Most wells drilled for water, oil, natural gas, information or other subsurface objectives are vertical wells – drilled straight down into the earth. However, drilling at an angle other than vertical can obtain information, hit targets, and stimulate reservoirs in ways that cannot be achieved with a vertical well. In these cases, an ability to accurately steer the well in directions and angles that depart from the vertical is a valuable ability.
When directional drilling is combined with hydraulic fracturing, some rock units which were unproductive when drilled vertically can become fantastic producers of oil or natural gas. Examples are the Marcellus Shale of the Appalachian Basin and the Bakken Formation of North Dakota.
(B) Drain a large area from one drill pad
horizontal drilling – minimize footprint
Minimize footprint: One drilling pad can be used to drill a number of wells. This reduces the footprint of drilling operations. In 2010 the University of Texas at Arlington drilled 22 wells on a single platform. These wells are draining the natural gas from about 1100 acres beneath the campus. Over a 25-year lifetime, the wells are expected to produce a total of 110 billion cubic feet of natural gas. The alternative would be to drill many wells, each requiring a drilling pad, pond, access road and gathering line.
Why Drill Wells That Are Non-Vertical?
Directional and horizontal drilling have been used to reach horizontal directional drilling targets beneath adjacent lands, reduce the footprint of gas field development, increase the length of the “pay zone” in a well, deliberately intersect fractures, construct relief wells, and install utility service beneath lands where excavation is impossible or extremely expensive.
Below is a list of six reasons for drilling non-vertical wells. They are graphically illustrated by the six drawings on this page.
A) Hit targets that cannot be reached by vertical drilling.
Sometimes a reservoir is located under a city or a park where drilling is impossible or forbidden. This reservoir might still be tapped if the drilling pad is located on the edge of the city or park and the well is drilled at an angle that will intersect the reservoir.
B) Drain a broad area from a single drilling pad.
This method has been used to reduce the surface footprint of a drilling operation. In 2010, the University of Texas at Arlington was featured in the news for drilling 22 wells on a single drill pad that will drain natural gas from 1100 acres beneath the campus. Over a 25-year lifetime, the wells are expected to produce a total of 110 billion cubic feet of gas. This method significantly reduced the footprint of natural gas development within the campus area.
(C) Increase the length of the “pay zone”
horizontal drilling – maximize pay zone
Maximize pay zone: If a vertical well is drilled through a 50-foot-thick reservoir rock, then natural gas or oil can seep into the well through 50 linear feet of “pay zone.” However, if the well is turned to horizontal (or the same inclination as the rock unit) and drilled within that rock unit, then the distance of penetration within the pay zone can be much greater. Some horizontal wells have over one mile of pay zone penetration.
C) Increase the length of the “pay zone” within the target rock unit.
If a rock unit is fifty feet thick, a vertical well drilled through it would have a pay zone that is fifty feet in length. However, if the well is turned and drilled horizontally through the rock unit for five thousand feet, then that single well will have a pay zone that is five thousand feet long – this will usually result in a significant productivity increase for the well. When combined with hydraulic fracturing, horizontal drilling can convert unproductive shales into fantastic reservoir rocks.
(D) Improved production in a fractured reservoir
Fractured reservoir: Some reservoirs have most of their pore spaces in the form of fractures. Successful wells must penetrate fractures to have a flow of natural gas into the well. In many geographic areas there is a dominant fracture direction along which most of the fractures are aligned. If the well is drilled perpendicular to the plane of these fractures, then a maximum number of fractures will be penetrated.
D) Improve the productivity of wells in a fractured reservoir.
This is done by drilling in a direction that intersects a maximum number of fractures. The drilling direction will normally be at right angles to the dominant fracture direction. Geothermal fields in granite bedrock usually get nearly all of their water exchange from fractures. Drilling at right angles to the dominant fracture direction will drive the well through a maximum number of fractures.
E) Seal or relieve pressure in an “out-of-control” well.
If a well is out of control, a “relief well” can be drilled to intersect it. The intersecting well can be used to seal the original well or to relieve pressure in the out-of-control well.
F) Install underground utilities where excavation is not possible.
Horizontal drilling has been used to install gas and electric lines that must cross a river, cross a road, or travel under a city.