Well drilling

Well drilling is the process of drilling a hole in the ground for the extraction of a natural resource such as ground water, brine, natural gas, or petroleum, for the injection of a fluid from surface to a subsurface reservoir or for subsurface formations evaluation or monitoring. Drilling for the exploration of the nature of the material underground (for instance in search of metallic ore) is best described as borehole drilling.

The earliest wells were water wells, shallow pits dug by hand in regions where the water table approached the surface, usually with masonry or wooden walls lining the interior to prevent collapse. Modern drilling techniques utilize long drill shafts, producing holes much narrower and deeper than could be produced by digging.

Well drilling can be done either manually or mechanically and the nature of required equipment varies from extremely simple and cheap to very sophisticated.

Managed Pressure Drilling (MPD) is defined by the International Association of Drilling Contractors (IADC) as “an adaptive drilling process used to more precisely control the annular pressure profile throughout the wellbore." The objectives of MPD are “to ascertain the downhole pressure environment limits and to manage the annular hydraulic pressure profile accordingly."

Several factors affect drill bit selection. Due to the high number of wells that have been drilled, information from an adjacent well is most often used to make the appropriate selection. Two different types of drill bits exist: fixed cutter and roller cone. A fixed cutter bit is one where there are no moving parts, but drilling occurs due to shearing, scraping or abrasion of the rock. Fixed cutter bits can be either polycrystalline diamond compact (PDC) or grit hotpressed inserts (GHI) or natural diamond. Roller cone bits can be either tungsten carbide inserts (TCI ) for harder formations or milled tooth (MT) for softer rock. The manufacturing process and composites used in each type of drill bit make them ideal for specific drilling situations. Additional enhancements can be made to any bit to increase the effectiveness for almost any drilling situation.

A major factor in drill bit selection is the type of formation to be drilled. The effectiveness of a drill bit varies by formation type. There are three types of formations: soft, medium and hard. A soft formation includes unconsolidated sands, clays, soft limestones, red beds and shale. Medium formations include calcites, dolomites, limestones, and hard shale. Hard formations include hard shale, calcites, mudstones, cherty lime stones and hard and abrasive formations.

...
Wikipedia