| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | November 17, 1998 |
| PATENT TITLE |
Wellbore sand control method |
| PATENT ABSTRACT |
A slug of solution, comprising a linear polymer such as polysulfone, polyethylene terephthalate or polyimide dissolved in a "good" solvent, such as morpholine, dimethylformamide m-cresol or dimethylsulfoxide, respectively, is emplaced in the near-wellbore region of an unconsolidated sand reservoir. A second slug comprising a poor solvent for the polymer, such as water or water+2-methanol is pumped into the region to contact the first slug. The linear polymer is precipitated and forms a three-dimensional network of interconnected strands extending through the fluid flow channels between the sand grains. The network functions to consolidate the sand without significantly damaging permeability. Petroleum and other fluids can then be produced without loose sand being entrained in the fluids. If necessary, this permeable network of plastic threads can be removed by re-injecting a slug of the good solvent to re-dissolve the plastic and reform the original linear polymer solution. |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | July 16, 1996 |
| PATENT PARENT CASE TEXT | This data is not available for free |
| PATENT CLAIMS |
The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 1. A process for consolidating the near-bore region of an unconsolidated subterranean reservoir containing reservoir fluid and being formed by discrete sand or gravel particles having communicating fluid flow channels extending therebetween to provide fluid flow permeability, comprising: (a) emplacing a liquid slug of a first solution in the near-bore region, said solution comprising a linear polymer dissolved in a good solvent for the polymer; (b) then Injecting a liquid slug of a poor solvent for the polymer, said poor solvent being miscible in the good solvent, into the near-bore region to contact the first slug and precipitate linear polymer to consolidate the particles of the near-bore region while retaining sufficient residual permeability to enable production of the reservoir fluid; the solvents and polymer having been selected on the following basis: (i) the good solvent being substantially non-reactive with the polymer, (ii) the polymer being non-miscible with water and with petroleum and substantially non-reactive with the reservoir solids and fluids, (iii) the combination of solvents and polymer used being operative to form a three-dimensional network of interconnected strands in a sand sample, said strands extending through fluid flow channels in the sample. 2. The process as set forth in claim 1 wherein: the good solvent is morpholine; the polymer is polysulfone; and the polymer is provided in an amount of 2.5 to 25% by weight of the first solution. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
FIELD OF THE INVENTION This invention relates to an in situ plastic consolidation method for constraining sand or gravel particles forming a subterranean reservoir. BACKGROUND OF THE INVENTION There are some hydrocarbon reservoirs that are referred to as unconsolidated sands. The sand grains are not cemented together or are only poorly cemented. When contained fluids are produced from the reservoir through a wellbore, there is some tendency for the sand grains to move with the flow and enter the wellbore. The sand can plug the wellbore or erode the producing equipment. The problem is significant and as a result extensive research has been carried out to develop ways to alleviate it. In situ plastic consolidation is one technique which has been applied for this purpose. In general, this technique involves emplacing a polymer oligomer in the flow channels of the near-wellbore region of the reservoir and then cross-linking or hardening the oligomer. (The "near-wellbore region" is a zone extending out radially from the wellbore a short distance--perhaps two or three feet.) More particularly, a slug of a first solution, comprising the polymer oligomer dissolved in a viscosity-reducing solvent, is displaced down the wellbore and into the near-wellbore region. A liquid slug containing a curing agent is then pumped into the region to contact the first slug. The well is then temporarily shut in, to allow the polymer to harden. The patent literature contains many examples of this general system. Typical polymers used are epoxy, furfuryl alcohol and phenol/formaldehyde. Now, there are a number of difficulties that require consideration in connection with plastic consolidation. Many of the prior art patents are specifically directed at proposing solutions for these difficulties. The difficulties include: Maintaining adequate residual permeability in the near-bore region. The hardened plastic can block the flow channels. Since the plastic is cross-linked, there is no effective way to remove it to restore permeability; Developing a consolidated near-wellbore sand/polymer matrix that has good compressive strength, which is an indicator of good resistance to erosion by the flow of produced fluids; Developing a plastic framework that has some residual structural strength in the event that sand grains are dissolved, which is a possibility in thermal projects where steam is being injected; Wetting the sand so that the cross-linked polymer resin binds sand grains together; and Developing a consolidated near-wellbore sand-polymer composite that does not shrink or disintegrate with time. It is therefore desirable to develop a novel process which yields a consolidated sand/polymer matrix that is characterized by good residual permeability, good