nach oben

1991 | Buch

Kapitel lesen Erstes Kapitel lesen

Polymer-Improved Oil Recovery

verfasst von: K. S. Sorbie, D. Phil.

Verlag: Springer Netherlands

Enthalten in: Professional Book Archive

Einloggen, um Zugang zu erhalten

Über dieses Buch

The importance of oil in the world economy cannot be overstated, and methods for recovering oil will be the subject of much scientific and engineering research for many years to come. Even after the application of primary depletion and secondary recovery processes (usually waterflooding), much oil usually remains in a reservoir, and indeed in some heterogeneous reservoir systems as much as 70% of the original oil may remain. Thus, there is an enormous incentive for the development of improved or enhanced methods of oil recovery, aimed at recovering some portion of this remainil)g oil. The techniques used range from 'improved' secondary flooding methods (including polymer and certain gas injection processes) through to 'enhanced' or 'tertiary' methods such as chemical (surfactant, caustic, foam), gas miscible (carbon dioxide, gas reinjection) and thermal (steam soak and drive, in-situ combustion). The distinction between the classification ofthe methods usually refers to the target oil that the process seeks to recover. That is, in 'improved' recovery we are usually aiming to increase the oil sweep efficiency, whereas in 'tertiary' recovery we aim to mobilise and recover residual or capillary trapped oil. There are a few books and collections of articles which give general overviews of improved and enhanced oil recovery methods. However, for each recovery method, there is such a wide range of interconnected issues concerning the chemistry, physics and fluid mechanics of flow in porous media, that rarely are these adequately reviewed.

Inhaltsverzeichnis

Frontmatter

1. Introduction to polymer flooding

Abstract

In the very early days of the oil industry, the general practice in land-based shallow reservoirs was to produce oil by primary depletion. In this method, the compressional energy of the reservoir was used to force oil to the producer wells, with a consequent drop in the reservoir pressure. However, it was recognised that reservoirs would ultimately drop below the bubble point pressure, such that dissolved gas would be released from the oil. As a result of the appearance of this extra phase, production impairment would occur. In order to maintain reservoir pressure and also to sweep out oil in a more efficient displacement process, waterflooding became the standard practice in many reservoir formations. An intense period of both field application and laboratory and theoretical research on waterflooding then followed. The main pre-eminence of waterflooding came to the fore in the 1950s and, during this period, the principal strengths and weaknesses of the methods were quite well established. In particular, the inefficiency of the waterflood oil displacement mechanism as a result of either an unfavourable mobility ratio or heterogeneity was largely identified. The general subject of waterflooding has been reviewed in some detail by Craig (1971) and Willhite (1986).

K. S. Sorbie

2. Structure of the main polymers used in improved oil recovery (IOR)

Abstract

This chapter will review what is known of the chemical structures of the main polymers used in improved oil recovery (IOR) applications. The overall objective is to establish the chemical structure of the various polymer molecules and their molecular conformation in solution. This will form the basis for explaining many of the properties associated with polymer flow through porous media which are discussed later in this work. Undoubtedly, the two most commonly used polymers in IOR applications are the synthetic material, Polyacrylamide, in its partially hydrolysed form (HPAM) and the biopolymer, xanthan. The historical reason for these two polymers being used in oil recovery operations is based on the fact that each one has extensive applications in other industries. Polyacrylamide is used in paper manufacturing, drag reduction and as a flocculant in other industrial processes; xanthan is used as a thickener in the food industry and is, in fact, UK food additive E415. It will be evident in the work presented later in this book that polymers with improved properties will be required in order to apply polymer flooding technology in more and more severe reservoir conditions. For this reason, structural information on some other polymers which may be used in the future for oil recovery processes will also be presented.

K. S. Sorbie

3. Properties of polymer solutions

Abstract

The main solution property which is of interest in polymer flooding applications is the viscosity of the polymer. Polymers are added to the injection brine in a waterflood in order to increase the viscosity of the drive fluid, which in turn improves the oil-water mobility ratio. This leads to improved areal and vertical sweep efficiency. In this chapter, how polymers actually viscosify in aqueous (or other) solutions is discussed. In addition, how this viscometric behaviour is related to the molecular weight of the macromolecule and to polymer-solvent interactions via some simple concepts such as the intrinsic viscosity, [η], the molecular expansion factor, α, etc. is considered. Polymeric solutions, unlike fluids such as water and oil, do not generally show the same viscosity at all flow rates either in a capillary or indeed in a porous medium; water and oil are said to be Newtonian fluids, whereas polymer solutions are generally non-Newtonian. The study of the flow behaviour of non-Newtonian fluids is known as rheology and is a vast area of study in itself (Walters, 1975; Schowalter, 1978; Bird et al., 1987a). In this chapter, a brief review of the properties of non-Newtonian fluids in so far as these properties relate to the flow of polymers in porous media will be presented.

K. S. Sorbie

4. Polymer stability

Abstract

When polymers are used in oil recovery operations, it is clearly important that the polymer properties are not rapidly degraded. The main property of interest in this respect is generally the polymer solution viscosity although, for some polymers, the ability of the polymer to reduce the permeability of the reservoir formation may also be of some importance. Polymer degradation refers to any process that will break down the molecular structure of the macromolecule and the main degradation pathways of concern in oil recovery applications are as follows:

(i)

Chemical degradation: this refers to the breakdown of the polymer molecules, either through short-term attack by contaminants, such as oxygen, or through longer term attack of the molecular backbone by processes such as hydrolysis.

(ii)

Mechanical degradation: this describes the breakdown of a molecule in the high flow rate region close to the well as a result of the high mechanical stresses on the macromolecule. This is a short-term effect and is only important in the reservoir near the well-bore (and also in some of the polymer handling equipment, in chokes, etc.).

