CA2037974C - Method and composition for delaying the gellation of borated gallactomannans - Google Patents

Abstract

A complexor and method of use are shown for providing controlled delay of the cross-linking reaction in an aqueous well fracturing fluid. A base fluid is first prepared by blending together an aqueous fluid and a hydratable polymer which is capable of gelling in the presence of borate ions.
The complexor is prepared by mixing a cross-linking additive capable of furnishing borate ions in solution with a delay additive. The delay additive is effective, within a selected pH range, to chemically bond with both boric acid and the borate ions produced by the cross-linking additive to thereby limit the number of borate ions initially available in solution for subsequent cross-linking of the hydratable polysaccharide. The subsequent rate of cross-linking of the polysaccharide can be controlled by adjusting the pH of the complexor solution.

Description

,\
1 BACKGROL;2tD OF THE INVENTION

3 1. Field of the Invention:

The present invention relates to methods and 6 compositions for controlling the gellation rate in an 7 aqueous well fracturing fluid. It particularly Yelates to a 8 novel lia_uid complexor used to obtain controlled delayed 9 gellation of borated polysaccharides.
to 11 2. Description of the Frior Art:

33 . During.hydraulic fracturing, a sand laden fluid is 14 injected into a well bore under high pressure. Once the natural reservoir pressures are exceeded, the fracturing 16 fluid initiates a fracture in the formation whic:: generally 17 conta.nues to grow during pumping. The treatment design 18 generally reauires t::e f't:id to reach maximum triscosity as 19 it enters the fracture which affects the fracture length and width. This viscosit_~ is normally obtained by td:e gellation 21 of suitable polymers, sue:: as a suitable polysaccaride. In 22 recent years, gellation has been achieved by cress-linking 23 these polymers with metal ions~including aluminum, antimony, 24 zirconium and titanium containing compounds including the so-called organotitanates. See, for instance, L'.S. Patent 26 No. 4,514,309, issued April 30.,1985, and assigned to the 27 assignee of the present invention.

29 The viscous fracturing fluid being pumped usually encounters high shear in the pipe string during pumping from 31, the surface to the fracture and after entering the fracture, 32 flows at low shear. Recent investigations indicate that the 33 high shear encountered in the pipe string causes extensive 1 degradation of the cross-linked fracturing fluid. Also, 2 high fluid viscosities cause excessive back or friction 3 pressures, limiting the pumping rate, which also affects 4 fracture geometry. These investigations have shown that by delaying the gellation for several minutes during most of 6 the high shear, higher pump rates can be obtained and the 7 fluid generally exhibits better stability. In the case of 8 the metal ion cross-linking systems, the delay in gellation 9 is normally achieved with a delaying additive that binds or chelates the metal ions in solution.

12 Recently, guar and guar derivaties cross-linked- with 13 borate ions have again become popular. In alkaline water 14 having a pH greater than about 7.8, cross-linking of the guar polymer is essentially instantaneous. This action is 16 probably due to the 'act that borates easily and readily 17 esterify with 1,2-cissoidial dialcohols or palyhedric 18 alcohols, such as those found on the guar polymer. This 19 esterification is readily reversible, especially at the elevated temperatures found in the well bore, so that free 21 borate ion is altaays available. As a result, the delay of 22 borate ion cross-linking sysuems is difficult to achieve.

24 Certain of the prior art borated guar systems have employed either slow dissolving metal .oxides which slowly 26 increase the fluid alkalinity, which in turn promotes cross-27 linking, or by using calcium borate salts having poor water 28 solubility, relying upon the slaw dissolution of borate ions 29 for delay. xn both cases, the delay action was based primarily on the slow dissolution of a solid in the aqueous 31 fracturing fluid, resulting in poor control of the delay 32 time and ultimate viscosity of the fluid. U.S. Patent No.
33 4,619,776, issued October 28, 1986, to Mondshine, is typical 1 of the prior art in teacing the use of a sparingly soluble 2 borate to achieve some degree of control over the cross-3 linking reaction.

A need exists for a composition and method for 6 providing more precise control over the cross-linking 7 reaction of a borated ac~,~eous fracturing fluid.

9 A need also exists for such a composition and method which does not rely upc~ the slow dissolution of a solid as 11 the basis of the delay Wechanism.

