Back. around of the Invention Various types of sealing compositions have been used in well completion operations, such as the primary bonding of casing and liner in the wellbore, preventing corrosion of steel pipes, or consolidating gravel packs or incompetent sands in formations. Sealing compositions have also been widely used for remedial operations, such as sealing leaks, cracks, or voids either in the formation or in casing and liners, placing plugs in the wellbore or in zones or formations containing undesirable fluids, placing temporary plugs in lieu of packers to isolate zones or formations, or filling external casing packers.

For example, in conventional well completion, a tubular liner or casing is run into the well after it is drilled, and a sealing composition such as cement is pumped between the casing and the wellbore wall and allowed to set. This isolates the various zones of the well from each other and so prevents, for example, oil from entering aquifers which might be at lower pressure. Where it is desired to allow formation fluids such as oil or gas to enter the well, the lining and cement are perforated by an explosive charge in order to provide a channel for the fluid to enter the lining so that it can pass more sensitive zones without causing damage or pollution.

This technique is generally effective in wells that are more or less vertical, since it is straightforward to run the liner into the well to ensure it is centralized, and to ensure that cement is placed evenly around the liner and bonds to both the liner and the wellbore.

However, when the well is deviated from vertical, particularly when highly deviated, even horizontal, certain problems can occur. In particular, it is often difficult to ensure that the liner remains central in the well before the cement is placed. There is a tendency for the liner to lie on the lower side of the well such that when the cement is pumped it flows easily along the high side of the liner, but little if any penetrates around the low side. This can lead to bonding problems. Also, the tools used to perforate the liner and cement can become eccentered and then do not operate efficiently.

The horizontal parts of deviated wells are often arranged so as to remain within a producing formation, and therefore in these sections good communication between the formation and the liner is required, whereas in the vertical sections leading to the surface, good zonal isolation is required. One technique that is used in such horizontal situations is to provide a perforate liner, for instance, a slotted, perforated, or predrilled liner, or a screen or a pre-packed screen, in the horizontal section of the well without any cementing to bond the liner to the wellbore. The annular gap between the liner and the wellbore is left"empty"or (as may be preferable in certain circumstances) is packed with suitably-sized gravel.

However, problems can arise if it is desired to perform a selective well treatment on, or produce selectively from, one zone in the horizontal section. This is because inside a perforate liner there is no way of isolating the zone in question from the remainder of the well by using packers, as with conventional liners, because fluid can by-pass the plug by exiting the liner into the annular space therearound.

One technique that has been proposed to overcome this problem is to provide one or more packers around the outside of the liner (external casing packers) which contact the wellbore and provide localized restriction to communication outside the liner. This then allows packers inside the liner to be used to isolate a portion of the well. Unfortunately, there are also problems associated with the use of external casing packers. For example, they must be sealed against the wellbore, they must be positioned in advance, and they substantially increase the cost of the completion.

For chemical sealing or isolation applications, various requirements must be met. For example, a sealing fluid composition useful in wellbore applications is preferably one that can be pumped through fine screens, gravel-packs, slotted liners, etc., in a one-way path, without flowing back. The sealing compositions are desirably solid-free or contain only very small particles that can pass through a fine screen or penetrating porous subterranean formation (i. e., are essentially solid-free). Materials that have been used to form solid-free gels include cross- linkable monomer or polymer solutions, however such materials do not generally provide sufficient gel strength for withstanding the high differential pressure experienced in a wellbore. To achieve higher gel strength, higher monomer or polymer concentrations are needed. However, at high polymer concentrations, the fluid becomes unacceptably viscous and can become waxy and/or semi-solid.

In view of the ever-increasing demand for improved well completion and remedial operation techniques, the present invention provides improved sealing compositions which overcome, or at least reduce the effects of, one or more of the aforementioned problems.

Sufnrraaf-vof the Iizvention In one aspect of the present invention, there are provided sealing compositions for completion or remedial operations in subterranean zones penetrated by a borehole. The sealing compositions comprise a polymer emulsion, e. g., a micellar, microemulsion, or cylindrical micelle solution, a phase inversion agent effective for controllably inverting the external and internal phases of said polymer emulsion, and a thixotropic agent.

