Horizontal drilling is a type of “directional drilling” where the wellbore is steered to almost, and sometimes more than, a 90 degree angle in order to access the “pay zone”. This is one of the more recent methods and has increased productivity immensely and relieved some issues from traditional vertical drilling (see below).
History of Horizontal Oil Drilling
Although the idea is modern and its only recently started to become more mainstream, the first idea and patent to develop horizontal oil drilling was given to Robert Lee in 1891. The drill did not make a 90 degree turn, but instead 2 holes were drilled, one vertically and one horizontally until they met. The advance in horizontal oil drilling was forced as they needed ways of accessing harder-to-reach pockets of oil in order to meet the rise in demand. That is the basis for why horizontal oil drilling was formed.
Below is an example of what horizontal oil drilling looks like. Source is from the USGS.
It shows a traditional oil well next to a horizontal drilling well.
The Practicalities of Horizontal Oil Drilling
The advantages of this method are numerous. Production rate is increased because the well bore has greater exposure to the “pay zone”. Reserves are recovered more efficiently because the drainage area is bigger. Issues that are more prevalent in vertical drilling and which are alleviated in horizontal drilling are pressures around the well bore and high fluid velocities. “Coning” is also mitigated to a degree by less drawdown in the reservoir.