Vol. 228 No. 10 LES SKINNER, PE, CONTRIBUTING EDITOR, LSKINNER@SBCGLOBAL.NET
Impression blocks, a.k.a. “confusion” blocks, have been around since people first started running any tool in a well. This simple device is designed to reveal the identity, size and shape of the top of a “fish,” any piece of equipment left in a hole that should be removed. These fish can range from pieces of broken pipe, to tools that have been dropped, to sand or scale fill in the bottom of the hole. The impression block has long been used to identify what the fish is and what needs to be done to remove it from the hole.
Impression blocks are simple devices. A piece of a soft metal, usually lead, about ⅜-in. (10-mm) thick, is mounted on a steel carrier block. The lead or babbitt is generally melted, poured into a mold that includes the block and filed or polished until it is circular with the same OD as the block with a flat face. The impression block is lowered on wireline, braided line or slickline to the top of the fish and allowed to strike the fish. When it is pulled back to the surface, an impression (often called a “picture”) of the top of the fish is left in the lead surface.
That’s how it’s supposed to work.
My experience is that this “picture” must be interpreted to be viable. I have spent hours in discussions with “experts” on reading impression blocks. One small dimple in the lead face can lead to lengthy expositions. Each participant in the discussion is convinced that his mental perception of the fish, based on the tiny mark, is absolutely correct. All of these experts claim to possess downhole vision.
I have generally tried to avoid espousing my opinion of the fish, unless the type of fish is obvious from the mark. For example, if the impression clearly shows the top of a fishing neck, I can pretty well guess that there is a tool in the hole, and it can be fished using common tools. However, what does one do when there is no mark?
In approximately 100 BCE, Publilius Syrus wrote in his Maxims that it is better to be ignorant of a matter than half know it. That’s the way I am about impression block interpretation. There are lots of folks out there who understand what those marks mean and how they can be related to the next step-how to remove the obstruction. Fishermen are paid to know what the impression block means and to select the proper fishing tool to recover the fish. If the other half of the fish has been recovered, such as one might expect if the drillpipe has twisted off, the job becomes much easier. If the top half of the fish is already out of the hole, why would one want to run and impression block anyway?
It’s the unknown fish that becomes the challenge. What was left in the hole that was not recorded in the well file? What is its diameter? How long and how heavy is it?
I have recently discovered that there is a better way to deal with unknown fish identification and removal. It’s called a downhole camera. These are marvelous devices that are run on wireline. The tool has a video camera and a light source. The video camera transmits a signal back up the wire to a recorder and to a screen where the operator can see, in real time, what’s in the hole. All the questions get answered in short order and fishing tool selection is accelerated.
The system is not without its drawbacks, however. First, oil in the well tends to coat the lens over the camera making it seem as though one is inside a chocolate milkshake. Another is that there must be an optically clear fluid in the wellbore. Drilling mud makes for poor picture quality. There are also bottomhole temperature and pressure limits on all of these cameras. So, what is one to do about the unknown fish in the HPHT well? Solution: impression block.
So, perhaps the solution is for those of us involved in such operations to learn how to interpret impressions. How is that done? Ever heard of a school to learn how to read impression blocks? Ever heard of one to select fishing tools once the impression block has been correctly interpreted?
There is another solution. An old West Texas toolpusher once tutored me on fishing. His idea was simple: if you don’t know what it is, run a three-blade junk mill and grind it up. If it’s cement, it can be milled-up. Scale or sand is easy. Logging tools are a bit harder since they want to turn with the mill (unless you cement it in place first). Packers, gas lift valves and previously run fishing tool strings are a bit harder to mill up. Then, there’s the real possibility of milling through the casing or sidetracking the well in the open hole.
In fact, there may not be a good solution, so we do what we can. We run every fishing tool imaginable and try to get a better idea what’s in the hole based on scratches and marks on these tools when they are pulled out of the hole. This practice becomes much like reading an impression block, but it’s much more difficult.
It seems to me that with all the imaging tools we have, someone could come up with a way to look at a fish without using impression blocks or visible light video cameras. What about all the medical imaging systems we have. They can identify and track arteries less than the width of the print on this page. Why can’t we get a downhole X-ray machine or Cat-Scan device that can tell us precisely what’s down there.
Oh well, we’re probably doomed to use impression blocks as long as there is an oil patch. If nothing else, we seem to be consistent in some areas, and this is one of them. The reluctance to run expensive downhole cameras in a well seems to be universal. I can’t imagine someone wanting to run a downhole NMR imager on wireline. Most operators couldn’t afford the tool insurance.
So, we move ahead until that bright inventor comes up with a real solution to this problem. In the meantime, we will muddle ahead, consistently striving to interpret impression block pictures. You see, consistency requires that you to be as ignorant today as you were a year ago, according to Bernard Berenson, an art critic of the early 1900s. Bernard got it right. I’ll bet he had a background in impression block interpretation.
Actually, this isn’t really about impression blocks only. This applies to the totality of our efforts and way we think in the oil patch. We simply must alter our thought paradigms and cast a wide net to capture new ideas, no matter how uncomfortable that may seem to us. Remember the last seven words of any organization, “We’ve never done it that way before.”?
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