to the surface through the string using a wireline or similar arrangement an isolation fluid 22 is then introduced into the first section 18 through the hybrid drill frac string to fill all or part of the first section 18 the isolation fluid 22 is one which can as stated previously withstand the pressure created at the fracture called fracture initiation pressure and that therefore does not allow significant movement of a fracturing fluid to another part of the well the isolation fluid 22 can be a suitable gel for example a fracturing fluid 24 is then introduced through the hybrid drill frac string into the first section 18 at a pressure sufficient to fracture the subterranean formation 4 in the vicinity of the end of the string in a manner similar to that shown in fig1 the fracturing fluid can once again be a slurry of for example mainly water and some sand and additives but can include any suitable fluid including but not limited to water salt water hydrocarbon acid methanol carbon dioxide nitrogen foam emulsions etc the isolation fluid is cleaned up by flowing it back out of well through the hybrid drill frac string annulus the hybrid drill frac string is then moved ahead and a second section beyond the first section is drilled to extend the wellbore the isolation and fracturing steps described above can then be repeated fig3 shows a plan view of a single leg horizontal wellbore 2 with fractures 28 propogated in a subterranean formation 4 in accordance with the methods of the present invention fig3 shows the plan view of fig3 with a grid overlay showing.