Even outside the drilling industry, the concept of directional drilling, whereby a drill is precisely guided through a particular target, is a fascinating one. This article will describe about applications of directional drilling in oil and gas industry. Later on, we will discuss in several aspects of directional drilling such as directional drilling tools, well path design, wellbore navigation tools, etc. Let’s get started.
Why Drill Directional Wells?
It is a fact that it is always more expensive to drill a deviated well to a target not directly below the rig location, as opposed to simply drilling down vertically to the target.
However, there is good reason why a directional well might be used: in some circumstances, it can actually lower the total cost of the project. Some potential reasons for this include:
Multiple exploration wells from a single wellbore
It is possible to drill a well to evaluate it, and then cement it back up. This well may then be deviated from its original path to an additional target. This may be done in order to evaluate multiple compartments in a single reservoir, if it is naturally split into several sections, or to extend the knowledge of the structure using a single well.
Single surface location for multiple wells
The effective draining of reservoirs needs wells located across multiple parts of the reservoir. If possible, these wells should align towards a single surface location, so that all necessary production facilities may be located there. This is usually cheaper than connecting multiple wells from various surface locations. Production staff are centralized, which lowers running costs, and the rig only requires a single location. This is how drilling from an offshore platform works; a single platform could potentially connect to over 30 wells which spread out beneath it, which unite at the platform.
Salt dome drilling
Some salt domes create additional structures that capture hydrocarbons. In these cases, it is often easier to drill around the salt dome instead of straight through it to reach a reservoir.
Onshore drilling to an offshore reservoir
It is more economically viable to reach an offshore target from onshore, rather than build an offshore platform. Wytch Farm (Figure 4), on the south coast of England, is one such example, and is in fact the largest onshore oil field in Western Europe, even though a great deal of it is located offshore. This area is environmentally sensitive, so operations were made economically possible by drilling from the land under the sea. Extended reach wells, which extend horizontally over twice their depth, allowed for this.
It might be necessary sometimes to cement a well back to a shallower depth, and for a new wellbore to be drilled away from this original bore. This could be due to drilling problems, such as stuck pipe, or because an old producing well is to be sidetracked to a new location.
Optimum orientation in the reservoir
Unfortunately, reservoirs are not neat and tidy. Some directions will be more permeable than others. If a reservoir is in fractured limestone, then the well should aim to intercept as many of these fractures as possible to maximize production. Such factors determine the direction that the well should be drilled into the reservoir. Some wells may require very complicated well paths in order to achieve their prime position.
In the worst possible scenario, a well is drilled as a relief well. This might be because, for instance, a well has suffered a blowout, and is blowing hydrocarbons into the surrounding environment, but the well cannot be killed from the surface. The relief well is then drilled to intercept the blowing well, and dense mud is forced into the blowing well to kill it from below. One example of this was the Deepwater Horizon disaster in the Gulf of Mexico in April 2010.
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Short, J.J.A. (1993) Introduction to directional and horizontal drilling. Tulsa, OK: PennWell Books.
Mitchell, R.F., Miska, S.Z. and Aadnoy, B.S. (2012) Fundamentals of drilling engineering. Richardson, TX: Society of Petroleum Engineers.
frackfreecv (2013) Why Wytch farm is a poor fracking comparison for Somerset. Available at: https://frackfreecv.wordpress.com/2013/12/16/why-wytch-farm-is-a-poor-fracking-comparison-for-somerset/ (Accessed: 15 February 2017).