Method and compositions for fracturing subterranean formations

An aqueous gel containing a retarded crosslinking composition comprising a zirconium salt or chelate and a polyhydroxyl-containing compound and optionally including an aqueous fluid or an alkanol. The gel is useful for fracturing and placing propping agents within a subterranean formation. The gel has a high viscosity in the formation and has pumping characteristics in turbulent flow similar to those of the base gel.

Background of the Invention 
1. Field of the Invention 
This invention relates to methods and compositions for the hydraulic 
fracturing of subterranean formations. It more particularly relates to 
methods and compositions for fracturing a subterranean formation 
penetrated by a well bore wherein a fluid composition having retarded 
crosslinking properties is injected into a formation through a suitable 
conduit at a rate and pressure sufficient to produce a fracture in the 
formation. 
2. Brief Description of the Prior Art 
In the drilling, completion and treatment of subterranean formations 
pentrated by well bores, viscous treating fluids commonly are utilized. In 
such operations, it often is desirable or necessary that the viscous 
treating fluids have relatively low initial viscosities, but when placed 
in the well bore or subterranean formation to be treated, the viscosities 
of the fluids increase. For example, in performing a subterranean 
fracturing process on a hydrocarbon-bearing formation to stimulate the 
production of hydrocarbons therefrom, a treating fluid which has a low 
viscosity and a low friction pressure when being pumped but which exhibits 
a high viscosity in the formation is desirable. 
Generally, in the art of hydraulic fracturing, a fluid is introduced 
through a conduit, such as tubing or casing, disposed in the well bore 
into a formation sought to be fractured. The fluid is introduced at a rate 
and pressure sufficient to produce a fracture or fractures in the 
formation and to extend the produced fracture or fractures from the well 
bore into the formation. Upon the creation of the fracture or fractures, 
additional fracturing fluid containing solid proppant materials can be 
introduced into the fracture or fractures in the event the initial fluid 
did not contain any proppant. Following this treatment, the introduced 
fluid is recovered from the formation, but the proppant remains in the 
produced fracture or fractures to thereby prevent the complete closure 
thereof. The propped fracture creates a conductive channel extending from 
the well bore into the formation. 
The conductivity of a propped fracture is effected by the particle size of 
the proppant material placed in the fracture. The particle size of the 
proppant that can be used depends upon the width to which the particular 
fracture can be opened during the introduction of the fracturing fluid. 
The fracture width normally is directly proportional to the viscosity of 
the fracturing fluid. In addition, the use of fracturing fluids having 
relatively high viscosities is advantageous since such fluids can support 
the proppant particles suspended therein without excessive settling. The 
use of such high viscosity fluids also permits the placement of relatively 
large-size proppant material in the fracture without a screenout 
occurring, that is, without the proppant bridging across the mouth of the 
fracture and preventing the introduction of proppant therein. 
The use of desirably high viscosity fracturing fluids undesirably is 
accompanied by the problem of high friction losses usually encountered 
during the introduction of such fluids into a formation through the 
conduit, such as tubing or casing, disposed in the well bore. Since the 
pumping equipment and tubular goods are limited in capacity and operating 
pressure, the viscosity of the fluid which can be pumped also is limited. 
The viscosity of the fluid must be low enough that excessive friction 
losses and high well head pumping pressures are not encountered. 
Summary of the Invention 
By the present invention there are provided methods of forming and using an 
improved viscous treating fluid. The treating fluid has an initial 
viscosity such that solid proppants can be suspended therein and carried 
thereby without excessive settling, but the viscosity of the fluid is not 
so high that excessive friction pressures are encountered in pumping the 
fluid. Thus, according to this invention, an aqueous gel is provided which 
contains a retarded crosslinking composition capable of effecting a 
delayed crosslinking of the gelling agent in the aqueous gelled fluid to 
produce a fluid of significantly higher viscosity. 
Brief Description of the Preferred Embodiment 
In accordance with the present invention an aqueous gel is provided 
comprising an aqueous fluid, a gelling agent, and a retarded crosslinking 
composition which is soluble in the aqueous fluid and capable of effecting 
a delayed crosslinking of the gelling agent. The aqueous gel has a 
non-Newtonian viscosity in laminar flow. However, during introduction of 
the aqueous gel into the formation through a conduit in which the fluid is 
in turbulent flow, the viscosity is no greater than that imparted by the 
gelling agent before crosslinking. The aqueous gel of the present 
invention can carry great quantities of proppants into a formation sought 
to be fractured and can be introduced into the formation at suitably high 
rates with pumping equipment and tubular goods normally available at the 
wellhead. 
