Before any equipment is moved to the drill site, the drilling company needs to design the well. A properly designed well is both safe and cost effective.
Many decisions need to be made when designing a well. These decisions are based on both the geology, engineering, and the economics of the oil or gas field where the well will be drilled.
Vertical Well vs. Directional/Horizontal Well
The first thing that needs to be decided once a drill site is selected is whether the well will be a vertical well or a directional/horizontal well. Although directional wells are more expensive to drill, they may be more profitable. The vast majority of new wells for shale and tight gas formations are horizontal.
One of the biggest hazards encountered during drilling operations is the presence of high-pressure fluids underground. A good well plan is designed to prevent zones of overpressure from overwhelming the ability of the well to contain them. If the pressure is released at the surface, a dangerous blowout could occur.
Containing High Pressures
There are two primary methods for containing high pressures: drilling mud and casing.
Drilling Mud
As discussed in Well Control, drilling mud contains the high pressures found underground by counteracting them with its hydrostatic pressure due to its density. As long as the pressure exerted by the mud is greater than the formation pressure, fluids will be unable to escape into the wellbore. When regions with different formation pressures are encountered, the density of the drilling mud must be adjusted by adding materials such as barite. The weight of one gallon of drilling mud can vary by several pounds depending on how many and what type of additives it contains.
Casing
The second pressure containment mechanism is casing. Casing consists of a string of steel pipe cemented into the well to great depths (and at great expense). Rather than holding back formation pressure by counteracting it with weight, as drilling mud does, casing contains the pressure by serving as a barrier to formation fluid movement, essentially turning the wellbore into a long, skinny metal tank with a hole at the bottom. A blowout preventer, or BOP, is attached to the top of the casing to contain abnormally high pressures. Blowout preventers are essentially extremely powerful valves that are capable of sealing off the entire borehole.
Like drilling mud, casing serves multiple purposes. It isolates the wellbore from the surrounding geologic formations, prevents contamination, and provides a clear conduit for fluids to pass through on their way to the surface. Most wells are surrounded by several layers of protective casing and cement, and almost all jurisdictions have regulations requiring casing in zones containing groundwater.
Casing Program Design
Assuring that the casing plan meets or exceeds the regulations, and that the plan is implemented correctly are two more opportunities for regulators to protect the environment, groundwater, and the people working on the rig.
Transcript
Casing Program Design – Alfred William Eustes – Colorado School of Mines
One of the most important parts of planning a well is designing a casing program – a plan for lining the wellbore with steel and cement.
Well casing serves a number of purposes. These include protecting zones of groundwater from production fluids, ensuring the pressure integrity of the well and isolating zones of high pore pressure or unstable lithology from the well. A good casing program will ensure that these goals, and the legal requirements that go along with them, are met in a cost effective manner.
To develop a casing program, drillers need to collect information about the geology of the well. This includes pore pressures, fracture pressures, the locations of ductile or weak zones that might squeeze into and onto the well bore, regions of permeable rock, and zones of groundwater storage, particularly fresh water aquifers.
Every casing program is legally required to install surface casing to protect water resources from drilling fluids. Because of this, the first step to developing a casing program is to determine the depth of the permeable groundwater zone.
Typically, the surface casing is required to run a certain distance beyond the bottom of this zone as designated by regulations.
The next step is to figure out how the mud weight will vary as the well is drilled.
Mud pressure at any point in the open section of a wellbore needs to be kept between the fracture strength of the rock and the formation pore pressure to prevent lost circulation or a kick which can lead to a blowout.
Since pressure typically increases with depth, only a certain portion of the well will be able to withstand a given mud pressure without the added strength of casing.
Once a mud weight pressure exceeds the strength of the rock in the open section of hole, a new string of casing needs to be added to protect those rocks from fracturing. Because of this requirement, casing depths can be determined by knowing the fracture pressure – and the pore pressure – of an entire well along with the planned mud weight.
So imagine a hypothetical case in which we’re going down to a total depth of 14 thousand feet and we believe we know the fracture pressure and pore pressure.
In this case, the well will need at least two sections of casing below the surface casing.
The deepest, highest mud weight section of the well doesn’t always need casing. Since the fracture prone rocks above are protected by casing, they aren’t affected by the high mud weight pressure needed for the deepest section.
Sometimes, however, zones of high pore pressure or exceptionally weak rock will be present even in these deep sections. Drillers might decide to run casing from the surface or hang liners in these sections to isolate them from the wellbore. Liners are just casing strings that do not reach the surface.
Once the casing set points have been determined, we’re ready to move on to the next step: casing selection. Since each length of casing is cemented into place within the previous length, it needs to be a smaller diameter. In the end, the size of the surface casing depends upon the size of the smallest pipe – the production tubing.
Production tubing is selected based upon the expected rate of production from a formation. It typically ranges in diameter between one and five inches.
In addition to sizes, casing also comes in different thicknesses and material properties. A general rule of thumb used by drillers is to use casing that can stand up to the worst possible conditions expected in a well.
Since casing is expensive, it pays to get as close to the cutoff as possible.
In some cases, certain thicknesses, materials and cementing techniques may be required by regulations, whether the casing plan calls for them or not.
Images: “Motor Parts” by Michael Black