Residual Resistance Factor Research Articles

Molecular Configuration and Flow Performance of Al3+ Cross-linked Polymer

According to the difference in compositions of Al3+ cross-linked polyacrylamide gel, there are two types of cross-linked reactions. One is the intramolecular cross-linked reaction that takes place in different chains of the same polymer molecule, and the other is the intermolecular cross-linked reaction, which takes place among different polymer molecules. Through changing the compositions of the cross-linked polymer and gelling environment, thorough research on the critical conditions and flow performance of intramolecular cross-linked polymer is carried out. The results show that the viscosity of the intramolecular cross-linked polymer almost remains the same over time, but during the flow performance experiments, the resistance factor and residual resistance factor are higher, and the residual resistance factor is larger than the resistance factor. The higher the salinity of solvent, the easier the intramolecular cross-linked polymer forms. In the scope of concentration of cross-linking agent, the higher the concentration of cross-linking agent, the easier the intramolecular cross-linked polymer forms. The higher the temperature, the easier the intramolecular cross-linked polymer forms. For the minerals that influence the molecular configuration of cross-linked polymer, the influence of Ca2+ is the biggest, then Mg2+, the least being Na+. A pilot test in the Pu I1-4 reservoir of the Daqing oilfield showed that the in-depth profile modification using intermolecular cross-linked polymer solution has characteristics of lower cost and easier injection technique, and can gradually form gel through the process of injection. When the effect of gel forming is at its best state, the injection pressure can reach the highest and retain relatively stable values. Therefore, comprehensive watercut would decrease and oil production rate would increase, which can be used as an important whole profile modification technique.

Stability and water control of nitrogen foam in bulk phase and porous media

Foam is widely used in the petroleum industry thanks to its unique properties and performance. Its application to water control in oil wells has received more and more attention. The stability of nitrogen foam was investigated in pressurized equipment and sand pack. The result indicated that with increasing pressure (0–2 MPa) the stability of foam in the pressurized equipment increased linearly. Increased nitrogen injection pressure caused better dispersion of nitrogen-foaming solution in porous media. The initial residual resistance factor to gas was an exponential function of injection pressure, but the residual resistance factor (to gas and liquid) decreased exponentially with time. The half-life of foam in porous media (expressed in residual resistance factor) was much longer than that in bulk phase (expressed in foam height). Pore model analysis indicated that good dispersion in porous media and the presence of thick liquid film during dispersion were the main reasons for high stability. Field test indicated that effective residence of foam in the formation not only resulted in much better heat insulation, but also improved steam stimulation by enhancing steam heating.

Comparison between a polyacrylamide and a hydrophobically modified polyacrylamide flood in a sandstone core

Polymer flooding in oil reservoirs to enhance oil recovery (EOR) has been used worldwide. However, the polymers applied, such as partially hydrolyzed polyacrylamides and xanthan gum, show a number of limitations related to their low salt tolerance and biological degradation, respectively. Therefore, the polymeric properties must be improved to maximize their application. With this goal in mind, a new class of polymers, named “hydrophobically modified water-soluble polymers”, has been developed. These polymers are very similar to conventional polymers used in EOR, except that they have a small number of hydrophobic groups incorporated into the polymer backbone. At incorporation levels of less than 1 mol% these hydrophobic groups can change polymer performance. In this work, a water-soluble polymer made of acrylamide and N, N-dihexylacrylamide was obtained. After characterization of the copolymer, its performance in a porous medium was evaluated through core flood experiments in Botucatu sandstone. In the presence of sodium chloride, the amphiphilic copolymer exhibited a significant increase in viscosity at low polymer concentrations, in addition to resistance factor and residual resistance factor values higher than those of a commercial polyacrylamide. This behavior can increase oil recovery, mainly in high salinity and high-permeability reservoirs, by improving waterflood sweep efficiency.

