Control of Particulate Flowback in Subterranean Wells

1. Field Of The Invention

This invention relates to means for recovering hydrocarbons from a subterranean formation and more particularly to a method and means for controlling particulate solids transport during the production of hydrocarbons from a subterranean formation.

2. Brief Description Of The Prior Art

Transport of particulate solids during the production of hydrocarbons from a subterranean formation is a continuing problem. The transported solids can erode or cause significant wear in the hydrocarbon production equipment used in the recovery process. The solids also can clog or plug the wellbore thereby limiting or completely stopping fluid production. Further, the transported particulates must be separated from the recovered hydrocarbons adding further expense to the processing.

The particulates which are available for transport may be present due to an unconsolidated nature of a subterranean formation and/or as a result of well treatments placing particulates in a wellbore or formation, such as, by gravel packing or propped fracturing.

In the treatment of subterranean formations, it is common to place particulate materials as a filter medium and/or a proppant in the near wellbore area and in fractures extending outwardly from the wellbore. In fracturing operations, proppant is carried into fractures created when hydraulic pressure is applied to these subterranean rock formations to a point where fractures are developed. Proppant suspended in a viscosified fracturing fluid is carried outwardly away from the wellbore within the fractures as they are created and extended with continued pumping. Upon release of pumping pressure, the proppant materials remain in the fractures holding the separated rock faces in an open position forming a channel for flow of formation fluids back to the wellbore.

Proppant flowback is the transport of proppant sand back into the wellbore with the production of formation fluids following fracturing. This undesirable result causes undue wear on production equipment, the need for separation of solids from the produced hydrocarbons and occasionally also decreases the efficiency of the fracturing operation since the proppant does not remain within the fracture and may limit the width or conductivity of the created flow channel.

Currently, the primary means for addressing the proppant flowback problem is to employ resin-coated proppants, resin consolidation of the proppant or forced closure techniques. The cost of resin-coated proppant is high, and is therefore used only as a tail-in in the last five to twenty five percent of the proppant sand placement. Resin-coated proppant is not always effective since there is some difficulty in placing it uniformly within the fractures and, additionally, the resin coating can have a deleterious effect on fracture conductivity. Resin coated proppant also may interact chemically with common fracturing fluid crosslinking systems such as guar or hydroxypropylguar with organo-metallics or borate crosslinkers. This interaction results in altered crosslinking and/or break times for the fluids thereby affecting placement. Another means showing reasonable effectiveness has been to gradually release fracturing pressure once the fracturing operation has been completed so that fracture closure pressure acting against the proppant builds slowly allowing the proppant particles to stabilize before flowback of the fracturing fluid and the beginning of hydrocarbon production. Such slow return is undesirable, however, since it reduces the production from the wellbore until the treatment fluid is removed.

In unconsolidated formations, it is common to place a filtration bed of gravel in the near-wellbore area in order to present a physical barrier to the transport of unconsolidated formation fines with the production of hydrocarbons. Typically, such so-called "gravel packing operations" involve the pumping and placement of a quantity of gravel and/or sand having a mesh size between about 10 and 60 mesh on the U.S. Standard Sieve Series into the unconsolidated formation adjacent to the wellbore. It is sometimes also desirable to bind the gravel particles together in order to form a porous matrix through which formation fluids can pass while straining out and retaining the bulk of the unconsolidated sand and/or fines transported to the near wellbore area by the formation fluids. The gravel particles may constitute a resin-coated gravel which is either pre-cured or can be cured by an overflush of a chemical binding agent once the gravel is in place. It has also been know to add various binding agents or adhesives directly to an overflush of unconsolidated gravel in order to bind the particles together.

U.S. Pat. No. 5,330,005 discloses a method for overcoming the difficulties of resin coating proppants or gravel packs by the incorporation of a fibrous material in the fluid with which the particulates are introduced into the subterranean formation. The fibers generally have a length ranging upwardly from about 2 millimeters and a diameter of from about 6 to about 200 microns. Fibrillated fibers of smaller diameter also may be used. The fibers are believed to act to bridge across constrictions and orifices in the proppant pack and form a mat or framework which holds the particulates in place thereby limiting particulate flowback.

While this technique may function to limit some flowback, it fails to secure the particulates to one another in the manner achieved by use of resin coated particulates. It would be desirable to provide a method which will physically bind particles of the particulate to one another whereby agglomerates may be formed which would further assist in preventing movement or flowback of particulates from a wellbore or formation.

The present invention provides a method and fluid for treating a subterranean formation and a resultant porous particulate pack which inhibits the flow of particulates back through the wellbore with the production of hydrocarbons.

