Upstream Oil and Gas Drilling Fundamentals
It is helpful to divide both the drilling fundamentals and production subjects into four basic themes around the key factors that drive operations complexity, including:
- Hydrocarbon type – whether oil or gas,
- Reservoir type – whether conventional or unconventional,
- Location – whether onshore or offshore, in remote or severe climate environments
- Equipment – for drilling this includes the type of drilling rig and subsurface equipment.
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Drilling – Where, What, and How
The actual process of drilling a well is much the same as using a power drill to perform a workshop task.
Before drilling, measurements are made to show where to drill. The material, the size of the hole and how deep to drill will determine the proper type of drill bit.
These characteristics will also determine how big the drill motor needs to be to drive the bit in a rotary motion.
The overall concept of the drilling process is relatively simple. There is a potential reservoir located deep beneath the earth’s surface. In order to gain access and extract oil or gas, a hole must be drilled through the various rocks, materials and layers to reach it.
In oil and gas drilling, the “how” to drill the well, is the subject of this module and, is often, the last element to be considered.
As discussed in the Exploration module, first the team must define Where to drill and have a good understanding of the What, or the actual geology of the subsurface conditions they will be drilling through.
Having defined the above, the drilling engineer can then develop the “how”, including:
- The optimal drilling rig
- A detailed well design and construction
- Needed operational safety and contingency planning
- The important “go” and “no-go” decision points in the operation
Hostile Environments
Wells today are exceeding 6,000 m (20,000 feet) in depth. At these well depths, temperatures on the drill bit can exceed 450 degrees F and pressures can exceed 15,000 psi. The presence of acid gases (H2S, CO2) and chlorides make drilling a technical challenge.
It’s important to know if hostile well conditions exist while drilling so that the proper downhole components can be selected.
Safety measures must also be put in place to protect the drilling rig staff.
Total Vertical Depth (TVD)
Two key dimensions are important in well design:
- The planned end of the well, measured by the length of drill pipe required to reach the pay-zone, is called the measured depth (MD).
- The total vertical depth (TVD) is the depth to the reservoir independent of the deviations and paths used to get to the pay zone.
All rig systems and equipment must be designed and setup to accommodate the measured depth (MD).
First and foremost, the proper rig must be selected. The key factor driving rig selection is its ability to turn the drill string – measured in horsepower.
TVD is important because it gives the drilling engineer an idea of the pressures and temperatures that will affect the drill string and the drill bit.
Related Resources:
What is the difference between Upstream and Downstream?
Drilling Wells for Oil and Gas and Offshore Drilling