GELLABLE AQUEOUS COMPOSITION AND ITS USE IN ENHANCED PETROLEUM RECOVERY

Abstract: A gellable aqueous composition able to reduce the permeability of high-permeability zones in an oil reservoir is a aqueous solution containing metered quantities of: - a) a water-soluble organic polymer cross-linkable by the effect of a polyvalent metal ion cross-linking agent; - b) a Cr(III) ion cross-linking agent; and - c) a ligand for the cross-linking agent chosen from water-soluble organic alpha-hydroxy acids and alpha-amino acids.

Description: This invention relates to a gellable aqueous composition with delayed gelling, which is able to reduce the permeability of high-permeability zones in an oil reservoir. The invention also relates to a method for reducing permeability in a reservoir using said gellable composition. The primary recovery methods for petroleum, make use of the natural energy of the reservoir or mechanical means to produce petroleum from the reservoir through the wells; however, these methods are known to allow only partial extraction of the crude contained in an oil reservoir. To increase the extractable petroleum quantity it is therefore usual to employ secondary recovery techniques, consisting essentially of introducing into the reservoir through selected injection wells a fluid, generally water or an aqueous polymer solution , which conveys the crude through the pores in the rock to producing wells. However, because of the non-homogeneous state of petroleum-bearing rock, which results in zones of different permeability, the injected fluid tends to be channelled through the zones of greater permeability. As a consequence, the zones of lower permeability or only partly are not flushed, by the injected fluid, which limits the total quantity of petroleum recovered from the reservoir. To solve this problem, use can be made of one of the techniques for enhanced petroleum recovery based on the partial or complete occlusion of the high permeability, watered-out zones. This is accomplished by feeding an aqueous solution of a gellable polymer into the reservoir through one or more wells, to form polymer gels in situ in the high permeability zones. In this manner it is possible to deviate the flow of fluids subsequently introduced into the reservoir towards the zones of lower permeability and thus recover the crude contained in them. The gellable solutions used for this purpose are normally an aqueous solution of a water-soluble polymer such as a polyacrylamide, a partly hydrolyzed polyacrylamide, or a biopolymer such as xanthan gum, cross-linkable by the effect of an ion of a polyvalent metal, normally Cr(III). Moreover, as the zones to be occluded can be very large and/or at a more or less great distance from the injection well, the gelling solution must be of delayed gelling type to allow the solution to reach the high permeability zones of the reservoir and to fill them completely. The problem of delayed gelling has been confronted in various ways. Thus, in one known method described for example in US-A-3,785,437, an aqueous solution containing the cross-linkable polymer and a hexavalent chromium salt unable in itself to cross-link the polymer is injected into the reservoir. The chromium is then reduced to the trivalent state at a slow rate by means of a reducing agent (such as thiourea or bisulphite) either contained in the gellable solution or injected into the reservoir in the form of an aqueous solution subsequent to the injection of the gellable solution, so as to induce gelling. This Cr (VI)/reductant method, which has the advantage of permitting the injection of a single, slow gelling solution into the reservoir, has however the drawback of toxicity of hexavalent chromium and its consequent impact on the environment. In the case of sequential injection, the drawbacks include the incomplete mixing of the injected solutions, so that the gel forms only at their interface, and is therefore thin and easily deteriorates. US-A-3,762,476 describes a gellable composition useful for correcting permeability in an underground reservoir, which comprises a cross-linkable polymer and a cross-linking agent in the form of an ion of a polyvalent metal complexed with certain anions having sequestering and delaying characteristics. According to the description of this patent, the solution of the complexed cross-linking agent is injected into the reservoir after injecting the polymeric aqueous solution, and thus the aforesaid drawbacks inherent in sequential injection are not overcome. US-A-4,683,949 describes gellable aqueous compositions useful in petroleum enhanced recovery methods which contain a water-soluble polyacrylamide and a cross-linking agent in the form of a Cr(III) complex with a carboxylate ion, and more particularly an aliphatic monocarboxylic acid ion, especially an acetate ion. The use of this composition avoids the problems relative to sequential injection, however the resultant delay in cross-linking is only modest. Consequently these compositions are unsuitable for positioning the gel in deep zones in the reservoir, as is often desired. US-A-4 772 397 is a document representative of the prior art, and discloses a method for completing a horizontally bored oil well, suggesting the use of an aqueous solution which contains a carboxylated polymer and a complex Cr III carboxylated cross-linking agent: exemplary of the latter are the Cr III carboxylates deriving from formic, acetic and lactic acid. Compositions of this kind, however, are typical quick-gelling compositions. GB-A-1 187 773 is another prior art document which discloses delayed-gelling aqueous