The polymer is usually in substantially contained (i. e. at least about 90% contained) within a, preferably aqueous, internal phase of the polymer emulsion. The external phase is preferably a non-aqueous phase, such as a hydrocarbon phase.

For completion or remedial operations in subterranean zones penetrated by a borehole, the polymer emulsion is preferably a polyacrylamide polymer emulsion. A preferred emulsion further comprises a cross-linking agent capable of cross-linking the polyacrylamide, a Cl-Cs alcohol phase inversion agent effective for controllably inverting the external and internal phases of said polymer emulsion, and a thixotropic agent comprising microfine clay particles.

According to another aspect of the present invention there is provided a method of forming a plug in a region in a well, in which method a volume of a sealing composition in accordance with this invention is pumped, conveniently via a suitable packer apparatus such as a pair of region delimiting packers, into the region, which volume is sufficient to displace substantially all other fluids (i. e. at least about 90% by volume) from the region to be plugged.

According to another aspect of the invention, there is provided a method of placing a plug in a region around a perforate liner in a wellbore, e. g., a horizontal wellbore, the method comprising: placing a pair of packers inside the liner where the plug is to be positioned, the packers being spaced apart so as to define both a chamber inside the liner which includes a perforation and also a region to be plugged outside the liner; and pumping a predetermined volume of sealing composition in accordance with this invention into the chamber and, via the perforation, into the region to be plugged, which predetermined volume of material is sufficient to displace substantially all other fluids from the region to be plugged.

According to another aspect of the invention, a method is provided for cementing a casing or pipe in a borehole, comprising introducing into the annular space between the outer surface of the casing or pipe and the wall of the borehole a sealing composition of this invention; and allowing the material to set.

Brief Descriptiota of tlze Drawifzs The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: Figure 1 illustrates the shear-thinning behavior of an illustrative sealing composition of the present invention. The sealing composition comprises a terpolymer containing monomer acrylamide, vinyl pyrrolidone, and sodium acrylamido propane sulfonate in mineral oil ("terpolymer").

Figure 2 illustrates the yield stress behavior of an illustrative sealing composition of the present invention. The sealing composition comprises a terpolymer containing monomer acrylamide, vinyl pyrrolidone, and sodium acrylamido propane sulfonate in mineral oil ("terpolymer").

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DetailedDescriPtion of Illustrative Embodiments Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers'specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

The sealing fluid compositions of the present invention find use generally in subterranean wellbore and formation isolation, such as those discussed above, and find particular application in known techniques for the completion of horizontal wells employing slotted or predrilled liners. The sealing compositions broadly employ the use of polymer emulsions in which a polymer is contained within an emulsion system, such as a micellar, microemulsion, or cylindrical micelle solution. The external phase of the emulsion generally comprises a non-aqueous liquid, e. g., a hydrocarbon oil, or a mixture of hydrocarbon oils and solvents. The internal phase of the emulsion system comprises a cross-linkable polymer.

Suitable polymers useful in this regard include, for example, acrylamide-based polymers, guar-based polymers, hydroxy ethyl cellulose (HEC) polymers, and the like.

In one particularly illustrative embodiment of this invention, the polymer emulsion comprises a polyacrylamide polymer present in a water-in-oil microemulsion. The polyacrylamide may be a modified or derivative form of polyacrylamide to suit the needs of a given application, such as the modified polyacrylamide water in oil microemulsion available from Drilling Specialties Company, Bartlesville, Oklahoma. This and similarly modified polyacrylamides are particularly useful in the polymer emulsions employed according to this invention. Of course, polymer science is a well developed art and numerous additional variations and/or modifications can be made to these and other like polymers, and/or to the methods for synthesizing the polymer emulsion, to suit the needs of a given implementation of this invention.

In one preferred embodiment of this invention, the polymer is a terpolymer containing monomer acrylamide, vinyl pyrrolidone, and sodium acrylamido propane sulfonate.

