Borehole

The structure of the developed borehole

BOREHOLE (EN: well, drilling hole; DE: Bohrloch; FR: trou de forage; ES: agujero, pozo de sondeo; RU: буровая скважина) is the mine working with the prevalently circular cross section (the diameter is 59-1000 millimetres), which is formed as the result of the drilling.

They sub-divide the boreholes into small, namely, with the depth of up to 2000 metres (the overwhelming majority of them is with the depth of up to several hundred metres), intermediate, namely, up to 4500 metres, deep, namely, up to 6000 metres, and ultradeep, namely, more than 6000 metres. Within the boreholes, they distinguish the mouth, shaft, and bottom (bottomhole). According to the position of the axis of the shaft, and according to the configuration, they sub-divide the boreholes into vertical, horizontal, and oblique; unbranched, and branched; single, and clustered. According to the purpose, they distinguish the researching boreholes, which are purposed for the research of the Earth's crust, the operational boreholes (developmental, see the Figure), namely, for the development of the deposits with the useful minerals, the construction boreholes, namely, for the construction of various built structures (of the bridges, piers, pile foundations and bases, underground storage depots for liquids and gases, and aqueducts), the mining-technological boreholes, namely, for the construction and operation of the mining built structures. The researching boreholes are sub-divided into mapping, structural-prospecting, reference-control geological, reference-control technological, engineering-geological, parametric, prospecting, and exploratory. They sub-divide the operational boreholes according to the type of the deposition, which is being developed, into the boreholes for the petroleum, gas, and water reservoir (see the "Petroleum borehole", "Gas borehole", and "Hydro-geological borehole" articles), according to the function, which is being performed, into extracting, injecting, evaluating, controlling (piezometric, observatory), according to the operational status, into active, repaired, idle, mothballed, and liquidated.

The mining-technological boreholes are sub-divided into blasting (they account for the largest quantities of the drilling, namely, approximately 50 million metres per year), freezing, tamping, ventilating, dewatering, and others. Depending on the depth and purpose of the boreholes, and on the conditions for the drilling, they fixate the walls of the boreholes, or leave the walls in the non-fixated state. They do not perform the fixation of the shaft for the mining-technical (for example, for the blasting types) and other boreholes of the small depth (up to 50 metres), which have been penetrated within the stable hard rock massifs. They fixate the boreholes, which are purposed for the operation and researches, during the process of the construction. These boreholes have the most complicated structure, which is determined by the dimensions of the parts of the shaft, of the casing pipe strings, and of the cement annulus within the space behind the casing pipe strings; by the type and quantity of the casing pipe strings; by the equipment of the casing pipe strings, mouth, and bottomhole of the boreholes.

The casing pipe strings (directing, conducting, intermediate, and operational) are purposed for the fixation of the wall of the parts of the shaft of the boreholes, and for the isolation of the zones of the various complications, and also of the productive stratum from the remaining part of the geological cross section. Usually, the casing pipe strings are screwed together (welded) of the steel pipes, while within the shallow boreholes, they use the casing pipes, which are made of the plastic and of the asbestos cement. The directing pipe string (the direction) is the first casing pipe string (with the length of up to 30 metres), which they lower into the upper (directing) part of the shaft, in order to isolate the upper alluvial layer of the soil, and in order to divert the upward flow of the drilling fluid from the shaft of the borehole into the cleaning system, this directing pipe string is cemented along the entire length. The conducting pipe string (the conductor) is the second casing pipe string, which is lowered into the shaft of the borehole, this pipe string is purposed for the blocking of the upper unstable sediments, of the aquifers and absorbing layers, of the zones with the permafrost rocks, and so on. They install onto this pipe string the blowout preventing equipment; they usually cement the annular space behind the pipe string along the entire length. They lower the intermediate casing pipe string in case of the necessity, after the conducting pipe string, for the fixation of the unstable rocks, for the separation of the zones with complications, and of the aquifers. They calculate the depth for the lowering of the intermediate and conducting pipe strings, with taking into account the prevention of the hydraulic fracturing within the layers, the stability of the wall within the shaft of the boreholes, the separation of the zones for the usage of different drilling fluids.

The quantity of the intermediate pipe strings depends on the depth of the boreholes, and on the complexity of the geological cross section. The last casing pipe string is purposed for the operation, and isolates the productive layers. For the extraction of the fluids from the productive layers, they lower into the operational pipe string the pumping-compressing pipe strings in various combinations, depending on the quantity of the layers, which are being developed, and on the method for the extraction, which is used. There may be lowered on the drilling pipes into the intermediate and operational part of the shaft of the boreholes, instead of the casing pipe string of the full length, the special casing pipe strings, namely, shanks, the top of which is fixated with the help of the special suspensions. After the completion of the construction of the borehole, they sometimes increase the shank casing pipe string by the extension pipe string up to the mouth of the borehole. For the facilitation of the descent, for the cementing of the casing pipe strings, and for the improving of the quality of these works, the casing pipe strings are equipped with the directing shoes, various valves, connecting and disconnecting devices, turbulators of the cement fluid, packers, centralizers, and scrapers. In case of the multi-stage cementing, they introduce into the structure of the casing pipe string the cementing couplings. According to the quantity of the casing pipe strings, which are lowered into the shaft of the boreholes after the conducting pipe string, they distinguish the constructions of the boreholes with one, two, three, and multiple pipe strings; according to the type of the equipment within the bottomhole zone, they distinguish the boreholes with the enclosed and non-enclosed bottomhole zone. The construction of the boreholes with the enclosed bottomhole zone may be obtained either in case of the lowering of the continuous operational pipe string into the borehole, with the subsequent cementing and perforating of this pipe string, of the cement stone, and of the productive layer, or in case of the lowering of the operational pipe string with the tail section, which is having the round or slit-shaped holes, which are placed at the level of the productive layer, into the borehole.

The structure of the gas boreholes is distinguished with the increased hermeticity of the casing pipe strings, which is achieved by the usage of the casing pipes with the special compounds and lubricants for them, by the lifting of the cement fluid behind all the pipe strings to the mouth of the boreholes, and so on. They equip the mouths of the developmental petroleum and gas boreholes with the special fitments. The structure of the borehole, which is purposed for the prospecting and exploration of the deposits with the hard useful minerals, is significantly simpler. The directing part of such boreholes has the length of several metres, and is fixated by the directing pipe, the conducting part has the length of 30-150 metres. They drill the shaft further with the complete retrieval of the drill core sample, and they perform the fixation of the unstable rocks with the quick-hardening mixtures.