Wells – Processes – Chemical inter-reaction of two or more introduced materials
Reexamination Certificate
2002-03-26
2003-11-04
Suchfield, George (Department: 3672)
Wells
Processes
Chemical inter-reaction of two or more introduced materials
C166S308400, C507S211000, C507S215000, C507S217000, C507S260000, C507S903000
Reexamination Certificate
active
06640898
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to seawater-based delayed cross-linking fracturing fluids and methods of using the fluids in fracturing subterranean formations penetrated by well bores.
2. Description of the Prior Art
Oil and gas wells often undergo hydraulic fracturing operations to increase the flow of oil and natural gas from subterranean formations. Hydraulic fracturing is accomplished by injecting a viscous fracturing fluid through the well bore into a subterranean formation to be fractured and applying sufficient fluid pressure on the formation to cause the production of one or more fractures therein. In offshore operations, the fracturing fluid can be prepared utilizing seawater to hydrate a gelling agent to form a viscous aqueous fluid. To promote adequate viscosity at increased well depths, cross-linking agents such as borate ion releasing compounds can be incorporated into the fracturing fluids.
Seawater-based borate cross-linked fracturing fluids perform satisfactorily in low temperature fracturing applications, typically about 200° F. or less. At these lower temperatures, the initial pH required to form a sufficiently cross-linked gel is about 9.5 or less. In general, a sufficiently cross-linked gel is defined as having a viscosity of about 200 centipoises or greater at 40/sec shear rate. In order to form a sufficiently cross-linked gel for use at formation temperatures in excess of 200° F., the initial pH of a borate cross-linked fracturing fluid must be greater than about 9.5. Elevation of the fracturing fluid pH to a level in excess of 9.5, however, poses several operational problems. For instance, seawater contains multivalent ions such as calcium and magnesium ions which form insoluble precipitates at a pH greater than about 9.5. The presence of solid precipitates reduces the proppant pack conductivity, and ultimately the productivity of the fracturing operation.
Furthermore, elevating the pH of the fracturing fluid to a pH greater than about 9.5 is difficult due to the formation of magnesium hydroxide. Hydroxyl ions needed to elevate the pH of the seawater are instead consumed in the formation of magnesium hydroxide. This reaction proceeds very slowly causing the pH change to be time-delayed and difficult to adjust. In addressing the problems associated with precipitate formation in high-temperature seawater-based fracturing fluids, prior art methods suggest the removal of solid precipitates by filtration. However, the proper disposal of magnesium hydroxide and other precipitates creates additional operational costs as well as environmental challenges.
In order to conduct fracturing operations at greater well depths, it is desirable to delay cross-linking of the fracturing fluid. In particular, a delayed cross-link is advantageous in the fracturing of offshore formations where such operations are usually performed at lower injection rates because of pumping equipment limitations. Reduced injection rates, typically about 10 barrels/minute or less, lead to increased pipe times. Pipe time refers to the time required for the fracturing fluid to make the transit from surface pumping equipment to the formation to be fractured. It is generally desirable to have cross-linking occur near the end of the pipe time as the fluid approaches the formation to be fractured. If cross-linking occurs too early, the increase in fracturing fluid viscosity will increase friction loss in the well bore and produce high pump pressures. To overcome these problems, the cross-linking of the fracturing fluid is delayed until the fluid approaches a location near or within the formation to be fractured.
For these reasons, there is a need for a seawater-based fracturing fluid that avoids precipitate formation and forms delayed cross-links in high temperature fracturing operations.
SUMMARY OF THE INVENTION
The present invention provides cross-linked fracturing fluids and methods of using the fluids to fracture subterranean formations which meet the needs described above and overcome the deficiencies of the prior art. The inventive fracturing fluids and methods are particularly useful for use in connection with offshore oil and gas wells. For example, the fluids are stable at temperatures above 200° F. and are fully functional at pH levels below the threshold for precipitate formation in seawater, e.g., at a pH of 9.5 or less. Even though the fluids are seawater based, cross-linking can be delayed and controlled in order to facilitate injection of the fluid and other aspects of the fracturing operation.
In one embodiment, the inventive composition is a high temperature delayed cross-linked fracturing fluid comprising:
a gelling agent;
seawater, present in at least an amount sufficient to hydrate the gelling agent, thereby forming a gelled aqueous fluid; and
a cross-linking agent, capable of causing delayed cross-linking of the gelling agent at a pH of about 9.5 or less, whereby the delay in cross-linking is about 5 minutes or more.
In another embodiment, the inventive composition is a high temperature delayed cross-linked fracturing fluid comprising:
a gelling agent;
seawater, present in at least an amount sufficient to hydrate said gelling agent, thereby forming a gelled aqueous fluid; and
a cross-linking agent capable of causing delayed cross-linking of the gelling agent at a pH of about 9.5 or less, said cross-linking agent being selected from the group consisting of ammonium titanyl citrate, ammonium titanyl tartarate, ammonium titanyl gluconate, and mixtures thereof.
In yet another embodiment, the inventive composition is a high temperature delayed cross-linked fracturing fluid comprising:
a gelling agent; and
seawater, present in at least an amount sufficient to hydrate the gelling agent, thereby forming a gelled aqueous fluid; and
a cross-linking agent capable of causing delayed cross-linking of the gelling agent at a pH of about 9.5 or less, said cross-linking agent being ammonium titanyl citrate.
In one embodiment, the inventive method of fracturing a subterranean formation penetrated by a well bore and having a temperature above about 200° F. basically comprises the following steps:
(a) preparing a seawater-based delayed cross-linked fracturing fluid comprising a gelling agent; seawater present in at least an amount sufficient to hydrate the gelling agent, thereby forming a gelled aqueous fluid; and a delayed cross-linking agent capable of causing delayed cross-linking of the gelling agent at a pH of about 9.5 or less, whereby the delay in the cross-linking is about 5 minutes or more; and
(b) introducing said fracturing fluid into a subterranean formation at a rate and pressure whereby fractures are formed in the subterranean formation.
In addition to fracturing subterranean formations, the inventive fracturing fluids are useful in connection with other operations as well. For example, the fluids can be used in combination fracturing/gravel packing operations.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A primary advantage of the cross-linked fracturing fluids of the instant invention is that the fluids can be prepared with seawater pumped from the ocean at the site of the fracturing operation, wherever the job happens to be. As a result, the present compositions are inexpensive and simple to prepare, using either batch mixing or on-the-fly procedures.
Another primary advantage is that the inventive cross-linked fracturing fluids are stable at temperatures above about 200°F. and at a pH of about 9.5 or less. Due to the lower pH, the fluids are compatible with enzyme breakers, and calcium and magnesium salts remain in solution. Even where the gelling agent has been hydrated with seawater, the fracturing fluid provides a delay in cross-linking, conducive to fracturing subterranean formations at greater depths and/or with lower injection rates. Thus, the fracturing fluid has an initial viscosity that is high enough to transport proppant but is not so high as to make pumping difficult.
In general, the cross-linked fracturing fluids of the
Lord Paul D.
Slabaugh Billy
Terracina John
Dougherty, Jr. C. Clark
Halliburton Energy Service,s Inc.
Kent Robert A.
Suchfield George
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