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Acrylamide-based water-soluble polymers are used in operations such as drilling, water flooding and water conformance. A drawback of these polymers is that the primary amide group is sensitive to hydrolysis at elevated temperatures. This fact sometimes becomes a problem in deep reservoirs, because the solutions properties change when the degree of hydrolysis of the polymer increases and, in a media of high salinity and water hardness, the precipitation of the polymer may occur. To improve the thermal stability different copolymers have been prepared. The comonomers used for this purpose are mainly N-alkyl substituted acrylamide derivatives and of particular importance N-vinyl pyrrolidone (NVP). The aim of the present work is to evaluate the improvement of the thermal stability of cationic water-soluble polymers based on acrylamide when NVP is introduced as a comonomer, in order to know the amount and kind of incorporation of NVP required to get a polymer useful for applications under severe conditions of temperature. Several copolymers and terpolymers of acrylamide, a cationic monomer and NVP were prepared. The products were evaluated in terms of the thermal stability of the primary amide group and the fraction of retention of the intrinsic viscosity in aging tests at 248°F. The introduction of NVP limited the hydrolysis of the primary amide groups. When the content of NVP increased in the terpolymers the hydrolysis degree reached after the aging was lower. In addition, the distribution of the monomers in the polymer chain also showed an influence in the improvement of the thermal stability. The results made possible to suggest a mechanism for the stabilization of the amide group by NVP. These results provide information to prepare a cationic water-soluble polymer that could be used for applications where severe temperature conditions are expected. Introduction A disadvantage of acrylamide based polyelectrolytes in aqueous solutions is the sensitivity of the amide group to the hydrolysis reaction1,2,3,4. The hydrolysis of the amide groups takes place readily when polyacrylamides are heated even under moderate pH and temperature conditions5. This can be a limitation for some applications in the oilfield because these polymers are usually employed in environments where temperatures between 170 and 350°F can be experienced in the presence of high levels of salts and water hardness. When the amide group is hydrolyzed, a carboxylate group is produced in the polymer chain (Fig. 1), which results in changes in solution properties. In environments of high hardness the carboxylate groups can bind to the divalent cations, causing a reduction in the molecular dimensions in solution and eventually it may give rises to the precipitation of the polymer3. Although hydrolysis of the amide groups is highly dependent on pH and temperature, this reaction is relatively slow at room temperature. However at 212°F almost all the amide groups in a polymer are known are hydrolyzed in a few days1,9. Under some conditions degradation of the polymeric chain may also occur. This leads to a reduction in the molecular weight of the polymer, which might affect even further the properties of the polymeric solutions. To improve the thermal stability many different copolymers have been prepared2,7,8,9. The two most important are sodium-2-acrylamido-2-methylpropane sulfonate (AMPS) and NVP. In the following paragraphs some studies carried out with copolymers of Am with these two monomers are summarized. AMPS is a secondary amide and it is known that these are less susceptible to hydrolysis than primary amides. In this monomer, the alkyl chain is a gem-dimethyl group, which can impart further protection due to steric hindrance10. Moradi-Araghi et. al. studied a copolymer of Am with AMPS with mole percent composition Am:AMPS of 63:37 at 121°C. They found that almost all the amide groups were hydrolyzed, and concluded that AMPS did not protect Am against thermal hydrolysis. However, the rate of the hydrolysis was a function of the amount of AMPS in the polymer; the polymers with higher amounts of AMPS hydrolyzed at a slower rate8. Parker et. al.11 determined the hydrolysis of acrylamide units in a copolymer of Am with AMPS, at 90, 120 and 150°C. They mention that the hydrolysis of the acrylamide units in these copolymers are unaffected by the presence of AMPS, even the hydrolysis of AMPS is facile over 120°C.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper describes the design, operation and results of two different water shut-off applications in a field located in Southern Venezuela. The production mechanism of the main reservoirs of the studied field is strong water drive. The presence of the active aquifer, the geological complexity and the high drawdown produced by the electric submersible pumps, it has abruptly increased the water-oil ratio causing premature shutting. The limitations of the traditional water shut-off applications in this field are the slotted liners gravel pack completion of the wells, the high formation permeability and the presence of caverns in the near wellbore due to sand production. A project was proposed to evaluate the gel technology, including the diagnosis of the high water production source and the design of applications to overcome these limitations. Two mayor causes were detected, water channeling through high permeability strakes at the top of the reservoir where thin shale barriers are present; and rapid coning at the base, where the sand has a higher vertical permeability and the water-oil contact is present. According to this, two different applications were designed and successfully evaluated in three wells of the field, two of them correctives and the other preventive. Relevant aspects of the design are the use of an elastic gel system of high consistency with easily controlled gelling times; and the use of placement techniques to guarantee the seal of offensive zones and the protection of the productive ones. In the corrective treatments the gelant was injected using mechanical isolation and the total volume was pumped in two batches, while dual injection was carried out in the preventive application. A reduction of 3000 barrels per day of water production was obtained, as well as a substantial increase in the productive life of the three wells.

