Method of fracturing a subterranean formation

Abstract: The crosslinking of galactomannan gum fracturing fluids by borate ion is delayed for high temperature applications by complexing boron in a basic solution with an organic polyol. Low pumping friction pressures are achieved by the delay in the crosslinking with the boron being available at high temperatures to add high temperature stability to the fracturing fluid at temperatures ranging upto about 300°F.

Description: This invention relates to the art of recovery of hydrocarbon fluids from subterranean formations, and particularly, to a fracturing process. BACKGROUND OF THE INVENTION In the art of recovering hydrocarbon values from subterranean formations, it is common, particularly in formations of low permeability, to hydraulically fracture the hydrocarbon-bearing formation to provide flow channels to facilitate production of the hydrocarbons to the wellbore. Fracturing fluids typically comprise a water or oil base fluid incorporating a polymeric thickening agent. The polymeric thickening agent helps to control leak-off of the fracturing fluid into the formation, aids in the transfer of hydraulic fracturing pressure to the rock surfaces and, primarily, permits the suspension of particulate proppant materials which remain in place within the fracture when fracturing pressure is released. Typical polymeric thickening agents for use in fracturing fluids comprise galactomannan gums such as guar and substituted guars such as hydroxypropyl guar or carboxymethylhydroxypropyl guar. Cellulosic polymers such as hydroxyethyl cellulose may also be used as well as synthetic polymers such as polyacrylamide. To increase the viscosity and, thus, the proppant carrying capacity as well as to increase the high temperature stability of the fracturing fluid, crosslinking of the polymers is also commonly practiced. Typical crosslinking agents comprise soluble boron, zirconium or titanium compounds. These metal ions provide for crosslinking or tying together of the polymer chains to increase the viscosity and improve the rheology of the fracturing fluid. Of necessity, fracturing fluids are prepared on the surface and then pumped through tubing in the wellbore to the hydrocarbon-bearing subterranean formation. While high viscosity is a desirable characteristic of a fluid within the formation in order to efficiently transfer fracturing pressures to the rock as well as to reduce fluid leak-off, large amounts of hydraulic horsepower are required to pump such high viscosity fluids through the well tubing to the formation. In order to reduce the friction pressure, various methods of delaying the crosslinking of the polymers in a fracturing fluid have been developed. This allows the pumping of a relatively less viscous fracturing fluid having relatively low friction pressures within the well tubing with crosslinking being effected at or near the subterranean formation so that the advantageous properties of the thickened crosslinked fluid are available at the rock face. One typical delayed crosslinking fracturing fluid system comprises borate crosslinked galactomannan gums such as guar or hydroxypropyl guar (HPG). The galactomannan polymers are generally provided to a blender in solid, powder form, or more typically, suspended in a hydrocarbon such as kerosene or diesel. When added to a neutral or acidic aqueous solution, the galactomannan gum hydrates to form a gel. Hydration of guar and HPG will only take place under neutral or acidic conditions, that is, at a pH of about 7 or less. Under these pH conditions, no crosslinking of guar or HPG will occur with borate ion. In order to effect borate crosslinking of guar and HPG, the pH must be raised to at least 9.5. It is this raising of the pH requirement which has been exploited in the prior art to effect a delay in the crosslinking of galactomannan gums by borate ion. One typical mechanism for delaying the elevation of the pH is to use a low-solubility base such as magnesium oxide (MgO). MgO is added to the hydrated, acidic galacatomannan gum solution along with a boron releasing compound. Since the solution is initially acidic, there is no crosslinking of the polymers effected by the presence of boron (or borate ion) in solution. As the MgO slowly solubilizes in the system, the pH is gradually raised according to the following reaction: (1) MgO + H 2 O yields Mg +2 + 2OH - It is also been suggested that the solubilization of the MgO be further delayed by dispersing solid particulate MgO in hydrocarbon droplets with a surfactant which further slows the solubilization of the MgO. The borate crosslinking of a galactomannan gum is, however, a reversible reaction should the pH of the solution drop below the required pH of about 9.5 overa period of time. At temperatures of above about 93° C (200° F), magnesium ion combines with hydroxide ion to produce insoluble magnesium hydroxide which causes a lowering of the pH of the fracturing fluid, and which in turn, destabilises the fluid through breaking of the borate crosslink. Thus the use of borate crosslinked galactomannan gums in fracturing high temperatures formations above about 93°C (200° F) is limited by the high pumping friction pressures required to pump a stable non-delayed borate-crosslinked fluid. The advantages of good clean up and removal of borate crosslinked galactomannan gums as well as their lower cost cannot be effectively employed above these temperatures. US-A-3215634 relates to viscous liquids prepared by cross-linking water-soluble polymers in aqueous solutions. According to this reference, a suspended polysaccharide is reacted with a crosslinking agent (borate ions) in the presence of a polyhydric alcohol of the type