Entrainment and Deposition of Fine Particles in Porous Media

Entrainment and redeposition of naturally occurring fine particles in porous media has been suggested as a mechanism leading to abnormal decline in productivity mechanism leading to abnormal decline in productivity of producing wells. This paper describes the results of studies conducted to determine factors affecting this phenomenon. Experimental work done as part of this study provides the basis for a proposed phenomenological theory of entrainment and deposition. The central concept of this theory is representation of both particle and pore size distributions by partitioning the porous medium at any cross section into parallel plugging and nonplugging pathways. This simple model appears to be completely adequate for describing a broad class of filtration and entrainment phenomena. We have shown that fines entrainment and deposition are mechanisms that can cause abnormal productivity decline and are phenomena restricted to the near-wellbore region. Introduction There is considerable evidence indicating that the excessive decline in productivity observed in many producing oil and gas wells results from a reduction in permeability near the wellbore arising from an accumulation of fines - i.e., small solid particles of sand and/or clay - which have become entrained in the flowing fluids and transported through the porous formation toward the well.1–4 This phenomenon, in some respects, is analogous to the process of deep-bed filtration, which is often employed in processing industries to remove suspended solids from fluids. In recent years, considerable literature has appeared on this subject. Muecke1 has studied the fines problem and discovered several important facts pertinent to fundamental understanding of the fines movement phenomenon. A major finding is that all natural porous materials contain particles that are classified as fines. This is demonstrated in microscopic photographs of a wide variety of granular materials. Another important finding is that in the undisturbed state these fines uniformly cover the interstitial solid surface, but when the fluid saturating the pores is set in motion these fines are entrained and subsequently are redeposited at preferred accumulation sites creating obstructions in the pores. If the pore-filling fluid is not homogeneous, but instead is composed of oil and water, for example, the fines tend to be entrained in one or the other fluid, depending on their wettability, and fines of mixed wettability accumulate in the oil/water interface. Other literature devoted to deep-bed filtration has contributed some understanding of the influence of such factors as particle size, fluid velocity, and physical properties of fluids and porous solids on the process.5–12 However, none of the studies reported in the literature provide an adequate basis for the design of a remedial treatment for wells of reduced productivity caused by fines accumulation. To determine where fines are entrained and deposited in the formation, local laws of deposition and entrainment must be known. Such knowledge could provide a basis for design of remedial treatments. The determination of such local laws was the objective of the research reported here. Outline of Research Program Our study of this problem consisted of (1) a sequence of experiments using synthetic fines/porous-media systems to identify fundamental processes and to provide guidelines for a phenomenological description, (2) construction of a theoretical description of the deposition and entrainment process, and (3) controlled laboratory experiments using field cores and naturally occurring fines to verify results of the earlier studies.