compressive strength, and a plastic framework that survives sand dissolution. In addition, it is desirable to use a plastic which can be reversibly dissolved in a solvent, so that, in the event of plugging, the plastic can be removed. At this point, it is appropriate to refer to a prior art technology which has been developed in connection with the manufacture of microporous plastic membranes used for pressure driven filtration. This technology is described in the Handbook of Industrial Membrane Technology, published by Noyes Publications, Chapter 1. The technology involves contacting a first solution, comprising a polymer dissolved in a "good" organic solvent, with a second "poor" solvent in which the polymer is insoluble. The polymer will precipitate in the form of a porous, permeable solid. To applicants' knowledge, this technology has not been applied in situ in a subterranean reservoir. The technology is applied in a specific manner in connection with the present invention. SUMMARY OF THE INVENTION The present invention is based on combining the following: emplacing, by displacement down a wellbore, a slug of polymer-carrying solution in the near-bore region of an unconsolidated sand or gravel reservoir, to locate the solution in the flow channels between the sand or gravel particles, the solution comprising a linear polymer dissolved in a first organic component that is a good solvent for the polymer, said good solvent preferably being miscible with water and substantially non-reactive with the polymer and, preferably, with the reservoir minerals and fluids; then injecting a slug of a poor solvent for the polymer into the near-bore region to contact the first slug, said poor solvent being miscible with the good solvent, and precipitating solid linear polymer from solution to form a three-dimensional network of interconnected strands, said strands extending through the fluid flow channels, to consolidate the particles while retaining residual permeability. The solvents and polymer need to "match" in order to achieve the required network. Stated otherwise, one needs to test combinations of polymer and good and poor solvents to determine if a combination yields the three-dimensional, fish net-like network. For example, in our best mode we have matched: morpholine as the good solvent, polysulfone as the polymer, with a polymer loading of 5 to 20% by weight of the solution, and water as the poor solvent to achieve the network in a consolidated sand that is characterized by residual permeability that is typically about 50% of the original permeability and a level of unconfined compressive strength such that failure occurs between 100 kPa and 4000 kPa. The viscosity of the polymer solution varies between 20 and 3800 centipoise over the given concentration range. In the product of this best mode embodiment, when tested in sand, one finds: that the strands have a slight clearance from the surfaces of the sand particles, which clearances appear to be the main contributing factor to a desirable level of residual permeability; that the network combines with the sand to create a compressively strong composite matrix, without bonding to the particles; and that if fluid is flowed through the consolidated product, the sand remains affixed in the composite matrix. It will be appreciated that it will be a difficult practical problem to sample a subterranean near-wellbore region to determine if the described network has been formed. The invention as claimed is therefore to be construed as being restricted with respect to reagents to those which perform, when applied to a sand sample to yield the network in a laboratory experiment carried out in accordance with a Standard Test Procedure set forth below. Also, it may be possible to collect downwell samples by a procedure known as side-track drilling. Broadly stated, the invention is a process for consolidating the near-bore region of an unconsolidated subterranean reservoir containing reservoir fluid and being formed by discrete sand or gravel particles having communicating fluid flow channels extending therebetween to provide fluid flow permeability, comprising: (a) emplacing a liquid slug of a first solution in the near-bore region, said solution comprising a linear polymer dissolved in a good solvent for the polymer; (B) then injecting a liquid slug of a poor solvent for the polymer, said poor solvent being miscible in the good solvent, into the near-bore region to contact the first slug and precipitate linear polymer to consolidate the particles of the near-bore region while retaining sufficient residual permeability to enable production of the reservoir fluid; the solvents and polymer having been selected on the following basis: (i) the good solvent being substantially non-reactive with the polymer, (ii) the polymer being non-miscible with water and with petroleum and substantially non-reactive with the reservoir solids and fluids, (iii) the combination of solvents and polymer used being operative, if tested under laboratory conditions in accordance with the Standard Test Procedure set forth in the disclosure, to form a three-dimensional network of interconnected strands in a sand sample, said strands extending through fluid flow channels in the sample. DESCRIPTION OF THE DRAWINGS FIG. 1 is a plot showing the increase in compressive strength of consolidated sand as the percentage of polymer (polysulfone) in the good solvent (morpholine) is increased; FIG. 2 is a plot showing the variance of residual permeability in consolidated sand as the polymer content in the good solvent is varied; FIG. 3 is a scanning