BAKER HUGHES - Drilling Fluids Reference Manual

Formation Mechanics
Selecting the proper size bridging agent is critical; if the size of the formation pores is known, a
bridging agent that will block the pores without passing through them can be selected. Abrams 1
presents guidelines for bridging agent use.
Other forms of formation damage are more complex, involving chemical reactions between the
drilling fluids and formations. Carbonate rocks (limestones and dolomites) are not very reactive with
most drilling fluids, so this type of damage is most likely to occur in clastic rocks such as sandstones.
Clastic rocks consist of a network of interconnected particles, usually quartz or feldspar, called
framework grains. The pore spaces between these grains may be empty or filled by other minerals.
The pore spaces in these rocks may contain clay minerals. Because clay minerals can be highly
reactive with drilling fluids, formation damage can occur in clay-bearing clastic rocks. “Clean”
sandstones that have no clay are not highly vulnerable to chemical formation damage since most
framework grains are not reactive.
If a sandstone contains expandable clays such as smectite or mixed-layer types, the potential for
formation damage exists. If a drilling fluid causes these clays to swell by cation exchange or further
hydration in their interlayer sites, this may cause them to detach, migrate, and plug the pores, causing
a loss of permeability (see Figure 2-10). In rocks whose framework grains are bound together by
expandable clays, disaggregation and borehole washout can also occur (see Figure 2-11).
Figure 2-10
Photomicrograph of Wilcox Sandstone
The particles labeled “P” are clay and other fine material that have migrated and plugged a pore
space. Mixed-layer clays are common in this sample, and exposure to a high pH lignosulfonate fluid
caused them to swell and detach, causing a 63% loss of permeability.
1 A. Abrams, “Mud Design to Minimize Rock Impairment Due to Particle Invasion”, SPE 5713, 1977
Reference Manual
Baker Hughes Drilling Fluids
2-16 Revised 2006