Wells are drilled holes in the ground that are used to obtain some natural resource from the subsurface. This natural resource may include water, gas, or oil. Another name for a well is a borehole.
Wells have been around for centuries, but the first modern wells were drilled around the mid-1800s.  Horizontal wells were introduced as a way to increase reservoir rate of return in low permeability rocks and in areas with thin reservoirs. It was also used as a way to prevent oil or water coning around the well bore during pumping.  In populated cities where vertical drilling is not possible or in areas with structural impediments, such as mountains, horizontal drilling may be used to reach a resource from another location.
Below is a brief history on the introduction of important first wells and advancements in seismic exploration. 
- 1848: First modern oil well in Asia
- 1858: First oil well drilled in North America
- 1859: First oil well drilled in United States
- 1929: First well logs collected, allowing better analysis of subsurface physical properties
- 1936: Introduction of seismic exploration, enabling better imaging of subsurface features
- 1980: First horizontal well, paving the path for unconventional plays
- 1990: Number of horizontal wells becomes more prominent, reaching almost 1200 wells
- 1990s: Introduction of 3D seismic acquisition and data pushes exploration and boosts confidence in identifying traps
Wells can be classified into different categories based on their design geometry or utility. Design geometry refers to how the well is drilled. Utility refers to the well’s purpose.
When planning the well design, it is important to note the structure of the target interval. Is it flat or tall? Is there gas updip that may be missed with a vertical well? Background knowledge of the vertical versus horizontal permeability of the target interval will also be helpful in determining the most efficient design for oil extraction.
Vertical wells are drilled such that the borehole is perpendicular to a locally-horizontal bedding plane. Often, it is hard to control the straightness of a well. Directional drilling using rotary steerable drilling technology, a system that seeks and maintains vertical dip, can help direct the position of the well. It is important to keep the well straight so that exploration and production costs are minimized and tools and casing can be dropped into the borehole with ease.  Vertical wells are most common for conventional oil and gas production.
Horizontal wells use directional drilling in order to drill parallel to the target interval’s boundaries. For horizontal drilling, three important pieces of equipment include bent housings to build well deviation, downhole adjustable stabilizer to maintain proper inclinations in straight, deviated sections of the well bore, and stabilizers to prevent the drill bit from getting stuck. The combination of the drill bit and the drill string rotation determines whether the path is straight or curved.  Horizontal wells are common in areas of unconventional oil drilling where oil can be extracted directly from source rocks. They are more expensive and difficult to drill than vertical wells. However, for areas where the target reservoir is thin, has natural vertical fractures, or contains lateral heterogeneities that may block fluid flow, horizontal wells may be more efficient in producing oil than a vertical well. 
Well utility is categorized by level of risk. The higher the uncertainty of reservoir presence, the higher the risk. The risk is determined by whether a particular reservoir is proven within the area of interest.
- Exploratory wells are used to gather data in a new area. Exploratory wells have higher risk because they search undeveloped areas for oil and gas potential.
- Wildcat wells are a type of exploratory well where little is known about the location's geology. These are drilled in unproved areas, where no previous drilling has occurred.
- Development wells are drilled in areas that exploratory wells have already identified as proven reservoirs. Development wells have lower risk because hydrocarbons are known to exist at that location. Their main purpose is to extract the hydrocarbons from the subsurface. 
In Oil and Gas Exploration
Wells are used to obtain data about the subsurface. Logging tools can utilize the 1D, depth-varying boreholes to record measurements during or after the drilling process. These measurements can help determine reservoir properties and map important horizons away from the well bore.
Identifying Geologic Properties
During or after drilling the well, several subsurface measurements can be made using logging tools, which are inserted into the well bore. These well logs record changes in measurements with depth. There are logging tools to collect a number of physical properties such as porosity, density, and resistivity of the subsurface layers. For example, the gamma-ray (GR) log measures radioactivity of a formation to determine the approximate shale volume. Sonic logs record the speed of sound waves through rocks, which can help calculate porosity and compactness. Resistivity logs use response to electricity to determine types of fluids present in the rocks.  The borehole diameter can be measured with a caliper log; this is useful for error analysis of other well data. A spike in data may match with a casing point in the hole.
Formation tops of important interval layers can be recorded using cuttings and biostratigraphic (fossil) data that are recovered from the borehole’s sidewall as the well is being drilled. LWD (logging while drilling) measurement tools are connected to the drill string, which allow real time data collection during drilling. 
Once the well is drilled and measurements are made, a well to seismic tie can be created from the density and velocity logs. The multiplication of velocity and density yields acoustic impedance. The changes in acoustic impedance between two overlying layers yield the reflection coefficient. This data provides information on the rock properties. By creating a well to seismic tie, a correlation can be made between physical properties obtained from the well log data and structures in the seismic data through a synthetic seismogram. By posting the synthetic seismogram that contains the time-to-depth relationship, the 1D depth log data can be correlated with the two-way traveltime seismic data.  A strong correlation between the synthetic seismogram and the seismic section yields a higher confidence tie.
After the synthetic seismogram and well formation tops are posted onto the seismic data, a well can act as a control point when mapping a target formation. For that one coordinate, the subsurface layer boundary depths and the physical properties of the rocks are known. Using multiple wells in an area with multiple synthetic seismograms and formation tops will provide higher confidence in mapping the subsurface.
- ↑ 1.0 1.1 San Joaquin Valley Geology, 2017, The History of the Oil Industry, http://www.sjvgeology.org/sjvgeology/history/index.html, accessed October 27, 2017.
- ↑ 2.0 2.1 Totten, George E., 2007, A Timeline of Highlights from the Histories of ASTM Committee D02 and the Petroleum Industry, https://www.astm.org/COMMIT/D02/1980_2004.html, accessed October 27, 2017.
- ↑ 3.0 3.1 3.2 3.3 Burgess, T. et al, 1991, Horizontal Drilling Comes of Age. Oil Field Review 2 (3): 22-23, http://www.slb.com/~/media/Files/resources/oilfield_review/ors90/jul90/4_drilling.pdf, accessed November 28, 2017.
- ↑ Brusco, G., Lewis, P., and M. Williams, 2004, Drilling Straight Down: Oilfield Review 16 (3): 14-17, https://www.slb.com/~/media/Files/resources/oilfield_review/ors04/aut04/02_drilling_straight_down.pdf, accessed October 28, 2017.
- ↑ 1996, Middle East Well Evaluation Review: Horizontal Highlights: Middle East & Asia Reservoir Review, 16, 7-25, http://www.slb.com/resources/publications/mearr/mewr16.aspx, accessed October 27, 2017.
- ↑ Wright, Charlotte J. and Rebecca A. Gallun, 2008, Fundamentals of Oil & Gas Accounting, e-book.
- ↑ 7.0 7.1 Zhou, Hua-Wei, 2014, Practical Seismic Data Analysis: Cambridge University Press.
- ↑ 8.0 8.1 Rigzone, 2017, How Does Well Logging Work?, http://www.rigzone.com/training/insight.asp?insight_id=298, accessed October 28, 2017.
- Schlumberger Oilfield Glossary - Wellbore
- Schlumberger - Horizontal Drilling Comes of Age
- Petroleum.co.uk - Oil Well Drilling