Objective— To determine the attitude of pregnant women to a proposal of con‐servative management of prolonged pregnancy. Design— A prospective questionnaire‐based survey. Setting— Louis Margaret Maternity Wing, Cambridge Military Hospital, Aldershot. Subjects— 500 pregnant women initially at 37 weeks gestation considered suitable for the potential conservative management of prolonged pregnancy. Results— Despite a stated obstetric preference for conservative management, only 45% of the women at 37 weeks gestation were agreeable to conservative management; of those undelivered by 41 weeks gestation 31% still desired conservative management. This significant decrease was unaffected by parity or certainty of gestational age. Conclusions— Most pregnant women are unwilling to accept the conservative management of prolonged pregnancy and become more reluctant to do so if undelivered by 41 weeks gestation. Women are not as favourably disposed towards the conservative management of pregnancy as has been suggested previously.
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Scott, T., SPE, U.K. Atomic Energy Authority, Winfrith Roberts, L.J., U.K. Atomic Energy Authority, Winfrith Sharp, S.R., U.K. Atomic Energy Authority, Winfrith Clifford, P.J., U.K. Atomic Energy Authority, Winfrith Sorbie, K.S., SPE, U.K. Atomic Energy Authority, Winfrith Summary. This paper presents results for a series of calculations on the deep emplacement of a polymer gel in a stratified reservoir model. These calculations were performed with a new chemical flooding simulator that has the facility to describe generalized chemical reactions between components. This code is described, and preliminary calculations on an in-situ gel treatment in a large model reservoir are presented. We find that to obtain significant amounts of incremental oil while avoiding very large pressure buildup, the polymer gel system must have the correct combination of long gelation times and good permeability-reducing properties in the high-permeability streak (residual resistance factor FR 10 to 40 in the cases studied here). No commercial polymer/crosslinker systems are currently available that have the very long gel times required to obtain deep emplacement in a large reservoir system. Introduction During recent years several chemical systems have been suggested for blocking off very-high-permeability channels in heterogeneous petroleum reservoirs. These systems involve the injection of petroleum reservoirs. These systems involve the injection of polymer [either polyacrylamide (PAM) or xanthan polymer] and polymer [either polyacrylamide (PAM) or xanthan polymer] and a crosslinking-ion or redox system to form a suitable gel. Reactants may be mixed just before injection into the reservoir, or they may be injected as alternating slugs of the two materials. To date, no calculations have appeared in the literature that attempt to quantify the flow patterns and oil recoveries expected when a polymer gel system is injected into a heterogeneous reservoir. In this paper, such calculations are presented, assuming a simple model of the gel kinetics and behavior in the porous medium. These calculations have been carried out with a new chemical flood simulator, Simulator for Chemical Oil Recovery and Polymer Injection (SCORPIO), developed at Winfrith. SCORPIO is a general-purpose, multiphase, multicomponent chemical-flood simulator that may be applied to polymer, surfactant, or caustic flooding on either the field or polymer, surfactant, or caustic flooding on either the field or laboratory scale. This paper includes a description of the simulator's underlying mathematical formulation, amplifying those features that deal with in-situ gelation. Results from in-situ gel calculations are discussed in some detail, particularly the effects of gel formation rate and pore blocking on oil recovery efficiency. Simulation Model SCORPIO is based on the method of finite differences and is designed to be flexible enough to handle a wide range of petroleum engineering problems. Up to 10 chemical components may be included. and these can be distributed among up to three liquid phases (aqueous, oleic, and micellar). Flow may be treated as either compressible or incompressible, making due allowance for rock compressibility. Two new features for a chemical flood simulator are included in SCORPIO:a generalized model of chemical reaction that defines the rates and stoichiometry for any series of coupled chemical reactions between components within a given phase anda heat-balance equation that allows calculation of temperature fronts and contours within a reservoir caused by injection of cool water into a hot reservoir. Heat flow across reservoir boundaries is treated with aquifer-type models. The calculated temperature may feed back onto such physical properties as fluid viscosity and reaction rates. Some calculations properties as fluid viscosity and reaction rates. Some calculations that use facilities of this type have been presented previously in a study of the effects of temperature on the chemical degradation of polymer in a stratified reservoir system. This module is used to describe the chemical reactions that take place in gel formations. In addition, modules are available to model the influence of coupled adsorption of components and the effects of frontal spreading through velocity dispersion and molecular diffusion. The code can accommodate several different rock types for relative permeability, adsorption, polymer residual resistance, and capillary pressure, in addition to allowing for regional variations in rock permeability. Injection and production wells can be operated under either pressure or rate constraints and may be completed in any number of layers of the reservoir. The compositional formulation used in SCORPIO is an extension of that introduced by Acs et al. and subsequently used by others in their code development work. This approach has been adapted to chemical flooding and has proved a good foundation on which to establish the framework of the simulator. Simulator Equations. In this section, the differential equations that govern fluid flow in the reservoir are presented, together with a statement of the physical significance of the various terms representing important phenomena. The corresponding difference equations and the solution strategy used to solve for the primary variables of the system are described later. SPERE P. 634