13 A need also exists for such a method and composition 14 which allows selective adjustment of the delay rate at a well site quickly and conveniently.
is 17 A need also exists for such a method and composition 18 which provides a reverse °eaction to reduce the viscosity of 19 the fracturing fluid wi4:: time to facilitate the cleanup of the treatment from the taell bore.
q, 1 SLIM~IARY OF TFi~ INV~NTICa 3 The cross-linking system of the invention utilizes a 4 novel complexor solution to control the gellation rate of an aqueous fracturing fluid containing a hydrated 6 polysaccharide polymer. The complexor solution comprises a 7 cross-linking additive and a delay additive which controls 8 the rate at which the cross-linking additive promotes 9 gellation of the hydrated polymer, the control rate being a function of the pH of the complexor solution. Preferably, 11 the cross-linking additive is a material ~ahich supplies free 12 borate ions in solution and the delay additive is a material 13 which attempts to bind chemically to ~e borate ions in 14 solution, whereby the hydrated polymer is forced to comuete with the delay additive far the free borate ions. Mast 16 preferably, the delay additive is selected from the group 17 consisting of dialdehydes having about 1 to 4 carbon atoms, 18 keto aldehydes having about 1 to 4 car: on atoms, hydroxyl 19 aldehydes having about 1-4 carbon atoms, ortho substituted aromatic dialdehydes and ortho substituted aromatic hydroxyl 21 aldehydes.

23 Tn the method of the invention, a hydratable polymer 24 capable of gelling in the presence of borate ions is blended with an aqueous fluid to form a base fluid and the polymer 26 is allowed to hydrate. A complexor solution is formed for 27 the base fluid by combining a cross-linki.~.g additive capable 28 of furnishing borate ions in solution wit:: a delay additive, 29 to chemically bond with both boric acid and the free borate ions produced by the cross-linking additive to thereby limit 31 the number of borate ions available in solution for 32 subsequent cross-linking of the hydrated polymer. The pH of 33 the complexor solution is adjusted in order to control the rate of the subsequent cross-linking of the hydratable polymer. The complexor solution is then added to the base fluid to cross-link the hydrated polymer.
In one preferred embodiment of the invention there is provided a method of controlling the cross-linking reaction of an aqueous fracturing fluid in fracturing a subterranean formation, comprising the steps of: blending together an aqueous fluid and a hydratable polymer capable of gelling in the presence of borate ions; allowing the polymer to hydrate to form a hydrated polymer sol; adding an alkaline buffer to thereby adjust the pH of the hydrated polymer sol in the range from about 8.0 to 11.5; forming a complexor solution for said hydrated polymer gel by combining a cross-linking additive capable of furnishing borate ions in solution with a delay additive selected from the group consisting of dialdehydes having about 2-4 carbon atoms, keto aldehydes having about 3-4 carbon atoms hydroxyl aldehydes having 2-4 carbon atoms, ortho substituted aromatic dialdehydes and ortho substituted aromatic hydroxyl aldehydes, the delay additive being effective, to chemically bond with the borate ions and boric acid produced by the cross-linking additive to thereby limit the number of borate ions available in solution for subsequent cross-linking of the hydrated polymer sol; adjusting the pH of the complexor solution to achieve a desired delay in the cross-linking reaction of the hydrated polymer gel, the delay achieved being a function of the complexor solution pH, wherein upward adjustment of the pH of the complexor solution serves to retard the rate of the subsequent cross-linking of the hydrated polymer sol while downward adjustment of the pH of the complexor solution serves to accelerate the rate of subsequent cross-linking; and adding the complexor solution to the hydrated polymer sol to cross-link the hydrated polymer sol.
In another preferred embodiment of the invention there is provided a method of fracturing a subterranean formation comprising the steps of: blending together an aqueous fluid - and a hydratable polymer capable of gelling in the presence of borate ions; allowing the polymer to hydrate to form a hydrated base fluid; adjusting the pH of the base fluid to above about 7.8; forming a complexor solution for the base fluid by combining a cross-linking additive capable of furnishing borate ions in solution with a liquid delay additive, the delay additive being added in an amount effective to chemically bond in the alkaline pH base fluid with the borate ions produced by the cross-linking additive to thereby limit the number of borate ions available in solution for subsequent cross-linking of the base fluid; and while maintaining an alkaline pH in the base fluid adding the complexor solution to the base fluid to cross-link the fluid.
In a further preferred embodiment there is provided a complexor solution when used to initiate a time controlled cross-linking reaction with an aqueous hydrated polysaccharide fracturing fluid, the complexor comprising: a mixture of cross-linking compound which supplies borate ions in solution and a liquid delay additive which controls the rate at which said cross-linking compound promotes cross-linking of said polysaccharide, the control rate being a function of the pH of the complexor solution, said liquid delay additive being selected from dialdehydes having 2-4 carbon atoms, keto aldehydes having 3-4 carbon atoms, hydroxyl aldehydes having 2-4 carbon atoms in the carbon chains, ortho substituted aromatic dialdehydes and ortho substituted aromatic hydroxyl aldehydes.
Additional objects, features, and advantages will be apparent in the written description which follows.
- 6a-DETAILED DESCRI?TION OF THE IPd'VENTION

