Migrations of Fines in Porous Media

Fines or fine particles are small particles that are present in porous media. The migration of fine particles in porous media is a challenging problem of both scientific and industrial importance. By migration of fine particles, here, we mean the entire sequence of occurrences of release or detachment of fine particles present in the porous media, their motion with the flow, and finally their capture at some pore sites or their migration out of the porous medium. Many applications can be found in fields such as of Petroleum, Geotechnical, Chemical, Environmental and Hydraulic engineering. The practical consequences of the migration of fines can be either beneficial or adverse in nature and will be discussed briefly in this chapter.

Knowledge of the size, shape, and other characteristics of pores and of the fine particles is essential to the understanding of the various processes that occur in the release and migration of fines in porous media. This knowledge can be obtained by pertinently characterizing the fines and the porous media. A detailed characterization of this complex system is extremely difficult and will not be attempted in this chapter. Instead, we intend to discuss only the most relevant aspects of migration, such as the size of fines, size of pore constrictions, surface charge of fines and porous surface, and the interactions between the permeating liquid and the porous media. Furthermore, in order to keep the task manageable, we confine our attention to migratory fines in two important classes of porous media: sandstones and soil. The procedures and the other aspects of this characterization, nevertheless, can be applied to fines migration in a number of other porous media such as ground water flows.

The release of fines from a pore surface is the first step in the phenomenon of migration of fines in porous media. A comprehensive understanding of this release process is necessary (1) to design treatments either to prevent or to induce/trigger the migration of fines, and (2) to analyze and develop a mathematical model to gain insights into the phenomenon. In general, two major types of forces are responsible for the release of fines: colloidal and hydrodynamic. This chapter focuses on colloidally induced release while the next chapter focuses on hydrodynamically induced release.

The fine particles adhering to the pore surfaces of the porous media of relatively higher porosity and permeability can be released or detached by the hydrodynamic forces. Hydrodynamically induced release of fines is found to be a more common occurrence in loose soil embankments and in packed beds than that in oil sandstone formations. There are two major reasons for this: (a) the flow velocity encountered is higher and (b) the fines are usually of large size and therefore experience a large drag force. We shall analyse these aspects later in this chapter.

In the previous chapters we have discussed the release of fine particles from the pore surface to the liquid stream flowing through the porous medium. These fine particles while flowing with the liquid phase, can either readhere to the pore surface or flow without capture or get entrapped at the pore constrictions. The later occurrence leads to the plugging of the pore constrictions. In the migration of fines, the later two occurrences are more common as the colloidal and hydrodynamic conditions that bring about their release are not likely to allow these particles to readhere back to the pore surface at the same conditions. Therefore, during the migration of fines in porous media, the released fine particles either get entrapped (entrapment of fines) or migrate without getting captured (piping or washout of fines). Whether entrapment or piping occurs will depend on parameters relating to the characteristics of the fine particles, the permeating liquid (suspending medium) and the porous medium. This chapter describes the factors affecting the entrapment or piping of fines. A mathematical model to predict whether entrapment or piping occurs or not is also discussed. Finally the model is applied to some important processes where migration of fines occurs.

In this chapter, we present mathematical models to describe the reductions in permeability resulting from migration of fines in porous media. First, we present the release and capture mechanisms followed by the model equations, describing the processes. Finally the solution procedures are discussed and the model predictions are compared with experimental observations.

The mathematical models for predicting permeability reduction of a porous medium due to fines entrapment discussed in the previous two chapters fall under the class of continuum models. In a continuum model, the porous medium is treated as a continuum within which quantities such as temperature, fluid species concentration (e.g., ionic strength), and solids concentration are defined as smooth functions of position. The use of such continuum models is appropriate when the porous medium under consideration is homogeneous in terms of the pore structure and the medium surface properties. If the above conditions are satisfied, continuum models are attractive to use because of their conceptual simplicity and their ability to provide useful insight into the process of fines release and capture, and subsequent evolution of the permeability of the porous medium (as seen in Chapters 5 and 6). Of course, the assumption of a suitable relationship between the permeability and the amount of retained fines is necessary. This assumption is typically based on empirical correlations and is a major limitation of continuum models. When a porous medium is spatially heterogeneous, however, it will not be possible to rely on continuum models for accurate prediction of the permeability reduction because the pore interconnectivity, which the continuum models generally ignore, becomes crucial.

A number of techniques to prevent the of migration of fine particles in sandstone oil reservoirs have been developed (Valey, 1969; Reed, 1972; Clementz, 1977; Sydansk, 1984; Borchard and Brown, 1984). Descriptions of these techniques, commonly known as clay stabilization techniques, are available in patented literature. This chapter will focus on the underlying mechanisms of these techniques and attempt to elucidate their influence on the retardation of the release of fines.

Many toxic chemicals adhere to fine particles; consequently the migration of fines containing contaminants can spread through our environment. There are two primary mechanisms by which the pollution can spread and grow: