Peh: Drilling Problems and Solutions Publication Information Petroleum Engineering Handbook

Fig. 10.10—Formation damage caused by
saturation.
Wettability Reversal. Reservoir rocks are water-wet in nature. It has been demonstrated that
while drilling with oil-based mud systems, excess surfactants in the mud filtrate that enter the
rock can cause wettability reversal. It has been reported from field experience and
demonstrated in laboratory tests that as much as 90% in production loss can be caused by this
mechanism. Therefore, to guard against this problem, the amount of excess surfactants used in
oil-based mud systems should be kept at a minimum. 
Emulsion Blockage. Inherent in oil-based mud systems is the use of excess surfactants. These
surfactants enter the rock and can form an emulsion within the pore spaces, which hinders
production through emulsion blockage. 
Aqueous-Filtrate Blockage. While drilling with water-based mud, the aqueous filtrate that
enters the reservoir can cause some blockage that will reduce the production potential of the
reservoir. 
Precipitation of Soluble Salts. Any precipitation of soluble salts, whether from the use of salt
mud systems or from formation water or both, can cause solids blockage and hinder production.
For more information, see the Formation Damage chapter in the Production Operations
Engineering volume of this Handbook.
Throughout the last decade, many studies have been conducted to gain understanding on hole
cleaning in directional-well drilling. Laboratory work has demonstrated that drilling at an
inclination angle greater than approximately 30° from vertical poses problems in cuttings
removal that are not encountered in vertical wells. Fig. 10.11 illustrates that the formation of a
moving or stationary cuttings bed becomes an apparent problem if the flow rate for a given mud
rheology is below a certain critical value.