electron micrograph showing a polymer network formed in sand using the polysulfone/morpholine system; FIG. 4 is a scanning electron micrograph of a polymer network formed in sand using the polysulfone/morpholine system, the network being revealed after dissolving the sand grains in aqueous hydrofluoric acid; and FIG. 5 is a photograph showing a consolidated sand "near-bore region" formed by injecting polysulfone/morpholine as a first slug and water as a second slug in a sand bed, after removal of the unconsolidated sand in the bed; DESCRIPTION OF THE PREFERRED EMBODIMENT Polymers which can be used with reservoir temperatures less than 100.degree. C. include polysulfone, polystyrene, polyvinyl/chloride, polymethyl/methacrylate, polyethylene terephthalate, polyimide and polyphenylene oxide. Good solvents which can be used which are miscible with water include acetone, acetonitrile, 2-butoxyethanol, dimethylformamide, dimethylsulfoxide, dioxane, ethylmethylketone, m-cresol, morpholine and tetrahydrafuran. Halogenated solvents may be used if they are miscible with water, provided that environmental problems are not an issue. The good solvent preferably should be non-reactive with respect to reservoir minerals or fluids or the polymer. The precipitating or poor solvent may be water or brine or a mixture of water with an alcohol such as 2-methanol. Alternatively, the poor solvent may be organic, such as white oil, kerosene or petroleum ether (also called hexanes). In the case of an organic poor solvent, useful good solvents include acetone acetonitrile, cyclohexanone, diethylether, dimethylformamide, dimethylsulfoxide, dioxane, m-cresol, methyl t-butyl ether, nitrobenzene, phenol, tetrahydrofuran, toluene, or xylene. Polymers which can be used for steam enhanced oil recover ("EOR") applications include such engineering plastics as polyethylene terephthalate and polyimides, such as those based on 1,4-phenylenediamine and 3,3', 4,4'-benzophenonetetracarboxylic acid. Good solvents which can be used with these polymers include dimethylformamide, trifluoracetic acid, m-cresol, phenol, resorcinol or a substituted phenol which is a liquid at reservoir conditions. In this case, the poor solvent may be a low carbon number alcohol such as 2-methanol or a mixture of the alcohol with water. Table I sets forth a group of recommended matched combinations which yield the desired network consolidation: TABLE I ______________________________________ Polymer Good Solvent Poor Solvent ______________________________________ Polyvinyl chloride dimethylformamide water polyvinyl chloride tetrahydrofuran water Polystyrene morpholine water Polymethylmethacrylate morpholine water Polysulfone tetrahydrofuran water Polysulfone morpholine water Polyethyleneterephthalate phenol water Polyethyleneterephthalate m-cresol 50% aq. methanol Polyimide dimethylsulfoxide 50% aq. methanol ______________________________________ In principle, mixtures of solvents, as well as mixtures of polymers may also be used. For example, a mixture of polysulfone and polyvinyl chloride will dissolve in a mixture of tetrahydrofuran and morpholine. However, workers skilled in the craft will be aware that not all solvents are chemically compatible with each other, and that some polymers are not chemically compatible with some solvents. We have found that the preferred polymer is a particular polysulfone based on bisphenol A and diphenylsulfone, (density=1.24 kg/L, molecular weight=50,000 Daltons; glass temperature=190.degree. C.). We have found that the preferred solvent is morpholine, (NHC.sub.4 H.sub.8 O, density=0.999; m.p. -6.degree. C.). The preferred poor solvent for this system is water or brine. The optimum concentration of polysulfone falls in the range 5-20% (wt/wt). The viscosity of the polysulfone solution varies between 20 and 3800 Cp over this concentration range. The exact best composition depends on the optimization for a particular application (based on compressive strength, final permeability, solution viscosity and cost). We have found empirically that 200 mL of 20% (wt/wt) polysulfone in morpholine will consolidate 160 mL of fine sand. With well-packed sand in an isotropic stress field, the final distribution of polymer is uniform around the injection port and homogeneous throughout the consolidated zone. The transition between consolidated and unconsolidated sand is sharp, (1-5 mm) (see FIG. 5). We have found that the specific gravities of the polymer solution and the poor solvent should be within 5% of each other. In experiments in which there was a significant difference in specific gravities, channelling of the polymer solution was observed. This resulted in the deposition of a dense polymer solid within a narrow zone, rather than evenly through the near-bore region of the sand bed. It is anticipated that this polymer consolidation treatment can be used in many applications, such as vertical and horizontal wells. Packers may be used to block off that section to be treated. The treatment should be effective in consolidating reservoirs producing conventional crude oil, heavy oil, natural gas and water. The treatment may also be used to help extend the life of injection wells. If a polymer is selected with a sufficiently high softening point (or melting point), and sufficiently good resistance to hydrolysis, the treatment can be used in steam EOR operations, such as those involving cyclic steam and steam drive. |
| PATENT EXAMPLES | Available on request |
| PATENT PHOTOCOPY | Available on request |
|
Want more information ? Interested in the hidden information ? Click here and do your request. |