3 The present invention provides a method for controlling 4 the cross-linking reaction of an aqueous fracturing fluid in fracturing a subterranean formation. In order to practice 6 the method, an aqueous (water ar brine) based fracturing 7 fluid is first prepared by blending a hydratable polymer 8 into the base fluid. Any suitable mixing apparatus may be 9 used for this procedure. _Tn the case of batch mixing, the hydratable polymer and aaueous fluid are blended for a 11 period of time which is sufficient to form a hydrated sol.
12 Once the hydration of the polymer is complete, a 13 predetermined quanity of complexor solution is added to the 14 base fluid sufficient to achieve a desired cross-linking reaction time. The mixture is pumped into the well bore as 16 the cross-linking reaction takes place.

18 Propping agents are typically added to the base fluid Z9 prior to the addition of the complexor. Propping agents include, for instance, quartz sand grains, glass and ceramic 21 beads, walnut shell fragments, aluminum pellets, nylon 22 pellets, and the like. The propping agents are normally 23 used in concentrations between about 1 to 8 pounds per 24 gallon of fracturing fluid composition, but higher or lower cancentrations can be used as required. The base fluid can 26 also contain other conventional additives common to the well 27 services industry such as surfactants, corrosion inhibitors, 28 buffers, and the like.

The hydratable polymer useful in the present invention 31 can be any of the hydratable polysaccharides familiar to 32 those in the well service industry which is capable of 33 gelling in the presence of borate ions to form a gelled base 1 fluid, for instance, suitable hydratable polysaccharides are 2 the galactomannan gums, glucomannan gums, guars, derived 3 guars and cellulose derivatives. Specific examples are guar 4 gum, guar gum derivatives, locust bean gum, karaya gum, carboxymethyl cellulose, carboxymethylhydroxyethyl 6 cellulose, and hydroxyethyl cellulose. The preferred 7 gelling agents are guar gum, hydroxypropyl guar, 8 carboxymethylhydroxypropyl guar, and carboxymethyl-9 hydroxyethyl cellulose. A suitable synthetic polymer is 7.0 polyvinyl alcohol. The most preferred hydratable polymers 11 for the present invention are guar gum and hydroxypropyl 12 guar.

14 The hydratable polymer is added to the aa_ueous base fluid in concentrations ranging from about 0.10% to 5.0% by 16 weight of the aqueous fluid. The most preferred range for 17 the present invention is about 0.24% to 0.72% by Weight.

19 The cross-linking system of the invention utilizes a novel complexor solution to control the cross-linking rate 21 of the base fluid ccntaining the hydrated polymer. The 22 complexor solution comprises a cross-linking additive and a 23 delay additive which controls the rate at which the cross-24 linking additive promotes gellation of the hydrated polymer, the control rate being a function of the pH of the complexor 26 solution. Preferably, the cross-linking additive is a 27 material which supplies borate ions in solutian. Thus, the 28 cross-linking additive can be any convenient source of 29 borate ions, for instance the alkali metal and the alkaline earth metal borates arid boric acid. A preferred cross-31 linking additive is sodium borate decahydrate. The cross-32 linking additive is preferably present in the range from g -1 about 5 to 25 % by weight, most preferably about l0 to 15 2 by weight of the complexor solution.

4 The delay additive used in the complexor solution is a material which attempts to bind chemically to the borate 6 ions produced by the cross-linking additive in solution, 7 whereby the hydrated polymer is forced to compete with the 8 delay additive for 'the borate ions. As will be explained, 9 the effectiveness of the delay additive in chemically bonding to the borate ions in the complexor solution is pFi 11 dependent. Thus, unlike the prior art systems which 12 utilized slow dissolving metal oxides or calcium borate 13 salts having poor water solubility, the present complexor 14 does not rely upon the slow dissolution of solids.
16 Preferably, the delay additive is selected from the 17 group consisting of dialdehydes having about 1-4 carbon 18 atoms, keto aldehydes having about 1-4 carbon atoms, hydroxy 19 aldehydes having about 1 to 4 carbon atoms, ortho substituted aromatic dialdehydes and ortho substituted 21 aromatic hydroxyl aldehydes. Preferred delay additives 22 include, for instance, glyoxal, propane dialdehyde, 2-keto 23 propanal, 1.4-butanedial, 2-keto butanal, 2.3-di keto 24 dibutanal, phthaldehyde, salicaldehyde, etc. The preferred delay additive is gly~xal due to its ready availability from 26 a number of commercial sources. Preferably, the delay 27 additive is present in the range from about 5 to 40 % by 28 weight, most preferably about 15 to 30 % by weight of the 29 compleaeor solution. The preferred ratio of glyoxal to sodium borate is from about 1:0.1 to 1:1, and most preferably is 31 about 1:0.516.

