Wells – Processes – Chemical inter-reaction of two or more introduced materials
Patent
1997-02-24
1998-09-29
Suchfield, George A.
Wells
Processes
Chemical inter-reaction of two or more introduced materials
166295, 166308, 252358, 507201, 507211, 507215, 507216, 507903, 507921, 405264, 523130, E21B 3313, E21B 33138, E21B 4322, E21B 4326
Patent
active
058134662
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the use of enzymes to generate acid to break crosslinked gels used in industrial applications for viscosifying or solidifying aqueous fluids.
2. Description of the Prior Art
Water-based gels are used for a variety of industrial applications, such as hydraulic fracturing of underground reservoirs and creating temporary plugs in underground rock formations or pipelines. They find application in a wide range of industries such as oil, water, gas, coal, mineral extraction.
Hydraulic fracturing comprises a variety of techniques used to stimulate the production of oil, gas and other fluids from underground formations. In hydraulic fracturing, a suitable fracturing fluid is introduced into an underground formation by way of a wellbore under conditions of flow rate and pressure which are at least sufficient to create and/or extend a fracture into a desired part of the formation. Hydraulic fractures tend to heal unless they are propped open and therefore the fracturing fluid normally carries a proppant (e.g. sand, bauxite, etc) which is forced into the fracture and keeps the broken formation from closing down upon itself once the pressure is released.
Various fluids under pressure have been used in hydraulic fracturing. Most of the fracturing fluids used today are aqueous-based liquids which have been either gelled or foamed. Fracturing fluids need to have a low leak-off rate, a low pumping friction loss and need to carry proppant. The rate of leak-off is dependent on the viscosity and wall-building properties of the fluid.
Aqueous gels are usually prepared by blending a polymeric gelling agent with an aqueous medium. Most frequently, the polymeric gelling agent of choice is a water soluble polysaccharide but may also be a polyacrylamide. The polymeric gelling agent can thus be a solvatable polysaccharide. These polysaccharides form a known class of compounds which include a variety of natural gums and certain cellulosic derivatives which have been rendered hydratable by hydrophilic substituents chemically attached to the polymer backbone. Examples of such polymers include guar, carboxyalkyl guar, hydroxyalkyl guar, carboxyalkyl hydroxyalkyl guar, galactomannan gums, glucomannan gums, xanthan gums and the like.
The water soluble polysaccharides have a remarkable capacity to thicken aqueous liquids. Even small amounts are sufficient to increase the viscosity of such aqueous liquids from 10 to 100 times or more. In many instances, the thickened aqueous liquid has sufficient viscosity to carry the proppant during the course of the fracturing process and represents a satisfactory fracturing fluid. In other instances, it is necessary or desirable to crosslink the polysaccharide in order to form a gel having sufficient strength and viscosity to carry the proppant. A variety of crosslinkers have been developed to achieve this result.
Most water soluble polysaccharides are typically crosslinkable in a basic aqueous medium (at a pH above 7) by a wide variety of organometallic compounds containing titanium or zirconium in a +4 oxidation (valance) state. The borate ion has been used extensively as a crosslinking agent for hydrated guar gums and other galactomannans to form aqueous gels used in fracturing and other areas. For example, a fracturing process which comprises crosslinking guar-containing compositions on-the-fly with borate ions was described by Free in U.S. Pat. No. 3,974,077. The borate crosslinked systems require a basic pH (e.g. 8.5 to 10) for crosslinking to occur.
Other crosslinking agents have been developed using certain transition metals. For example aqueous solutions of galactomannan gums have been crosslinked at pH 6-13 with antimony, bismuth, titanium, zirconium, chromium and iron compounds.
Most water-based drilling fluids have built-in breaker systems that gradually reduce gels to low viscosity solutions at reservoir temperatures and pressures. This allows the fluid to be produced back from the reservoir and prevent
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Harris Ralph E.
Hodgson Richard J.
Cleansorb Limited
Suchfield George A.
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