Workover Equipment

This is the slightly delayed Sunday Technical Post, and follows on the series that is listed on the right side of the page.

When we began this series of technical posts a drilling rig was anything that punched a hole through the ground, to get at the oil or natural gas underneath. Once a hole is drilled, however, there is often other work that needs to be done on the well, but now the infrastructure that helped when we drilled the well starts to get in the way when we need to other things. And so the tool will change to what is known as a workover rig. (Though these could be the old rigs left in place on a platform after the production wells are drilled - just to keep life clear).

See there you are, having sunk your kid's inheritance into this oilwell, and it just isn't producing the way you were promised. Sure it's making oil, but the supply seems to be dropping faster than it should, or perhaps there is too much sand coming out with the oil, or one of a variety of reasons.

And suddenly the partnership is talking about hiring an oilwell service company to bring out a workover unit to come out and fix the problem. You might have heard of one of the two of the small companies that carry out this sort of work, the two more prominent are Halliburton and Schlumberger, although the latter came into the business first as a company that helped log or survey the hole to determine the types of rock that the drill had gone through. (And in true MSM tradition I should admit that I have consulted for both these companies, and that “small” was meant as a joke).

Work-overs can deal with a wide variety of problems, but they come at the situation from a different perspective than the original well drilling. To begin with there is a cased hole that often goes all the way down to the original pay. Further the tools that will be used are not going, in large measure, to be used to drill new segments of holes, but rather to treat the original well, replace parts that have failed, or change the layer of rock that the well is getting the oil from.

Now there is a word of caution here. To work on the well the first thing that you are likely to do is stop it pumping oil. That is known as shutting the well in or killing the well. Then you bring in the work-over rig, do what needed to be done, and it leaves, and you start the well producing again. Here is the caution. Because you stopped the well producing for a while, when it restarts, in most cases, and regardless of whether the action or treatment that the company applied really worked, the well will begin by producing more oil than it did just before it was shut in.

Because they may have paid quite a bit of money for a treatment, it is sometimes amazing to me how well educated folk will see that immediate gain and believe that a treatment that in other circumstances they would find incredible, has created an improvement in production. Techniques, for example, that promise the ability to drill lateral wells out from the main bore at high rates of speed – without discussion of where the cuttings that were so miraculously removed went to may, at some time, be the subject of another post. The well behavior has also to be monitored over a period of time to validate the improvement (`nuff said).

Some of the treatments that need to be carried out are not very complicated. Perhaps when the well was first drilled it was not effectively acidized or perhaps the oil might have precipitated out some of its contents into the drill pipe as it moved from the completion zone up to the surface. Remember that the oil starts out usually more than a mile or two deep in the ground, where the temperatures can be quite hot. Then as it flows up though the pipe the oil can, for example, suddenly enter a section of the pipe that is being cooled by the North Sea that lies all around it and is a great heat sink. Suddenly any dissolved minerals in the oil might reach saturation and then drop below the saturation temperature, and start to crystallize out. (I have seen pipe sections where the hole diameter has been cut by more than half by crystals that grew in from the wall and were more than an inch long.) Paraffin or similar waxes that were in the oil might similarly, for example, have started to clog the pipe , or some of the carbonate and sulphates in the oil might form a precipitate or scale on the pipeline wall as the oil flows upwards.

A casing scraper (National Oilwell Varco)

These deposits can be removed by putting a scraper onto either the drilling rod of the workover unit, or from a wireline (a wire line) that is run from the surface, generally from a winch. A wireline can be either a slickline or single strand cable, or a braided line which has a number of strands and is capable of carrying a higher payload. These lines can be run into the well very quickly (and in smaller cases do not need to have the well killed to be used).

In a slightly more complicated case an electrical control cable or power cable can be added to the wire to power down-hole operations, particularly when packers or plugs are being used. Depending on purpose these might also be fielded using a more conventional drill string, or through coiled tubing, which I am going to talk about in a different post.

Packers are devices that are lowered into the well to isolate the well zone in which the work is to be carried out. For example if one were going to seal off the old production zone and move to another one, one might pack off the old zone, first before pumping cement into the sealed-off segment to fill it with cement.

