Sunday, July 20, 2014

Tech Talk - and things continue to get worse

It is difficult to see any positive interpretation of the changes and conflicts that are increasingly filling the headlines of the press. Fluctuating optimism over the return to credible export production from Libya, to take but one example, is no sooner reported when the news comes of increased fighting in Tripoli, including the international airport. At the same time violence is spreading towards Egypt. Without a strong central government it is likely that the conflicts in that country will continue into the foreseeable future, with continued negative impacts on the export of oil from the country.

Transient attempts to maintain a cease-fire and stabilize South Sudan have apparently failed again. The fighting has shut down local oil production, while overall production from South Sudan has been cut to 165 kbd.

Capital continues to leave Russia (h/t Nick) and that flight is only likely to accelerate as the tensions over the shooting down of the Malaysia Airlines plane continue to grow. Given that investment continues to be required to sustain Russian oil production against the current transition into decline, and that such cash is not being spent only magnifies the concern that Russian export decline will be faster and sooner than the world anticipates. (And given the critical value of Russian oil and gas exports to their economy – it provides about half the budget revenue - President Putin desperately needs a scapegoat to blame as the economic gains of the past, and future growth targets of over 5% become unrealistic dreams for that future).

With the emphasis on the daily events in all these countries (not forgetting Iraq) it is more difficult to discern the overall medium term impact that this is likely to have on oil availability, and consequently on oil and gas prices. Europe cannot function at current economic levels without the 30% of its energy that it gets from Russian natural gas, which has to be a big consideration as they discuss whether to impose more sanctions on Russia. While a recent Total study shows that, with Gazprom co-operation, Europe could cope if flows through Ukraine were stopped, without that co-operation the EU would not be able to adequately replace the lost fuel. And the conflict in Ukraine is unlikely to be resolved fairly soon, so the degree of co-operation that Western Europe can expect from Gazprom next winter is likely to lead to some fairly tense negotiations over the next few months.

One of the frustrations with watching TV pundits muse on this is that there seems to be an assumption that wells, pipelines and other necessary infrastructure will magically appear to provide immediate solutions should things start to get worse. One such today commented that President Putin is now in total control, since should the west decide not to take all of the Russian oil and natural gas that they currently consume, that he could immediately increase sales to China to replace the lost income.

That neglects the time that it is going to take to get the wells drilled in Siberia, the pipeline connections made and the receiving network in place to meet the current amount that has been sold. Even with the current agreement to increase Russian exports to China it is going to take some four years for the new gas to flow, and it took years for this agreement to be signed.

By the same token Europe can’t turn around and expect the US to be able to replace any significant amount of Russian natural gas for about a similar period of time. Facilities cannot be created overnight, and permitting and construction take finite amounts of time.

I would expect that, if anything, the price that is charged for Russian oil and gas is going to go up for the Europeans, even as the oil supply starts to decline. As Euan Mearns has noted all the significant producers of natural gas in Western Europe are seeing declines in production and while the fall last year was not that significant, overall the continued cumulative decline will make the need for Russian gas that more critical, given that the pipelines are in place to deliver it.

Unfortunately as oil and natural gas supplies continue to tighten, the natural consequence is going to be an increase in price. And this will, in turn, affect the economic growth of the different countries around the world. The current price has slowed economic growth, but as it continues to ratchet up then the impact on global growth will become rapidly obvious, although differentiated by country depending on how dependent they are on fuel imports.

Complacency within the United States, given the assumptions of indigenous supply availabilities, is likely to be shaken as internal oil supplies stop there unsustainable growth rates, while the current low prices for natural gas will disappear as the available funds for future wells reduce on the increasing evidence that most of these wells are unprofitable at current gas prices.

It is difficult – well, to be honest, impossible - for most of us to be able to see how almost any of the growing conflicts around the world can be resolved in any short-term period. The consequent impact on oil production in the countries of the Middle East and North Africa (MENA) is going to lead to a tightening of the surplus between available supply and demand, particularly at current levels. And, unfortunately, when economic circumstances grow colder political rhetoric gets hotter, and there is less chance for negotiation and diplomacy to resolve the situation.

The main surprise, at the moment, is how rapidly the situation is deteriorating in so many of the countries that supply oil and gas to the world. Sadly the headlines will only cover one or two of these at a time. As a result the overall trends are missed as headlines instead focus on the very small changes driven more by sentiment and political perspective than by the realities of the medium, and even short-term oil and gas supply situation.