The polymer emulsion can be produced by essentially any known technique for producing micellar, microemulsion and/or cylindrical micelle solution. In one illustrative approach, the polymer emulsion is produced by a process of emulsion polymerization.

The sealing compositions of the present invention further comprise one or more cross- linking agents. The cross-linking agent used in the sealing compositions of this invention is selected so as to be capable of cross-linking the polymer present in the polymer emulsion.

Many cross-linking systems are well known in the art and may include, without limitation, known metallic and/or organic cross-linkers, e. g., aluminum, zirconium, chromium, and their chelates and complexes; aldehyde-phenolic and aldehyde-alcohol functional cross-linking agents, aldehyde and phenolic precursors, imines and polyethyleneimines, urea-aldehydes, substituted urea-aldehydes, ureas and substituted urea-aldehyde precursors, amine-aldehydes, and their precursors. The compositions may further contain cross-linking accelerators and retarders, as well as gel stabilizers, as desired.

In one illustrative embodiment, the cross-linking agent is selected for use in conjunction with the polymer. Agents suitable for cross-linking polyacrylamide-based polymers or some derivative thereof are known in the art and can include, for example, chromium acetates and propionates, zirconium lactates, zirconium hydroxy alkyl amines, aluminum lactates, acetates and propionates. Other cross-linking agents include organic cross-linkers, such as phenol and aldehyde combinations (e. g. phenol, phenyl acetates, phenyl succinates, naphthalene diols, furan, or furfuryl alcohols) in combination with formaldehyde, acetaldehyde, glyoxal, trioxane, hexamethylenetetramine, acetaldehyde-ammonia trimer, or glycerol formal.

The sealing compositions of the present invention will further comprise one or more phase inversion agents. The function of the phase inversion agent is to controllably effect the release of polymer molecules from the polymer emulsion. Preferably, the phase inversion agent is one that can cause phase inversion of the polymer emulsion and thereby release of polymer molecules at a location within the wellbore. Suitable phase inversion agents useful in this regard can include, for example, short hydrocarbon chain alcohols such as Cl-C8 alcohols, ethoxylated alcohols, surfactants, and other surface active agents. A preferred class of phase inversion agents includes linear, branched or cyclic Cl-C$ alcohols. Propanols (in particular iso-propanol), butanols (in particular n-butanol) and the isomers of pentanol are most preferred inversion agents. The ability of such agents to cause phase inversion of the polymer emulsions and thereby effect release of polymer from the polymer emulsion is temperature-dependent. Thus, such agents are selected so as to not cause substantial phase inversion at ambient temperatures, such as at the surface of a well, but instead cause phase inversion at the elevated temperatures, for example between about 75°C and about 300°C, experienced within the wellbore.

The sealing compositions will generally be essentially solid-free and therefore readily pumpable through screens and openings less than about 75 microns. The sealing compositions are generally pumpable through pipe or tubing as small as about 4 mm in diameter. Moreover, the sealing compositions will also typically have high initial yield stress and can retain the form last pumped with little or no backflow through the screens and openings under normal wellbore pressure conditions. Also, the sealing compositions have high gel strength in the bore hole or casing annulus, yielding higher extrusion pressure gradients than other gel systems in comparable wellbore completion and flow geometries.

In order for the sealing composition to meet these requirements, it is desirable in many applications that it be thixotropic in nature. During pumping, the confining pressure keeps the fluid tight against the borehole walls and liner surface but once pumping stops there are no containment pressures. Thus, the plug must become substantially"self-supporting"very quickly so as not to move significantly along the annulus.

Therefore, the sealing compositions of this invention will generally further comprise one or more thixotropic agents. Such agents are particularly useful in applications where it is desired to prevent the compositions from sagging or slumping under gravity, such as for operations in extended horizontal and/or other wells deviated from vertical. Such agents are well known and can include, for example, chemical systems containing microfine clay particles, chemically treated clay particles, mixture of these clay particles, and other like materials. By thixotropic, it is meant that the shear yield strength must be such that under the likely ambient conditions the fluid flows readily when being pumped and yet rapidly gels when pumping stops. Naturally, acceptable values will depend upon the physical parameters of the well (of the wellbore and of any tubing therein). In certain illustrative embodiments, the preferred thixotropic agents will be cationic surfactant-treated microfine clay particles.

