight of the polymer and on environmental conditions such as fluid
pH and ionic composition and concentration.
[0045] Examples of polymers having low critical solution
temperatures includes, but is not limited to, ethylene/vinyl
alcohol copolymers; ethylene oxide/propylene oxide copolymers;
copolymers of N,N-dimethylacrylamide with methyl acrylate, ethyl
acrylate, propyl acrylate, butyl acrylate, 2-ethoxyethyl acrylate,
and/or 2-methoxyethyl acrylate; hydroxypropyl cellulose;
N-isopropylacrylamide/acrylamide copolymers; copolymers of
N-isopropylacrylamide with 1-deoxy-1-methacrylamido-D-glucitol;
N-isopropylmethacrylamide; methylcellulose (having various
concentrations of methyl substitution);
methylcellulose/hydroxypropylcellulose copolymers; polyphosphazene
polymers, including poly[bis(2,3-dimethoxypropanoxy)phosphazene],
poly[bis(2-(2'-methoxyethoxy)ethoxy)phosphazene],
poly[bis(2,3-bis(2-methoxyethoxy)propanoxy)phosphazene],
poly[bis(2,3-bis(2-(2'-methoxyethoxy)ethoxy)propanoxy)phosphazene],
and
poly[bis(2,3-bis(2-(2'-(2''-dimethoxyethoxy)ethoxy)ethoxy)propanoxy)phosp-
hazene]; poly(ethylene glycol); poly(ethylene
oxide)-b-poly[bis(methoxyethoxyethoxy)-phosphazene] block
copolymers; poly(ethylene
oxide)-b-poly(propyleneoxide)-b-poly(ethylene oxide) triblock
copolymer; poly(N-isopropylacrylamide);
poly(N-isopropylacrylamide)-poly[(N-acetylimino)ethylene] block
copolymers; poly(N-isopropylmethacrylamide); poly(propylene
glycol); poly(vinyl alcohol); poly(N-vinyl caprolactam);
poly(N-vinylisobutyramide); poly(vinyl methyl ether);
poly(N-vinyl-N-propylacetamide); N-vinylacetamide/vinyl acetate
copolymers; N-vinylcaprolactam/N-vinylamine copolymers; vinyl
alcohol/vinyl butyrate copolymers; N-vinylformamide/vinyl acetate
copolymers and combinations of these.
[0046] Thermo-responsive polymer flocculants can be used for
aggregation of proppant particulates in a fracture. The schematic
below shows a representation of the aggregation mechanism of
proppants using polymers having LCST behavior, FIG. 5. The
mechanism of particle agglomeration includes adsorption of polymer
onto the surface of particles at temperatures below the LCST. Under
these conditions, polymers are soluble in water, so there is
hydrogen bonding between the polymer and water molecules; the
polymer chains have an extended random coil conformation. When the
temperature is increased above the LCST the hydrogen bonding is
weakened, resulting in phase separation of the polymer and water,
whereupon the polymer chains collapse and precipitate, entrapping
proppant particulates.
[0047] The formation of LCST precipitates is also a way to induce
or trigger the aggregation/agglomeration of proppant. However, if
the formation of LCST precipitates leaves a water-like matrix
fluid, leak-off will be high, leaving only the clots and proppant
in the fracture. On the other hand, a useful system is obtained if
the LCST precipitates are formed in a way such that the residual
matrix is a high viscosity low leak-off fluid.
[0048] There are two ways of practicing the invention: by injecting
a single composition containing a trigger or delay agent or by
injecting two or more compositions which mix and react
downhole.
[0049] For polyelectrolyte complex (PEC)-induced agglomeration, the
treatment sequence is typically as follows: inject a pad; inject a
proppant-laden slurry containing at least one polyelectrolyte
already in charged form and at least one non-ionic polymer, which
can be converted to a polyelectrolyte with a charge opposite to
that of the first polymer by a trigger or a delay agent; allow
proppant aggregation; and allow fracture closure. The concentration
of the polyelectrolytes and polyelectrolyte precursors is in the
range of from about 0.005 to about 5 weight percent. Suitable
triggering mechanisms for PEC formation are listed above. The
slurry may further contain oilfield additives, known to those
skilled in the art, such as viscosifiers, surfactants, clay
stabilizers, bactericides, fibers, etc.
[0050] For the syneresis embodiment of causing agglomeration, the
sequence would be as follows: pump a pad stage for fracture
initiation; pump proppant-laden fluid that undergoes syneresis at
downhole conditions; allow agglomeration of proppant; and allow the
fracture to close on the aggregates formed. In a preferred
embodiment, the fluid formulation additionally contains fibers for
agglomerate stabilization and settling prevention.