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Saturday, July 19, 2014

Waterjetting 23c - Abrasive Slurry cutting range

In an early section of these notes on high-pressure water and its uses, there was a review of some of the ways in which jet power could be assessed. For the most part the best way to see how changes in a system alter the way the jets cut is to run a simple cutting test and in many cases it will give the answer that is needed. However, sometimes it is important to go beyond the simple assessment of whether condition (a) is better than condition (b) and to try and explain why it is.

One of the early discoveries in trying to explain abrasive jet behavior was made by my colleagues Marian Mazurkiewicz and Greg Galecki who showed that, when creating an abrasive waterjet system in the conventional way, that a large amount of the abrasive was being fragmented in the mixing chamber of the nozzle.

Figure 1. Size distribution of garnet particles after being fed through the mixing chamber of an abrasive waterjet nozzle, AFR 0.6 lb/min, at 30,000 psi.

Before entering the chamber the particles had been screened to be very close to an average 200 micron size. After going through the chamber the average particle size (50%) was 140 microns, with roughly 25% of the particles being smaller than 100 microns, at which point we had found that the cutting performance gets significantly poorer.

The way in which we found the particle size, (and also assessed how fast the particles were moving after they left the nozzle) was to set the nozzle horizontally, and then to fire the jet down the center of a large plastic tube.

Figure 2. Plastic tube set up to capture the abrasive particles from an abrasive waterjet nozzle. (The nozzle is on the left of the tube).

Barriers were placed at 1-ft intervals along the tube, so that as the particles lost energy and fell to the bottom of the pipe they could be collected into the small dark blue containers under the tube, and then dried, sized and weighed. During a test the top half of the pipe sections are replaced, so that the jet is contained over the pipe length, which was just over 20-ft, and this was long enough to capture, within the length, all the abrasives from all the tests of abrasive waterjet nozzles that we carried out (which included all those commercially available at the time).

It was based in part on this test that we were able to understand why some abrasive waterjet nozzle designs work better than others, and also to begin to understand more of the mechanisms that drive the cutting process.

As the abrasive slurry system (ASJ) started to enter the American market we were thus ready to test the way in which it worked and to see if we could find any improvement, as had been reported by those who first used the system.

When we set the system up so that the ASJ system fed a nozzle in the same arrangement as with the AWJ system we got a surprise. Much of the abrasive was collecting at the far end of the pipe, and it was starting to poke a hole through the end piece.

Over time we extended the tube, and eventually moved it outside to ensure that we could capture all the abrasive in the same way as earlier.

Figure 3. The test set-up needed to capture all the abrasive when using an abrasive slurry jet rather than an abrasive waterjet. (The pipe is roughly 50-ft long).

Even at lower pressures the ASJ was carrying the abrasive much further than was the case with the conventional AWJ system, showing that the particles were retaining more energy over greater distances. In retrospect this is not surprising, since there is sensibly no break-up of the abrasive particles with the ASJ system during the mixing and acceleration of the particles.

This is because the particles are mixed in with the water before the water accelerates, and when it does the abrasive is already mixed throughout the jet, rather than trying to force its way into the jet, while at relatively slow velocity relative to the water. There is, as a result, no break-up of the particles, and larger particles will lose energy more slowly than smaller ones. (One of the findings of the AWJ tests carried out earlier).

In addition because there is no air in the ASJ jet stream there is no active component trying to disrupt the jet, and as a result the water remains coherent to a greater distance from the nozzle, and has, as a result, a much greater capability of transferring the energy to the particles to accelerate them to their full potential, given the design of the system.

There are various different ways in which this benefit can be illustrated, using lab data, but the clearest demonstration is to take a conventional waterjet system, and to run the triangle test and then, with the same amount of abrasive in the jet stream, and a roughly equivalent amount of water, to run the same test with an abrasive slurry system.

Figure 4. Comparison of the cuts achieved with an abrasive slurry system (upper) and an abrasive waterjet system (lower).

The picture shows that the two systems are cutting to sensibly the same depth, and the ASJ system is being operated at a quarter of the pressure of the equivalent AWJ system.

There have been many comparisons between the two systems in the time since this initial evaluation was made, and there is a rough consistency in the results that have been obtained, on the order of that shown in figure 4. The comparisons are not all equivalent to this, since the tests compare different attributes of the two systems, and, for example, there are additional advantages of the lower pressure ASJ system that can enhance the relative performance. One way, when one compares equivalent horsepower, is to increase the diameter of the ASJ nozzle. This allows use of a larger abrasive particle that, in turn, will give a further increase in achievable depth of cut.

