Wells – Processes – Placing fluid into the formation
Reexamination Certificate
2000-02-22
2001-12-04
Bagnell, David (Department: 3672)
Wells
Processes
Placing fluid into the formation
C166S278000, C507S266000
Reexamination Certificate
active
06325149
ABSTRACT:
The invention relates to improvements in a fluid for and a process of controlling fluid loss to a formation during workover and completion operations, and in a fluid for and a process of preventing the mixing of fluids within a borehole.
BACKGROUND OF THE INVENTION
In the drilling and servicing of oil and gas wells, there are numerous operations that require a viscous fluid, such as the control of the loss of fluid from the fluid present in the borehole of the well to the surrounding subterranean formations contacted by the borehole, or the mixing of fluids within the borehole. Such operations include drilling, fracturing, gravel packing, workover, and other completion operations.
The present invention relates to the use of certain novel pills to control the fluid loss in such operations.
PRIOR ART
Situations arise in completion operations requiring effective fluid loss control. In the past, controlling fluid loss during completions in unconsolidated reservoirs has been an art and not a science. Industry has induced major formation damage in the interest in economy and safety of well operations. As well productivity issues receive more attention, engineers and scientists have turned a more critical eye to the behavior of fluid loss control systems.
Completion fluids are designed based upon the anticipated reservoir pressure to create a determined overbalanced condition. The presence of high reservoir permeability can result in significant losses of completion fluids under these conditions. Before coming out of the hole with a bottom hole assembly, it is necessary to minimize fluid loss to less than five barrels per hour, preferably less than one barrel per hour. The concept of fluid loss involves the reduction of the effective permeability and/or an increase in the viscosity of the leak off fluid. A combination of a particle and a viscous fluid is the widely accepted approach to controlling fluid loss.
Difficulty arises when it is desired to regain the permeability in the formation. Clean up efficiency depends upon both the fluid and the particle. Materials such as salt, oil soluble resins and calcium carbonate suspended in viscosified polymer solutions are the most common completion fluids used in fluid loss control. Removal of these materials in order to reestablish fluid loss for gravel packing or increase hydrocarbon production typically involves treatment with an acid or hydrocarbon fluid to dissolve the solids material thereon. At best, clean up efficiency is in the range of 10 to 50 percent for typical systems. Such leak off control materials have been found to be nearly impossible to remove from perforation tunnels against the formation in the reinjection direction. Filter cakes deposited by these particulate systems provide leak off rates that are too low to permit contact by solvents or solids in order to affect their removal. Polymer systems alone allow more effective removal but the fluid loss efficiency is considerably lower thereby allowing unacceptable fluid loss rates when leak off control is desired.
The following papers published by the Society of Petroleum Engineers provide further background for the invention and the prior art: SPE 39438, “Development of a New Crosslinked-HEC Fluid Loss Control Pill for Highly-Overbalanced, High Permeability and/or High Temperature Formations,” F. F. Chang et al.; SPE 10666, “Guidelines for Using HEC Polymers for Viscosifying Solids-Free Completion and Workover Brines”, R. F. Scheuerman, both incorporated herein by reference.
Situations also arise in which a viscous fluid is required to prevent the mixing of fluids within a borehole, so-called displacement or spacer fluids.
SUMMARY OF THE INVENTION
The invention provides solids-free viscous fluids, typically pills, which are less damaging to a producing formation during workover and completion operations. The fluids comprise an aqueous liquid comprising a solution of one or more soluble bromide salts having dissolved therein a polyethylene glycol at a concentration such that a viscous solution is obtained.
Thus it is an object of the invention to provide aqueous solids-free viscous fluids containing a viscosifying concentration of a polyethylene glycol.
It is another object of the invention to provide aqueous solids-free viscous fluids which are less damaging to a producing formation.
It is another object of the invention to provide aqueous, solids free, viscous fluids which are useful in various oil and gas well workover and completion operations wherein the aqueous fluid comprises a brine having a density from about 11.0 to about 20 lbm/gal.
It is still another object of the invention to provide a process for reducing the pressure differential required to remove a fluid loss pill from a producing formation which comprises utilizing as the fluid loss pill a viscous aqueous solution of one or more soluble bromide salts and a polyethylene glycol.
Another object of the invention is to provide a method of decreasing the loss of fluid to a hydrocarbon producing formation during workover or completion operations, particularly such operations which require a post gravel pack in-screen fluid, a fluid loss control pill, perforation sealing, and the like.
Yet another object of the invention is to provide a method of decreasing the mixing of fluids within a borehole.
These and other objects of the invention will be obvious to one skilled in the art on reading this specification and the appended claims.
While the invention is susceptible of various modifications and alternative forms, specific embodiments thereof will hereinafter be described in detail and shown by way of example. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications and alternatives falling within the spirit and scope of the invention as expressed in the appended claims.
The compositions can comprises, consist essentially of, or consist of the stated materials. The method can comprise, consist essentially of, or consist of the stated steps with the stated materials.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Solids-free fluid loss control pills are typically compounded with a viscosifying polysaccharide such as hydroxyethyl cellulose. Sheared and hydrated into various brines, these pills are pumped into porous media such as reservoir or gravel pack sands where the viscosity reduces the rate of fluid loss. However, the polysaccharides used in such pills have proven to be damaging to the permeability of the sands and difficult to remove therefrom.
The present invention provides a solids-free, polymer-free, viscous pill which can be utilized in the same manner as the prior polysaccharide containing viscous pill. The viscous pill of the present invention comprises an aqueous liquid comprising a solution of one or more soluble bromide salts having dissolved therein a polyethylene glycol having a molecular weight from about 1000 to about 8,000,000.
By the term “solids-free” as used herein is meant that the viscous fluids of the invention contain no added water-insoluble solids. Thus the fluids may contain water soluble salts dissolved in the aqueous phase of the fluid. By the term “polymer-free” as used herein is meant that the fluids contain no polymers hydrated therein, such as polysaccharides and the like, i.e., the well known so-called “water soluble polymers”, other than the polyethylene glycols of this invention.
The solids-free pills of this invention are less damaging to the hydrocarbon-containing formations contacted by the pill. The polyethylene glycol adsorbs onto the surface of clays within the formations, stabilizing the clays as the pill is placed, thus reducing damage to the formation. The polyethylene glycol readily degrades over time, particularly at the higher temperatures within the formation. The viscous pill thermally loses viscosity with time, and also upon dilution with produced formation water, such that no acid clean up is required. Moreover, the pressure differential requir
Dobson, Jr. James W.
Harrison, III Jesse Clark
Muijs Herman Mathieu
Bagnell David
Dougherty Jennifer R.
House Roy F.
Texas United Chemical Company LLC.
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