Wells – Processes – Distinct – separate injection and producing wells
Reexamination Certificate
2000-09-21
2002-11-05
Bagnell, David (Department: 3672)
Wells
Processes
Distinct, separate injection and producing wells
C166S295000, C166S400000
Reexamination Certificate
active
06474413
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the selective and controlled reduction of the relative permeability to water in oil-bearing formations, and more specifically to such a process whereby said reduction is achieved with the aid of successive polymer slugs, which are adsorbed to the rock surface and among themselves, creating a hydrophilic film which swells in the presence of water and strongly reduces the permeability to water. On the contrary, in the presence of oil the film keeps its original thickness, so that the permeability to oil is only slightly reduced.
BACKGROUND INFORMATION
The production of water associated to the production of oil has been one of the major drawbacks for oil companies worldwide. The breakthrough of the formation or injection water results in a strong decline of oil production, increasing the operation costs for pumping, treatment and discarding of huge volumes of water.
Different techniques may be used to reduce the production of water or to prevent its anticipated breakthrough, the choice of the best technique being a function of the origin of the produced water. Therefore, the analysis of the well and the reservoir should precisely determine the origin of such water.
Water coning or channeling of injected water frequently causes high water/oil ratios so that the use of flow barriers is successful. Such barriers may have a plugging effect or they may reduce permeability, that is, the systems may be respectively blocking or selective.
The plugging barriers, which block simultaneously the flow of gas, oil and water, may be formed by cements, resins, solid particle suspensions, silicates, wax, or water soluble polymers crosslinked in situ in the reservoir.
On the contrary, selective systems block mainly the flow of water, while the flow of hydrocarbons is only slightly restricted. These selective barriers are generally formed by water-soluble copolymers, which adsorb to the surface of the rock, forming a hydrophilic film, which reduces water mobility.
The preparation of selective polymer systems is a function of the mineralogy and permeability of the rock, reservoir temperature and salinity of the formation water. The kind of rock, sandstone or limestone, defines the ionic character of the polymer to be used as the first slug to contact the formation, since such first slug promotes a better adsorption of the next plug of polymer. Thus, the adsorption sites mainly negatively charged on the surface of sandstones require cationic polymers. Inversely, limestones are favored by the anionic character of polymers to enhance the adsorption ability and thus the formation of the hydrophilic film in charge of the selective reduction of water permeability.
High formation temperatures may inhibit the effect of applied treatments. Under these circumstances, the polymer tolerance may be improved by adding products such as AMPS (2-acrylamido-2-methyl propanosulfonic acid) and/or vinyl pyrrolidone to the molecular structure of the base polymer.
On its turn, salinity may inhibit the swelling of the hydrophilic film and thus hinder the effect of permeability reduction to water.
As regards the rock permeability, the higher this parameter, the thicker should be the hydrophilic film. This may be obtained through the increase in the molecular weight of the polymer (monolayer configuration), or through the piling of cationic and anionic polymers and crosslinking agents (multilayer configuration).
Further main advantages of the selective systems are:
The treatment is injected through the annulus or through the string, dispensing with the need to modify the equipment normally in the production of the well; perforation or completion rigs are dispensed with.
On the contrary, plugging treatments may require the isolation of the zone to be treated. Rigs are then required and the operation cost is considerably increased.
U.S. Pat. No. 4,409,110 teaches a process for the secondary or tertiary recovery of oil using a mixture of non-ionic and polycationic polymers. The mixture leads to a reduction in viscosity in a first pumping phase while after the injection of the mixture, the adsorption of the cationic polymer on the rock causes an increase in viscosity, so as to improve the sweep efficiency. At the same time, the observed adsorption causes the inhibition of the swelling of the clays. On the other hand, under certain circumstances of pH and temperature the non-ionic polymer undergoes hydrolysis, and becomes anionic. By contacting the cationic polymer, the anionic polymer forms a solid precipitated which would also be adequate for use in producing wells as plugging agent.
U.S. Pat. No. 4,498,539 teaches a process for the correction of permeability to water and a composition for the process, to improve the efficacy of the water injection. Such process involves the sequential injection of i) one optional aqueous slug in order to adjust the salinity of the formation water; ii) an aqueous thickened slug; iii) an aqueous entrainment fluid. The aqueous thickened slug is made up of a polyvalent metal cation, such as aluminum citrate, a gellable polymeric viscosifier such as polyacrylamide, and a hydrolizable ester suitable for the pH adjustment which is effective for the delayed reduction of the pH of the polymer slug, so as to provide the ideal condition for in situ gellification.
U.S. Pat. No. 5,079,278 teaches a gel-forming polymer aqueous composition, which is able to selectively plug high permeability zones in subterranean oil-bearing formations. The composition comprises an aqueous solution of a high molecular anionic terpolymer of acrylamide, which contains of between 5 to 95 weight % 2-acrylamido-2-methyl propane sulfonic acid, of between 2 and 95 weight % N-vinyl-N-methyl acetamide and of between 5 to 93 weight % acrylamide, and a crosslinking agent selected among the group comprising transition metal ions, phenolic resins and amino resins. The described composition forms stable gels in brines of a wide range of salinity, the gels being effective at the pH levels encountered in carbon dioxide and water injection.
U.S. Pat. No. 5,379,841 teaches the reduction or the elimination of the permeability to water by injecting a copolymer, which contains anionic and cationic portions in its structure. It is alleged that by varying the amount of types of comonomers present in the copolymer it is possible to achieve improved adsorption of the polymers to various kind of rocks, from sandstones to limestones. In spite of the alleged advantage, that the polymers may be used in various kinds of rocks, the synthesis of polymers having complex structures, which should be tested for each particular situation, is certainly time-consuming and expensive. Besides, said US patent uses high molecular weight polymers in one single layer, this being only effective for high salinity, and low to medium permeability reservoirs. In case of high permeability, the efficacy of the process using a high molecular weight polymer as a monolayer may be critically impaired.
On the contrary, the present invention may be extremely flexible by only using ready commercial products, this rendering the present process cheap and versatile, since the need of the particular formation is met by varying the number of successive polymer layers. The flexibility of the applications according to the present invention is also consequent to the various molecular weights and hydrolysis degrees of the commercial products used, which may be dimensioned as a function of the reservoir conditions (kind of rock, temperature, salinity and permeability).
European EP 0197684 teaches reducing the permeability of sandstones by injecting an aqueous solution of a water-soluble anionic polymer having a molecular weight higher than 100,000, followed by the contact of said polymer with a stabilizing fluid which comprises a water-soluble cationic polymer having molecular weight higher than 1,000. In spite of the alleged results, it should be noted that sandstones are rocks having mostly negatively charged rocks, and
Barbosa Luiz Cesar Ferreira
Coelho Sérgio Luiz Pereira De Freitas
De Godoy Geraldo Maria Ramos
De Holleben Carlos Roberto Carvalho
Bagnell David
Petroleo Brasileiro S.A. - Petrobras
Walker Zakiya
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