Preparation, morphology and properties of nanocomposites of polyacrylamide copolymers with monodisperse silica

To improve oil recovery (IOR) performance of polyacrylamide polymer media, the paper presented the nanocomposites (PA-B-S) of acrylamide–styrene–AMPS copolymers (PA-S) with monodisperse SiO 2 particles. The monodisperse particles from 17 to 100 nm with low size deviation were adopted as an inorganic phase, and their nanocomposite properties and morphology were investigated with viscosity measurements, thermal degradation (TGA), flooding test and transmission electron microscopy (TEM) techniques. For 66.7 nm SiO 2 particles at 0.5 wt% load, the nanocomposites produced viscosity enhancement at critical concentration, high salt-tolerance behavior, and the high degradation temperature at 411 °C, which were obviously higher than those of pure PA-S copolymers. These inorganic–organic synergistic or nano size effects were shown in a series of prepared nanocomposite samples. TEM morphology proved that PA-B-S solution at LCST formed uniform dispersion of the SiO 2 particles encapsulated with this associating copolymer and formed stable drop-like emulsion patterns. In flooding experiments, the PA-B-S solutions at critical viscosity gave the resistance factor of 9.38 and residual resistance factor of 3.39, compared with those of 5.20, 1.51 for pure PA-S, respectively. Such improved properties of PA-B-S were suitable for producing high shearing behavior and sweep volume in IOR or EOR. As the controllable characters of monodisperse SiO 2 particles, the results from their nanocomposites were the good references to the multi-disperse particles acted as IOR media.

Effect of Cross Linking Agent on Alkali/Surfactant/Polymer

AbstractAlkali/surfactant/polymer (ASP) multisystem flooding technique, which has an expansive application prospect, is one of the enhancing oil recovery (EOR) methods. By adding the organic chromium to the ASP, the molecular structure of polymer was made to change, and the capability of controlling mobility coefficient of ASP was improved. The results showed that multisystem could still keep ultra‐low interfacial tension between the multisystem and crude oil after addition of Cr3+. The resistance factor and residual resistance factor, the indicator which describes the capability of controlling mobility, upgraded strikingly. However its storage modulus and loss modulus, the indicator which describes viscoelasticity, increased. The results of physical simulation experiment indicated that this type of improved ASP could increase the recovery ratio by 4.3% compared to common ASP multisystem.

Preformed Particle Gel for Conformance Control: Transport Mechanism Through Porous Media

Summary Preformed particle gel (PPG) has been successfully synthesized and applied to control excess water production in most of the mature, waterflooded oil fields in China. This paper reports on laboratory experiments carried out to investigate PPG transport mechanisms through porous media. Visual observations in etched-glass micromodels demonstrate that PPG propagation through porous media exhibits six patterns of behavior: direct pass, adsorption, deform and pass, snap-off and pass, shrink and pass, and trap. At the macroscopic scale, PPG propagation through porous media can be described by three patterns: pass, broken and pass, and plug. The dominant pattern is determined by the pressure change with time along a tested core (as measured at specific points), the particle-size ratio of injected and produced particles from the core outlet, and the residual resistance factor of each segment along the core. Measurements from micromodel and routine coreflooding experiments show that a swollen PPG particle can pass through a pore throat with a diameter that is smaller than the particle diameter owing to the elasticity and deformability of the swollen PPG particle. The largest diameter ratio of a PPG particle and a pore throat that the PPG particle can pass through depends on the swollen PPG strength. PPG particles can pass through porous media only if the driving pressure gradient is higher than the threshold pressure gradient. The threshold pressure depends on the strength of the swollen PPG and the ratio of the particle diameter and the average pore diameter.