You can imagine that almost every type of repair must be carried out in this fashion. If something goes wrong down hole, then because of the limits of access it is going to take some imagination to deal with the problem, and the oilwell service companies have now provided that for a number of years.

The more simple jobs, such as sealing off a zone that has stopped being productive, or lowering a new perforating system down the well to stimulate production from the layer of rock, to cleaning the screens at the bottom of the well that keep the rock in place, while allowing the oil to flow into the well, are all somewhat obvious once named, though perhaps not so obviously needed until you see the effect of the problem on well production.

The more common other workover uses, for stimulating production, will be in another post. And once again this has been but a short summary of what can go on, just to describe some of the basic ideas. Comments and questions are welcome.

Completing and Perforating a well

This is another in the technical series in which I talk about the different aspects of getting fossil fuels out of the ground, Earlier posts are listed to the right. They normally run on Sundays, but since I am travelling over the weekend, this has been put up a little early. Since I will be at Conferences for the next two weeks, posts will also be a little more sparse, since I am not sure whether I will always have internet access - but when I can, I'll add more.

It would nice, once the drill hits the oil-bearing rock, to say that you were done. That having connected the feed line from the well through a choke valve (that controls the outflow from the well), we could proceed to tie the outflow into some kind of collection network, and then we could sit back and count the money as it flowed by.

Well not quite. There are a number of different steps that we have yet to go through before we can finish what is commonly called, the completion, of the well. At this point in the process the bottom of the well is still an open hole. And one of the first things that we do is to flush out the drilling fluid, and then clean the walls of the well – firstly be washing the remaining mud from the well down in the production zone. That means that the rock wall is exposed, just as it was drilled. There are several issues that can come about as a result of this. The first is that the rock we have drilled into can be fairly weak. This is one of the peculiarities of geology. To a degree the richer in oil the rock is, the weaker the rock will be. (And that also holds true for oil shale - of which more at a later date). Why is that?

Well let's talk a little about the rock structure, particularly in this post the porosity that it has. (I’ll talk about permeability next time). There are, simplistically, two types of rock, that oil is usually found in and for now, to make a simple generalization, I am going to call them sandstone and carbonate (as I said holding shale until a later time). Sandstone rock is made up of relatively large grains that are glued together at the edges with various different types of natural cement. The grains do not fit that well together (think apples filling up a room, and connected where they touch). We call the gaps between the grains, the pore space of the rock, and it is these gaps that the oil fills up to form the reservoir. And so we can calculate the "free volume", as it were, of the rock as the (relative amount of free space in the rock, you can get this by subtracting the weight of the rock from the weight of the same sized piece cut from solid quartz and it will tell you how much empty space there is in the rock, and thus, how much oil there could be in that volume.

Section of sand with oil in the pores –this is actually an oil sand, so the grains aren’t that well cemented together. (Syncrude)

So say we had a core that weighed 144 lb/cu ft and the weight of solid quartz (flint) is 220 lb/cu ft. Then only 65% of the rock (144/220) is solid rock and the remainder is what is known as pore space. Now these holes can be connected or totally separated, with each pore surrounded by a solid piece of rock. Normally the percentage given is reversed, i.e porosity = proportion of void space to total volume, or in this case 35% of the total volume is not rock. (Another picture showing porosity of a sandstone can be found here. Now in the reservoir rock this space is going to be filled with a fluid, either gas, oil or water. For now let us assume that it is filled with oil.

What I have described so far is known as primary porosity,i.e. that which is created by this initial structure of the rock. With carbonates more than sandstone there is a secondary porosity, and this is the porosity induced by rock movement and the dissolving of channels and holes in the rock by the movement of fluid over the rock through the millennia. Again put simply the oil found in a sandstone will occur between the grains of the rock. In the case of the carbonates, which normally have a much smaller individual particle size, the oil is more often usefully found in the cracks and joints formed were the rock bedding planes were created (and which can be seen in exposed rock in a lot of road cuts along the highway).

