An Engineering Appraisal of Small Diameter Hole Drilling in Soft Rock Operations

Abstract

Published in Petroleum Transactions, AIME, Vol. 207, 1956, pages 80–87.Paper presented at Petroleum Branch Fall Meeting in New Orleans, Oct. 2–5, 1955. Abstract An examination is made of the various factors influencing drilling rate and drill bit life in various diameter holes. It is concluded that drill bits in the size range 4 3/4 to 6 1/4 in can be used to attain substantially the same daily drilling progress as conventional size bits of soft rock drilling operations. An examination of the most important factors influencing "hazard of losing the hole" in the different hole sizes results in the conclusion that the smaller hole sizes can probably be drilled at reasonably low risk. However, this risk will be somewhat greater in the smaller diameter holes, requiring that special drilling and mud engineering practices be used to make small diameter holes practicable in soft rock drilling operations. An engineering analysis is made of the power and hydraulic requirements of several drill string-hole size combinations to test the feasibility of drilling small diameter holes in soft rock areas. Large economies in rig power requirements appear possible by drilling smaller size holes in these operations. Introduction For many years the industry has talked about and made sporadic attempts to exploit small diameter holes. Lack of sufficient technological progress was apparently responsible for many earlier failures. The industry now is re-examining the utility of small diameter holes in the light of substantial technical advances made in recent years. Faced with increasing cost of finding reserves and with proration after developing them, the industry must either develop methods of economizing or receive a lower rate of return on investments. Small diameter holes appear to involve an attractive method of economizing. Our objective is to determine if this is technically possible in soft rock operations. Soft rock, as used in this report refers to those formations of a "drillability" corresponding to that of all Gulf Coast formations shallower than and frequently including much of the Eocene rock system.