Unfortunately the difficulties in achieving consistent performance with some ASJ systems, over long operating periods, has made it more difficult for this relatively new system to penetrate fully into the market place as yet.

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Sunday, July 13, 2014

Tech Talk - Here we go again, again

A couple of posts or so ago I mentioned that there are three major problems sitting relatively un-noticed as we head into the mess of Peak Oil. Of these, perhaps the one that gets the least attention is the steady decline in production from existing wells. We are just about at the point where the Alaskan Pipeline will tip over into feeding less than half-a-million barrels a day down from the North Slope. (It sent 501 kbd down the pipe in June with a 98.6% reliability factor). At the same time those in control of the oilfields in the Russia are reporting that Russian exports have fallen to the lowest level in 6 years. This brings back the relatively unrecognized reality of the Export Land Model which Jeffrey Brown first introduced on The Oil Drum back in 2007.

It is worth resurrecting that thinking (which time has proven to be only too true) as we look at the continued declines in production from the UK, as an example. It is not easily discernable from the official Department of Energy and Climate Change, which plots oil production on a monthly basis (with different months having a variety of days):

Figure 1. Monthly production of oil from the fields of the UK continental shelf (DECC ).

Euan Mearns has, however, done the necessary arithmetic and clearly shows the reality of the situation once one converts it back to barrels per day:

Figure 2. UK production of oil and natural gas over the past decade (Euan Mearns)

The steady decline has also been noted by the EIA who commented that UK production fell by 9% from 2012 to 2013. There was a time, back in the days of The Oil Drum, where we debated whether an estimate of 5% for field decline rates was or was not too high. Obviously those days are now behind us, and reality is starting to show numbers that far exceed the rates that, at the time, some thought rather pessimistic. To continue the UK numbers, as OPEC recently anticipated, the decline this year will take the total down to 800 kbd with an 8% decline expected for this year.

The Export Land Model, in its simplest form, can be illustrated with the following plot:

Figure 3. A simplified illustration of the changing production, internal use and exports for an oil producing country, once it reaches a peak in production (Sam Foucher )

The argument that produces the above plot goes along the lines that, as an oil producer (think for a moment of Russia for eg) produces larger volumes of oil, so the economy of that country starts to grow. As that growth continues it demands an increasing amount of energy to sustain the increased internal demand (the green line). However, once production stabilizes or starts to decline (the blue line above) so the amount available for export becomes reduced (the red line).

The three top producers of petroleum products in the world are the United States, Russia and the Kingdom of Saudi Arabia. The United States consumes far more than it produces, and thus is already a net importer of petroleum products, although in the short term, as I noted earlier production gains from the Bakken and Eagle Ford are hiding the problems of decline rate. It is increasingly unlikely that any significant volume of US oil will make it onto the world market.

Saudi Arabia has, for years, controlled the amount of oil that it puts on the market, based on the anticipated global demand, and the supply available from the rest of the world - so that the global price remains at a level to sustain OPEC economies. That has been illustrated over the past couple of years by the increase in production from the KSA to cover the decline from Libya as about a million barrels a day disappeared from the global market. The gains in production from the US helped in meeting global demand and the strain in supply was thus relatively easily hidden. But the KSA has an imminent problem that has largely disappeared from public view now that the eyes of The Oil Drum correspondents have lost that focus.

The major oilfields of the Kingdom are old, and to sustain production perimeter wells were located around the oilfields that injected millions of barrels of seawater a day, to drive the oil towards the center of the fields, where it could be relatively easily recovered from Maximum Reservoir Contact wells drilled along the very top of the reservoirs. But as folk such as JoulesBurn have noted, those wells slowly change in nature, over time, as the oil migration continues, and water injection must move inwards to ensure continued production.

Figure 4. Layout of initial wells at the Haradh III development in the Ghawar oilfield in Saudi Arabia (JoulesBurn at The Oil Drum)

He noted, in the original post, that Aramco had to drill some 52 wells, rather than the estimated 32, to get the production they needed, and that was back in 2010. Since then Ghawar has continued to produce for the Kingdom, but with daily levels of up around 10 mbd, the volumes in the crests of the anticlines along which the oil wells sit within the Ghawar field have been steadily contracting, and although they have carried out some of the most advanced oilwell engineering to sustain production from the attic oil in the older parts of the fields, there are only so many ways you can squeeze a rock before you get out all the oil that you will – and those days are approaching fast.