Effects of shear fracture on in-depth profile modification of weak gels

Two sand packs were filled with fine glass beads and quartz sand respectively. The characteristics of crosslinked polymer flowing through the sand packs as well as the influence of shear fracture of porous media on the indepth profile modification of the weak gel generated from the crosslinked polymer were investigated. The results indicated that under the dynamic condition crosslinking reaction happened in both sand packs, and the weak gels in these two cases became small gel particles after water flooding. The differences were: the dynamic gelation time in the quartz sand pack was longer than that in the glass bead pack. Residual resistance factor (FRR) caused by the weak gel in the quartz sand pack was smaller that that in the glass bead pack. The weak gel became gel particles after being scoured by subsequent flood water. A weak gel with uniform apparent viscosity and sealing characteristics was generated in every part of the glass bead pack, which could not only move deeply into the sand pack but also seal the high capacity channels again when it reached the deep part. The weak gel performed in-depth profile modification in the glass bead pack, while in the quartz sand pack, the weak gel was concentrated with 100cm from the entrance of the sand pack. When propelled by the subsequent flood water, the weak gel could move towards the deep part of the sand pack but then became tiny gel particles and could not effectively seal the high capacity channels there. The in-depth profile modification of the weak gel was very weak in the quartz sand pack. It was the shear fracture of porous media that mainly affected the properties and weakened the in-depth profile modification of the weak gel.

Discussion and Reply to "Advances in Polymer Flooding and Alkaline/Surfactant/Polymer Processes as Developed and Applied in the People's Republic of China"

In the February 2006 issue of JPT, H.L. Chang et al. reported on some of the pilot and commercial-scale field activities of polymer flooding and alkaline/surfactant/polymer (ASP) flooding that were performed in China (Chang et al. 2006). Unquestionably, polymer flooding and ASP flooding are proven processes, and China should be proud of their work in these areas. Unfortunately, the article also advocated a controversial technology [flooding with aqueous colloidal-dispersion gels (CDGs)] as being superior to polymer flooding. I submit that this claim is misleading and generally incorrect. CDGs [i.e., relatively low concentrations of partially hydrolyzed polyacrylamide (HPAM) crosslinked with aluminum citrate] should not be applied without carefully examining the purported science and engineering behind this process. Chang et al. speculated that low-concentration aluminum citrate/HPAM microgels propagate through porous rock like superpolymer solutions. Specifically, they suggested that these CDG formulations penetrate deep into porous matrix reservoir rock and subsequently provide higher resistance factors (effective viscosities in porous media) and residual resistance factors (permeability-reduction factors) than comparable HPAM polymer solutions without cross-linker. However, independent results from three university research laboratories demonstrated conclusively that this assertion is not correct (see Seright 2006 and Wang et al. 2006 for details). Another assertion made in the February article (Conclusion 3 on page 87) is that "a large amount of CDG would preferentially enter the high-permeability or thief zones and divert polymer or water into medium- and low-permeability zones." This assertion contradicts basic calculations with Darcy's law (Seright 2006). Furthermore, the field evidence given to support CDG gels can readily be attributed to other, more plausible, factors. In many cases, questions arise whether the aluminum citrate had a positive effect on the field results. More details can be found in Seright 2006 and Wang et al. 2006.

Application of In-Depth Gel Placement for Water and Carbon Dioxide Conformance Control in Carbonate Porous Media

