5. LA ESPERANZA COMO ACTITUD VITAL: CUATRO CUARTETOS
5.1. Burnt Norton
Obviously from the above charts we select the hole size for our particular casing and that automatically sets our bit size too. While that is true, there is another aspect to the bit sizes that should be mentioned. Those charts are based on the most commonly available bit sizes. There are special cases where it will be necessary to use an unusually thick wall casing and you find that the common bit used in that casing will not work – it is too large. There are many other diameters of bits available for special applications.
In general they tend to cost more, but the biggest problem is that often there is a limited choice of types when it comes to unusual bit sizes. For instance for one common size we may have a choice of twenty-five different tooth and hardness characteristics just from a single manufacturer, and maybe 50 to 100 choices if we include all manufacturers. However, with some odd size bit we may be limited to six choices and only one manufacturer. That may be acceptable for some special case, but it should always be considered.
Actual Bit Clearance
To determine the bit clearance we look at the casing tables for the internal diameter and see if it is larger than the diameter of the bit. But in the table we see two diameters listed. One is the internal diameter and the other is the internal drift diameter which is slightly smaller than the internal diameter. The internal diameter is the diameter to which the tube is supposedly manufactured. Once it has gone through the milling process it is inspected for final diameter by passing a mandrel through it of the diameter listed as the internal drift diameter. So its internal diameter might be the same as that listed or it might be slightly smaller, but we know for sure (assuming the manufacturer does its job) that it is at least as large as the drift diameter. We normally then assume that the drift diameter is the maximum bit diameter we can be assured will pass through the casing. But in many cases bits greater than the drift diameter have been used. The only thing is that you have to drift the casing with a mandrel the size of the bit first and cull out those joints that are undersized. Some steel mills will actually do this for customers (for extra $$$).
An Unfortunate Case History
To illustrate the consequences of making poor choices when it comes to casing selection here is a case history. A well was planned such that an 8 ½ in. hole was drilled below 9 5/8 in. casing set at 10000 ft with 7 in. casing to be set at 14000 ft. A 6 1/8 in. hole would be drilled below that to 14800 ft and a 4 ½ production liner would be cemented in place.
At about 13000 ft serious lost circulation and borehole stability problems were encountered. Now the operator had no contingency plan for such an occurrence. It appeared that it would be necessary to set the 7 in. casing at 13000 ft and the 4 ½ in.
liner would have to be set at 14000 ft, and now the last 800 ft of hole would have to be drilled with a 3 3/4" in. bit and a 2 7/8 in. liner would be the final string. This was unfortunate, but that was the only good choice the operator had left. But that is not the choice they made.
Thinking that the 2 7/8 in. liner would not give acceptable production rates the operator decided to run 7 5/8 in. casing in the 8 ½ in. hole hoping to finish the hole with a 3½ in.
liner. That size is not recommended for unconsolidated formations, but the operator had done that many times in hard rock areas where it is common. The reasoning was that in unconsolidated formations the hole is probably over gage anyway so there should be even more clearance than in hard formations. (Don’t ever make this foolish mistake!) So they ran 7 5/8 in. casing in the well and it stuck 600 feet off bottom. There was nothing left to do at that point, so it was cemented in place. They drilled out the shoe and tested it to the equivalent mud density that would be required to drill to the next casing point at 14000 ft. They lost circulation immediately.
Two squeeze jobs were performed with no success. Now the situation looked very discouraging. The only choice left was to drill the hole back to 13000 with a 6 3/4 in. bit and set a 5 in. liner. (The operator had earlier thought that they could set a 5 ½ in. liner below the 7 5/8 in. but now they were beginning to believe the charts and tables.) Then they would drill a 3 7/8 in. hole to 14000 ft and set a 2 7/8 liner. Then they would drill a 2 ¼ in. hole to 14800 ft and set a ??? You get the picture now. There were no more options; the well was plugged. Now it may not have been a really bad well plan in the
beginning because the hole problems at 13000 ft were totally unexpected. There was still a way to reach total depth though not with the size liner the operator wanted.
Unfortunately that final liner size became a priority and the operator made a very foolish and uniformed decision. It cost them the well.
Alternative Approaches
There are additional approaches to allow for more clearance for the casing. One method is to under ream the open hole below the current casing string. This allows additional clearance and is a proven method where the expense of the extra time and reaming can be justified. A similar result can be obtained with a bi-centered bit for drilling below the current string of casing. Such a bit will drill a larger diameter hole than its nominal diameter. This technique can eliminate the extra expense of under reaming and accomplish the same result.
Another option is the use of expandable casing. This is a relatively new technology and has proven itself successful in a number of applications. The hole is typically drilled with a bi-center bit or under-reamed to give more clearance. The casing itself is run just like a conventional liner but with an expander device on bottom. Once in place cement is displaced into the annulus then the expander mandrel is forced through the casing from the bottom up and it expands the casing to its final size. The expander also expands a liner hanger and pack-off. The expandable casing is an ERW tube so as to maintain a constant wall thickness.
Figure 3 - 3. Expandable open hole liner.
Next is an illustration of a program showing a proposed well plan utilizing expandable casing to optimize the hole size.
Figure 3 - 4. Alternative casing program using expandable casing strings.
There are some potential problems with the process.
• The pipe itself or one of its connections can split during the expansion process.
• The cement must be placed before the expansion operation commences.
• The expanding tool can get stuck and thus plug the expandable casing.
• Once expanded the casing has a much lower collapse resistance than conventional casing.
• The expandable casing is not readily available on short notice as an emergency alternative in a well already drilling.
Because of the limitations it appears that the most appropriate application is for those occasions in which unforeseen well conditions require a change of the program to run an unanticipated string of casing or liner. However, the lack of availability seems to offset that advantage, at least at the present. Despite these limitations this technology has a lot of promise for the future.