g ~~~'~~"~~~
1 Glyoxal, a 1.2- dialdehyde, hydrates to form 1.1.2.2-2 tetrahydroxyethane which favorably binds to the borate ions 3 provided by the cross-linking additive of the complexor. As 4 the pH of the complexor solution increases, the rate of gellation declines. As the pH of the camplexor solution 6 decreases, the rate of gellation increases. Thus, by 7 adausting the ,pH of the complexor solution within a 8 preselected range, extremely accurate control of the cross-9 linking delay time can be achieved. Experimental delay times have ranged from 10 to 300 seconds by varying the pH
11 of the complexor solution from about 5.0 to 11.50, 12 respecti~rely.

14 The complexor can also contain a stabilizer which increases the shelf life of the complexor and can serve to 16 enhance the delay time. Suitable stabilizers include, for 17 instance, polyhedric alcohols such as pentaerythritol 18 glycerin, lanolin, mono and oligosaccharides having multiple 19 hydroxyl groups, and the like. The preferred stabilizer is sorbitol, a reduced sugar. Ths stabilizer is preferably 21 present in the range from about 5 to 20 % by weigh, most 22 preferably about 8 to 10% by weight of the complexor 23 solution.

The complexor mixture is heated to a temperature 26 ranging from ambient to 105°C for 1 to 5 hours. Most 27 preferably heating should range from 65-80'C for 2 to 4 28 hours.

The complexor of the invention can be used to control 31 the delay time of a cross-linked fracturing fluid being 32 pumped into a well bore traversing the subterranean 33 formation to be fractured. The fracturing fluid is pumped 1 at a rate sufficient to fracture the formation and to place 2 propping agents into the fracture. A typical fracturing 3 treatment would be conducted by hydrating a 0.24 to 0.72%
4 galactomannan based polymer, such as a guar, in a 2%
(wt/vol) KC1 solution at a pH ranging from about 5.0 to 8.5.
6 The pH of the complexor would be adjusted with caustic prior 7 to the treatment to provide the desired delay time. During 8 actual pumping, a buffer would be added to increase the 9 hydrated polymer pH to above 8.0, followed by addition of the complexor, and typically a breaker and proppant. During 11 the treatment, the area close to the well bore will 12 typically begin cooling gradually, resulting in a decreasing 13 gellation rate. The delay tine can be easily readjusted to 14 accommodate the cooling by acidifying the complexor.

15 In addition to a precisely controlled delay, the novel 17 complexor of the invention provides another useful junction.
18 After the fracture is formed and the pumping is terminated, 19 the viscosity of the fluid must be reduced below about 10 centipoise. At this viscosity, the fluid can be recovered 21 while leaving the proppant in the fracture. As previously 22 discussed, borate cross-linked galactomannans are pH
23 dependent, requiring an alkaline base fluid having a pH
24 above about 7.8. Glyoxol, in alkaline water, slowly converts to alpha-hydrnxy acetic acid, a strong acid, which 26 decreases the pH of the hydrated polymer gel with time.
27 This in turn reduces tre amount of available borate ion, 28 since the borate ion is converted to boric acid which does 2~ not cross-link, and thus reduces the viscosity of the fracturing fluid.

32 The following examples of the cross-linked fracturing 33 fluid of the present invention embodying the novel complexor '~ H
1 discussed above are intended to serve primarily for the 2 purpose of illustration. The invention, in its broader 3 aspects, is not to be construed as limited thereto.
4 Included are examples of glyoxol/borate formulation, data relating gellation times to complexor pH and gellation 6 stability after cross-linking.

