Wellbore Skin: Why Its Presence and Properties Are So Difficult to Predict

The presence of positive wellbore skin, that is, deposits of fine-grained particles from the drilling fluid on the borehole wall, significantly affects the efficiency of water wells. Previous studies of skin samples have shown a significant variability in typology, thickness, and composition but were largely unable to explain the differences. In order to overcome this problem, we therefore (1) significantly expanded the sample data base by investigating skin samples from nine wells with very similar geological and technical conditions and (2) investigated the evolution of the density of drilling fluids during the drilling. The former is done in order to evaluate differences in skin thickness and composition, and the latter to study the differential mobilization of particles. Incohesive and poorly sorted layers form the source of the particles, while the thickest accumulation of particles occurs in highly permeable layers, where the highest exfiltration rates initially occur. For well drillers, we recommend continuous monitoring of drilling fluid density to obtain a measure of the presence of particle-providing layers and the probability of wellbore skin formation.