Well is a circular mine opening of over 5 m depth and usually of 75-300 mm diameter, made by drilling rig.  Wells go from land surface and from mine workings at any angle to the horizontal. There is distinguished well datum (well head), bottom (unit) and shaft. Well depth ranges from several meters to 9 km and deeper. Solid minerals tests diameter is customary 59 and 76 mm, oil and gas tests diameter is 100—400 mm.

By purpose wells are divided into wild-cats for geological targets, geotechnical investigations and hydrologic studies, structures study, geophysical works, minerals prospecting; operating holes – for oil and gas extraction, ground waters, mineral salts, etc.; service wells – injection holes, observation, piezometric, air holes dewatering, methane drainage boreholes; special wells — freezing, grouting, drain holes, etc.; blast holes — for explosive blasts placing. 

Well is created by rocks sequential destruction (see Drilling), drill cuttings removal, hole walls consolidation when required. Drill cuttings removal is realized with mud fluid, gas or mechanical devices. In wild cats with unstable head ground thin walled casing pipes are used (without cementing if the formation is competent (tight)). Rough ground and loss zones are grouted with quick-setting mixtures. Operating holes and deep tests are fixed with metal casing pipes and grouted. Casing pipes are screwed or welded, in shallow water wells plastic, asbestos cement and other casing pipes are used. Oil and gas wells cementing technology includes installation of soil pipe (called guide) that is usually up to 20 m length in the well head (picture).

To assure following shaft vertical or inclined directivity and to unstable head ground closure and water and gas shut-off the second casing string is landed, it is so called conductor with the length from tens to hundreds of meters. With mud or flush liquid cement via conductor is injected in annulus between well walls and conductor. After drilling to projected depth completion practice and geophysics with productive horizons (oil, gas, etc.) revealed carried out, production string is inserted into the well. In order to avoid oil and gas cross-flow to the overlying beds and water cross-flow to productive horizon the well area behind production string is also cement grouted. If geological conditions are complicated (water-bearing formation or lost circulation horizon etc.), when it is not possible to drive a well without additional lining then between conductor and production string an intermediate string (service casing) is landed. In case after conductor production string only is landed the well design is called as single casing string well program (if there is one or two intermediate strings then well design is called as two-casing or three-casing well program).

There are different methods of well penetration and bottom hole equipment for liquid and gaseous minerals layers extraction. In most cases a row of holes is perforated in the casing wall and cement lining in the production string lower cemented part located in the pay. Wellbottom zone in stable grounds is equipped with different types of filters without grouting or casing is landed up to the top of pay and its drilling over and use is done without well cementing. Depending on the well function well head is equipped with fittings (surface wellhead, latchings, flowcross and others).