The casing is placed in the open hole completion type and cemented above the productive interval(s). Further drilling extends the well bore into and/or through the reservoir(s) left uncased, hence the term ‘Open Hole’; See Figure below.
All open-hole completions avoid the cost and complexity of perforating but have their complications.
This type of completion is standard and ﬁnd application in competent formations such as dolomite & limestones. They have several advantages beyond their obvious low cost.
- Water or gas shut-off is difﬁcult in any completion but is more accessible in a barefoot well than in a pre-drilled casing liner well. (by an open-hole bridge plug backed up by cement).
- They are especially applicable to techniques such as through-tubing drilling (TTRD).
- They have wide use in simple multilateral wells.
- No formation damage from cement
- We can quickly deepen or sidetrack the well without screens or liners.
- We can convert it to liner-type completion.
- Large flow area.
Disadvantages Of Open Hole Completion
Compared to other completion types, the main disadvantages are:
- The inability to deploy upfront zonal isolation techniques such as swellable wells packer or ECP (external casing packers).
- Hole collapse probability
- We can’t control producing excessive water or gas.
- It is challenging to deploy zonal stimulation.
Formation Damage In Open Hole Completion
This type of completion is inherently prone to formation damage caused by drilling. The ﬁltrate should generally be designed to avoid chemical interactions with the reservoir ﬂuid or rock – particularly clays- and should be free of ‘plugging’ particles. In general, we should consider that all solids in any ﬂuid that will contact the production zone should either be of bridge type or don’t cause plugging. This process is shown in Figure 3.
External ﬁlter cake is easier to back produce than internal ﬁlter cake. Removing the ﬁlter cake in an open hole, non-sand control completion should require opening the well to production and ﬂowing the ﬁlter cake through the predrilled liner or other reservoir completion.
Filter cake clean-up with a pre-drilled liner should be easier than for a sand control completion as the ﬁlter cake can be designed to ﬂow back through the completion – no gravel or screens in the way. Drawdowns with consolidated formations are usually higher than the equivalent poorly consolidated reservoirs.