(iii)

Biological degradation: this refers to the microbial breakdown of macromolecules—both biopolymers and synthetics—by bacteria during storage or in the reservoir. This is only important at lower temperatures or in the absence of effective biocides.

K. S. Sorbie

5. Polymer retention in porous media

Abstract

When polymers are added to displacement fluids, the objective is usually to viscosify the injection brine using the properties of the transported polymer as discussed in Chapter 3. However, there may be significant interactions between the transported polymer molecules and the porous medium. Such interactions will cause the polymer to be retained by the porous medium and will lead to the formation of a bank of injection fluid wholly or partially denuded of polymer. Clearly, this bank of fluid will have a viscosity which is much lower than the injected polymer solution, and this will generally lead to a reduction in the efficiency of the polymer flood. However, this polymer retention on the porous medium may also cause some reduction of the rock permeability, which can contribute to the oil recovery mechanism, as is discussed further below. However, overall, the retention of polymer tends to reduce oil recovery despite the permeability reduction contribution. In fact, it is the author’s observation that the level of polymer retention is one of the key factors in determining the economic viability of a polymer flood. Thus, it is of great importance to establish the correct retention levels for a given proposed field polymer flood. The conditions under which such laboratory measurements should be made are extremely important so that relevant figures for retention are available for the simulation assessment of the polymer flood.

K. S. Sorbie

6. Polymer rheology in porous media

Abstract

In this chapter, the important aspects of the rheology of non-Newtonian polymeric solutions as they flow through porous media will be described. This is sometimes referred to as the in-situ rheology of polymers, and one of the objectives here will be to compare and contrast this behaviour with the rheology of the bulk polymer solutions described in Chapter 3. It was noted in Chapter 3 that the bulk rheological behaviour of xanthan and HPAM is related to the molecular structures of these polymers. That is, the more rigid rod-type xanthan structure gave a purely pseudoplastic, inelastic solution at lower concentrations, whereas the flexible coil molecular structure of HPAM leads to solutions which show elastic behaviour even when fairly dilute. Again, as might be expected, the molecular structure plays an important role in determining the in-situ rheological behaviour. Another important factor is the microscopic structure and geometry of the porous medium itself. Clearly, the flows through a porous medium will be much more tortuous and complex than those found in rheometers where flows are well-defined (e.g. in capillaries, Couette flow, constant shear cone and plate flow, etc.). Thus, some aspects of the structure of porous media and how it may be represented by different mathematical approaches will be discussed. Prominent in the history of representing porous media is the simple idea of the capillary bundle (Dullien, 1979).

K. S. Sorbie

7. Polymer transport in porous media

Abstract

This chapter complements the previous chapter in which polymer rheology in porous media was discussed. Here, the phenomena involved when polymer flows through a porous medium are considered, concentrating only on single-phase flow in which polymer and/or tracer is transported in the aqueous phase. The basic idea is to contrast the flow of polymer with that of an inert tracer such as chloride, tritium-labelled water or sodium ions, and to develop the appropriate transport equations for describing all of the observed behaviour. It is the multiphase generalisations of these transport equations which must be solved to describe the flow of polymer solutions in oil displace¬ment processes either in the laboratory or in the reservoir; these will be dealt with in the next chapter. The basic transport behaviour which is described is obtained from the analysis of effluent profiles in one-dimensional (1-D) core flooding experiments using polymer and tracer solutions. For this reason the following section gives a brief outline of tracer flow in a 1-D core and the convection equation which is generally used to describe tracer transport. Some additional effects, such as adsorption and velocity enhancement, which are more relevant to polymer transport are also included in the basic tracer transport equation.

K. S. Sorbie

8. Oil displacement using polymers

Abstract

Earlier parts of this book have discussed the various aspects of polymer structure, stability, solution behaviour, in-situ rheology and transport in porous media that are relevant to their ultimate task of improving oil recovery. In this chapter, an attempt is made to pull these strands together by describing the main mechanisms of polymer oil displacement processes in reservoir systems. For this purpose, the main multiphase flow equations that may be used in the design and simulation of polymer floods are developed, along with some simpler solutions for certain limiting cases.

K. S. Sorbie

9. Application and planning of field polymer floods

Abstract

In this chapter, it is intended firstly to discuss the process of identifying candidate reservoirs for polymer flooding via screening criteria and then to describe the assessment procedure when a likely candidate reservoir has been selected for further study. In the screening phase, the various rules of thumb which assist in the original selection of the candidate reservoir for polymer flooding will be enumerated. The subsequent study of the prospect involves more detailed assessment of the reservoir, the planning and carrying out of laboratory work and the development and carrying out of a programme of reservoir simulation. This more detailed assessment is very similar to the planning stage of other improved oil recovery projects.

K. S. Sorbie

Backmatter

Titel: Polymer-Improved Oil Recovery
verfasst von: K. S. Sorbie, D. Phil.
Verlag: Springer Netherlands
Electronic ISBN: 978-94-011-3044-8
Print ISBN: 978-94-010-5354-9
DOI: https://doi.org/10.1007/978-94-011-3044-8

Springer Professional

Über dieses Buch

Inhaltsverzeichnis

Frontmatter

1. Introduction to polymer flooding

2. Structure of the main polymers used in improved oil recovery (IOR)

3. Properties of polymer solutions

4. Polymer stability

5. Polymer retention in porous media

6. Polymer rheology in porous media

7. Polymer transport in porous media

8. Oil displacement using polymers

9. Application and planning of field polymer floods

Backmatter