[0051] For using the LCST approach to agglomeration a suitable
sequence of steps is the following: pump a pad stage for fracture
initiation; pump proppant-laden fluid that undergo phase transition
at downhole conditions (for example upon heating to downhole
temperatures); allow agglomeration of proppant; and allow the
fracture to close on the aggregates formed.
[0052] For the approach of using and mixing two different fluids,
agglomeration is induced by pumping a pad stage for fracture
initiation followed by pumping the two fluids to the perforation
region by different flow paths, for example by pumping one fluid
down coiled tubing and pumping the other fluid down the annulus
between the coiled tubing and the wellbore. Mixing of the two
fluids, in the perforations or after the perforations, induces
agglomeration of proppant. Agglomerated particles are transported
to the fracture. After the treatment the fracture closes on the
agglomerates.
[0053] The method of the invention can be used in fractures of any
size and orientation. It is particularly suitable for fractures in
horizontal wellbores and/or in soft formations. The agglomeration
and resulting heterogeneous proppant placement should occur during
the pumping or during an optional shut in period; it should occur
before flowback. [0054] The present invention can be further
understood from the following examples.
EXAMPLE 1
[0055] A linear gel slurry containing 3.6 g/l (30 lb/mgal) of guar
and 406 g/l (4 ppa) of sand 0.300-0.106 mm (50/140 US) was prepared
with deionized water. The gel was then crosslinked with different
crosslinker concentrations (see Table 1). The crosslinker consisted
of a H.sub.3BO.sub.3:NaOH:CaCl.sub.2 mixture in a weight ratio of
3.1:1:1.3 in which the solids content was 50 weight percent in
water. Noticeable gel collapse was observed in the sample with the
highest borate concentration in 3 hours, while no change was found
in the gel sample with the lowest crosslinker concentration. The
volumes of water expelled from the gel sample were measured after
24 hours of storage at room temperature. It was found that after
syneresis all the proppant particles remained in the gel phase,
where the proppant concentration was increased up to 2 times.
TABLE-US-00001 TABLE 1 Water phase Proppant Sample Crosslinker
volume after concentration after # added, ml 24 hours, % syneresis,
g/L 1 2 0 406 2 4 51 705 3 7 61 826 4 10 64 881
EXAMPLE 2
[0056] Slickwater sample 1 was prepared from a water-in-oil
emulsion of anionic polyacrylamide-AMPS copolymer useful as a
friction reducer at a concentration of 0.1 weight percent (1 gpt)
in deionized water; about 10 mg of methylene blue dye was added to
the sample. Slickwater sample 2 was-prepared from a water-in-oil
emulsion of cationic polyacrylamide also useful as a friction
reducer at the same concentration in deionized water; about 10 mg
of methyl orange was added.
[0057] In one experiment, 20 ml of sample 1 was placed in a Petri
dish and 20 ml of sample 2 was added to it. The Petri dish was
shaken by hand to mix the two samples. After about 1 min of
shaking, a net of fine green lines started to appear and grow,
which was a polyelectrolyte complex colored by the mixture of dyes.
The net appeared to be sticky and its further growth with shaking
resulted in the formation of a clot. In a second experiment, 4.8 g
of 400-800 micron (20/40 US sieve) sand was dispersed in 20 ml of
sample 1 to give a proppant concentration of about 240 g/l (2 ppa).
Then 20 ml of sample 2 was slowly added to the proppant slurry and
the two were thoroughly mixed. The 20/40 sand grains, originally
evenly dispersed, became assembled into green aggregates.
EXAMPLE 3
[0058] Syneresis of gels made from 3.6 g/L of guar, 0.5 g/L of
boric acid, and 3 ml/L of 5 weight percent NaOH with various
amounts of Ca(OH).sub.2 in deionized water was studied. The Ca(OH)
.sub.2 used was 0.6-0.3 mm (30/50 mesh). Syneresis that took about
a day at room temperature took several hours at 50.degree. C. The
concentration of cations controlled the degree and speed of
syneresis. FIG. 1 shows the dependency of syneresis vs. time for
borate crosslinked guar gel samples having different concentrations
of Ca(OH).sub.2 at room temperature. Syneresis started at about
0.014 mol/L of Ca(OH).sub.2 in the systems.
