kelly is connected to the hook through a rotary swivel 224 which permits rotation of the drill string relative to the hook alternatively the drill string 214 may be rotated from the surface by a u201c top drive u201d type of drilling rig drilling fluid or mud 226 is contained in a mud pit 228 adjacent to the derrick 210 a pump 230 pumps the drilling fluid into the drill string via a port in the swivel 224 to flow downward as indicated by the flow arrow 232 through the center of drill string 214 the drilling fluid exits the drill string via ports in the drill bit and then circulates upward in the annulus between the outside of the drill string and the periphery of the borehole as indicated by the flow arrows 234 the drilling fluid thereby lubricates the bit and carries formation cuttings to the surface of the earth at the surface the drilling fluid is returned to the mud pit 228 for recirculation in the present embodiment as will be described a well known directional drilling assembly with a steerable motor is employed as shown in fig2 b which shows downhole portions of wells 10 and 20 mounted near the drill bit 216 is a bottom hole assembly.