In one illustrative embodiment of the invention, the sealing composition comprises about 80- 98 wt. %, more preferably about 85-95 wt. %, of one or more polymer emulsions; about 0.5-5 wt. %, preferably about 1-3 wt. %, of one or more cross-linking agents; about 1-8 wt. %, preferably from about 3 to 6 wt. %, of one or more phase inversion agents; and about 0.5-5 wt. %, preferably from about 2-3 wt. %, of one or more thixotropic agents. These levels are intended to be illustrative only; an individual skilled in this area of technology will recognize that the levels of each component present in the sealing compositions can be readily varied to suit the needs of a given implementation.

Numerous additives for use in sealing compositions are known in the art and may also be formulated in the sealing compositions of this invention if desired. Of course, it will be preferred that any additives employed do not adversely impact the ability of the sealing compositions to function in their intended applications. Additives that one skilled in the art might consider employing in the sealing compositions include, for example, stabilizers, delaying agents, oxygen scavengers, pH buffering agents, cross-linking resin, and metallic coupling agents.

The sealing compositions disclosed herein are suitable for use in any of a variety of subterranean applications, such as the primary bonding of casing and liner in the wellbore, preventing corrosion of steel pipes, or consolidating gravel packs or incompetent sands in formations. They are particularly well suited for various remedial operations, such as sealing leaks, cracks, or voids either in the formation or in casing and liners, placing plugs in the wellbore or in zones or formations containing undesirable fluids, placing temporary plugs in lieu of packers to isolate zones or formations, or filling external casing packers. It will be appreciated, of course, that it may be necessary to formulate a sealing composition that is suited to the variety of liner being employed, for certain sealing compositions will have better compatibility with certain types of liners. Which sealing compositions are suitable for which liner variety will be evident to those skilled in the art.

Therefore, another aspect of this invention provides a means for achieving zonal isolation in a well with a perforate liner. The perforate liner can take any suitable form. Typically, it is a slotted liner or a pre-packed screen. This is achieved by pumping a fluid sealing composition of the present invention into the relevant annular region between the liner and the wellbore and allowing the sealing composition to set to form a plug that prevents communication from one side to the other, except via the liner. The sealing compositions can be conveniently pumped either from the surface to the chamber via a tube or by means of a downhole pump from a reservoir located, for example, near the packers in the wellbore.

In one particularly illustrative embodiment, the present invention provides a method of placing a plug in a region around a perforate liner in a wellbore, the method comprising placing a pair of packers inside the liner where the plug is to be positioned, the packers being spaced apart so as to define a chamber inside the liner which includes a perforation and also a region to be plugged outside the liner. A predetermined volume of sealing composition is then pumped into the chamber and, via the perforation, into the region to be plugged, which predetermined volume of sealing composition fluid is sufficient to displace substantially all other-fluids from the region to be plugged.

Additionally, the volume of the fluid sealing composition employed is preferably such that when in the annulus it does not extend beyond the limit of the packers, but nevertheless it is possible for the fluid to extend past them. However, to prevent the fluid from then re-entering the liner, it is often preferred that the critical pressure drop along the annular region being plugged does not exceed the pressure drop across the slots in the liner, i. e. no fluid enters the liner beyond the packer.

Thus, the sealing composition preferably will have rheological properties to displace other fluids when pumped into the region and to remain there while it sets so as to seal against the wellbore and the casing and form an impermeable plug.

The sealing compositions preferably have a characteristic gelling time that is shorter than or comparable to the time taken to displace the fluid into the region, and the gel strength or viscosity of the material is sufficient to eliminate gravity-induced flows. The required gel strength and gelling time are calculated to achieve optimum displacement for the specific geometry of the region, the pumping time and the density difference between the fluid and the oil/water initially filling the region for each job.