EXAMPLE 4
[0059] The kinetics of syneresis in the presence of Mg ions was
investigated. Gels were prepared in deionized water that contained
3.6 g/L of guar, 3.6 g/L of H.sub.3BO.sub.3, and 23 ml/L of 5
weight percent NaOH doped with 0.142-1.3 g/L of MgCl.sub.2 and the
appropriate quantity of NaOH to create Mg(OH).sub.2. FIG. 2 shows
the syneresis of the borate crosslinked guar gels in samples having
different concentration of Mg(OH).sub.2 at room temperature.
Syneresis started at about 0.005 mol/L of Mg(OH).sub.2 in the
systems.
EXAMPLE 5
[0060] Syneresis kinetics was investigated in the presence of Al
ions. Gels were prepared in deionized water that contained 3.6 g/L
of guar, 3.6 g/L of H.sub.3BO.sub.3, and 23 ml/L of 5 weight
percent NaOH doped with 0.178-3.26 g/L of AlCl.sub.3.6H.sub.2O and
the corresponding quantity of NaOH to create Al(OH).sub.3. FIG. 3
shows the syneresis of the borate crosslinked gel samples having
varying concentrations of AlCl.sub.3.6H.sub.2O. Formation of
Al(OH).sub.4.sup.- is believed to have occurred; syneresis started
at a concentration of about 0.004 mol/L.
EXAMPLE 6
[0061] Gels with 0.014 mol/L of aluminum (3.6 g/L of guar, 3.6 g/L
of H.sub.3BO.sub.3, 3.26 g/L of AlCl.sub.3.6H.sub.2O, and 55.4 ml/L
of 5 weight percent NaOH) were prepared in deionized water and
various amounts of 6-8 mm polylactic acid fiber were added to the
samples. Fiber concentrations ranged from 0 to 10.3 g/L. After
syneresis, the volume of shrunken gel was a function of the fiber
concentration; the more fibers added, the greater the volume of the
gel up to about 3.6 g/L fiber. From 3.6 to 10.3 g/L of fiber there
was little apparent change in gel syneresis.
EXAMPLE 7
[0062] The influence of borate crosslinking site density on a
syneresis level was examined. Two samples of crosslinked gel with
added copper ions were prepared with different concentration of
borate. The first sample was made from 3.6 g/L of guar, 0.652 g/L
of CuCl.sub.2.2H.sub.2O, 3.6 g/L of H.sub.3BO.sub.3 and 29.1 ml/L
of 5 weight percent NaOH in deionized water. The second sample was
made from 3.6 g/L of guar, 0.652 g/L of CuCl.sub.2.2H.sub.2O, 0.5
g/L H.sub.3BO.sub.3 and 9.1 ml/L of 5 weight percent NaOH in
deionized water. After 2 hours at room temperature, the syneresis
levels were 70 percent in the sample with high boric acid
concentration and 9 percent in the sample with low boric acid
content.
EXAMPLE 8
[0063] Three gm of 0.212-0.106 mm (70/140 mesh) sand (d.sub.50 of
169 .mu.m by Malvern Mastersizer analysis) was placed in a Petri
dish with 20 ml of deionized water and 0.8 gm of
poly(N-isopropylacrylamide) (average M.sub.n of about
20,000-25,000); the polymer has an LCST of 32.degree. C. The slurry
was mixed vigorously at room temperature for one minute with a
magnetic stirrer. No agglomeration was observed. The suspension was
then heated to 40.degree. C. while still being stirred. When the
temperature reached 40.degree. C., the stirring was stopped and a
number of agglomerates were observed. The mean sizes of the
agglomerates which formed were estimated by measurement on
photographs of the sample bottle laid alongside a graduated scale.
The mean size of the agglomerates obtained under dynamic conditions
(intensive agitation) was about 0.9 cm. Analysis of the
agglomerates showed that they consisted of sand and precipitated
polymer.