At the same time (relating back to the ELM) while Saudi production has remained at just under 10 mbd for the past few years, internal demand has been rising at a steadily more rapid rate.

Figure 5. Internal consumption of oil in Saudi Arabia (Index Mundi ).

Hoping to transition some of the current internal demands to natural gas, the KSA has been looking for internal resources to allow it to move away from oil. However the search has not been as successful as hoped, particularly with the search for natural gas, Shell having backed out of the program as a result of the poor results to date.

With internal consumption continuing to rise at more than twice the rate anticipated by the ELM shown in Figure 3, and, at best, stable production, global exports from the Kingdom are of increasing concern.

Which brings us back to Russia, where the new fields that must be exploited to sustain production are in remote parts of Eastern Siberia and the Yamal Peninsula – if not offshore in the Arctic.

Russian oil production has been peaking for some time (falling from 3.4% growth in 2012 to 1.3% in 2013) and is now reported to likely fall by 6.3% over the next two years. Since this implies that Russia is now at peak, the decline in overall production initially will fall below that of Figure 3, though likely only for a year or so, before the rate will be, at minimum, that shown. (The reason for this conclusion comes from the lack of enough investment in the fields where growth can be expected). At the same time internal demand is rising at around 100 kbd or 3% pa slightly above the value assumed for Figure 3.

If none of the three largest producers can even sustain exports, and the ELM explains why they can’t, and world demand continues to rise at the rates projected, then, in even the short-term, something is going to have to give. The logical weakest link is price, with the consequence, that invalidates a lot of the other arguments, of a significant impact on global economic health. As we have seen before, significant increases in price lowers the demand for oil, and thus demand from the various nations will become even more skewed.

The only problem, with this next iteration, is that there isn’t another Bakken or Eagle Ford conveniently sitting waiting to be tapped.

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Saturday, July 12, 2014

Waterjetting 23b - From DIAjet to Abrasive Slurry Jetting (ASJ)

When the Direct Injection of Abrasive jet (DIAjet) was first introduced to the general public, back in 1986, there was some initial skepticism as to the overall market potential for the system. Certainly, as the next post will discuss, the ability to transfer higher levels of energy from the pressurized water to the entrained abrasive with no particle fragmentation in the mixing chamber, had a number of advantages, that I will spell out below, but the long-term problem was to develop a method of sustaining a constant abrasive feed in the mix, critical to high precision cutting, while concurrently having a system that can run continuously both day and night. To my own knowledge there have been at least two different designs of ASJ system that have solved this problem, but the market was damaged by the early problems in sustaining continuous flow to the point that customers shied away from this advanced technology, even where it showed some considerable commercial advantage.

The first, most obvious advantage to this new tool was in the smaller cut width that it generated, relative to conventional Abrasive Waterjet Cutting (AWJ). And because DIAjet became known as the original BHRA technology, and there were competitors over the years, let me re-name the technology (as the WJTA did some years ago) as Abrasive Slurry Jetting (ASJ).

Figure 1. Cut size difference between ASJ and AWJ as an illustration. The cuts are in Plexiglas, with the ASJ cut on the left.

The reason for the difference in cut width is, as explained in the last post, that the volume of the cutting jet is cut by 90% when the air carrier for the abrasive is no longer necessary or present in the jet stream. This increase in jet “delicacy” can be illustrated by a small example, and a humorous competition between Don Miller and ourselves back some years ago.

Don was, as the technology evolved, one of the master players in moving the technology toward the micro-cutting market that, to this day, remains remarkably under-exploited.

Because the abrasive particles accelerate to a large extent with the water that is both the cutting and carrier fluid, the cutting ability of the jet is significantly less sensitive to the diameter of the nozzle than is the case with the conventional AWJ. And, because the waterjet is not disrupted within the mixing chamber as a way of helping mix the abrasive with the water, so the jet stream can be kept convergent away from the nozzle, increasing the range, as I will discuss further in a later section.

The high precision cutting, using a finer abrasive since the nozzle diameter is smaller, can create very delicate pieces. We had been asked to use our system to cut jewelry out of silver, since while the ASJ could cut this easily and quickly to the desired shapes (matching necklace and earings) trials with laser cutting had been less successful because of the high conductivity of the metal.

This led on to a demonstration of the precision of the cut that can be achieved. One of the early models cut with an AWJ had been of a dragon, it seemed to be a good idea to match this with a knight on a trusty steed.