Abstract This paper presents the results of an investigation of the application of gel placement in an attempt to reduce the effective permeability of a carbonate porous medium to water and supercritical carbon dioxide, as encountered in the CO2 flooding of carbonate reservoirs. Three gel systems based on a high and a low molecular weight polyacrylamide polymer with chromium(III), as crosslinker, were used for this study. Since sodium lactate is commonly used for increasing gelation time at elevated temperatures, experiments were conducted by adding sodium lactate to the gel solution with a ratio of polymer to sodium lactate equal to one. The higher molecular weight polyacrylamide gel system was composed of 7,200 ppm Alcoflood 935 and 300 ppm Cr(III), while the other gel system tested was composed of 5% low molecular weight polyacrylamide (Alcoflood 254S) with a ratio of 1:12 chromium(III)-acetate to polymer as crosslinker. Experiments were conducted at 8,274 KPa and 40 °C with and without the presence of residual oil. Performance and stability of the above gel systems for reducing the permeability of the carbonate medium to the injected water and carbon dioxide was tested in a series of flow experiments by alternately injecting several pore volumes of water and carbon dioxide into the porous media in several cycles. The porous medium used was crushed carbonate with an initial permeability of over 9.86 µm2. For all experiments, the presence of Sor led to lower residual resistance factors (RRF). Nevertheless, RRFs were between 100 and a few thousands for all experiments conducted. The results obtained are a clear indication of the effectiveness of these gel systems for conformance control purposes during carbon dioxide flooding projects in carbonate reservoirs. Introduction From the pore-scale point of view, dense carbon dioxide is an ideal displacement fluid for many crude oils. Carbon dioxide gains miscibility with oil and is able to achieve very high recovery efficiencies at reservoir pressures near or above minimum miscibility pressure. However, even when pressure conditions for miscibility are met, this high microscopic sweep efficiency is not often observed in field tests. The combined effect of heterogeneity of the reservoir and the low viscosity of the injected carbon dioxide leads to the viscous fingering of carbon dioxide through fractures, channels, or high-permeable streaks in the reservoir. This causes early breakthrough of injected carbon dioxide into production wells, which reduces oil recovery efficiency. Different techniques have been investigated for reducing channelling through fractures or high-permeability zones. One such technique is in-depth gel placement. This process has been applied successfully in many reservoirs, resulting in fracture sealing, water and gas shut-off, and permeability modification. The objective of gel placement is to reduce fingering of the injected fluid through fractures or high-permeability zones of formation while minimizing gel penetration in hydrocarbon bearing regions of the reservoir. The main advantages of using gels over the other methods, such as cements or mechanical plugs, is their flexibility for pumping without a workover rig, high control of setting time, a deeper penetration into the formation, ease of cleaning, and an easy removal from the wellbore by water recirculation(1, 2).

Evaluation of Polymer-Injection Projects in Brazil

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 94898, "Evaluation of Polymer-Injection Projects in Brazil," by M.A. de Melo, C.R.C. de Holleben, I.P.G. da Silva, A. de Barros Correia, G.A. da Silva, A.J. Rosa, A.G. Lins, and J.C. de Lima, Petrobras, prepared for the 2005 SPE Latin American and Caribbean Petroleum Engineering Conference, Rio de Janeiro, 20-23 June. Petrobras has implemented three polymer-injection pilot projects during the last 2 decades in the Canto do Amaro, Carmópolis, and Buracica onshore fields. The first one is in the middle of the operation, and the last two are finished. The main objective of the pilot projects was to gain practical knowledge of the process for possible future expansion to other reservoirs and even to offshore fields. Introduction The objective of polymer-injection projects is to increase the oil-recovery factor by reducing the mobility ratio between the water and the oil. The reduction in mobility ratio is achieved by increasing the viscosity of the aqueous phase, which increases the sweep efficiency and, hence, the oil recovery. Depending on the type of polymer used, this increase in viscosity also can cause a reduction in the effective permeability to water in the swept areas. This reduction in permeability has a secondary effect of restoring part of the reservoir pressure after polymer passage. The technical and economic success of the process depends on the correct selection of the reservoir and the specification and design of the polymer bank to be injected. Reservoirs and Pilot Areas The selection of Carmópolis, Buracica, and Canto do Amaro reservoirs was based on the criteria defined in the literature. By comparing average reservoir properties with the adopted screening guide, it can be seen that they fit all the criteria except for heterogeneity and clay content. Temperature and water salinity are two important process parameters. If the temperature and water salinity are greater than the specified values, the polymer can degrade. Oil saturation has to be above a minimum value for the process to be economically feasible. High clay content can increase adsorption of the polymer in the rock. This problem can be eliminated or minimized through laboratory tests to find a polymer compatible with rock conditions. The worst problem is the presence of faults, fractures, and preferential-flow channels. These heterogeneities can be identified by use of a tracer before using the polymer. After identification, these channels can be plugged by use of blocking polymers. Mobility ratio is the most important parameter to be considered because it will be affected directly by the polymer solution to increase the sweep efficiency and oil recovery. Polymer Selection In this phase, market research was performed to verify available products for this application. Synthetic and natural polymers were analyzed, and on the basis of cost, a partially hydrolyzed polyacrylamide was selected. After selection of the polymer, laboratory tests were performed to select the proper molar mass and degree of hydrolyzation on the basis of rock/fluid-interaction parameters. These parameters should be defined for each reservoir through specific laboratory tests. These tests were performed to evaluate compatibility of the polymeric solution with the reservoir rock and fluids and to deter-mine the parameters of rock/fluids interaction [e.g., adsorption, resistance factor (RF), and residual resistance factor (RRF)].

