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.
Cased Hole Completions
In cased hole completions, production casing is run along the entire length of the well and through the reservoir. The cased hole effectively acts as a control mechanism for safe production of desired hydrocarbons and as a barrier preventing the reintroduction of unwanted fluids, gases, and solids into the wellbore.
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How is a cased hole completion performed?
After the drill string has been removed, a continuous coupling of casings of different diameters is run into the well at varying depths and secured to the formation in a process known as Cementing. ‘Cement’ here refers to a mixture of cement and certain additives which is pumped into the well and fills the vacuum between the casing and surrounding formation.
After the wellbore has been completely insulated from the surrounding formation, the casing must be perforated to stimulate production from viable sections of the reservoir called pay zones. Perforation is performed using Perforating guns which set off controlled explosions that blast holes through specific sections of the casing (and into the reservoir) for controlled production of hydrocarbons.
Advantages of cased hole completions
The following are the main advantages of cased hole:
- The casing effectively screens out contaminants by controlling the production of associated sand, water, gas, and other formation fluids
- Enhances well integrity
- Offers better zonal isolation of the reservoir
- Makes it easier for multiple zones to be completed within the same well
- Work over (remedial) processes is easier to carry out in a cased hole
Drawbacks of cased hole completions
The following are the main disadvantages of cased hole:
- More expensive than an open hole completion due to costs of procuring casings, perforation guns, cement, and other completions equipment
- Limited exposure to the reservoir could affect the volume of production
Open Hole Completion vs. Cased Hole Completion – Cost Implications
The most significant distinction between cased and open hole completions is the amount of equipment and manpower required in each procedure.
In open hole completions, casing is only required to run through the well to the top of the reservoir, implying cost savings for procuring additional equipment and workforce. Special fluids are utilized to prevent the well from collapsing, but this is a less costly procedure.
In cased hole completions, production casing is run through the entire length of the well, up to the various zones where hydrocarbons can be tapped. This implies significant costs – to procure casings, cement, manpower, and equipment (such as Perforation guns) to stimulate production.
Both open hole and cased hole completion methods can be technically feasible and economically viable, depending on the unique characteristics of the formation and prevailing cost considerations.
Open hole completions are often favored in horizontal wells, where running a production casing along the entire length of the well might be too expensive, or technically unfeasible. Cased hole completions, on the other hand, might be the better option for vertical wells when there is low formation integrity.
In the same vein, the difference between open hole and cased hole completion lies in the absence or presence of a production casing while critical data about the formation is being obtained.
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