OIL WELL DRILLING
An oil well is a boring in the Earth that is designed to bring petroleum oil hydrocarbons to the surface. Usually some natural gas is released along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well. Before a well is drilled, a geologic target is identified by a geologist or geophysicist to meet the objectives of the well.
An oil well is a boring in the Earth that is designed to bring petroleum oil hydrocarbons to the surface. Usually some natural gas is released along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well. Before a well is drilled, a geologic target is identified by a geologist or geophysicist to meet the objectives of the well.
- For a production well, the target is picked to optimize production from the well and manage reservoir drainage.
- For an exploration or appraisal well, the target is chosen to confirm the existence of a viable hydrocarbon reservoir or to ascertain its extent.
- For an injection well, the target is selected to locate the point of injection in a permeable zone, which may support disposing of water or gas and /or pushing hydrocarbons into nearby production wells.
The target (the end point of the well) will be matched with a surface location (the starting point of the well), and a trajectory between the two will be designed. When the well path is identified, a team of geoscientists and engineers will develop a set of presumed properties of the subsurface that will be drilled through to reach the target. These properties include pore pressure, fracture gradient, wellbore stability, porosity, permeability, lithology, faults, and clay content.
This set of assumptions is used by a well engineering team to perform the casing design and completion design for the well, and then detailed planning, where, for example, the drill bits are selected, a BHA is designed, the drilling fluid is selected, and step-by-step procedures are written to provide instruction for executing the well in a safe and cost-efficient manner.
DRILLING
The well is created by drilling a hole 12 cm to 1 meter (5 in to 40 in) in diameter into the earth with a drilling rig that rotates a drill string with a bit attached. After the hole is drilled, sections of steel pipe (casing), slightly smaller in diameter than the borehole, are placed in the hole.
Cement may be placed between the outside of the casing and the borehole known as the annulus. The casing provides structural integrity to the newly drilled wellbore, in addition to isolating potentially dangerous high pressure zones from each other and from the surface.
With these zones safely isolated and the formation protected by the casing, the well can be drilled deeper (into potentially more-unstable and violent formations) with a smaller bit, and also cased with a smaller size casing. Modern wells often have two to five sets of subsequently smaller hole sizes drilled inside one another, each cemented with casing.