In this work a gel of an acrylamide based polymer crosslinked with organic reagents was studied. This gelling system was one of the commonly used in water conformance treatments. Once formed in bottle tests the gel was placed in contact in several proportions with water of various salinities. The tests where carried out at 266°F, and the changes were registered as a function of time. It was found that the gel swelled until a constant final volume. This value depende the difference between the salinity of the water used to prepare the gel and the added water. The performance of this gel in porous media was evaluated by coreflood experiments in Berea sandstone at 266°F with water prepared with the same compositions evaluated in the bottle tests. The results indicated that the RRF values are lower if the salinity of the reservoir water is similar to water salinity used to prepare the gelling mixture; while the Residual Resistance Factor (RRF) increased if the salinity of the reservoir water was much higher than the salinity of the preparation water. These findings show that it is important to take into account the difference between the water salinity used to prepare the gelling system and the water reservoir in the design of water conformance treatments with polymeric gels. Introduction The aqueous gels prepared from crosslinking reactions of water-soluble polymers are nowadays used to reduce the formation permeability in water conformance treatments. Despite, there are still many questions about the behavior of the gels inside the porous media. One of these questions is how the fluids present in the reservoir affect this gel. The aim of this work, as a contribution to the available information, is to study the influence of the difference between the salinity of the reservoir water and the water in which the gelling system has been prepared. The gels are an intermediate state between a liquid and a solid. As a result of this dual character they can show intermediate properties between the cohesion of the solid and the transport of the liquids. The polymer network holds the liquid in place, and the liquid prevents the networks from collapsing into a compact mass2,4,6. They have the capacity of swell or shrink according to the external media against they are put in contact. A gel can swell or shrink when there are changes in the variables of the external environment like pH, temperature, salinity, etc.1,2,4. The studies on gel swelling have demonstrated that small changes in the external conditions as ionic strength, and external electric field can induce drastic changes in the state of the swollen network. In particular, it is well known that under certain conditions polyelectrolyte gels may undergo a discontinuous volume change. The volume of gel is influenced by osmotic pressure7,8. The osmotic pressure is the sum of the following three components; interactions between the polymer strands and the solvent, the pressure relates to the elasticity of the individual resistance to stretching or bunching, and the hydrogen ion pressure, which is associated with the ionization degree of the polymer network9. The magnitude and direction of the components of osmotic pressure in cnjunction each other govern whether a gel swells or shrinks. In this work the interaction between the reservoir water of reservoir and the water in which the gel was prepared was studied in bottle tests and by coreflood experiments in Berea sandstone. Experimental Gelling mixture preparation. The gelling system was prepared dissolving a water-soluble polymer in water of the composition shown in column A in Table 1. Then a mixture of organic crosslinkers was added and the pH was adjusted adding NaOH.