Ultrasonic characterization of formation damage: Effect of particle sizes

Permeability of a geological formation such as an oil field can be altered locally during drilling operations through penetration of particulates from the drilling fluid into the formation pores. This can adversely affect the overall production rate by constricting the pores. The composition of these fluids, particularly the sizes of the particulates therein, can be critical from the point of view of controlling the extent of 'damage' to the formation. Using our recently reported ultrasonic mapping technique, we have investigated the depth of particle penetration as a function of particle size and contamination time. An important correlation is observed between the diameters of the 'mud' particles and the pore size distribution in the formation. It is further noted that a substantial layer of 'mud cake' is formed on the surface beyond a certain critical particle size and this subsequently acts as a filter allowing only some finer particles to continue penetrating into the formation. The results are discussed in the context of bridging action at the pores.