8 Example 1 Complexor Preparation:

12 rnto 300 parts of 40% aaueous glyoxal are added, with 13 stirring, 130 parts of sodium borate decahydrate yielding a 14 milky white suspension. Then, 65 parts of 25% aqueous sodium hydroxide are slowly added resulting in a clear, pale 16 yellow solution. The solution pH can range from 4.90 to 17 6.50. Afterward, 71.4 carts of 70% aqueous sorbitol are 18 added to the solution followed by heating to 95°C far 3 19 hours. During heating, the solution color changes from pale yellow to amber. After cooling to ambient, the solution pH
21 ranges between 4.50 and 5.00.

23 Example 2 Gellation Rate:

27 The base sol used to determine the gellation rate is 28 prepared by adding, with vigorous stirring, 2.4 parts of a 29 0.4 D.S. hydroxypropyl guar gum and 0.18 parts of sodium bicarbonate to 500 parts of 2% aqueous potassium chloride 31 solution. After the addition, the stirring rate is reduced 32 to provide mild agitation to the sot for 2 hr. Then, 3.2 1 parts of 30~ aqueous potassium carbonate are added which 2 buffers the sol to about pF3 10Ø

4 Meanwhile, the complexor prepared in Example 1 is blended with 0,4,8 and 12 parts of 25% aq sodium hydroxide 6 per 100 parts of complexor. The pHs of the treated 7 complexors are shown in Table 1.

Then, 250 parts of hydrated sol are transferred to a one liter blaring blender jar and sheared at a rate 11 sufficient to create a vortex exposing the hub nut on the 12 blender blades. Next, 0.98 parts of the treated complexors 13 are added to the sol vortex. The time required for the 14 fluid to viscosity and cover the hub nut is defined as the vortex closure time. These data are also shown in Table 1.

