Formation considerations
Homogenous
Heterogeneous
Naturally Fractured
Homogenous-found in heavy oil sands. Easy to accommodate in completion and don’t require wellbore segregation. Can be treated with the entire wellbore exposed using coiled tubing or perforated technology.
Heterogeneous-reservoir quality and pressure vary between sections of the wellbore. These segments of the wellbore need to be isolated to accommodate selective stimulation.
Naturally Fractured-if proper wellbore orientation is picked and natural fractures are in the right density then a perforated liner or open wellbore is all that is needed. If the fracture intensity is low or the wellbore misaligned the completion is more involved. The main focus of a horizontal completion in a naturally fractured reservoir is to leave the fractures in an undamaged state. If the fractures are not present in sufficient quantities to make the well economically viable then near wellbore stimulation (acid) will not work. Hydraulic stimulation will be a better choice. This requires a liner and cementing, this would facilitate segregation of sequentially induced fractures.
The initial completions should allow for the horizontal interval to be evaluated in an open hole state. Cementing and stimulation can be done later. Centralizing of the casing is essential.
Casing – 2 major alternatives open hole or cased and cemented.
Open hole completions – 1 True open hole
2 Slotted or perforated liner/casing
3 Segmented un-cemented liner/casing
Un-cemented completion – used when stimulation is not needed. Difficult to control the placement of any sort of completion fluids with any degree of accuracy.
True Open hole – is used in medium to high permeability, competent formations, with no water or gas concerns. Can be used in competent formations as an intermediate step in evaluating reservoir quality. Depending on evaluation stimulation will be decided upon.
Slotted or perforated liners can be run to protect the borehole from collapse. Slots can be used in unconsolidated formations and pre-drilled holes are used in consolidated formations. Pre-drilled holes are cheaper than slots.
The un-cemented slotted or pre-perforated approach is best if no stimulation is anticipated. Really there is no way to control stimulation fluids in an un-cemented liner.
Segmented un-cemented liners are the most sophisticated of the “pseudo” open hole completions. It allows open hole evaluation of the wellbore and subsequent selective stimulation. Can also isolate the segments of the wellbore. You can use external casing packers (ECP) to selectively isolate zones.
Cased and Cemented Completions – A premium completions approach. There is a high degree of cost associated with this choice. To go with this would mean that there is no other option. The problems with the formation can’t be addressed in any other way. The cased and cemented wellbore offers the best isolation to allow for sequential transverse fracturing. Only recommended in extenuating circumstances.
