... on the part of gate protecting, and rducing the friction from oil well wall; improving quality of well wall, protecting ... GHJ series tri-cone rock bits for well drilling: 1, Very soft with low compressive strength, such as soft marl, clay, unconsolidated sandstorm, ... speed, 220~60 r/m.3, Medium soft with hard abrasive treak, such as hard shale, soft limestone, sandstone, etc. Recommended Bit type: ...