19 Parts of 25% as NaOH Vortex Closure Complexor per 100 warts comnlexor Ti~ae (sec.) 22 0 22 4.92 5.80 24 8 121 6.09 12 275 8.28 27 Example 3 29 Shear and thermal stability of borated galactomannans:
31 The preparation of the base sol used in this example is 32 mixed as described in Example 2. After hydrating for 2 33 hears, the 500 parts of base sol are treated with 4.5 parts 1 of 30$ aqueous potassium carbonate which buffers the sol to 2 about pH 10.3. Afterward, 2.28 parts of complexor 3 containing 0.17 parts of 25~ aqueous sodium hydroxide are 4 added to the vigorously stirring sol. After 100 seconds, 42 parts of gel are syringed into a Fann 50~ cup. The sample 6 is sheared at 102 sec-1, using an R1B1 cup and bob 7 combination, while heating to 190°F in a preset bath and 8 pressuring to 110 psi with nitrogen. The sample is heated 9 and sheared fox 20 minutes followed by a rate sweep using 170, 128, 85 and 42 sec-1 while recording stress. These 11 sweeps are repeated about every 30 minutes and the interim 12 rate between sweeps is 102 sec-1. After 359 minutes; the 13 shearing is stopped while heating continues overnight. A
14 final sweep is made after 22 hours and 21 minutes. The rates and stresses are used to calculate the Power Law 16 indices, n° and K, described in the API bulletin RP-39.
17 From the calculated indices, the viscosity of the gel at 18 various shear rates can be calculated and are shown in Table 19 2 at 170 and 85 sec-1 over time.

~~~'.~~'~~

Time Temp n K V9.scos~ty(cp) i ~t (m ~
n) lbm/ft2 170 s 85 s F

6 20 183 0.7005 0.0497 512 630 7 51 191 0.7090 0.0420 451 552 8 81 191 0.6631 0.0456 387 489 9 112 1.92 0.8411 0.0144 306 341 141 192 1.0?62 0.0040 286 271 11 172 190 1.1220 0.0028 252 231 12 202 191 1.1981 0.0016 210 183 13 232 191 1.1293 0.0020 185 169 14 262 192 1.1020 0.0022 181 169 292 192 1.0589 0.0025 160 155 16 359 193 0.9811 0.0020 86 87 17 1341 192 0.5486 0.0034 16 20 19 Example 4 21 Shear and thermal stability of borated galactomannans:

23 The experiment in Example 3 is repeated using 4.0 parts 24 30% aqueous potassium onate and 1.62 parts of untreated carb complexor prepared in Example 1. After 60 secands, 42 parts 26 of gel are syringed intothe Farm 50C cup. The fluid is ' 27 sheared at 102 sec1 whileheating to 160F in a preset bath 28 and pressuring to 110 with nitrogen. The rate sweeps psi 29 are conducted as describedin Example 3. After 233 minutes of heating and shearing,the shearing is stopped while 31 heating overnight continues.
A final sweep is made after 32 heating for l9 hours and 40 minutes. These data_are shown 33 in Table 3.

_ 15 _ 20"'l~"l4 4 Time Temp n' K Viscos~ty(c ) t (mln) °F lbm/ft2 170 sr~- 85ps°~

7 20 160 0.4708 0.1844 583 841 8 51 164 0.4824 0.1530 513 ?35 9 80 163 0.5501 0.1038 493 674 111 163 0.5143 0.1143 452 632 11 141 164 0.5275 0.1047 443 614 12 171 163 0.5224 0.1044 430 599 13 203 163 0.6097 0.0625 403 529 14 233 162 0.6572 0.0419 345 437 1180 163 0.7992 0.0011 19 21 is 17 Example 5 19 Shear and thermal stability of borated galactomannans:

21 The polymer used in Examples 3 and 4 is a hydroxypropyl 22 guar gum. The polymer used in this example is 3.0 parts of 23 a nonderivatized guar gum in 500 parts of 2~ aqueous 24 potassium chloride solution mixed as described in Example 2.
The sol is stirred for 2 hours prior to adding 4.5 parts of 26 30~ aqueous potassium carbonate and 1.12 parts of 27 triethanolamine, a temperature stabilizer. Then with 28 vigorous stirring, 1.30 parts of untreated complexor 29 prepared in~Example 1 are added. After 60 sec~l of shear, 42 parts of gel are syringed into a Fann 50C cup. The gel 31 is then sheared at 102 sec°1 while heating to 245°F in a 32 preset bath and pressuring to 110 psi with nitrogen. The 33 rate sweeps are routinely made as described in Example 3.
_ 16 _ ~~~"l~'~j~
1 The final sweep is made after shearing and heating for 149 2 minutes. These data are shown in Table 4.

6 Time Temp n' K Viscos~.ty(cp) t 7 (min) 'F lbm/ft2 17o s-1 8s s-~
s 9 20 239 0.4516 0.1763 505 738 50 244 0.7736 0.0298 446 521 11 83 . 245 1.1109 0.0046 389 . 360 12 119 245 1.3101 0.0008 194 157 13 149 245 1.3858 0.0003 102 78 An invention has beer, shown with several advantages.
16 The cross-linking system of the present invention provides 17 an increase in viscosity in an aqueous well fracturing fluid 18 by a method which is si:~ple and economical. The cross-19 linking system provides a fracturing fluid which is shear stable at normal fractur'_::g pump rates. The delayed borate 21 cross-linking of the hydrated polymer occurs without the use 22 of suspended solids. Because the delay mechanism does not 23 rely upon the dissolution of solids in solution, the delay 24 time can be precisely ad;usted. Since the rate of cross-linking is a function of the pH of the complexor, the rate 26 can be adjusted while the fob is running at the well site by 27 the addition of caustic or acid to the complexar solution.
28 Because glyoxol, in alkaline water, slowly converts to an 29 acid, it serves to decrease the pH of the polymer gel with time, thereby decreasing the viscosity of the fluid for 31 easier cleanup.

33 While the invention has been shown in only one of its 34 forms, it is not thus limited but is susceptible to various 1 changes and modifications without departing from the spirit 2 thereof.

Claims (22)

1. A method of controlling the cross-linking reaction of an aqueous fracturing fluid in fracturing a subterranean formation, comprising the steps of:
blending together an aqueous fluid and a hydratable polymer capable of gelling in the presence of borate ions;
