Well is a mining tunnel of a round hole in diameter many times less than its length, which is formed in a rock mass by means of drilling and casing without people access with a pre-designated 3D position. The upper part of a well is called a wellhead, the bottom – a downhole, the side surface – a wellbore wall. Rock particles formed during wellbore progressing are called cuttings. The distance from the wellhead to the downhole along the wellbore axis determines a well length. The length of the vertical axis projection determines a well depth.

During exploration as well as for production of liquid and gaseous underground resources in the Earth crust rock mass, small diameter and considerably long cylindrical holes are made, which are called drilling wells. Drilling wells are made both from the surface and subsurface tunnels. They may have various dimensional forms (linear, spiral) and directions (upward, downward, horizontal, inclined).

Test wells are used to study the geological and hydrological conditions of major regions, for defining a common patterns set of occurrence of deposits with features favorable for oil and gas accumulation with the purpose to select the exploration areas with the best potentials.

Parametric (appraisal) wells are used to study deep geology aspects, to compare hydrocarbons potential of possible oil and gas accumulation zones. Such wells help to identify the areas with the highest potential for detailed geological studies and to get the necessary data about geological and geophysical features of the section to specify the results of seismic and other geophysical studies.

Structural well (core hole) is drilled with the purpose to identify the most promising areas and to prepare them for prospecting and exploratory drilling. Based on data obtained during such drilling mode of occurrence, lithological composition and stratigraphical position of the studied reservoirs are defined and then used to develop geological profiles of the given area.

Prospecting wells (“wildcats”) are drilled in the area prepared through geological exploration work with the purpose of discovering new fields and identifying new accumulations in previously discovered fields.

Exploratory wells are drilled in the areas with already identified commercial HC potential with the purpose of preparing commercial oil and gas reserves and collecting initial data for reservoir development planning.

Producing wells are used for oil and gas fields and deposits development. There are several categories of producing wells:

• Producer – an operational facility providing for transporting gas from reservoir to the surface at the expense of its natural formation energy.
• Test wells are used for evaluation of producing reservoirs. Injection wells are necessary for pumping water, gas, air and other agents into producing horizons.
• Monitoring and pressure-observation wells are used for monitoring changes of reservoir pressure, temperature, for gas and water sampling, for moving of gas-water and oil-water contacts, and for geophysical studies. Physical wells and inspection wells also belong to monitoring wells; they are used for producing reservoir completion when building underground gas storage facilities.
• Wells for thermal bed stimulation are used for developing high-viscosity oil fields.

Special well is drilled to collect produced water (injection well), to liquidate open oil and gas flows; exploration and production of process water. Back-up well for gas off-take from UGS facility during peak demand and for replacing producing well also belongs to special well.