Prior to use of the sealing compositions, it may be desirable to pump a wash fluid through the chamber and region to be plugged. These wash fluids and their pumping rates are well-known in the field of sealing and well treatment, and are designed according to the particular nature of the job at hand.

Moreover, after the sealing composition is in place in the annulus it will normally be the case that the residue of the fluid in the chamber (and possibly in the liner outside the packer pair) needs to be washed out. Again, suitable wash fluids and their pumping rates are well-known in the field of cementing and well treatment, and need no further description here.

The techniques described above mostly require that the sealing fluid be pumped from the surface to the region in question. However, in various alternative embodiments, the fluid can be held in a downhole reservoir near the region and pumped through the chamber using a downhole pump. This lessens the strict rheological requirement of the fluid and allows downhole mixing of two-part fluids or the like which can set rapidly in the region without causing problems in the tubing itself. In some instances, a downhole source of radiation such as UV or heat might be provided near the region to trigger or aid, for example, in facilitating the phase inversion agent and/or in the setting of the sealing composition. Moreover, triggering of downhole pumps or sources can be achieved by in situ measurements--for example, the conductivity of fluids passing through the tool.

The techniques described above provide a single plug around the liner. To effect treatment to a particular zone of a well having a perforate liner, it is typically necessary to set two or more plugs, such that there is one plug on either side of the zone in question. Packers can then be used and sealed against the plugs so as to isolate the zone therebetween and allow a selective treatment to be applied to that zone. It will be appreciated that if the treatment zone is near the bottom of the well or another plug, it may only be necessary to set a single plug to define the zone.

Therefore, in a further embodiment of the invention, a method is provided for isolating a zone of a well which is lined with a perforate liner. This method comprises placing plugs on either side of the zone and then setting a packer in the liner adjacent each plug.

There are occasions other than when dealing with the special problems posed by perforate liners when it may be desirable to form a plug in the well (perhaps in the well itself, or possibly in the annular region between the wellbore wall and some tubing within the wellbore), and the compositions and techniques disclosed herein may be useful for these purposes as well. Such occasions include, for example, selective abandonment of a section of the well, as well as the regulated fill of a washout.

Accordingly, in a further aspect the invention provides a method of forming a plug in a region in a well, in which method a volume of sealing composition is pumped into the region, conveniently via suitable packer apparatus such as a pair of region-delimiting packers, which volume is sufficient to displace substantially all other fluids from the region to be plugged.

The following examples are provided to demonstrate certain illustrative embodiments of this invention. It should be appreciated by those skilled in the art that the techniques disclosed in the examples which follow represent those found by the inventors to function in the practice of the invention and thus can be considered to constitute examples of illustrative modes for its practice. However, those skilled in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention.

Examples The following represents an illustrative sealing composition of this invention, conveniently formulated using commercially available products: Description Weight Percent Remarks Terpolymer containing 89 to 90 % of total Primary polymer monomer acrylamide, vinyl fluid pyrrolidone, and sodium acrylamido propane sulfonate in mineral oil Hexamethylene tetramine 1 to 3 % of HE300 Cross-linker Isopropyl Alcohol 3 to 6 % of HE300 Phase inversion agent Quarternary amine-treated 2 to 3 % of HE300 Thixotropic Agent fine clays The terpolymer containing monomer acrylamide, vinyl pyrrolidone, and sodium acrylamido propane sulfonate in mineral oil is commercially available as HE300 from Drilling Specialties Company, Bartlesville, Oklahoma.

Recorded lab tests showed the fluid was injectable through pipe and tubing.

Witnessed injection and screen squeeze tests demonstrated that the formulation fluid could be squeezed through a 200 Mesh screen (75 micron or 0.003" openings) at a rate of 20 ml/min and at 50 psi back pressure via a 4 mm ID tubing. The pipe extrusion measurements showed that the set gels were solid and could withstand 30 to 50 psi/ft pressure gradient in 1"ID pipe.