Peh: Drilling Problems and Solutions Publication Information Petroleum Engineering Handbook

Fig. 10.8—Formation skin damage.
Borehole fluids are classified as drilling fluids, completion fluids, or workover fluids. Drilling
fluids are categorized as mud, gas, or gasified mud. There are two types of mud: water-based
(pure polymer, pure bentonite, bentonite/polymer) and oil-based (invert emulsion, oil).
Completion and workover fluids are mostly brines and are solids free.
Formation damage is a combination of several mechanisms including solids plugging, clay-
particle swelling or dispersion, saturation changes, wettability reversal, emulsion blockage,
aqueous-filtrate blockage, and mutual precipitation of soluble salts in wellbore-fluid filtrate and
formation water. 
Producing Formation Damage
Introduction
Borehole Fluids
Damage Mechanisms

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Solids Plugging. Fig. 10.9 shows that the plugging of the reservoir-rock pore spaces can be
caused by the fine solids in the mud filtrate or solids dislodged by the filtrate within the rock
matrix. To minimize this form of damage, minimize the amount of fine solids in the mud system
and fluid loss. 
Fig. 10.9—Formation damage caused by
solids plugging.
Clay-Particle Swelling. This is an inherent problem in sandstone that contains water-sensitive
clays. When a fresh-water filtrate invades the reservoir rock, it will cause the clay to swell and
thus reduce or totally block the throat areas. 
Saturation Change. Production is predicated on the amount of saturation within the reservoir
rock. When a mud-system filtrate enters the reservoir, it will cause some change in water
saturation and, therefore, potential reduction in production. Fig. 10.10 shows that high fluid loss
causes water saturation to increase, which results in a decrease of rock relative permeability. See
the chapter on transport properties in the General Engineering volume of this Handbook for
additional information. 

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