Formation Fines and Factors Controlling Their Movement in Porous Media

Abstract

Microscopic observations of fine-particle movement in micromodels of porous media have shown that transport of these particles by fluids porous media have shown that transport of these particles by fluids moving through pores is controlled by several factors. Besides mechanical bridging at pore restrictions, fines movement also is influenced strongly by particle wettability and the relative amounts of fluids flowing through the pores when two or more immiscible fluids are present. Introduction Very small particles of loose solid materials are present in the pore spaces of all sandstone reservoirs. These particles, called formation fines, can be incorporated in the particles, called formation fines, can be incorporated in the formation as it is deposited during geologic tune, or they can be introduced into the formation during drilling and completion operations. Regardless of their mode of entry, they long have been recognized to cause severe formation damage. This is because these particles are not held physically in place by the natural cementatious material that binds larger sand grains together, but instead are individual particles located on the interior surfaces of the porous matrix. Thus, these particles are free to migrate through the pores along with any fluids that flow in the reservoir. If these particles do migrate, but are not carried all the way through the formation by produced fluids, they can concentrate at pore restrictions, causing severe plugging and large reductions in permeability.Numerous studies have been conducted to determine the composition, physical characteristics, and other factors controlling the behavior of these materials while the industry has tried to develop techniques for overcoming their undesirable effects. Although these studies have contributed significantly to our understanding of fines and their movement in porous media, there is still considerable need to extend this knowledge if satisfactory remedies for the fines-plugging problems are to be developed. This paper discusses research studies that focused on determining the effects of both single- and multi-phase fluid flow on fine-particle movement in porous media. Most conclusions in this study were reached after observing (through an optical microscope) the movement of particles when carried by various fluids that were injected into a micromodel of porous media. Some experiments also were conducted by flowing fluids through linear sand packs and by monitoring the particle content of the effluent to verify the observations made in the micromodel. Results show that several factors affect particle migration. As has been recognized, mechanical bridging at pore restrictions does occur if certain conditions, such as pore restrictions does occur if certain conditions, such as particle size and concentration, are met. However, this is particle size and concentration, are met. However, this is by no means the only mechanism that is important for controlling fine-particle mobility. When immiscible fluid phases are present in pore spaces, wettability of the phases are present in pore spaces, wettability of the particles and the relative amounts of fluids flowing particles and the relative amounts of fluids flowing through the pores also influence particle movement strongly.The observations made here help explain why the onset of water or gas production can cause fines to move and can lead to well productivity problems, and why injection of seemingly nonreactive fluids (such as water, oil, or solvent/surfactant solutions) often leads to at least temporary removal of near-wellbore damage. JPT P. 144