Figure 2. Knight vs dragon, in this case the knight was cut with an ASJ, while the dragon was cut with an AWJ system.

Don Miller had put together his precision system in his garage, and was able to control the cutting ability with an on-off switch located upstream of the nozzle, using sliding diamond coated plates to ensure a seal, without the wear problems which are common when trying to valve an abrasive laden flow.

Figure 3. Don Miller’s cutting equipment with precision table. Don Miller).

The need for the rapid on-off design comes where a series of holes must be punched into the target metal in order to effectively create a screen, or similar device. Pratt and Whitney engineers had used a somewhat different concept, and had held the abrasive in a polymer. This is sometimes necessary when using the ASJ system, when the cut-off in flow to the nozzle occurs with abrasive in the feed line from the reservoir to the nozzle itself. If there is any significant delay before flow restarts then the abrasive will settle to the bottom of the containing pipe. When flow then restarts this initial plug of abrasive is picked up by water flow and can block the nozzle when it reaches it. The use of the polymer holds the abrasive in suspension to prevent this happening. (We have seen abrasive held in suspension for over a week, using relatively low concentrations of polymers such as those used to suspend fracking sand for the oil industry).

Figure 4. The diamond-coated valve used by Don Miller to control flow in an ASJ system. (Don Miller)

The risk of abrasive build-up is increased when the shut-off valve is directly behind the nozzle, or where it is mounted vertically, when a polymer is not used, but where the jet is cycling rapidly to drill a precise series of holes in a rapid sequence, then the issue of nozzle blockage doesn’t arise as much.

Figure 5. A grid of 85-micron diameter holes drilled at 2.5 holes/sec at a jet pressure of 10,000 psi (Don Miller).

When we discussed the relative scaling that could be achieved with the technique, Don’s answer to the thickness of the lance that we had given the knight was to put scales on his dragon.

Figure 6. The scales on Don Miller’s dragon. The picture width is around 1 mm.

I had promised to go back and put eyelashes on the horse, but somehow we never managed to find the time.

The delicacy and accuracy of the technique is in marked contrast to manufacturing techniques other than those using abrasive-laden water as a cutting medium. Not only is it possible to cut through metals and other materials without distortion, even with very narrow webs left holding the pieces together, but since there is no heat involved in the cutting process, the precision is retained over the cut and part, after completion. This is illustrated with Don’s construction of a butterfly wing through 150 micron thick stainless steel. (The scale on the illustration shows mm).

Figure 7. Detail of a butterfly wing cut by Don Miller, using his ASJ system (Don Miller)

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Tuesday, July 8, 2014

Myrdalsjokull - is Katla getting ready to rumble again?

This is likely, again, not likely to amount to much, but as Jón Frímann has noted, the Katla volcano in Iceland is getting a bit antsy again. There were a couple of deep quakes nearly two weeks ago and there have been shallower and more numerous quakes since.

These might indicate that magma is making its way upwards. We saw this a few months ago, and it did not amount to anything in the end, but it is a little early to tell yet whether this pattern will be the same, or more serious.

Earthquakes in the Myrdalsjokull caldera July 7, 2014 (Icelandic Met Office)

For those who don't know, Katla has often erupted after Eyjafjallajokull, which noticeably erupted back in 2010, shutting down air traffic at the time. And Katla is often the more violent of the two - hence the current interest.

I will update this if it develops into something that may turn out to be more serious.

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Sunday, July 6, 2014

Tech Talk - of longer wells and drawdown pressure

There are, simply, three major parts to the coming global economic mess that will be created as we enter into the period of Peak Oil. The first of these comes from the current rising demand for oil, particularly emphasized by those countries, such as China and India, where demand is rising fastest. The second part is the declining production from existing fields as their reserves are drawn down. (Though it should be remembered that even when “exhausted” the fields will still contain vast quantities of oil, but oil which is at present not economically recoverable). And finally there is the oil in the undeveloped, and undiscovered wells and fields that can be added to the existing reserve to help ameliorate the imbalance between demand and supply from existing wells.

The high decline rates from long horizontal wells drilled into, and along the shale deposits in the United States, most particularly the Bakken and the Eagle Ford, mean that there is a constant need to drill new wells to sustain existing production. The EIA has taken note of this and calculated based on some assumptions, the number of rigs that must be operating in these fields, so that they will drill enough new wells to sustain current production.