allowing the polymer to hydrate to form a hydrated polymer sol;
adding an alkaline buffer to thereby adjust the pH of the hydrated polymer sol in the range from about 8.0 to 11.5;
forming a complexor solution for said hydrated polymer gel by combining a cross-linking additive capable of furnishing borate ions in solution with a delay additive selected from the group consisting of dialdehydes having about 2-4 carbon atoms, keto aldehydes having about 3-4 carbon atoms hydroxyl aldehydes having 2-4 carbon atoms, ortho substituted aromatic dialdehydes and ortho substituted aromatic hydroxyl aldehydes, the delay additive being effective, to chemically bond with the borate ions and boric acid produced by the cross-linking additive to thereby limit the number of borate ions available in solution for subsequent cross-linking of the hydrated polymer sol;
adjusting the pH of the complexor solution to achieve a desired delay in the cross-linking reaction of the hydrated polymer gel, the delay achieved being a function of the complexor solution pH, wherein upward adjustment of the pH
of the complexor solution serves to retard the rate of the subsequent cross-linking of the hydrated polymer sol while downward adjustment of the pH of the complexor solution serves to accelerate the rate of subsequent cross-linking;
and adding the complexor solution to the hydrated polymer sol to cross-link the hydrated polymer sol.

2. A method of controlling the cross-linking reaction of an aqueous fracturing fluid in fracturing a subterranean formation, comprising the steps of:
blending together an aqueous fluid and a hydratable polymer capable of gelling in the presence of borate ions;
allowing the polymer to hydrate to form a hydrated polymer sol;
forming a liquid complexor solution for the hydrated polymer sol by combining a cross-linking additive capable of furnishing borate ions in solution with a delay additive, the delay additive being effective, to chemically bond with both boric acid and the borate ions produced by the cross-linking additive to thereby limit the number of borate ions available in solution for subsequent cross-linking of the hydrated polymer sol;
adjusting the pH of the complexor solution in order to control the rate of the subsequent cross-linking of the hydrated polymer sol wherein upward adjustment of the pH of the complexor solution serves to retard the rate of the subsequent cross-linking of the hydrated polymer sol while downward adjustment of the pH of the complexor solution serves to accelerate the rate of subsequent cross-linking;
and adding the complexor solution to the hydrated polymer sol to cross-link the hydrated polymer sol.

3. The method of controlling the cross-linking reaction of an aqueous fracturing fluid of claim 2, wherein the hydratable polymer comprises at least one of:

guars and derivatized guars, locust bean gum, karaya gum, carboxymethyl cellulose, carboxymethylhydroxyethyl cellulose, hydroxyethyl cellulose and polyvinyl alcohol.

4. The method of controlling the cross-linking reaction of an aqueous fracturing fluid of claim 3, wherein the cross-linking additive comprises at least one of:
alkali metal borates, alkaline earth metal borates and boric acid.

5. The method of controlling the cross-linking reaction of an aqueous fracturing fluid of claim 4, wherein said delay additive comprises at least one of:
dialdehydes having about 2-4 carbon atoms, keto aldehydes having about 3-4 carbon atoms, hydroxyl aldehydes having 2-4 carbon atoms, ortho substituted aromatic dialdehydes, and ortho substituted aromatic hydroxyl aldehydes.

6. The method of controlling the cross-linking reaction of an aqueous fracturing fluid of claim 5, wherein the delay additive is glyoxal.

7. A method of fracturing a subterranean formation comprising the steps of:
blending together an aqueous fluid and a hydratable polymer capable of gelling in the presence of borate ions;
allowing the polymer to hydrate to form a hydrated base fluid;
adjusting the pH of the base fluid to above about 7.8;
forming a complexor solution for the base fluid by combining a cross-linking additive capable of furnishing borate ions in solution with a liquid delay additive, the delay additive being added in an amount effective to chemically bond in the alkaline pH base fluid with the borate ions produced by the cross-linking additive to thereby limit the number of borate ions available in solution for subsequent cross-linking of the base fluid; and while maintaining an alkaline pH in the base fluid adding the complexor solution to the base fluid to cross-link the fluid.

8. The method of controlling the cross-linking reaction of an aqueous fracturing fluid of claim 7, wherein the cross-linking additive comprises at least one of:
alkali metal borates, alkaline earth metal borates and boric acid.

9. The method of controlling the cross-linking reaction of an aqueous fracturing fluid of claim 8, wherein said delay additive comprises at least one of:
dialdehydes having about 2-4 carbon atoms, keto aldehydes having about 3-4 carbon atoms, hydroxyl aldehydes having 2-4 carbon atoms, ortho substituted aromatic dialdehydes and ortho substituted aromatic hydroxyl aldehydes.

10. The method of controlling the cross-linking reaction of an aqueous fracturing fluid of claim 9, wherein the delay additive is glyoxal.

11. A method of fracturing a subterranean formation comprising the steps of:
blending together an aqueous fluid and a hydratable polymer capable of gelling in the presence of borate ions, thereby allowing the polymer to hydrate to form a base fluid;