The aqueous fluid utilized herein is defined as a water-alcohol solution 
having from 0 to 80 percent and preferably from about 0 to 40 percent and 
most preferably from about 0 to 10 percent alcohol by volume of the 
solution. The preferred alcohols are alkanols having from 1 to 5 carbon 
atoms. Examples of alcohols believed to be useful in the aqueous fluid 
include methanol, ethanol, propanol, isopropanol, butanol, isobutanol, 
pentanol, furfuryl alcohol, ethylene glycol, and ethoxylated derivatives 
thereof. 
The aqueous fluid is used to solvate the gelling agent. The solvated 
gelling agent is referred to hereinafter as a "base gel". The pH of the 
aqueous fluid can be adjusted, if necessary, to render the fluid 
compatible with the crosslinking agent used to crosslink the solvated 
gelling agent. The pH adjusting material can be added to the aqueous fluid 
before, after, or during addition of the gelling agent to the aqueous 
fluid. 
The gelling agent useful in the present invention is selected from 
solvatable polysaccharides having molecular weights of at least 100,000. 
Examples of polysaccharides useful herein include the galactomannan gums, 
glucomannan gums, and their derivatives. Solvatable galactomannan and 
glucomannan gums are naturally occurring. The galactomannan gums and 
glucomannan gums also can be reacted with hydrophilic constituents to 
thereby produce gelling agents useful herein. 
Solvatable polysaccharides having molecular weights of less than about 
100,000 do not form crosslinked gels which are useful herein. The most 
preferred solvatable polysaccharides useful herein have molecular weights 
in the range of from about 200,000 to about 3,000,000. 
Guar gum, locust bean gum, karaya gum, sodium carboxymethylguar, 
hydroxyethylguar, sodium carboxymethylhydroxyethylguar, hydroxypropylguar 
and sodium carboxymethylhydroxypropylguar are examples of gelling agents 
useful herein. 
The preferred gelling agents are guar gum, hydroxypropylguar and sodium 
carboxymethylhydroxypropylguar. The most preferred gelling agent is 
hydroxypropylguar. 
The gelling agent useful herein is present in the aqueous fluid in a 
concentration in the range of from about 0.2 to about 1.25 percent, 
preferably from about 0.2 to about 1.0 percent and most preferably from 
about 0.4 to about 0.7 percent by weight of the aqueous fluid. A gelling 
agent concentration of less than about 0.2 percent by weight of the 
aqueous fluid is not a sufficient quantity of gelling agent to permit 
effective crosslinking of the gel within the formation. 
The discovery now has been made that the introduction of a retarded 
crosslinking composition comprising an admixture of a crosslinking 
compound and a polyhydroxyl-containing compound to the base gel will 
provide a controllable delay in the rate of the crosslinking reaction. 
This retarded aqueous gel readily can be introduced through a conduit into 
a subterranean formation sought to be fractured as a result of its 
relatively low initial viscosity. The significant increase in the 
viscosity of the gel through crosslinking as it reaches the lower portion 
of the conduit or upon entry into the formation facilitates the fracturing 
process through a reduction in the hydraulic horsepower necessary to 
effect the fracture. 
The retarded crosslinking composition also can contain an aqueous fluid or 
an alkanol having from about 1 to 6 carbon atoms. The presence of 
controlled amounts of the aqueous or alkanol in the retarded crosslinking 
composition has been found to further delay the rate of the crosslinking 
reaction. 
The retarded crosslinking composition of the present invention utilizes 
crosslinking compounds which feature the presence of zirconium in the +4 
oxidation state and are referred to as zirconium salts or chelates. An 
example of a zirconium (IV)-containing crosslinking compound useful herein 
is zirconium (IV) acetylacetonate chelate which is available from Kay 
Fries, Rockleigh, N.J. Another example of a zirconium salt or chelate 
useful herein is zirconium carbonate available from Magnesium Electron, 
Starret, Tex. Yet another example is zirconium lactate and zirconium 
diisopropylamine lactate which are available from Zirconium Technology, 
Gainsville, Fla. 
The crosslinking mechanism is not fully understood. However, it is believed 
that the zirconium does not experience any sort of valence change during 
the crosslinking reaction. 
The amount of crosslinking compound useful to crosslink the gelling agent 
of this invention is that which provides a zirconium ion concentration in 
the range of from about 0.0005 percent to in excess of about 0.01 percent 
by weight of the aqueous gelled fluid. The preferred concentration is in 
the range of from about 0.001 percent to about 0.01 percent by weight of 
the aqueous gelled fluid. 
The rate of the unretarded crosslinking reaction is extremely rapid. At 
ambient temperature conditions, the zirconium chelates comprising the 
crosslinking compound can crosslink the polysaccharides, comprising the 
gelling agent in as little as 10 to 15 seconds. When the aqueous fluid of 
the base gel is maintained at an elevated temperature, such as when 
preheated solutions are employed having a temperature above 100.degree. 
