Well Completion 101: Unlocking Downhole Treasure - Casing

Well Completion 101: Unlocking Downhole Treasure 

Well Completion 101: Unlocking Downhole Treasure Part 1 Casing

Aug 7, 2014 | Engineering | 0 Comments

Once upon a time the success or lack of success of an oil and gas well had to do with the comparative fortune of finding the right sort of producing field. (Here’s a great article on East Texas wildcatter Columbus Earl Joiner that gives a solid flavor of that experience.)
But once you had struck oil, how would you deliver that oil to market?
Well Completion is the process of making a drilled well ready for production.
In the days of Mr. Joiner, well completion consisted of running a steel pipe down the hole, casing that pipe in cement (this cementing was relatively new technology in the 1920s – at the time more for preventing ground water from contaminating the oil than vice versa), and then hooking the appropriate valves and pump fixtures at the surface for delivery into pipeline or truck delivery to refineries.
Fast forward to today and well completion is the special sauce. As our tight oil and gas plays explore the source rock from the gushers of yore and provide us with more economic and energy stability, the art and science of well completion is what enables this new era of E&P.
Well completion is generally broken down into three phases:

• Casing – where the piping is run and the cement casing is pumped in.
• Perforation – where holes are blasted through the casing at precise locations for stimulation and production flow. Often done in conjunction with tubing, packing, and setting up the Christmas tree.
• Stimulation – Hydraulic Fracturing, Acidizing. Preparing the rock formation for optimal flow.

For Part 1 of this series I will focus on Casing.

The pressure builds

When discussing downhole operations it is very important to understand how very complex the calculations are to ensure a proper balance between your drilling mud weight (and cement and other drilling fluids) and the shifting downhole environment. With increasing depth comes increasing pressure and temperature; different geological formations have different pore pressures; and faulting can shift the formations you are working through unexpectedly. I am not an engineer and this post is just an overview, but if you have a flair for math and physics, Petrowiki gets into the necessary calculations.

Casing the joint

After you’ve drilled your well, done any wireline testing and coring, and you feel there appears to be a sufficient quantity of oil, you are done with the open hole portion of your operation. (Dry holes are plugged and abandoned (P&A) with cement plugs).
Broadly there are three types of casing – surface, intermediate and production casing.
Surface casing runs from the surface down to some distance below the deepest known aquifer. Primarily surface casing is designed to protect the hole from contaminates and protect surface water and other formations from erosion and contamination from the well. The way surface casing is structured also provides stability for the downhole drilling operation and prevents collapse of loose soil near the operation.
Intermediate casing, also known as protection pipe, may then be run at various intervals to balance the effects of pore pressure versus fracture pressures over the open column of increasing mud weights.
Production casing protects the production zone, and isolates the producing intervals.

Nesting, nesting, 1, 2, 3…

One of the key ideas about how drilling and casing are set up is the idea of nesting. Near the surface the hole and casing is comparatively large – sometimes up to 24” in diameter. As the hole progresses downward the size of the hole steps down periodically such that the next section of well will be drilled by a bit that fits within the previous section. Like Russian dolls or a telescope. This animation illustrates the nesting concept well.

Time for your annulus review

As you reach the desired length of your new nested section, you have to case that section before moving on. There are different proprietary methods for flowing the cement in (Erle Halliburton made his original fortune from a technique for cementing that he patented in 1920), but in most cases the stages involved are:

1. Run continuous steel (or other material appropriate to the downhole environmental conditions) piping the length of the new section, with a float valve attached to the bottom of the section. (This will likely be anchored to the previous section)
2. Start pumping cement in the piping, down through the valve, and up through the annulus until the appropriate volume of cement has been pumped in.
3. Place a wiper plug into the piping and continue to pump drilling mud behind the wiper until the wiper plug has reached the original float valve.
4. Allow the cement to set up completely (in most cases this takes about 90 minutes).
5. Send your next stage drill bit through the piping and drill out through the float valve and concrete plug and on into the next section.

Materials and operations

Because of the segmented nature of the casing operation, it occurs mostly in sequence with the drilling operation, and hence is largely done with the drilling rig in place. (Final production casing may be done with a workover rig.)
As previously indicated the piping is usually steel, although in highly caustic situations different alloys such as nickel may be substituted. From an economic standpoint, steel is preferred. For the vertical surface and intermediate sections these would be fixed pieces of pipe joined together. As the situation downhole becomes more directional, operators may use rolls of “coiled tubing” (up to 3.5”!) that are designed to make the directional changes required.
The cement slurry is mixed at the surface and again is doctored with additives to adjust density and curing time and weight based on the downhole environmental conditions. Portland cement is the most common base material and other components like sand and iron oxide may be added to that as well.