Geological exploration wells, pre-exploration wells, detailed exploration wells (evaluation wells), and development wells (including inspection data wells, production wells, water injection wells, adjustment wells, and so on) are the most common types of oil drilling. It can be separated into vertical wells, horizontal wells, and some other specific structure wells based on the well structure.

Production Well

Production wells, also known as development wells, are wells drilled during the oilfield development stage for oil and gas production, and include oil (gas) wells, water injection (gas) wells, adjustment wells, and so on. Drilling production wells rarely takes cores since the formation conditions are clear, the drilling speed is fast, and the cost is low.

(1) Oil (gas) wells: During the development of an oil field, wells drilled for the extraction of oil and natural gas are classified as oil-producing wells or gas-producing wells.

(2) Water (gas) injection wells: wells drilled to inject water and gas into the oil field to supplement and rationalise formation energy in order to increase recovery rate, and can be classified as positive injection wells (wells that inject water from the tubing to the formation) or reverse injection wells (wells that inject water from the casing into the formation).

(3) Adjustment wells: The supplemental drilling to improve the oilfield development effect refers to wells drilled in favourable positions during the oil and gas field development process to better develop the oil and gas fields based on the original well pattern.

Exploration well

Exploration wells, which include geological wells, pre-exploration wells, detailed exploration wells, and evaluation wells, are wells drilled for the goal of acquiring geological data. In most cases, exploration wells are drilled vertically.

(1) Pre-exploration wells: These are wells drilled to identify oil and gas reservoirs in favourable traps defined by intensive seismic examination and extensive geological research, as well as wells drilled to discover new oil and gas reservoirs in established oil fields.

(2) Detailed exploration wells: These are wells that are dug in identified oil and gas traps to prove the boundaries and reserves of oil and gas, as well as to comprehend the structure and oil production capability of oil and gas layers.

(3) Evaluation well: a well drilled to demonstrate the oil and gas area and geological reserves of the trap after the oil and gas reservoir is located in the preexploration well.

Drilling exploratory wells takes a long time and costs a lot because of the unknown formation conditions and the requirement to take cores and measure geological data.

Exploration wells include geological exploration wells, pre-exploration wells, and detailed exploration wells (evaluation wells). Oil production wells, gas production wells, water injection wells, and gas injection wells dug for oil production are one type of development well; the other is adjustment wells, supplemental wells, inspection of resource wells, and so on.

Directional well

In oil drilling, directional wells, which include horizontal and deviated wells, are wells drilled following the design trajectory for specific reasons and requirements, in addition to vertical wells. Since the 1990s, as drilling technology has advanced and improved, highly deviated wells, horizontal wells, and extended reach wells have been the development direction to satisfy the needs of oil and gas field exploitation.

(1) Horizontal well

A horizontal well is one that is drilled along the (nearly) horizontal direction to a fixed length and has a large well inclination of between 86° and 120°. Horizontal well technology has been developed since the 1990s because it can expand the exposed area of the produced oil layer, raise the oil output of the oil layer, and increase the recovery rate of the oil field. Horizontal wells have four curvature radius types: long, medium, medium-short, and short. Horizontal well drilling is a relatively challenging task that necessitates the use of specialised equipment, drilling tools, tools, instruments, and processes.

(2) Deviated well

Deviated wells refer to wells whose designed large inclination angle does not exceed 85°, and can be divided into low-inclination wells, medium-inclination wells, and high-inclination wells, among which medium-inclination wells are used more.

(3) Cluster wells

Due to the constraints of the ground, environment, and other conditions, as well as the requirements for rational use of land resources, it is often necessary to drill several or dozens of directional wells and one vertical well in a planned manner on a well site or platform during the development of an oil field. Cluster wells are the collective name for these wells. The average distance between cluster wells' wellheads (well spacing) is 23m. Cluster wells are frequently employed in offshore oilfield development because they lower the cost of building well sites or platforms and make oil well production management easier.

Deep and ultra-deep wells

It can be split into shallow wells (drilling depth less than 2000 metres) and deep wells (drilling depth greater than 2000 metres).

Wells with a drilling depth of 2000-4500 metres

Deep wells (drilling depths ranging from 4500 to 6000 metres)

Ultra-deep wells (wells drilled to depths greater than 6,000 metres)

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