gellable compositions which contain a water soluble high-molecular-weight polymer, a metallic carboxylated cross-linking agent, and a retarder, which is a carboxylic acid such as formic, acetic, propionic and lactic acid, preferred being the acetic acid: compositions of this kind show a certain gelling delay, but they cannot be pumped easily when the gelation rate, G', attains values in the 1 to 10 range. It has now been found that the aforesaid drawbacks can be obviated by the gellable compositions of the present invention, which use particular ligands for the cross-linking agent and have gelling delay times which can be adjusted within a wide range of values useful for practical purposes. Moreover, such compositions are able to undergo transformation into a compact gel at ambient or higher temperature, so being very useful in petroleum assisted recovery methods in which the permeability values within the reservoir are corrected. In accordance therewith, one object of the present invention is the provision of a gellable aqueous composition with an adjustable gelling delay time. A further object of the present invention is the the use of such a composition in petroleum assisted recovery methods. Further objects of the invention will be apparent from the ensuing description and experimental examples. Thus, according to one aspect, the present invention provides a delayed-gelling, permeability-reducing, gellable aqueous composition, consisting of an aqueous solution containing a gellable organic polymer, a Cr III -ion-based cross-linking agent therefor, and a ligand for the cross-linking agent, characterized in that: a) the gellable organic polymer is a water soluble organic polymer selected from polyacrylamides, and partially hydrolyzed polyacrylamides having a weight average molecular weight of from 100.000 to 20.000.000; b) the ligand, other than lactic acid, is selected from those having the general formula: wherein: R is a hydrogen atom, a straight-line or a branched C₁-C₁₅ alkyl, a cycloalkyl or an aryl, and X is a hydroxyl or an amino group, the a) component being present in an amount of from 1.000 ppm to 50.000 ppm on a weight basis, the weight ratio of a) to the Cr III -ion-based cross-linking agent being from 1 to 1 to 1.000 to 1, the b) component being present in an amount of from 5 mol/gram atom to 500 mol/gram atom relative to the Cr III -ion-based cross-linking agent, the maximum pH of the composition being 7,0. In the present description, ppm means ports per million by weight. Water-soluble polymers useful for the purposes of the present invention are polyacrylamide, partially hydrolyzed polyacrylamide and the copolymers of acrylamide with one or more copolymerizable monomers. The weight average molecular weight of these polymers and copolymers can vary from 100,000 to 20,000,000 and their concentration in the composition can vary from 1000 to 50,000 ppm. In the preferred embodiment, polyacrylamide polymers or their partial hydrolysis products with a weight average molecular weight of between 200,000 and 12,000,000 can be used, in a concentration varying from 4000 to 30,000 ppm. The cross-linking agent useful for the purposes of the present invention is the Cr(III) ion and therefore the water-soluble organic salts of this metal such as the chloride, nitrate or sulphate are used. The concentration of the cross-linking agent in the composition, expressed as metal, can vary between 10 and 5000 ppm and preferably between 25 and 500 ppm. The weight ratio of the water-soluble organic polymer to the cross-linking agent is kept between 1:1 and 1.000:1, and preferably between 5:1 and 500:1. The ligands for the cross-linking agent useful for the purposes of the present invention are chosen from the water-soluble organic alpha-hydroxy acids and alpha-amino acids other than lactic acid, having general formula: wherein R and X are as hereinbefore defined. Examples of such ligands are glycolic acid, glycine, serine and 2-aminobutyric acid. These ligands can be added to the gellable aqueous composition as salts, especially salts with an alkaline metal or ammonium. In the composition of the present invention the Cr(III) ligand and the cross-linking agent can be present in a molar ratio of between 5:1 and 500:1, and preferably between 5:1 and 100:1. In addition, the composition of the present invention has a pH within the composition gelling range, which generally is between about 2.0 to 7.0. Consequently, when necessary or desirable, the pH value of the composition is adjusted by adding a mineral acid such as hydrochloric acid or perchloric acid, or an inorganic base such as sodium hydroxide. A neutral or acid pH is essential for inducing gelling of the gellable compositions of the present invention. In this respect, at alkaline pH values colloidal chromium hydroxide precipitates, this being inactive for crosslinking purposes. Although the mechanism by which the ligands of the present invention exercise their delaying effect on the gelling rate of the gels cross-linked by Cr(III) is not known with certainty, it is considered that this effect depends on the chemistry of their complexing with the Cr(III) in solution. In particular, it is considered that in the complexed or sequestered form, ie Cr(III)-L (L = ligand), the Cr(III) is not available for reaction with the reactive carboxylic acid groups of the polymer chain, and therefore no gelling occurs. In accordance with this theory, the Cr(III) becomes available for cross-linking only when the ligand L dissociates, as represented by the following equation: Basically, this implies that the rate of cross-linking depends