The penetration rates using gas, foam, and mud in an area are 10 ft/hr, 5 ft/hr, and 1 ft/hr. respectively. If gas is used, each water zone en- countered must be sealed off. The cost of the plugging treatment is $2,000, and 25 hours of rig time are required to complete the sealing operation. The normal operating cost for air drilling is $200/hr. The use of foaming agents requires an additional $60/hr. The normal operating cost when using mud is $160/hr. Regardless of the drilling fluid used, the average bit cost is $1,000. The average bit life is 25 hours and the average trip time is 6 hours, Determine which drilling fluid yields the lowest drilling cost if one water zone is encountered per 1,000 ft drilled and if five water zones are encountered per 1,000 ft drilled. Answer: gas is best for both assumptions ($35.80/ft and $63.80/ft).