the combination or alternation at intervals of different methods and types of drilling (percussion and rotary drilling, coreless and core drilling, and so on) in sinking a borehole. The main advantage of combined drilling over any other drilling method is that it permits efficient use of several types of drilling, each of which is most favorable under certain drilling and geological conditions, during the sinking of a hole.
The combined method is often used for drilling shallow holes during geological engineering studies, surveying and prospecting work, and prospecting for nonmetallic building materials, because the rock and soils near the surface of the earth are particularly diverse in their physical and mechanical properties. Portable and self-propelled rigs that make possible the combination of several drilling methods are manufactured for sinking such holes (usually to depths of 40–50 m). For detailed deep-hole prospecting for many minerals, coreless drilling is often combined with core drilling. In this case thick rock series having no mineral content are drilled through at high speed by the coreless method, and the rock strata with mineral content are drilled through by the core method, with careful recovery of the core. The combined drilling method is used for deep gas and oil drilling: thick viscous-plastic rock strata are drilled by the rotary method, but hard rock is drilled by means of turbodrills and electrodrills.
REFERENCEVozdvizhenskii, B. I., A. K. Sidorenko, and A. L. Skorniakov. Sovremennye sposoby bureniia skvazhin. Moscow, 1970.
B. I. VOZDVIZHENSKII