EXAMPLE 9
[0064] The use of polyelectrolyte complexes for agglomeration of
proppant particles was demonstrated. Agglomeration of 0.850-0.425
mm (20/40 mesh) sand was studied in a polyelectrolyte complex (PEC)
formed from partially hydrolyzed polyacrylamide (PHPA) (a random
anionic copolymer made from 40 mol percent sodium acrylate and 60
mol percent acrylamide, having an average molecular weight of about
10.times.10.sup.6 g/mol) and polyethyleneimine PEI (a highly
branched cationic polymer having an average molecular weight of
about 8.times.10.sup.5 g/mol). A representative structure of the
PEI is:
##STR00001##
[0065] A suspension of 10 g 0.850-0.425 mm (20/40) mesh sand was
mixed with 100 g of deionised water in a 250 mL beaker using a
flat-two-blade impeller driven at 270 rpm by an overhead mechanical
stirrer. With continuous mixing (270 rpm), 25 g of a 1 weight
percent PHPA solution (dissolved in 2 weight percent KCl) was
added. After a further 10 minutes of continuous mixing, 2.5 g of a
10 weight percent PEI solution (dissolved in deionized water) was
added and mixing was continued for 15 minutes. At this point, the
aqueous phase of the mixture contained 0.196 weight percent PHPA
polymer, 0.196 eight percent PEI polymer and 0.39 weight percent
KCl. The pH of the aqueous phase was sufficiently alkaline that the
PEI polymer was uncharged, which inhibited precipitation of the
PEC. Then acid was added to induce protonation of the PEI polymer
and precipitation of the PEC; again with continuous mixing (270
rpm), 2 g of 1 molar HCl was added to the mixture using a Pasteur
pipette to introduce the acid solution at the base of the agitating
mixture. After a few minutes of continuous mixing, a voluminous and
sticky PEC precipitate was formed. After a few more minutes, the
PEC precipitate shrank to a small fraction of the total volume and
had completely encapsulated (agglomerated) all of the 10 g of sand;
there was no residual sand at the bottom of the beaker. The
experiment resulted in 100 percent agglomeration efficiency (AE)
where AE is defined by the weight percent of the sand
encapsulated/agglomerated by the PEC. After acid addition and
subsequent thorough mixing, the pH of the aqueous phase was 9.5. At
this pH, the PEI was sufficiently protonated (cationic) to interact
strongly, by electrostatic attraction, with the anionic carboxylate
sites on the PHPA polymer. This resulted in the observed formation
of a sticky PEC precipitate. The same 100 percent agglomeration
efficiency (AE) of the same sand was achieved in similar
experiments with end-point pH's 8.5 and 10.0.
EXAMPLE 10
[0066] As a further demonstration of agglomeration of 0.850-0.425
mm sand (20/40 mesh), experiments were performed in polyelectrolyte
complexes (PEC) formed from the same PEI and PHPA as in Example 9.
The experiments were as described for example 9, but this time the
ratios of PEI and PHPA were varied and 100 g of 2 weight percent
KCl aqueous solution was used in place of the deionized water. As
was expected, the higher ionic strength of the 2 weight percent KCl
aqueous phase somewhat screened the strong electrostatic
interaction between the oppositely charge polymers. The results
shown in table 2 were obtained. Again, a high AE was observed even
in the presence of salt, but it was not 100 percent.
TABLE-US-00002 TABLE 2 Aqueous phase pH after acid composition
(before Base addition and acid addition) fluid mixing AE 0.2 wt %
PHPA, 0.1 wt % 2 wt % 9.1 66 PEI KCl 0.2 wt % PHPA, 0.2 wt % 2 wt %
9.2 71 PEI KCl 0.2 wt % PHPA, 0.3 wt % 2 wt % 9.3 80 PEI KCl
[0067] It can be seen that there was a slight increase in AE with
increasing amounts of PEI.
EXAMPLE 11
[0068] Agglomeration of 0.850-0.425 mm (20/40 mesh) sand was
studied in a polyelectrolyte complex (PEC) formed from borate
complexed guar (which is an anionic polymer) and a cationic
copolymer of acrylamide and DADMAC. A suspension of 0.850-0.425 mm
(20/40 mesh) sand was mixed with 100 g of linear gel that contained
1.2 g/L of guar, 0.46 g/L of boric acid, and 0.1 g/L of the
copolymer of acrylamide and DADMAC in a 250 mL beaker using an
overhead mixer driven at 500 rpm. The pH of the aqueous phase was
approximately neutral. After 5 minutes of continuous mixing, 6 ml/L
of 5 weight percent NaOH solution was added, which induced
formation of the guar borate complex and PEC precipitation.
Essentially all of the sand was in the precipitated phase.
EXAMPLE 12
[0069] The influence of ionic strength on the level of syneresis
was examined. Two samples of crosslinked gel were prepared with
different concentration of potassium chloride. The first sample was
made from 3.6 g/L of guar, 7 g/L of H.sub.3BO.sub.3 and 42 ml/L of
5 weight percent NaOH in deionized water. The second sample was
made from 3.6 g/L of guar, 7 g/L H.sub.3BO.sub.3, 42 ml/L of 5
weight percent NaOH in deionized water, and 20 g/L of KCl. After 2
hours at room temperature, the syneresis levels were 94 percent for
the sample with potassium chloride and 0 percent for the sample
without the salt.
* * * * *