Hard formations,especially gravel layers with high compressive strength and abrasion,are generally not suitable for PDC bit application.Under such extreme conditions,the impact and abrasion will damage the PDC cutters at a much higher rate than common.A patent of new PDC bit design—te fully embedded back-up curer technology,can remarkably enhance the durability of PDC bit in these extreme applications.The new technology is of successive back-up cutting structure based on the ordinary cutters layout.The back-up cutters,which are placed beneath the primary cutters and fully embedded into the matrix body,in conjunction with primary cutters form a complex cutting structure capable of penetrating rocks successively.These cutters start to work only after the primary cutters overhead are worn out,therefore dramatically extend PDC bit life.The patented technology has been applied successfully in drilling basal conglomerate formationof Luda Oilfield located in Bohai Bay of China,which greatly improved drilling efficiency and reduced drilling cost.