The voids and spaces in the rock are also formed from the spaces from what might have been old coral reefs, or where water dissolved holes through the rock. But sometimes the two methods of formation mix, and I would like to quote from Kenneth Deffeyes book "Hubbert's Peak" (my favorite text as an explanation of the geological case).

Fine grained calcium carbonate mud usually gets consolidated into massive limestones, usually with little or no porosity. . . . . . . .About 10 percent of ancient limestones do have porosity. . . . . . .Most massive and nonporous limestones contain textures made by invertebrate animals that ingest sediment and turn out fecal pellets. Usually the pellets get squished into the mud. Rarely do the fecal pellets themselves form a porous sedimentary rock. . . .I twisted Aramco's collective arm for samples from the supergiant Ghawar field. . . . .Examining the reservoir rock of the world's biggest oil field . . .a small part of the reservoir was dolomite, but most of it turned out to be fecal pellet limestone. I had to go home that evening and explain to my family that the reservoir rock in the world's biggest oil field was made of shit.

So there you have it. And the reason for the quote is that the rock at the bottom of our well can be very weak, and may be left in poor shape by the oil drilling bit that just passed it by. Now remember it is this wall around the hole that is the barrier through which all the oil in that rock must pass to get into the well. So before we leave it we have to ensure that it is in as good a condition to allow that flow as possible. (Hence the reason for the removal of the mud and the cleaning of the wall). We also have to isolate the production zone from the rest of the well, and we do this with what is known as a completion or production packer.

Production packer (B.J. Services ) - the three rings swell out and fill the gap (pack it) between the tool and the rock wall of the hole acting as a seal to separate the well below, from the well above – note the internal pipe to allow flow from the underlying part of the well.

One of the problems is that the drill bit may have overly crushed the rock, so that fine carbonate particles are pushed into the cracks and pores of the rock, right around the bore. These can block the passages that will allow the oil to enter the well. And so, in order to get rid of these particles, a strong acid can be poured into the bottom of the well. This acidizing dissolves these fine particles and opens up the cracks leading out into the surrounding rock, so that the oil can flow into the well bore more easily.

Process of isolating and acidizing the formation.

Another problem is that the rock may be very weak, since a lot of its strength comes from the oil that fills the holes within it. This oil only provides strength as long as the rock is totally confined on all sides, but when the pressure is removed on one side (think of popping a champagne cork) then the oil can flow away, taking the support for the surrounding rock with it. If the rock bridges that are left are weak then they can crush. This will cause the crushed rock (sand) to mix with the oil, which will require a de-sanding process at the surface, but it will also close some of the passages through which the oil is flowing to the well. A well operator that speeds the flow of oil out from the rock around the well, can reduce the support that the oil gives to the surrounding rock to the point that it crushes, and permanently reduces oil flow into the well. We can put in a screen that will hold the rock in place, but allow the oil to seep through slots in the screen wall.

Completion screen

Or, to stop that rock crushing from happening and to reinforce the rock , we can pump a layer of concrete into the bottom of the well, cementing a steel liner into the rock, just as we cased the well higher up the well. The steel liner, or production casing, has, however, one problem. Once it is cemented into place, there is this hollow tube all the way to the surface, but there is no way that the oil can get through the cement and the steel into that passage.

And this is where Her Majesty's Explosive comes in. Small, specially designed, explosive charges, known as shaped charges are now put together into specifically designed charge packages, and lowered down into the well into the completion zone.

Arrangement of shaped charges (the yellow cylinders) – when the explosive goes off the cones collapse and small liquid metal jets shoot out of the open end, through the casing, concrete and into the rock, creating a channel. (Core Labs)

Here they are detonated, sending small jets of metal against the wall of the casing and perforating the steel and concrete into the surrounding rock. There is an animation that shows the jet being produced (see also information here) .

Representation of shaped charges firing and penetrating the casing, cement and wall (OSHA

This gives the passage for the well to flow out of the rock and into the well bore. We have finally completed the first stage of our oil production.

Again this is a very simplified explanation of a quite complex process, but it moves us along to consider some of the other issues relating to how the oil now flows to the well, that will be next.

Since this is simplified additional comments and questions are welcome.