Investigation of Polymer Adsorption on Rock Surface of Highly Saline Reservoirs

Blocking or reducing water production from oil wells is a serious problem in oil industry. Two types of polymers, namely, polyacrylamide (PAA) and polysaccharides (xanthan) have been investigated in this paper. The viscosity of both polymer solutions was first evaluated at different salinities, shear rates and concentrations. Afterwards, the solutions were injected into core samples to examine the adsorption on the rock surface by calculating the resistance factor as well as the residual resistance factor. Also, the effect of the injection rate of the polymer solutions has been studied. The results show that xanthan solution is tolerant of high salinity (20 %), while PAA solution is very sensitive to salt. Both polymer solutions show a pseudoplastic flow as a function of the shear rate. The core sample experiments show that both polymer solutions suffer a reduction in the adsorption rate with salinity increase. However, xanthan shows acceptable values even with a salinity up to 20 % and a temperature of 60 °C. Therefore, xanthan can be recommended to shut off water in high salinity and high temperature reservoirs. It was also found that the lower the injection rate the higher the adsorption on the rock surface.

MATHEMATICAL MODEL OF OIL DISPLACEMENT BY WATER WITH THE USE OF POLYMER-DISPERSE SYSTEMS

A mathematical model describing the process of displacement of oil by water with the use of polymer‐disperse systems is proposed. The point of the technology consists of pumping sequentially water, a low‐concentration polymer solution, and water with disperse particles. The porous medium is given in the form of two interpenetrating continua, one of which is associated with mobile phases and the other — with immobile phases. The corresponding expressions for dynamic porosity and the residual resistance factor have been obtained. In describing the mass transfer between the continua, an ideal model of a porous medium in the form of a bundle of capillaries of different radii and the pore‐ and particle‐size distribution functions have been used.

In-vitro Evaluation of Doxorubicin Cytotoxicity and Cellular Uptake in the Presence and Absence of Multidrug Resistance Modulators

The cytotoxicity and cellular uptake of doxorubicin was evaluated in the presence and absence of several multidrug resistance (MDR) modulators in the P-glycoprotein overexpressing cell lines, P388/ADR murine lymphocytic leukaemia and MCF7/ADR human breast carcinoma. MDR fold-reversal and the residual resistance factor were used to analyse the cytotoxicity results. Newer second generation modulators such as PSC 833, dexniguldipine and Rol 1–2933, exhibited higher fold-reversal and lower residual resistance values than first generation modulators, such as verapamil. Cytotoxicity data correlated with cellular uptake indicating the blockade of the drug efflux pump. These results emphasise the importance of estimating residual resistance in conjunction with fold-reversal when screening MDR reversing agents for in-vivo application.