F., the unretarded crosslinking reaction occurs almost instantaneously 
upon introduction of the crosslinking compound into the base gel. Such 
rapid reaction rates do not permit the gelled fluid to be pumped into the 
subterranean formation before a signifcant increase in the viscosity of 
the fluid occurs. 
The surprising discovery now has been made that admixing the crosslinking 
compound with at least a polyhydroxyl-containing compound in predetermined 
amounts produces a composition which can be used to delay the rate of the 
crosslinking reaction for a period of time sufficient to permit pumping of 
the aqueous gel through the conduit to the subterranean formation. The 
crosslinking compound and polyhydroxyl-containing compound also can be 
admixed with an aqueous fluid or a selected alkanol. Typically, this time 
can be from several minutes to hours in extremely deep formations. 
The polyhydroxyl-containing compound useful in the present invention is 
selected from the polyhydroxyl-containing compounds having from 3 to 7 
carbon atoms. Examples of compounds useful herein include glycerol, 
erythritol, threitol, ribitol, arabinitol, xylitol, allitol, altritol, 
sorbitol, mannitol, dulcitol, iditol, perseitol, and the like. The 
preferred polyhydroxyl-containing compound for use in the invention is 
glycerol. The compound can be in solid or liquid form when admixed with 
the aqueous and complexing compound of the present invention. 
The polyhydroxyl-containing compound useful herein is admixed with an 
aqueous fluid and the crosslinking compound in an amount sufficient to 
provide a controlled delay in the crosslinking rate of the base gel. The 
particular amount of polyhydroxyl-containing compound necessary to delay 
the crosslinking reaction will depend upon the specific gelling agent, 
crosslinking compound and polyhydroxyl-containing compound utilized as 
well as the equipment available at the wellhead and tubular goods which 
will affect the pumping rate of the aqueous gel into the formation. 
The aqueous fluid utilized to formulate the retarded complexing composition 
can comprise substantially any aqueous solution which does not adversely 
react with the gelling agent, crosslinking compound or 
polyhydroxyl-containing compound. Preferably, the aqueous fluid comprises 
water. 
The alkanol utilized to formulate the retarded complexing composition can 
comprise alcohols having from 1 to 6 carbon atoms. Examples of alcohols 
believed to be useful in the composition include methanol, ethanol, 
propanol, isopropanol, butanol, pentanol, hexanol, ethylene glycol and 
ethoxylated derivatives thereof. 
The retarded crosslinking composition is prepared by admixing the 
crosslinking compound and polyhydroxyl-containing compound in 
predetermined amounts. The constituents are admixed in a volumetric ratio 
of crosslinking compound to polyhydroxyl-containing compound in the range 
of from about 0.01:1 to about 100:1. Preferably, the volumetric ratio is 
in the range of from about 0.1:1 to about 10:1 and, most preferably, the 
volumetric ratio is about 0.5:1 to about 5:1. 
When an aqueous fluid or alkanol is present, the constituents are admixed 
in a volumetric ratio of crosslinking compound to polyhydroxyl-containing 
compound to aqueous fluid or alkanol in the range of from about 1:10:10 to 
about 1:0.1:0.1. Preferably, the volumetric ratio is in the range of from 
about 1:0.5:0.5 to about 1:2:2. 
The constituents of the retarded crosslinking composition can be admixed in 
any order in any conventional mixing apparatus, such as for example, a 
batch mixer. When an aqueous-containing solution of the crosslinking 
compound is utilized, the aqueous portion is included in determining the 
total aqueous fluid content of the retarded crosslinking composition. The 
retarded crosslinking composition can be admixed with the aqueous gel in 
an amount of from about 0.01 gallon to about 0.5 gallon per 10 pounds of 
gelling agent. 
Surprisingly, it has been found that the high temperature rheological 
properties of the aqueous gels formed with the retarded crosslinking 
composition of the present invention improve when the retarded 
crosslinking composition is "aged" prior to use. The term "aged" as used 
herein is intended to mean that the admixture comprising the retarded 
crosslinking composition is held in an appropriate container after 
formulation for a period of from a few minutes to over several weeks prior 
to use. Preferably, the retarded crosslinking composition is aged for from 
about 8 hours to about 25 weeks. It has been found that when the retarded 
crosslinking composition is aged at a generally constant temperature, the 
low-temperature crosslinking reaction rate declines while the high 
temperature viscosity of an aqueous gelled fluid crosslinked with the 
retarded crosslinking composition increases. When the retarded 
crosslinking composition is aged at a temperature above ambient 
temperature, such as for example, an elevated temperature such as from 
about 100.degree. F. to 180.degree. F., the rate of decline in the 
crosslinking reaction rate and rate of increase in the high temperature 
viscosity of the aqueous gelled fluid are enhanced. This permits the 
production of retarded crosslinking compositions having preselected 
properties by controlling the time and temperature of the aging. 
