After drilling operations have been concluded, it becomes necessary for the wellbore to be Completed – the process of preparing the well for the production phase. The entire well completion process includes everything from casing the wellbore to cementing, plug and perf operations, gravel packing, and installation of a Production tree (or ‘Christmas Tree’) at the wellhead.
Broadly speaking, there are two main well completion methods typically utilized: open hole and cased hole. Each of these methods has its peculiarities, applications, and benefits over the life cycle of the well. The system employed ultimately depends on the unique characteristics of the formation, technical feasibility, and cost considerations.
Open Hole Completions
In open hole completions, drilling mud of a specific weight, viscosity, density, and inhibition is used to match downhole conditions to prevent the wellbore from caving in on itself (well control) by utilizing hydrostatic pressure difference.
After the drill string is removed, a production casing is inserted into the freshly drilled hole but does not run through the reservoir.
How is an open hole completion performed?
In open hole completions, a production casing is run through the wellbore until it sits directly on top of the reservoir, but the freshly drilled hole is left ‘uncapped’ at the bottom.
Advantages of open hole Completions
The following are the main advantages of open hole completions:
- Saves the cost of cementing operations which is an expensive process
- Saves cost on perforation operations which is expensive, requires more manpower and is technically challenging, particularly in deviated wells.
Drawbacks of open hole completions
The following are the main drawbacks of open hole completions:
- It is difficult to control the production of associated sand, water, gas, and other formation fluids since the wellbore is not completely shielded from the surrounding formation.
- Special fluids are required to prevent the well from collapsing due to a lack of support for the wellbore.
- Remedial work on the well is more difficult after production has commenced.