In accordance with the invention, a method of treating a subterranean formation penetrated by a wellbore is provided comprising the steps of providing a fluid suspension including a mixture of particulate material and another material in fiber, particulate, ribbon or flake form having an adhesive coating thereon, pumping the fluid suspension including the mixture of particulate and other coated material through the wellbore and depositing the mixture in the formation. Upon deposition of the particulate and other coated material mixture in the formation the adhesive coating causes particulate adjacent the material to adhere to the adhesive coated material thereby creating agglomerates which bridge against other agglomerates in the formation to prevent particulate flowback. The adhesive coated material may comprise ground material having an average particle size of from about 10 to 150 mesh on the U.S. Sieve Series. When in ribbon form it may be from 2.5 to 250 microns thick, 0.4 millimeters to about 6.5 millimeters wide and have a length of from about 5 millimeters to in excess of 50 millimeters. When in flake form, the material may have a thickness of from about 2.5 to about 250 microns and an average surface area of from about 2 to about 325 square millimeters. The flakes may be produced by shredding, cutting, chopping or otherwise grinding the adhesive coated material prior to or subsequent to addition of the adhesive coating. When in fiber form, the material may have an average or effective diameter of from about 2 to about 200 microns and a length of at least 2 millimeters.

The adhesive coated material is effective in inhibiting the flowback of particulate in a porous pack having a size ranging from about 2 to about 400 mesh in intimate admixture with the thermoplastic material.

The adhesive coated material is effective in consolidating particulate in the form of agglomerates in a formation as a result of a fracturing or gravel packing treatment performed on a subterranean formation without the use of complicated and expensive resin consolidating formulations or precoated particulates.

In accordance with the present invention, an adhesive coated material is incorporated in an intimate mixture with a particulate material such as conventional proppants or gravel packing materials and introduced into a subterranean formation.

As used in this specification, the term "intimate mixture" will be understood to mean a substantially uniform dispersion of the components in the mixture. The term "simultaneous mixture" will be understood to mean a mixture of components that are blended together in the initial steps of the subterranean formation treatment process or the preparation for the performance of the treatment process.

The adhesive coated material may comprise substantially any substrate material that does not undesirable chemically interact with other components used in treating the subterranean formation. The material may comprise thermoplastic material capable of softening upon heating or softening at the temperature of the subterranean formation whereby it may adhere to the particulates with which it is introduced. Examples of suitable materials include polyolefins including polyethylene, polypropylene, polybutylene polymers and fluoropolymers and copolymers, polyimides, polyurethanes, polysulfones, polycarbonates and cellulose derivatives. The material also may comprise glass, ceramic, carbon natural or synthetic polymers or metal filaments and the like.

The adhesive coated material may be utilized in substantially any physical form, such as for example, ground material, ribbons, flakes, fibers and the like.

The adhesive coated material may comprise ground material having an average particle size of from about 10 to about 150 mesh on the U.S. Sieve Series. When the material is in the form of a ribbon, it may be from about 2.5 to about 250 microns thick, from about 0.4 to about 6.5 millimeters wide and from about 5 to in excess of from about 50 millimeters in length. When in flake form the adhesive coated material may have a thickness of from about 2.5 to about 250 microns and may have either a regular of irregular shape and will have a surface area in the range of from about 2 to about 325 square millimeters. When in fiber form, it may have an average or effective diameter of from about 2 to about 200 microns and a length of at least about 2 millimeters. The length of the fibers is limited only by the practical applications of handling, pumping and the like. The current practical limit to the length is about 100 millimeters. The quantity of adhesive coated material used in the intimate mixture can range from about 0.01% to about 15 percent by weight of the particulate. Preferably, the adhesive coated material concentration ranges from about 0.1 to about 5 percent by weight of the particulate.

The adhesive coating placed on the substrate material comprising flakes, fibers, ribbons and the like described herein may be incorporated on the material during manufacture or subsequent thereto. The coating may be applied to the entire substrate material or only one side or even only various spots on the substrate. It is not necessary that the adhesive coating be uniform on the substrate. It is only necessary that sufficient adhesive be present to facilitate particulate adhesion in the subterranean formation. The coating may be sprayed or otherwise applied to the material during the coating process. One form of adhesive which may be used is one which will set over time after the adhesive coated material has been introduced into the subterranean formation. An alternate adhesive which might be used is one which will set upon treatment with a catalyst which is either introduced with the adhesive coated material into the subterranean formation or introduced prior to or subsequent to introduction of the adhesive coated material whereby the catalyst contacts the adhesive coated material within the subterranean formation.

The adhesive may comprise compounds of the type described in U.S. Pat. Nos. 5,218,038; 5,425,994; 5,420,174; 4,888,240 and the references cited therein which are incorporated herein in their entirety by reference. The adhesive may comprise thermoplastic materials such as those previously described herein for the substrate material to which the adhesive is applied as well as the various well known phenolic or furan resins described in the above cited references.

