Well Drilling Basics
It is believed the first wells ever “drilled” were water wells. These were crude and shallow dugouts that gave access to the watershed that hid just beneath the dry surface. Wells as we understand them truly began when these excavations were bolstered by rock, wooden, or stone walls to protect the dig from collapsing and allowing for deeper and wider excavations. Today, the oil and gas industry has made well drilling literally a science. Well drilling now occurs in the arctic ice, thousands of feet beneath the ocean surface, and virtually anywhere on land that is accessible.
Measured Depth (MD) and Total Vertical Depth (TVD): Before drilling, these data points must be determined.
(b)Measured Depth is the length of the borehole and pipe required to reach the “payzone”. This is closely calculated by geologists and engineers since most drill operations are not strictly vertical, careful measurements need to be made about the deviations in the drill path.
By definition, anything that isn’t considered vertical drilling is deviated drilling. There can be some confusion when coming across other terms such as directional or horizontal drilling. Any wellbore that purposely changes direction to avoid an obstruction, formation, or to access a payzone is a type of deviated drilling. If the deviation in the wellbore is not significant or approaching 90 degrees it is generally described as being directional.
Horizontal drilling has become common practice in the industry despite the fact that 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 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 developed.
This type of well is most common when drilling into sedimentary rock, which is the most common kind of formation in oil and gas operations. Sandstone, coal, slate and limestone are examples of sedimentary rock. Carbonate rocks (sedimentary) are believed to hold more than half of the remaining global oil reserves. These formations, and indeed most oil and gas reserves, are found as wide and irregular “strata”. Horizontal drilling is the most effective way of accessing these formations.
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. Because more is extracted from a single site, there is less infrastructure required at the surface as well as below the surface. This reduction in the number of wells needed is of course a huge benefit both environmentally and financially.
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 draw-down in the reservoir.
As you might guess, vertical drilling is when the boring direction does not deviate from a mostly vertical axis. It is less expensive to develop a vertical well but is not nearly as efficient or lucrative because it cannot access reserves that are beneath obstructions and is limited in the amount of “pay zone” it can penetrate. This was the drilling method for nearly 100 years and is still very much used today when applicable.