Earth boring – well treating – and oil field chemistry – Well treating – Contains organic component
Reexamination Certificate
1998-07-31
2001-06-05
Tucker, Philip (Department: 1712)
Earth boring, well treating, and oil field chemistry
Well treating
Contains organic component
C507S209000, C507S269000, C507S271000, C507S274000, C507S277000, C507S922000, C166S308400
Reexamination Certificate
active
06242390
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to polymer returns during flowback after proppant placement with a conventional polymer gel; more particularly, it relates to using chelants to increase the return of crosslinked polymers.
DESCRIPTION OF THE PRIOR ART
In the recovery of hydrocarbons from subterranean formations, it is common practice to subject the formation to a fracturing treatment. The fracturing treatment is designed to provide flow channels in the formation and enhance recovery of the hydrocarbons.
During hydraulic fracturing, a fracturing fluid is injected down a wellbore under pressure and forced against the formation strata to penetrate the formation and create flow channels. The fracturing fluid generally contains at least a thickened or gelled aqueous solution and proppant particles. The thickened or gelled aqueous solution must be of sufficient viscosity that it can penetrate the formation strata under the applied pressure.
Natural polymers including polysaccharides have been crosslinked to yield the thickened or gelled aqueous solution. The crosslinker binds the molecular strands of the polymer into a network which provides needed viscosity to carry proppants into the formation. Borate, titanate, and zirconate are preferred crosslinkers.
Proppants are carried with the solution into the created fractures or conductive channels. When the fracturing pressure is removed, the proppants remain in the fractures and prevent the fractures from closing—they prop open the fractures. Typical proppants include sand, walnut shells, and sintered bauxite. Others are known in the art.
After the fracturing is complete, it is desirable to remove the thickened or gelled aqueous solution. The recovery of the fracturing fluid is accomplished by reducing its viscosity to a low value such that it flows naturally from the formation under the influence of formation fluids. If the thickened or gelled aqueous solution is a conventional polymer gel, there are typically two ways to remove the polymer gel (or at least to increase the polymer return during flowback): (1) break down the polymer and (2) break down the crosslinking bonds.
Conventional breakers break down the polymer. They reduce the polymer's molecular weight by the action of an acid, an oxidizer, an enzyme, or some combination of these on the polymer itself. Thermal energy may also be used to break down conventional polymer gels.
However, if the conventional polymer gel is guar or a substituted guar, an important risk arises when conventional breakers or thermal energy is used to break down the polymer. Guars are made water soluble by the galactose side chains but have a mannose backbone which is insoluble. Because of the mannose backbone, the breaker or thermal reactions may create smaller but insoluble fragments (or residues). These insoluble residues can prevent the complete opening of the fractures and may occlude previously open pathways.
With regard to breaking down crosslinks, pH variations may be used. This is particularly true with borate-crosslinked gels. Borate crosslinks are reversibly created by increasing the pH and therefore increasing the effective concentration of the active crosslinker, the borate anion. The borate/polymer bonds can just as easily be eliminated by lowering the pH. At a high pH above 8, the borate ion exists and is available to crosslink and cause gelling. At lower pH, the borate is tied up by hydrogen and is not available for crosslinking, thus gellation caused by borate ion is reversible. The borate/polymer bond is ionic.
Other crosslinks are more difficult to break down and require extra care. For example, zirconate crosslinked polymers are known to have poor clean-up properties, i.e., they leave behind excess polymer in the formation. Other metal crosslinks are also difficult to eliminate. It is believed that compounds sufficient to break down metal crosslinks will generally also produce insoluble residues. More insoluble residue is produced after the break if there is more crosslinker present initially. It is noteworthy that zirconate crosslinked polymers are a component in major hydraulic fracturing fluids for wells with bottom hole temperatures of 200-375° F. The bond between the zirconium ion and the polymer is ionic and forms a coordination complex. Since this complex is quite stable at the bottom hole temperature noted above, the zirconium bond acts as though it is essentially covalent.
Fortunately, chelating or complexing agents can break down some metal crosslinks. Traditionally conventional polymer gels can be degelled by a process consisting of bringing the gel in the well into contact with an aqueous solution of a complexing agent able to form with the polyvalent metal ion a coordination complex which is thermodynamically more stable than that which the ion forms with the polymer. Some compounds attempt to disrupt the metal crosslink by undergoing hydrolysis to form a free, active ligand or chelator. But, zirconium crosslinked polymers continue to present problems for breaking down crosslinks. Accordingly, there is a need to provide an improved composition and method for cleaning up zirconate crosslinked polymers and thereby, increasing the final fracture conductivity.
It is also important to manage the timing of the viscosity reduction. Polymer gels which break down prematurely can cause suspended proppant material to settle out of the gel before being introduced a sufficient distance into the produced fracture. Premature break down can also result in a less than desirable fracture width in the fracture being created. On the other hand, polymer gels which break down too slowly can cause slow recovery of the polymer from the produced fracture. It is desirable for the polymer gel to retain its fracturing viscosity for the time necessary to complete the fracturing fluid injection. It is further desirable to achieve sufficient polymer returns within 2-5 days of injecting the fracturing fluid. Therefore, a need also exists for providing a composition and method with time-controlled break down of crosslinking bonds.
SUMMARY OF THE INVENTION
According to the present invention, a composition and method for hydraulically fracturing a subterranean formation and increasing the return of crosslinked polymers is provided. More specifically, there is provided a composition comprising an aqueous mixture of a hydrated polysaccharide, preferably a galactomannangum, the hydrated polysaccharide having a plurality of bonding sites; a crosslinking agent for crosslinking the hydrated polysaccharide at the bonding sites at the conditions of the subterranean formation with a polyvalent metal ion to form a polyvalent metal crosslink, thereby increasing the viscosity of the hydrated polysaccharide; and a controlled solubility compound for releasing a chelating agent for controllably breaking the polyvalent metal crosslink and bonding with the polyvalent metal ion resulting from breaking said crosslink, thereby decreasing the viscosity of the hydrated polysaccharide.
The controlled solubility compound is the cleanup additive. The cleanup additive breaks down the polyvalent metal crosslink to reduce the viscosity of the polymer gel, wherein the polysaccharide is the polymer, and facilitate the removal of the polymer gel after the fracture operation is completed. The controlled solubility compound produces a complexing agent which is able to form a coordination complex with the polyvalent metal ion which is thermodynamically more stable than that which the ion forms with the polymer.
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patent: 52
Card Roger J.
Gomtsyan Arthur
Mitchell Thomas O.
Mitchell Thomas
Nava Robin C.
Ryberg John J.
Schlumberger Technology Corporation
Tucker Philip
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