The adhesive coated material interacts mechanically with the particles of particulate introduced into the subterranean formation to limit or prevent the flowback of particulates to the wellbore.

An important additional feature of the material is the chemical interaction that occurs upon activation of the adhesive coating on the materials within the formation. The adhesive coating on the material interacts within the formation to adhesively bind particles of the particulate into larger adhered agglomerates which are locked into place through bridging with the other particulates and agglomerates to prevent flowback of particulates to the wellbore.

The adhesive coated material may be incorporated with the particulate in any of the conventional fracturing or gravel packing fluids comprised of an aqueous fluid, a hydrocarbon fluid or an emulsion, a viscosifying agent and any of the various known breakers, buffers, surfactants, clay stabilizers or the like.

The adhesive coated material is incorporated with the particulate as a simultaneous mixture by introduction into the fracturing or gravel packing fluid along with the particulate. The material may be introduced into the fluid before, after or simultaneously with introduction of the particulate into the fluid. The adhesive coated material may be pre-blended as a mixture of dry discrete components prior to introduction into the fluid. The adhesive coated material may be incorporated with the entire quantity of particulate introduced into the subterranean formation or it may be introduced with only a portion of the particulate, such as in the final stages of the treatment to place the intimate mixture in the formation in the vicinity of the wellbore. For example, the adhesive coated material may be added to only the final 20 to 30 percent of the fluid introduced into the formation. In this instance, the intimate mixture will form a tail-in to the treatment which upon interaction within the formation with the particulate will cause the other particles to bridge on the agglomerates formed therein and prevent movement of the particles into the wellbore with any produced fluids.

The adhesive coated material upon introduction into the formation is heated to a temperature above the temperature at which the material is admixed as a simultaneous mixture. In one embodiment, the adhesive coating on the material softens as it is heated and generally becomes tacky or adhesive whereupon it adheres to or binds particles of the particulate in which it is in contact within the formation. When the material is a thermoplastic that has been subjected to either uniaxial or biaxial stress prior to addition to the particulate, the thermoplastic material may exhibit the additional property of shrinking and twisting further binding the material to the particulate. In one embodiment the material is a thermoplastic material that is subjected to biaxial stress through the formation of a film of the material. The film may be cut into ribbons, chopped or slit or shredded into flakes or small pieces and also may be comprised of multiple layers of different thermoplastic materials to vary the properties of the material. In this instance, the adhesive coating may comprise a layer of thermoplastic material bonded to another layer of material which functions as a substrate. The layered material then may be cut or shredded as desired. Thus, an adhesive coating can be combined with a material that will readily shrink or distort upon heating to improve the agglomeration properties.

To further illustrate the present invention and not by way of limitation, the following examples are provided.

The stabilization properties of the method of the present invention are determined by comparison to a viscosified fluid containing particulate in a flow cell. Proppant conductivity and critical proppant flowback velocity is measure in an American Petroleum Institute approved simulated fracture flow cell. The cell has a proppant bed size of about 1.5 inches in height, about 7 inches in length and about 0.25 inches in width. The bed is initially prepacked with 20-40 mesh Brady Sand by introducing the sand into the cell in a borate crosslinked guar fluid containing 30 pounds of guar per 1000 gallons of aqueous fluid. The fluid also included a quantity of an oxidizing breaker with a low temperature catalyst to break the gel. The cell was maintained at a temperature of 250.degree. F. for approximately 16-18 hours and the fracture conductivity was determined at a simulated closure stress of 2000 psi using a 2% KC1 solution. Once conductivity was established, the screen in the end of the flow cell was removed and the cell was fitted with a full open slot about 1.5 inches high and 0.25 inches wide visible through a sight glass. Fluid flow of a 2% KC1 solution then was initiated at steadily increasing flow rates through the proppant bed until proppant production was noted to occur through the slot. The cell then is cleaned and packed with another proppant material for testing. The test was repeated using a proppant containing 1 percent by weight of glass fibers having a 12 micron diameter and 1/2 inch length and a phenolic coating that becomes adhesive at a temperature above 150.degree. F. and a proppant containing 0.5% by weight of a thermoplastic material comprising polyethylene film having a polyvinyl alcohol coating in ribbons 0.25 inch long, 0.18 inch wide and 0.002 inch thick.

Sand production occurred in the test of proppant alone at a flow rate of 150 cc per minute. No sand production was found to occur in the test using phenolic coated glass fibers or polyvinyl alcohol coated polyethylene ribbons at a flow rate of 500 cc per minute which was the maximum pump rate achievable by the test equipment.

While the present invention has been described with regard to that which is currently considered to comprise the preferred embodiments of the invention, other embodiments have been suggested and still other embodiments will occur to those individuals skilled in the art upon receiving the foregoing specification. It is intended that all such embodiments shall be included within the scope of the present invention as defined by the claims appended hereto.