EMPLOYEE WAS SETTING UP TO CHANGE OUT A DRILL STEM ON 5-10 HEATER. THIS PROCEDURE REQUIRES THE USE OF A PIPE WRENCH TO BRACE AND HOLD THE DRILL STEM AND A U BOLT TO SECURE THE STEM. WHILE SECU RING THE U BOLT ONTO THE STEM, THE WRENCH SLIPPED OFF CAUSING THE STEM TO MOVE DOWNWARD, PINCHING THE EMPLOYEE'S RIGHT INDEX FINGER. HE SUFFEREDA LACERATION WHEN HE PULLED HIS FINGER OUT.