Abstract: Since the geological conditions are complicated in FN block of Block VI in Sudan, so accidents occur frequently in the production of the two horizontal wells that have been completed. As a result, the drilling cycle is prolonged, the production cost is increased, and certain damage to oil and gas zones is caused. To overcome the problem, the calcium chloride clay-free drilling fluids system is developed. It has been applied to the FN-H3 and FN-H4 wells. Application results show that the drilling fluids system features in security, high efficiency, and environmental protection. Laboratory evaluation and field application show that the system has strong inhibition and can effectively prevent well collapse. What’s more, it has excellent carrying capabilities, which will enable the drilling fluids to obtain high dynamic plastic ratio. This can solve the problem of unsmooth returning of cuttings completely. In addition, calcium chloride is used as the weighting agent of the drilling fluids instead of heavy spar, which strengthens the protection of oil and gas zones and can be widely applied.