strongly on the concentration and chemical nature of the particular ligand added to the gellable composition. While certain ligands, in particular those described in the present invention, are effective, others will be much less so, depending on the rate at which they release the Cr(III) for cross-linking purposes. In this context it should be noted that the acetate ion is a weak complexing agent for Cr(III) [in this respect see Irving H.N.M.H. "The XVth Proctor Memorial Lecture. Fact or Fiction? How Much Do We Really Know About the Chemistry of Chromium Today?", J. Soc. Leather Techn. Chemists (1974) 58, 51] and is therefore not effective in obtaining long delays in gelling at high temperature. As according to this mechanism the effect obtained by adding the ligand L is due to the formation of the complex Cr(III)-L, the invention can be implemented either by mixing the components in solution (as described in the following examples) or by using preformed complexes. In this latter case, the complexes could be wed either with or without excess ligand in solution. According to a further aspect, the present invention provides a method for reducing the permeability of a high-permeability zone of an oil-reservoir, comprising the steps of: 1) Preparing above ground a delayed-gelling, permeability-reducing, gellable aqueous composition as defined hereinbefore ; 2) Injecting said gellable aqueous composition into said high-permeability zone through a; leas; one wellbore in fluid communication therewith; 3) Allowing said gellable aqueous composition to propagate through said high pemeability zone, and 4) Allowing said gellable aqueous composition to gel. The gellable aqueous composition is prepared by simply mixing the constituents, operating under ambient temperature conditions. The order of addition of the constituents is not particularly critical, however in a preferred embodiment an aqueous solution of the water-soluble organic polymer and ligand is firstly prepared, to which the solution of cross-linking agent is then added. Finally, if necessary or desired, the pH is adjusted by adding a mineral acid or an inorganic base. As will be apparent from the experimental examples given hereinafter, the use of the alpha-hydroxy acid or alpha-amino acid ligand delays gelling by a time adjustable within a wide range of values, particularly as a function of the ratio of ligand to cross-linking agent. Moreover, gelling occurs both at ambient temperature and at higher than ambient temperature, with the production of stable, compact gels. It is therefore possible to prepare gellable aqueous solutions suitable for the most diverse conditions which can be encountered in an oil reservoir. In the following experimental examples, given by way of non-limiting illustration, the following general procedure is used. Two separate solutions in deionized water are prepared, containing respectively 10,000 ppm of a commercial polyacrylamide (1% hydrolysis, weight average molecular weight 5,000,000-6,000,000) and 1000 ppm (1.9x10⁻² M) of Cr(III) in the form of Cr(III) nitrate or chloride. Aliquots of these solutions are mixed together to obtain a final polyacrylamide concentration of 8,000 ppm, a final Cr(III) concentration of 50 ppm and variable quantities of ligand with molar Cr(III)/ligand ratios between 0.5:1 and 50:1. The solution pH is adjusted to 5.5 ± 0.2 by the addition of an aqueous sodium hydroxide or hydrochloric acid solution as required. Equal samples of each solution are prepared in order to follow the gelling progress at 25, 60 and in some cases 85°C. EXAMPLE 1 Glycolic acid is used as ligand (L) in various molar ratios to the Cr(III), the time in days being measured for complete gelling of the solution to occur at the various temperatures indicated. The test results are given below. EXAMPLE 2 Glycine is used as ligand (L) in various molar ratios to the Cr(III), the time in days being measured for complete gelling of the solution to occur at the various temperatures indicated. The test results are given below. EXAMPLE 3 Serine is used as ligand (L) in various molar ratios to the Cr(III), the time in days being measured for complete gelling of the solution to occur at the various temperatures indicated. The test results are given below. EXAMPLE 4. 2-aminobutyric acid is used as ligand (L) in various molar ratios to the Cr(III), the time in days being measured for complete gelling of the solution to occur at the various temperatures indicated. The test results are given below. EXAMPLE 5 (comparison) Acetic acid is used as ligand (L) in various molar ratios to the Cr(III), the time in days being measured for complete gelling of the solution to occur at the various temperatures indicated. The test results are given below. EXAMPLE 6 Three gelling tests are carried out using glycolic acid as ligand with a molar ligand /Cr(III) ratio of 5/1 at different pH values. The results are given in the following table. Tests 2 and 3 are comparison tests. In test 2 the pH value is adjusted after mixing all the reagents together. In test 3 the pH value is adjusted to 9.0 after mixing the polymer solution and ligand solution together but before adding the Cr(III). After adding the Cr(III) the pH is adjusted to 9.5. In the comparison examples the presence of Cr(OH)₃.3H₂O, a substance of very low water solubility, can be noted. The determination can be made by turbidity measurements on the aqueous solutions, even in the presence of the water-soluble polymers. Under these conditions, the ligand is evidently inactive towards the chromium, and this latter is inactive in cross-linking the polymer solutions.

