We, are a borehole drilling company operating in West Africa, around Burkina Faso, Niger and Mali. In Niger and Mali we have come across very deep boreholes to be dug in unconsolidated formations using rotary drilling to depths upto 600mts.
We do not have sufficient experience in taking up tasks of this magnitude, we have drilled in hard formations to depths of 180mts, look forward for suggestions into type of mudpump, mudpump chemicals and the drilling rig needed for the task.  We are aware of Bentonite as drilling clays but all projects in West Africa prohibit use of bentonite specifically, please enlighten on alternate drilling muds that can be used, some times we had used guar gum but again some projects do not want as it had potential for bacterial growth.

Venkatesh.K.N.
saira_pps@yahoo.co.in