forming a complexor solution for the base fluid by combining a cross-linking additive capable of furnishing borate ions in solution with a delay additive, the delay additive being effective to chemically bond with the borate ions produced by the cross-linking additive to thereby limit the number of borate ions available in solution for subsequent cross-linking of the base fluid; and adding the complexor solution to the base fluid to cross-link the fluid.

12. A method of controlling the cross-linking reaction of an aqueous fracturing fluid in fracturing a subterranean formation, comprising the steps of:
blending together an aqueous fluid and a hydratable polysaccharide capable of gelling in the presence of borate ions, thereby allowing the polysaccharide to hydrate to form a hydrated polymer sol;
forming a liquid complexor solution for the hydrated polymer sol by combining a cross-linking additive capable of furnishing borate ions in solution with a delay additive, the delay additive being effective, to chemically bond with both boric acid and the borate ions produced by the cross-linking additive to thereby limit the number of borate ions available in solution for subsequent cross-linking of the hydrated polymer sol;
adjusting the pH of the complexor solution in order to control the rate of the subsequent cross-linking of the hydrated polymer sol; and adding the complexor solution to the hydrated polymer sol to cross-link the hydrated polymer sol.

13. The method of controlling the cross-linking reaction of an aqueous fracturing fluid of claim 12, wherein the hydratable polysaccharide comprises at least one of:

guars and derivatized guars, locust bean gum, karaya gum, carboxymethyl cellulose, carboxymethylhydroxyethyl cellulose, hydroxyethyl cellulose and polyvinyl alcohol.

14. The method of controlling the cross-linking reaction of an aqueous fracturing fluid of claim 13, wherein the cross-linking additive comprises at least one of:
alkali metal borates, alkaline earth metal borates and boric acid.

15. The method of controlling the cross-linking reaction of an aqueous fracturing fluid of claim 14, wherein said delay additive comprises at least one of:
dialdehydes having 2-4 carbon atoms in the carbon chain, keto aldehydes having 3-4 carbon atoms in the carbon chain, hydroxyl aldehydes having 2-4 carbon atoms in the carbon chains, ortho substituted aromatic dialdehydes and ortho substituted aromatic hydroxyl aldehydes.

16. The method of controlling the cross-linking reaction of an aqueous fracturing fluid of claim 15, wherein the delay additive is glyoxal.

17. A method of controlling the cross-linking reaction of an aqueous fracturing fluid in fracturing a subterranean formation, comprising the steps of:
blending together an aqueous fluid and a hydratable polysaccharide capable of gelling in the presence of borate ions, thereby allowing the polysaccharide to hydrate to form a hydrated polymer sol;
adding an alkaline buffer to thereby adjust the pH of the hydrated polymer sol in the range from 8.0 to 11.5;
forming a complexor solution for said hydrated polymer gel by combining a cross-linking additive capable of furnishing borate ions in solution with a delay additive selected from dialdehydes having 2-4 carbon atoms in the carbon chain, keto aldehydes having 3-4 carbon atoms in the carbon chain, hydroxyl aldehydes having 2-4 carbon atoms in the carbon chains, ortho substituted aromatic dialdehydes and ortho substituted aromatic hydroxyl aldehydes, the delay additive being effective, to chemically bond with the borate ions and boric acid produced by the cross-linking additive to thereby limit the number of borate ions available in solution for subsequent cross-linking of the hydrated polymer sol;
adjusting the pH of the complexor solution to achieve a desired delay in the cross-linking reaction of the hydrated polymer gel, the delay achieved being a function of the complexor solution pH.

18. The method of controlling the cross-linking reaction of an aqueous fracturing fluid in fracturing a subterranean formation of claim 17, further comprising the steps of:
selecting a delay additive which slowly converts to a strong acid in an alkaline solution, thereby reducing the viscosity of the fracturing fluid over time.

19. A complexor solution when used to initiate a time controlled cross-linking reaction with an aqueous hydrated polysaccharide fracturing fluid, the complexor comprising:
a mixture of cross-linking compound which supplies borate ions in solution and a liquid delay additive which controls the rate at which said cross-linking compound promotes cross-linking of said polysaccharide, the control rate being a function of the pH of the complexor solution, said liquid delay additive being selected from dialdehydes having 2-4 carbon atoms, keto aldehydes having 3-4 carbon atoms, hydroxyl aldehydes having 2-4 carbon atoms in the carbon chains, ortho substituted aromatic dialdehydes and ortho substituted aromatic hydroxyl aldehydes.

20. The composition of claim 19, wherein said cross-linking compound comprises at least one of:
alkali metal borates, alkaline earth metal borates and boric acid.

21. The composition of claim 20, when used with a cross-linked hydratable polysaccharide comprises at least one of:
guars and derivatized guars, locust bean gum, karaya gum, carboxymethyl cellulose, carboxymethylhydroxyethyl cellulose, hydroxyethyl cellulose and polyvinyl alcohol.

22. The composition of claim 21, wherein said liquid delay additive is glyoxal.