Experimental Study of the Gelation Behavior of a Polyacrylamide/Aluminum Citrate Colloidal-Dispersion Gel System

Summary The gelation behavior of a polyacrylamide/aluminum citrate colloidal-dispersion gel (CDG) system was investigated in sandpacks at frontal advance rates of 2 ft/D. A gelatinous mass formed where the gelant encountered a change in flow medium. Permeabilities of homogeneous media were reduced only by factors that were similar to those obtained with a polymer displacement alone. A laboratory investigation of a polyacrylamide/aluminum citrate CDG system was conducted to determine whether the system develops in-depth permeability modification in unconsolidated sandpacks. The study includes flow of the polymer and in-situ gelation behavior of the gelant in porous media and aggregate growth during the gelation reaction in beakers. Flow experiments were conducted in long unconsolidated sandpacks in which the gel solution was mixed in-line before injection. Injection rates were designed to provide adequate residence time for the gel solution to develop in-situ flow resistance during displacements based on bulk gel characterization tests. Residual resistance factors were determined following a static rest period in which the sandpack was left saturated with the injected gel solution. For comparison, apparent viscosity and residual resistance factors were also determined for the polymer flowing through unconsolidated sandpacks. Membrane dialysis was used to study the aggregate size distribution of the gel system at selected times after mixing. In-depth in-situ flow resistance did not develop when the gelling solution was injected into the sandpacks at frontal advance rates of 2 ft/D. Propagation through the sandpack was similar to a polymer solution. Flow resistance was characterized by the formation of a gelatinous filter cake, which formed when the injected solution encountered a change in flow medium, such as a screen placed at the ends to retain sand in place, the interface between the 50-mesh coarse sand layer at the inlet and the rest of the sandpack, and void spaces. Delaying injection by 2 hours after mixing in-line (to simulate field conditions) resulted in severe front-end stripping of gel aggregates. In all cases, residual resistance factors for the gel solution were similar to those obtained with a polymer displacement alone. Effluent fractions from gel solution displacements never developed a gel structure, and their viscosities were significantly lower than the injected solution. In the study of gel size distribution, aggregates were not detected at reaction times of 4 and 8 hours.

A Relative Permeability Modifier for Water Control of Gas Wells in a Low-Permeability Reservoir

Summary Water control in gas wells is a major measure to enhance gas recovery. The work is concentrated on finding a highly selective polymer to reduce water production without affecting gas production from gas wells in low-permeability reservoirs. This paper presents the conceptions of residual resistance factors (RRF's) to both wetting and non-wetting phases and the laboratory experimental and field trial results of relative permeability modifiers for water control in gas wells.

EOR Polymer Screening for an Oil Field With High Salinity Brines

Abstract Ten Polyacrylamides of different molecular weight and in different forms (powder, broth, emulsions) and three biopolymers were tested in order to evaluate their suitability for a EOR project in Catriel Oeste oil field in Rio Neg4ro province, with very high water salinity. Screening involved: a) Resistance (RF) and Residual Resistance Factor (RRF) through fritted glass filters and viscosity measurement of solutions with different polymer concentrations, b) RF and RRF in actual pay zones cores with the selected polymers, c) ageing test at reservoir temperature, d) and oil recovery flood tests with final selected poliacrylamide. Laboratory tests allowed a suitable Polyacrylamide to be selected and an optimal concentration of the solutions to be injected in the field chosen, which have been used for pilot test simulation and detailed design. RF in actual cores gave satisfactory results.

Enhanced cytotoxicity of doxorubicin encapsulated in polyisohexylcyanoacrylate nanospheres against multidrug-resistant tumour cells in culture

We have studied the cytotoxicity and accumulation of doxorubicin encapsulated in polyisohexylcyanoacrylate nanospheres in a model of doxorubicin-resistant rat glioblastoma variants differing by their degree of resistance to this drug. We observed that the particulate form of doxorubicin was always more cytotoxic than free doxorubicin, whereas coadministration of drug-unloaded nanospheres with free doxorubicin did not modify significantly doxorubicin cytotoxicity. In C6 0.001 cells, which were 6-fold resistant and present a pure multidrug-resistant phenotype, the reversal of doxorubicin resistance was complete. In C6 0.1 cells, which were 60-fold resistant, as with C6 1V cells (selected with vincristine), the reversal of doxorubicin resistance was almost complete, with a residual resistance factor of 2-3. In C6 0.5 cells, which were 600-fold resistant to doxorubicin, the reversal of resistance was only partial and, in all cases, not above the expected participation of P-glycoprotein to the phenotype of resistance. Intracellular drug accumulation after 2-h exposure to 17.2 mumol/l doxorubicin was systematically reduced by a factor of 2-3 when doxorubicin was incubated under the form of nanospheres; doxorubicin accumulation after a 2-h exposure to IC50 was also highly reduced in all cell lines for doxorubicin-loaded nanospheres. This work shows that association of doxorubicin with nanoparticles could provide a useful tool for circumventing multidrug resistance, probably by a bypass of P-glycoprotein rather than by an inhibition of this pump.