comprising from 2 to 5 carbon atoms and having at least two adjacent hydroxyl groups. Due to the polyhydric alcohol, the borate ions are not readily available for crosslinking purposes at low temperatures (about 30° c) but become available at higher temperature (about 60° C). US-A-3058909 relates to a typical fracturing fluid system where the crosslinking of the gum is adjusted by lowering or raising the pH. EP-132 044 relates to compositions useful for delaying the zirconium-crosslinking of guar gum. It mentions sorbitol within a large list of polyhydroxyl-containing compounds. The gel is made over a large pH range including acidic or neutral pH. SUMMARY OF THE INVENTION The present invention provides a method of fracturing which allows the delay of borate crosslinking of a galactomannan gum fracturing fluid for use at temperatures well above the former 93° C (200° F) temperature limitation of prior art systems. In accordance with the invention, a method of fracturing a selected subterranean formation at temperature above 93° C (200° F) comprises the steps of adding to an aqueous solution a galactomannan gum, allowing the gum to hydrate to form a base fluid, adding to the base fluid a boron compound capable of furnishing borate ions in solution (said borate ions capable of crosslinking the galactomannan gum), further adding to the base fluid an organic polyol capable of complexing the borate ions and a base and pumping the fluid to a subterranean formation whereas the complexing reaction of the organic polyol with the borate ions prevents syneresis at low mixing and temperatures but allows the release of borate to effect crosslinking at fracturing pressures, at temperatures of up to about 148° C (300° F). Still further in accordance with the invention, the organic polyol is selected sodium gluconate, glucose, sorbitol and dextrose. BRIEF DESCRIPTION OF THE DRAWING These and other objects of the invention are accomplished through the manner and form of the present invention to be described more fully hereinafter and in conjunction with the accompanying drawing forming a part of this specification and in which the sole Figure is a graphic illustration of the delay in the crosslink time at various concentration levels for exemplary boron complexing agents in accordance with the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS AND THE DRAWINGS The invention will now be more fully described in the more limited aspects of preferred embodiments thereof. In accordance with the invention, the crosslinking of a galactomannan gum with borate ion is delayed by complexing boron in an aqueous alkaline solution using a complexing agent comprising an organic polyol in accordance with equilibrium reactions (2) below: As used in this specification, the term "polyol" will be understood to mean an organic compound having adjacent alcohol functions as set forth in 3 below: Thus, a polyol may comprise such materials as glycols, glycerin, saccharides such as glucose, sorbitol, dextrose, mannose, mannitol and the like as well as other carbohydrates and polysaccharides including natural and synthetic gums. Also included in the term "polyol" are acid, acid salt, ester and amine derivatives of a polyol. The complex is formed by adding a base and a polyol to an aqueous solution of boric acid or other boron containing compound. Since reaction (2) is an equilibrium reaction, the materials may be added at least in a stoichiometric ratio or, more preferably, more alkaline material and/or more polyol may be added to form additional amounts of complexed boron. The more complexed boron present with the equilibrium reaction, the longer the crosslinking reaction of boron ion with a galactomannan gum is delayed. Those skilled in the art are easily capable of determining the various desired ratios of these three components in order to effect the desired delay time in the crosslinking reaction. When sea water is used to makeup the fracturing fluid, i.e. when the base fluid in which the galactomannan gum is dispersed and hydrated is sea water, the solution comprising a boron-containing compound, a polyol and a base, of necessity includes a significantly larger amount of base in order to raise the pH of sea water at the elevated temperatures. Any common base such as an alkali metal or ammonium hydroxide or an alkaline earth metal hydroxide may be used to adjust the pH. It is contemplated within the scope of the invention that the boron, the pH adjusting base and the complexing polyol be added to an aqueous hydrated galactomannan gum solution as a single additive solution although it is certainly possible to add these materials individually. In a preferred embodiment, a continuous mix or on-the-fly mixing procedure is followed wherein the galactomannan gum is supplied to the makeup water such as fresh water, brine or sea water and is passed through a continuous mixer allowing sufficient residence time for substantially complete hydration of the galactomannan gum followed by the addition of the delayed crosslinking agent as described above. The fracturing fluid is then in a condition to be pumped into the wellbore to the formation to be fractured either as a pad or with the addition of proppant material in normal loadings of up to 7.2576 kg or more (sixteen or more pounds) of proppant per 3.785 liters (one gallon) of fracturing fluid. As an hydrated galactomannan gum solution immediately prior to pumping. The following examples illustrate several preferred embodiments of a fracturing fluid showing various delay times in the crosslinking of a typical galactomannan gum fracturing fluid using several different types of polyols materials, said fracturing fluids appropriate for the performance of the