Conventional propping agents can be employed with the fracturing fluid 
compositions of the present invention, examples of which are quartz sand 
grains, tempered glass beads, rounded walnut shell fragments, aluminum 
pellets, sintered bauxite, nylon pellets, and similar materials. Propping 
agents generally are used in concentrations in the range of from about 1 
to about 10 pounds per gallon of the aqueous fluid; however, higher or 
lower concentrations may be used as required. The particle size of 
propping agent employed is a function of the nature of the formation to be 
fractured, the pressure required to produce the fracture, and pumping 
fluid flow rates available, as well as other known factors. However, 
particle sizes in the range of from about 200 to about 2 mesh on the U.S. 
Sieve Series scale can be employed in fracturing well formations with the 
compositions of the present invention. 
The aqueous gel of the present invention can be prepared for use by mixing 
a predetermined quantity of the solvatable polysaccharide gelling agent 
with a quantity of aqueous fluid to form a solvated gel. Any conventional 
batch mixing apparatus can be employed for this purpose. After the gelling 
agent and aqueous fluid have been mixed for a time sufficient to dissolve 
the gelling agent and form the base gel, a quantity of the retarded 
crosslinking composition is mixed with the gel. The mixture then is pumped 
into the wellbore and into the formation as the retarded crosslinking 
reaction takes place. Proppant generally is added to the base gel prior to 
addition of the retarded crosslinking composition as the gel is introduced 
into the wellbore. 
The aqueous gel of this invention can be made over a wide pH range and be 
useful for fracturing subterranean formations. The rate at which the 
crosslinking reaction proceeds at normal temperatures (about 60.degree. F. 
to about 120.degree. F.) is a function of the pH of the base gel. To 
assure that the crosslinking reaction takes place in the desired period of 
time, the pH of the aqueous fluid or of the base gel can be adjusted to a 
desired level within the range of from about pH 5.0 to about 11.0 and, 
preferably, to a level within the range of from about 9 to about 10.5 by 
the addition of a pH adjusting chemical. Since water from most sources is 
substantially neutral, the chemical or chemicals used for this purpose can 
be acids, acid buffers, mixtures thereof, or mixtures of acids and bases. 
Examples of suitable acids are hydrochloric acid, formic acid, acetic 
acid, fumaric acid, and phthalic acid. Examples of suitable buffers are 
potassium biphthalate, sodium hydrogen fumarate, sodium bicarbonate and 
sodium carbonate. Examples of mixtures of acids and bases are fumaric acid 
and sodium fumarate, adipic acid and sodium bicarbonate, and fumaric acid 
and sodium carbonate. 
A presently preferred process for fracturing a subterranean formation 
penetrated by a well bore comprises injecting down the well bore and into 
the formation, at a pressure sufficient to fracture the formation, a fluid 
comprising an aqueous gel which is prepared by adding from about 30 to 
about 70 pounds of gelling agent comprising hydroxypropylguar to each 
1,000 gallons of aqueous fluid containing about 0 to about 40 percent by 
volume methanol. If desired, the pH of the aqueous fluid can be adjusted 
by the addition of a sufficient quantity of a buffering agent such as 
fumaric acid, formic acid, sodium carbonate or sodium bicarbonate. The 
base gel is introduced into the well bore and, as it is introduced, a sand 
proppant is introduced in an amount of from about 1 pound to about 10 
pounds per gallon and the retarded crosslinking composition then is 
introduced. The retarded crosslinking composition is comprised of 
zirconium (IV) lactate, glycerol and methanol in a volumetric ratio of 
about 1:1:1 and is introduced at the rate of 0.1 to about 0.25 gallon per 
each 10 pounds of gelling agent per each 1,000 gallons of aqueous fluid. 
After the aqueous gel has been pumped into the subterranean formation and a 
fracture has been formed, it is desirable to convert the gel into a low 
viscosity fluid so that it can be recovered from the formation through the 
well bore. This conversion often is referred to as "breaking" the gel. 
There are various methods available for breaking the aqueous gel of the 
present invention. The gels of the present invention break after either or 
both the passage of time and prolonged exposure to high temperatures. 
However, it is desirable to be able to predict breaking time within 
relatively narrow limits. Therefore, breakers optionally can be included 
in the crosslinked gel of the present invention. Mild oxidizing agents are 
useful as breakers when a gel is used in a relatively high temperature 
formation, although formation temperatures of 200.degree. F. or greater 
will generally break the gel relatively quickly without the aid of an 
oxidizing agent. A suitable oxidizing agent is ammonium persulfate. For 
crosslinked gels used at temperatures below about 140.degree. F., enzymes 
are generally used as breakers. Suitable enzymes for such use are alpha 
and beta amylases, amyloglucosidase, oligoglucosidase, invertase, maltase, 
cellulase, and hemicellulase.