Impact of Gelation pH, Rock Permeability, and Lithology on the Performance of a Monomer-Based Gel

Summary Resorcinol/formaldehyde gels are used to show that gel performance in porous rocks depends critically on the pH at which gelation occurs. The gels generally reduced the permeability of low-permeability sandstone more than in high-permeability sandstone. However, residual resistance factors can be greater in sandstones than in less permeable carbonate cores. A simple mathematical model is used to assess whether pH effects can be exploited to optimize gel placement in injection wells.

Reversal of multidrug resistance by a new lipophilic cationic molecule, S9788. Comparison with 11 other MDR-modulating agents in a model of doxorubicin-resistant rat glioblastoma cells

We have compared the properties of the novel multidrug resistance modulator, S9788, to a panel of 11 well-known modulators in a model of rat glioblastoma cells resistant to doxorubicin and displaying a P-glycoprotein-mediated multidrug-resistance phenotype complemented by a mechanism of intracellular drug tolerance not yet identified ( Br J Cancer 1992, 65, 538–544). S9788, like most modulators, was able to completely restore drug accumulation in the resistant line to the level obtained in the sensitive cells. This was obtained with 10 μmol/l of modulator, which is slightly higher than required for cyclosporine A (3 μmol/l) verapamil and nicardipine (6 μmol/l), but lower than for amiodarone, trifluoperazine and dipyridamole (20 μmol/l), tamoxifen and diltiazem (40 μmol/l), quinine, quinidine and nifedipine (> 100 μmol/l). Complete restoration of drug cytotoxicity was, however, obtained only with amiodarone, and a residual resistance factor of 4 could not be overcome by cyclosporine A or S9788, while other modulators gave residual resistance factors of 5–20 (trifluoperazine, tamoxifen, verapamil, quinine, nicardipine, dipyridamole) or even higher (diltiazem, quinidine, nifedipine). When studying doxorubicin accumulation obtained for an exposure to the ic 50 of this drug, it appeared that some modulators were able to decrease this “intracellular ic 50” independently of their efficiency in resistance reversal (cyclosporine A, S9788, amiodarone, trifluoperazine, quinine, tamoxifen), thus reversing intracellular drug tolerance, whereas other modulators could not reduce this parameter (verapamil, nicardipine, dipyridamole, diltiazem, quinidine). It is suggested that drugs of the first group could be able to segregate doxorubicin in subcellular compartments from which it could not reach its nuclear targets.

Degradation and Stabilization of Polyacrylamide

Abstract The thermooxidizing stability of aqueous solutions of polyacrylamide in the temperature range of from 20°C to 70°C in water with different degrees of mineralization containing H2S and Fe2+ is researched. The affect of some of the above-mentioned factors on the change in molecular weight and molecular weight distribution is studied. Some aspects of the mechanism of thermooxidative destruction of polyacrylamide are investigated. To stabilize aqueous polyacrylamide solutions a number of water-soluble antioxidants phenolic, aminophenolic and amine types, metal-deactivator, containing phosphorus, nitrogen and sulfur, are studied. The optimal proportion of components and their total concentration in aqueous solutions is established for composite stabilizers. For aqueous polyacrylamide solutions, containing the most effective stabilizers, rheological characteristics and oil-driving ability are studied in an oil-stratum model. The applications of stabilizers considerably increases the resistance factor, residual resistance factor and oil-driving ability of polyacrylamide solutions.