After drilling and casing your well, the next phase in well completion is perforation. Just as drilling and casing happen together, best practices in today’s oil field involve an “underbalanced” downhole environment, and thus, have the perforation happening in tandem with the installation of production tubing and packing.
Workover Rigs
This phase of the completion is usually done with a “workover” rig. Workover rigs are smaller rigs that more easily, efficiently and cheaply can deal with downhole operations both on tubing and wireline. This video shows a workover rig, and gives a good idea of the theory and scale of using workover rigs.
Caveat
Before I get too far into this post, I do need to point out that I am not a petroleum engineer – I am simply someone who is very interested in the engineering that goes into wells so I can be more knowledgeable about the industry.
At Drillinginfo we have a lot of data and analyses that focus on operator performance and efficiency – as we should. The days of the 3 million dollar wildcat gusher are gone, and have been replaced with quarter of a million dollar pad development. Rather than counting on exceptional results for better ROI, we know that predictable efficiency and the associated cost savings are where the smart money is to be made in the unconventional world.
So when the next “game-changing efficiency” gets developed, for my job I need to know 1: what game is being changed, and 2: what is functionally different. This post is some of the information I have gathered from my research – if you would like a more detailed look into perforation design, petrowiki.org has a great breakdown.
Well Perforation
You need to perforate the casing and do the appropriate amount of damage to the surrounding formation – too much damage can create excess debris (which can cause problems with downhole operations) and not enough can limit your productivity.
Normally, there are 4 to 8 holes perforated per foot and those perforations are done in a certain spread around the diameter of the hole – most commonly 60, 90 and 120 degrees (resulting in six, four or three different directions).
Image Source: https://petrowiki.org/File%3AVol4_Page_158_Image_0001.png
The process consists of plugging the target section, flooding the casing with the appropriate fluid at the appropriate pressure, and then exploding the charges. “Underbalanced” indicates that the pressure is slightly less than the surrounding formation, which is advantageous in the process as it immediately starts removing debris from the area around the perf. Here is a great animation of a perforation gun in action:
“Perforated Length” and “Entrance Hole Diameter” are two commonly discussed elements of perforating design. To control these elements, the shape and blasting power of the charges are attenuated.
Image: A Variety of Shape Charges from https://www.slb.com/services/completions/perforating.aspx#
Tubing and Packing
As I indicated above, other processes are undertaken during the perforation phase, most notably running your production tubing and then gravel packing.
This animation shows coiled tubing on a special workover rig, and you can see how much more efficient coiled tubing can be in some cases.
After you have run production tubing (through which the hydrocarbons will flow to the surface) into the casing, you must pack the space in between this tubing and the casing.
This video from Baker Hughes shows the packing better than I can explain:
Natural completion in Conventional Drilling
In Conventional Drilling in a porous formation, the completion is now done, and you can rig up your Christmas tree or pump and start flowing – this is what they call a “natural completion.” In unconventional exploration or tight formations, you’re ready to move on to the next stage – well stimulation – which we will cover in our next post.
