Water Well Construction Technology - Part 2 - Rock Drillability - 1972

Hydrogeology, Geology, & Environmental Issues / Field Methods & Procedures

Campbell reported in 1972 that drilling technology is as old as water well drill­ing itself. It has been generally recognized that the art of drilling and casing was invented, per­fected, and extensively practiced by the ancient Chinese. Using percussion drilling, bamboo cas­ing, and patience, they drilled to depths rivaling those reached in modern times with steam.

However, it has only been during the last cen­tury that water well drilling, as we know it, has been perfected. Until recently, well drilling was strictly an art instead of a science. We still do not know all of the answers, but a great deal of work now has practical field application.

The need to relate drilling penetration rate and other factors to the properties of the rock is well known. Only after such a correlation has been made, can the ideal conditions be determined. Then, actual performance can he expressed as a percentage of the ideal, and various types of drilling systems can be compared against each other.

In the development of any drilling system, ­either rotary or cable tool-composition, prop­erties, and behavior of the rock control the drill­ing process. The science of rock mechanics de­scribes such rock composition, properties, and behavior. However, much of the data now avail­able from rock mechanics studies do not apply directly to well drilling. The information must be interpreted for use by the water well in­dustry.

Rock drillability is defined as the inverse of the resistance of a rock to be broken up. This re­sistance is related not only to the rock material, but to the rock environment, which includes such things as formation fluid and depth of the rock. Under certain given conditions, the behavior of drill bits can be mathematically predicted on the basis of compressive rock strengths, although rock drillability does not appear to depend only on compression. Basic indentation tests show, for instance, that rock compressive strength, shear strength, and impact resistance probably are the major controls on rock drillability. 

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