Drilling rigs create a hole called a wellbore that targets a geological formation where oil and/or gas may be present. Vertical wells that are straight up and down were typical in the past, but the industry now drills mostly horizontal or directional wells, where the well curves and exposes the wellbore to more of the target formation. In most shale basins, typical oil and gas wells range from 3,000 to 15,000 feet in depth. The horizontal part of the well can extend for up to two miles, or longer.
To protect the wellbore and any drinking water formations, a steel pipe called “casing” is placed in the wellbore, and cement is pumped through the casing. The cement pushes out the bottom of the casing and flows up the space between the wellbore and casing (or through the “annulus”) back to the surface. When the cement hardens, it forms a bond between the walls of the wellbore and the outside of the casing, thus sealing that space off from the flow of fluids (water, oil, or gases). The casing and cementing are then tested to ensure its integrity. This bond protects groundwater and oil and gas reservoirs from contamination. Typically, there are three separate layers of both casing and cement placed between drinking water aquifers and the actual pipe containing crude oil and/or natural gas. To produce oil and gas, holes are made in the casing in the reservoir interval, allowing hydrocarbons to flow into the well and up to the surface where they are processed and transported to the market. Once a well is drilled, cased, and cemented, many also undergo stimulation processes, such as hydraulic fracturing, to boost production and minimize the amount of residual oil remaining in the formation.
What is a Horizontal Well
A horizontal well is an oil or gas well dug at an angle of at least eighty degrees to a vertical wellbore. This technique has become increasingly common and productive in recent years. The horizontal well is a type of directional drilling technique. Operators use it to retrieve oil and natural gas in situations in which the shape of the reservoir is abnormal or difficult to access.
Most horizontal wells are started by drilling a vertical well. After drilling down to the target rock, the pipe is pulled out of the well and a motor is attached to the drill bit. The motor is fuelled by a flow of drilling mud down the drill pipe, rotating the bit without rotating the entire pipe. This allows the drill bit to create a path that is different from the orientation of the drill pipe.
Even though horizontal drilling is more expensive, there are numerous reasons as to why they are drilled. Vertical wells are able to drain rocks that have high permeability, however low permeability rocks do not let fluids flow quickly, and thus using a vertical well for these rocks would not be economically viable.
OFF IN ALL DIRECTIONS: MULTILATERAL WELLS
An added benefit of drilling horizontally in one direction is that it’s then possible to drill in another direction from the same platform. These multilateral wells can produce more than four times the crude than conventional horizontal wells and can reduce production costs substantially.
Eliminating the need for just one additional platform can save millions of dollars in development costs, but multilateral wells are more costly and complex.
However, challenges remain, particularly for onshore wells. Knorr says the multiple wellbores must be sealed and selectively accessible through the joints (or side connections), which is not easy to design.
Further, the equipment needed for well completions is complex, which can be expensive. Nonetheless, multilateral well design and technology continue to develop, with applications being extended throughout the industry and costs becoming more economical.