The search for new sources of oil, and particularly gas, is leading the industry to drill ever deeper wells. A depth of 15,000 ft was first passed in 1938, 20, 000 ft was reached in 1939, followed by 25, 000 ft in 1958, and 30, 000 ft in 1972. The current US record depth is 31,441 ft. As the total depth increases, not only does the rock to be drilled become stronger, but increasing pressure and temperature induce plasticity and "chip hold-down" effects that make it more difficult to remove cuttings from the workfront. In addition to the reduction in rate of the drilling process itself, other activities become more complex and time-consuming, for example, tripping, running and cementing casing, and logging and coring activities.
We have analyzed the different tasks involved in drilling deep wells, in order to identify those activities that contribute most to the overall cost. These are therefore expected to be the activities where future efforts in research and development should provide the greatest reductions in total cost.
OVERALL EFFECTS OF INCREARTNR DRPTH
We have obtained information from a large drilling database that contains data on over 3000 wells, drilled in all parts of the world, both on land and offshore, during the period 1977-1988. The majority of these wells were for exploration rather than production. Some of the major findings include the following: