Abstract
PHPA mud recommended to drill shaly formation to prevent shale swelling and keep hole stability especially in horizontal wells is tested for formation damage. The results show, that PHPA mud yields high formation damage. About 45% of the original absolute permeability of Berea sandstone ,ill be lost. This reduction is actually lower than that caused by bentonite mud or bentonite non barite mild, it is recommended to add bentonite and/or barite to PHPA mud because adding bentonite and/or barite to PHPA mud lowers the damage process and improves the permeability reduction.