This paper comprises studies of the kinetics of the xanthan/chromium gel system. The central objective of this work is to perform well-characterized core flow experiments with a simple gelling system that may then be mathematically simulated to obtain a detailed knowledge of the processes that are occurring. Gamma-labeled 51Cr is used so that in-situ chromium profiles may be observed during gel emplacement. These are required to distinguish between different kinetic/transport models because effluent profiles alone are insufficient for this purpose. A generalized multicomponent transport equation including terms describing the crosslinking reaction is used to simulate the experiments. IntroductionIn recent years, there has been considerable interest in the application of polymer/crosslinker technology to remedy problems associated with macroscopic reservoir heterogeneity. Several different types of systems come under this description, but the underlying idea is the same in all cases: a mobile fluid is injected that chemically reacts to produce a fluid having a very high apparent viscosity within the porous medium. The chemical and physical processes that occur during in-situ gelation are complex and not well understood.In this study, a simple time-setting polymer gel system is investigated to study some of the basic phenomena that occur when reacting systems flow in porous media. A series of nonsteady-state experiments is described in which xanthan biopolymer and chromium (as Cr+ 3 ) are mixed at the inlet of a sandstone core. The crosslinking behavior is studied by analyzing effluents and monitoring pressure along the core during the flood. The in-situ measurement of total chromium concentration along the core is also determined using 51Cr, which is a gamma ray emitter.Results from the experiments are analyzed with a set of transport equations and a range of limiting models ofthe reaction kinetics. In applying the mathematical model to the experimental results, we obtain a much clearer picture of the behavior of the system within the porous medium. In this respect, our work is more quantitative than previous experimental studies of gelling systems. It is through the understanding of this type of experiment that we hope to be able to predict the behavior of polymer/crosslinker systems in oil reservoirs.
Polymers are frequently injected into oil reservoirs in order to improve recovery. As they reduce the in-situ mobility of the aqueous phase (either by viscosity increase or permeability reduction), the fluid injectivity generally drops. It is very useful to be able to estimate in advance from a few laboratory measured quantities (a) the injectivity of the polymer and (b) whether the polymer is likely to be seriously degraded by the high shear experienced in the near-wellbore region. It is difficult to calculate the injectivity of the polymer solutions due to their complex rheological behaviour within porous media, especially when the polymer mechanically degrades. In this paper, we investigate one approach to calculating the injectivity of polymers in the general case where mechanical degradation occurs. A kinetic model for polymer degradation is proposed which is used to obtain the radial viscosity profile of the degrading polymer. This may in turn be used to calculate the steady-state pressure drops associated with the degrading polymer. The model is based on a discrete multicomponent representation of the polymer molecular weight distribution (MWD). During mechanical degradation, the MWD changes as higher components degrade into lower molecular weight fragments. The degradation rate of a given component of the MWD is related to the local shear/elongational stress within the porous medium and the concentration of the component (ci). The model is used to match the results of experiments studying the shear degradation of polyacrylamide (PAM) in radial sandstone cores. The quantitative predictions of the model are very satisfactory. In addition, the model gives insight into the mechanism of shear degradation of polymers in porous media. Introduction When an oil reservoir is flooded with either water or EOR fluids, it is important to know how easily the fluids may be injected. In practice, the pressure drop associated with a given rate of fluid injection must not be too large. For a Newtonian fluid, the viscosity is independent of flow rate and the pressure drop, Î"Pe, between the well radius, rw, and the effective drainage radius, re, can be found exactly for incompressible, steady-state injection into a homogeneous formation by integrating the radial Darcy equation. However, non-Newtonian fluids such as polymer solutions, do not have a constant viscosity under all rates of deformation either in shearing or elongational flow (1–4). For the divergent flow around injection wells, the shear rate decreases with radial distance, r. Hence, the polymer viscosity is a function of r and the pressure drop is given by:Equation 1 where η(r) is the r-dependent apparent polymer viscosity for a given polymer concentration and formation temperature. At low shear rates, most polymers used in EOR behave in a Newtonian manner (5,6) and their apparent viscosities in a given porous medium depend only on concentration. This apparent viscosity may be higher than the capillary viscosity at the same concentration, due to adsorbed / retained polymer (7,8).
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