BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Primary Factors Causing Formation Damage
Solids Invasion
Solids can enter or penetrate the producing zone due to differential pressure placed across the interval.
These solids can plug internal pores and flow channels, thus increasing the potential for formation
damage. Factors which influence the migration or entrance of particles into the formation are as
follows.
Size and shape of particulates
Permeability and porosity of the formation
Differential pressure applied
Compressibility or incompressibility of solids
Generally, compressible solids such as polymers can be more damaging because they tend to deform
the shape of the pore throat, thereby building an internal bridge that could eventually seal off pore
channels connecting the pore spaces. Incompressible solids, on the other hand, become wedged, thus
leaving some communication between the pore spaces. Also, depending on the size, quantity, and
shape, an instantaneous bridge could occur at the pore openings, thus delivering minimal invasion of
particulates.
Filtrate Invasion, Hydration Inhibition, and Emulsions
Producing zones are seldom homogeneous; therefore the detrimental effects of filtrate invasion are
often encountered. The most frequent effect is the alteration of clays by the influx or invasion of
freshwater. Freshwater can cause damage by the hydration and swelling of clays in the rock matrix,
thus causing particle plugging by clay dispersion. Freshwater can cause further damage by increasing
pore channel capillary pressure or by creating an emulsion block.
Smectite or montmorillonite is the most important clay mineral associated with swelling. This clay is
capable of expanding by interlayer adsorption of water and can expand as much as 10 times its normal
occupied volume. The degree of swelling is dependent upon the type of cation present within the
interlayer. Thus, introducing an appropriate type and quantity of cation can reduce the amount of
swelling a particular clay undergoes.
This reduction occurs because the positive ions have a stronger attraction or force on the negatively
charged surfaces of the silica and alumina particles than the displaced ion. This base exchange can be
induced by either the introduction of highly active cations, the mass action of cations, or cations with
sufficient size to fill all available bonding sites with the structure of the clay. Thus, understanding the
mechanisms of cation exchange is beneficial in minimizing formation damage by the hydration of
clays within the pore throats of producing zones.
Note:
See Chapter 3, Water Base Fluids and Chapter 7, Borehole Problems, for a thorough
explanation of formation clays and their associated properties.
In addition to formation damage, freshwater invasion can cause the misinterpretation of logs, coring
samples, and drillstem tests.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 1-45