Claims: Claims for the following Contracting States : DK, FR, GB, NL A delayed-gelling, permeability-reducing, gellable aqueous composition, consisting of an aqueous solution containing a gellable organic polymer, a Cr III -ion-based cross-linking agent therefor, and a ligand for the cross-linking agent, characterized in that: a) the gellable organic polymer is a water soluble organic polymer selected from polyacrylamides, and partially hydrolyzed polyacrylamides having a weight average molecular weight of from 100.000 to 20.000.000; b) the ligand, other than lactic acid, is selected from those having the general formula: wherein: R is a hydrogen atom, a straight-line or a branched C₁-C₁₅ alkyl, a cycloalkyl or an aryl, and X is a hydroxyl or an amino group, the a) component being present in an amount of from 1.000 ppm to 50.000 ppm on a weight basis, the weight ratio of a) to the Cr III -ion-based cross-linking agent being from 1 to 1 to 1.000 to 1, the b) component being present in an amount of from 5 mol/gram atom to 500 mol/gram atom relative to the Cr III -ion-based cross-linking agent, the maximum pH of the composition being 7,0. Composition according to Claim 1, wherein the molecular weight of a) is from 200.000 to 12.000.000. Composition according to Claim 1, or Claim 2, wherein the concentration of a) is from 4.000 ppm to 30.000 ppm. Composition according to Claim 1, wherein the Cr III -ion-based cross-linking agent is selected from the chlorides, the nitrates and the sulphates of trivalent chromium. Composition according to Claim 1, or Claim 4, wherein the concentration of the Cr III -ion-based cross-linking agent is from 25 ppm to 500 ppm. Composition according to Claim 1, wherein the ligand b) is selected from glycolic acid, glycine, serine and 2-aminobutyric acid. Composition according to to Claim 1, wherein the pH range of the solution is from 2,0 to 7,0. Method for reducing the permeability of a high-permeability zone of an oil-reservoir, comprising the steps of: 1) Preparing above ground a delayed-gelling, permeability-reducing, gellable aqueous composition according to any of the Claims 1 to 8; 2) Injecting said gellable aqueous composition into said high-permeability zone through at least one wellbore in fluid communication therewith; 3) Allowing said gellable aqueous composition to propagate through said high pemeability zone, and 4) Allowing said gellable aqueous composition to gel. Claims for the following Contracting State : ES Process for preparing a delayed-gelling, permeability reducing, gellable composition, consisting of an aqueous solution containing a gellable organic polymer, a Cr III -ion-based cross-linking agent therefor, and a ligand for the cross-linking agent, characterized in that it comprises the step of combining together: a) a gellable organic polymer which is a water soluble organic polymer selected from polyacrylamides, and partially hydrolyzed polyacrylamides having a weight average molecular weight of from 100.000 to 20.000.000; b) a ligand, other than lactic acid, which is selected from those having the general formula: wherein: R is a hydrogen atom, a straight-line or a branched C₁-C₁₅ alkyl, a cycloalkyl or an aryl, and X is a hydroxyl or an amino group, the a) component being present in an amount of from 1.000 ppm to 50.000 ppm on a weight basis, the weight ratio of a) to the Cr III -ion-based cross-linking agent being from 1 to 1 to 1.000 to 1, the b) component being present in an amount of from 5 mol/gram atom to 500 mol/gram atom relative to the Cr III -ion-based cross-linking agent, the maximum pH of the composition being 7,0. Process according to Claim 1, wherein the molecular weight of a) is from 200.000 to 12.000.000. Process according to Claim 1, or Claim 2, wherein the concentration of a) is from 4.000 ppm to 30.000 ppm. Process according to Claim 1, wherein the Cr III -ion-based cross-linking agent is selected from the chlorides, the nitrates and the sulphates of trivalent chromium. Process according to Claim 1, or Claim 4, wherein the concentration of the Cr III -ion-based cross-linking agent is from 25 ppm to 500 ppm. Process according to Claim 1, wherein the ligand b) is selected from glycolic acid, glycine, serine and 2-aminobutyric acid. Process according to to Claim 1, wherein the pH range of the solution is from 2,0 to 7,0. Method for reducing the permeability of a high-permeability zone of an oil-reservoir, comprising the steps of: 1) Preparing above ground a delayed-gelling, permeability-reducing, gellable aqueous composition according to any of the Claims 1 to 8; 2) Injecting said gellable aqueous composition into said high-permeability zone through at least one wellbore in fluid communication therewith; 3) Allowing said gellable aqueous composition to propagate through said high pemeability zone, and 4) Allowing said gellable aqueous composition to gel.

Inventors: BURRAFATO, GIOVANNI, LOCKHART, THOMAS PAUL
Applicants: AGIP S.P.A., ENIRICERCHE S.P.A.
Application year: 1990
Main classification: E21B33
IPC codes: C09K8, B01J13