invention. EXAMPLE 1 A base fracturing fluid comprising 18.144 kg (forty pounds) of guar per 3785 liters (one thousand gallons) of fracturing fluid was prepared and was allowed to hydrate. The fracturingfluid contained minor amounts of normal, nonactive (from the standpoint of crosslinking activity) fracturing fluid additives such as surfactant, a biocide and a defoamer. This base fracturing fluid will be used in all subsequent examples made up, as stated, with either fresh water or sea water. To this base fluid, 2.268 kg (five pounds) of boric acid and 15.1416 liters (four gallons) of 30% sodium hydroxide solution per 3785 liters (thousand gallons) of base fluid were mixed together and then aded to the base fluid. Crosslinking of the guar was effected in a period of approximately 10 to 15 seconds. EXAMPLE 2 To the base fluid of Example 1 made up with sea water, 2.268 kg (five pounds) of boric acid, 11.34 kg (twenty-five pounds) of sodium gluconate and 15.1416 liters (four gallons) of 30% sodium hydroxide solution (molar ratio 1:1.4:4) were mixed together and then added per 3785 liters (thousand gallons) base fluid. A delay in the crosslinking of the base fluid of three to four minutes was obtained. EXAMPLE 3 Using the base fracturing fluid of Example 1 made up with sea water, 2.268 kg (five pounds) of boric acid, 16.329 kg (thirty-six pounds) of sodium gluconate and 4.536 kg (ten pounds) of sodium hydroxide (molar ration 1:2:3) mixed together and then were added per 3785 liters (thousand gallons) of base fluid. A crosslinking delay time of approximately ten minutes was obtained. EXAMPLE 4 Using the base fluid formulation of Example 1 made up with fresh water, a four minute crosslinking delay was achieved by adding a mixture of 2.268 kg (five pounds) boric acid, 13.608 kg (thirty pounds) glucose or dextrose and 4.536 kg (ten pounds) of sodium hydroxide (molar ratio 1:2:3) per 3785 liters (thousand gallons) of base fluid. EXAMPLE 5 A one hundred eighty minute delay in crosslinking was achieved with the addition of a mixture of 2.268 kg (five pounds) boric acid, 13.608 kg (thirty pounds) sorbitol and 4.536 kg (ten pounds) sodium hydroxide all per 3785 liters (thousand gallons) of base fluid using fresh water in the base fluid of Example 1. EXAMPLE 6 A three minute delay in gel crosslinking was achieved with the addition of 2.268 kg (five pounds) boric acid, 6.804 kg (fifteen pounds) sorbitol and 4:536 kg (ten pounds) sodium hydroxide (molar ratio 1:1:3) to a fresh water base fluid as used in Example 1. The Figure illustrates additional testing in a 18.144 kg (forty pound) per 3785 liters (thousand gallons) guar solution illustrating the various delay times with various amounts of several different polyol complexing agents such as illustrated in the forgoing examples. From this Figure and the examples given, one skilled in the art can clearly select the desired delay time in the crosslinking action of the particular complexing agent. Crosslink delay times are chosen based on pumping parameters of the fluid through the wellbore and, thus, vary with the pumping flow rate and the diameter and length of the well tubing. Typical desired delay times are in the range of 3-6 minutes. The complexing of the borate using the polyol complexing agent of the present invention prevents syneresis at low mixing and pumping temperatures but allows the release of borate to effect crosslinking at high temperatures to give the fluid high temperature stability in a range of up to about 148°C (300°F). The complexing agent permits the formulation of high temperature system at low temperatures and the delay permits low pumping friction pressure during the pumping of the fracturing fluid to the formation with the desired crosslinking and its attendant viscosity and rheology benefits being available at high formation temperatures following fluid heat up in the formation.

Claims: A method of fracturing a selected subterranean formation at temperature above 93° C (200° F) comprising the steps of: adding to an aqueous solution a galactomannan gum, allowing the gum to hydrate to form a base fluid; adding to the base fluid a boron compound capable of furnishing borate ions in solution; said borate ions capable of crosslinking the galactomannan gum; further adding to the base fluid an organic polyol capable of complexing the borate ions and a base, the organic polyol comprising an organic compound having adjacent alcohol functions as set forth below: pumping the fluid to a subterranean formation whereby the complexing reaction of the organic polyol with the borate ions prevents syneresis at low mixing and pumping temperatures but allows the release of borate to effect crosslinking at fracturing pressures at temperatures of up to 148° C (300° F). A method as claimed in claim 1, wherein the organic polyol complexes the boron in the manner shown below: A method as claimed in claim 1 or 2, wherein the organic polyol is selected from glycols, glycerine, saccharides inlcuding glucose, sorbitol, dextrose, marinose, mannitol, other carbohydrates, polysaccharides including narutal and synthetic gums. A method as claimed in claim 1, 2 or 3, further including the step of adding a particulate proppant material prior to the step of pumping. A method as claimed in any of the preceding claims, wherein the step of providing a hydrated galactomannan gum comprises providing a hydrated guar solution, or a hydrated hydroxypropyl guar solution.

Inventors: AINLEY BRIAN R, MCCONNELL S BRUCE, AINLEY, BRIAN R., MCCONNELL, S. BRUCE
Applicants: DOWELL SCHLUMBERGER SERVICES, PUMPTECH NV, COMPAGNIE DES SERVICES DOWELL SCHLUMBERGER S.A., PUMPTECH N.V.
Application year: 1992
Main classification: E21B43
IPC codes: C09K8, C08K5