Core Drilling

rotary drilling in which the rock is cut by an annular bit, with retention of the interior portion in the form of a core, rather than over the entire face of the hole. During core drilling, the pieces of crushed rock are removed from the face and brought to the surface by the mud injected into the drill string by a mud pump. The drilled-out core passes into the core barrel and fills it as the borehole is deepened. The core is periodically wedged, separated from the face of the hole, and raised to the surface. Core drilling is done with rigs that lower and hoist the boring tool and rotate and feed the coring tool.

The crowns or bits used for drilling depend on the hardness and abrasive properties of the rock. Core drilling is done in vertical, slanted, rising, and multiface boreholes in rock with extremely diverse physicomechanical properties. It is generally used in prospecting for hard minerals and in geological-engineering surveys.

(drilling of orientation wells), a method of studying the earth’s crust and, in most cases, of long-term evaluation of oil and gas capacity by drilling deep wells during regional geological exploration.

Orientation wells, or test holes, are drilled to study the geological structure and history of major geostructural elements that have been little studied, and also to provide a scientific grounding for the most promising trends in geological prospecting for oil, gas, and other minerals. Test holes are usually located according to data supplied by regional aeromagnetic, gravimetric, or seismic geophysical research, under the most favorable structural conditions. The drilling of test holes proceeds with the extraction of a core, usually down to the crystalline foundation or, in areas where the foundation lies at great depths, as deep as is technically feasible.

The deposits revealed during core drilling are subjected to complete and integrated geological and geophysical study (determination of age, lithology, the reservoir characteristics of the rock, and the presence of oil, gas, and other minerals in the log).

The volume and type of subsequent regional and prospecting operations are planned on the basis of data from core drilling.

A layer-by-layer description of the log revealed by the well, with the basic laboratory analysis and research data (including brief conclusions on the geological structure and long-term oil and gas capacity of the region), is published by the Ministry of Geology of the USSR in the series Opornye skvazhiny SSSR (Test Holes of the USSR).

The idea of studying the geological structure of oil and gas provinces by drilling deep single wells was expressed by I. M. Gubkin in the mid-1930’s. By 1973, 289 test holes had been drilled in the USSR, including 169 in the European part of the country, 54 in Middle Asia and Kazakhstan, 34 in Western Siberia, 18 in Eastern Siberia, and 14 in the Far East. The drilling of test holes has facilitated the discovery of major oil and gas provinces in the USSR, among them the Volga-Ural Oil-Gas Region and the Western Siberian Oil and Gas Basin. Test holes are now being drilled in poorly studied areas of Eastern Siberia, the Far East, and the northern European part of the USSR.

In other countries, single wells called stratigraphic wells are drilled, primarily for the study of stratigraphy and oil and gas capacity, when studies of new regions are required.