1. n. 
Alteration of the far-field or virgin characteristics of a producing formation, usually by exposure to drilling fluids. The water or solid particles in the drilling fluids, or both, tend to decrease the pore volume and effective permeability of the producible formation in the near-wellbore region. At least two mechanisms are at work. First, solid particles from the drilling fluid physically plug or bridge across flowpaths in the porous formation. Second, when water contacts certain clay minerals in the formation, the clay typically swells, increasing in volume and decreasing the pore volume. Third, chemical reactions between the drilling fluid and the formation rock and fluids can precipitate solids or semisolids that plug pore spaces. One approach to minimize formation damage is to use drill-in or completion fluids that are specially formulated to avoid damage to the formation when drilling pay zones, rather than ordinary drilling fluids.
2. n. 
A reduction in the natural capability of a reservoir to produce its fluids, such as a decrease in porosity or permeability, or both. Damage can occur near the wellbore face (easier to repair) or deep into the rock (harder to repair). Damage is caused by several mechanisms: (1) physical plugging of pores by mud solids, (2) alteration of reservoir rock wettability, (3) precipitation of insoluble materials in pore spaces, (4) clay swelling in pore spaces, (5) migration of fines into pore throats, (6) introduction of an immobile phase, and (7) emulsion formation and blockage. Damage can occur when sensitive formations are exposed to drilling fluids.
3. n. 
A general term to describe the reduction in permeability to the near-wellbore area of a reservoir formation. There are several recognized damage mechanisms, such as the invasion of incompatible fluids swelling the formation clays, or fine solids from dirty fluids plugging the formation matrix. Because formation damage can significantly affect the productivity of any well, adequate precautions should be exercised to avoid damage during all phases in the life of a well.
4. n. 
Natural or induced production impairments that can develop in the reservoir, the near-wellbore area or the perforations. Natural damage occurs as produced reservoir fluids move through the reservoir, while induced damage is the result of external operations and fluids in the well, such as drilling, well completion, workover operations or stimulation treatments. Some induced damage triggers natural damage mechanisms. Natural damage includes phenomena such as fines migration, clay swelling, scale formation, organic deposition, including paraffins or asphaltenes, and mixed organic and inorganic deposition. Induced damage includes plugging caused by foreign particles in the injected fluid, wettability changes, emulsions, precipitates or sludges caused by acid reactions, bacterial activity and water blocks. Wellbore cleanup or matrix stimulation treatments are two different operations that can remove natural or induced damage. Selecting the proper operation depends on the location and nature of the damage.