Figure 1. The number of rigs required in the Bakken and Eagle Ford formations to sustain production at the level of the previous month (EIA).

Should the need be to increase production (which is the current assumption by most prognosticators of future equilibrium between demand and supply) then these numbers need to be significantly higher, perhaps by as many as 50 additional rigs. At present the Bakken rig count is running at around 176 rigs while there are around 270 rigs drilling in the Eagle Ford.

One of the ways in which production is anticipated to expand above earlier estimates for the wells drilled in both fields comes from the ability to drill longer horizontal wells and to increase the fracture density along these wells.

However, as the Kingdom of Saudi Arabia discovered some years ago, longer wells can only be viably effective out to a certain distance, beyond which there is no gain in productivity. As a result they have changed their drilling patterns so that the wells are shorter, with multiple laterals spreading from the original wells to more thoroughly cover the rock within the formation. Initially wells were drilled out to distances of up to 12 km, but over time the KSA found that this was too long.

Since there is a somewhat similar argument to be made for the wells in the United States, as they move to longer distances, I thought I would go over the explanation as to why this is not a very productive idea a second time.

To begin consider that regardless of whether I put a tiny glass of water or a huge glass of soda in front of you, if I glue it to the table then the amount that you can drink at one time becomes limited by the size of the straw that I give you to drink the liquid, rather than the amount in the container. And to get that liquid into the straw and up into your mouth requires that you suck on the straw.

What you are doing is reducing the pressure at the bottom of the straw, while the pressure from the atmosphere on the top of the liquid remains the same. By creating this differential pressure there is now a force to move the liquid into the straw and thence up into your mouth.

But, as Fishbuch et al showed, as the horizontal well bore gets longer the pressure at the back of the hole declines as then does the difference in pressure between the oil in the rock and the well (the drawdown pressure), and while the longer hole gives an overall increase in production to a certain point this seems to maximize at a length of around 6,000 ft. Beyond that distance the differential pressure between the formation and the well falls to a point where there is less benefit to the additional cost of drilling to that distance.

Figure 2. Drop in well pressure with increased well length, while increasing overall oil flow (Simulation by Fishbuch et al )

The answer which Aramco came up with to get around this problem was to use a main lateral from which a number of shorter laterals could then be drilled out into the formation, providing higher drawdown pressures within the wells and making it also easier to isolate any well section where the underlying water broke through into the well.

Figure 3. Schematic of a Maximum Reservoir Contact well as used in Saudi Arabia (Aramco).

The optimum length at which a well can produce is a function of the rock type and structure as well as the nature of the oil/natural gas that it contains and the water content (to name by a few of the parameters). Thus there are limits to the analogy, nevertheless it does show, even in the much more productive rocks of the fields in KSA that there are limits to how far a well can be productively driven, and these limits will also exist in the shales of the United States, although the oil locations and the optimal ways of extracting it are somewhat different.

The extraction of oil and natural gas in these shales is more sensitive to the levels of drawdown pressure, since much of the oil and gas is found in natural fractures that are not that wide (although they may be spread further apart by the fracking process itself). With exposure to the lower well pressure thus being restricted to a relatively small volume, significant reduction in the pressure because of the location relative to the heel of the well can have significant effects on lowering the overall well production.

Further as a general rule the complex valve systems used in KSA are not installed in the shale wells of the United States, making it less practical to focus the relative pressure differentials at different points along the well bore as a means of increasing production sequentially along the well.

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Saturday, July 5, 2014

Waterjettting 23a - Injecting abrasive into a waterjet

In an earlier part of this series I wrote about the introduction of abrasive into waterjets, and the loss in energy that occurs when the abrasive and the air that transports it are accelerated into the waterjet stream in the mixing chamber of a conventional abrasive waterjet nozzle assembly.

Figure 1. Conventional mixing of abrasive into a waterjet cutting stream.

Because air is conventionally used to carry the abrasive into the mixing chamber, and due to the relatively high volumes that are entrained it is often the case, as Tabitz* and others have shown, that the abrasive velocity exiting the jet is reduced as air volume increases.

Figure 1. Simulation of the effect of increasing air volume and abrasive feed rate on the particle velocity issuing from a conventional abrasive waterjet nozzle. (Tabitz et al*)

Using a higher density fluid to carry the particles into the mixing chamber is a self-defeating exercise, since the heavier fluids also will have to be accelerated to the final velocity, so that if a carrier fluid is to be used, then air is a logical choice. But it can make up some 90% of the jet leaving the nozzle, the water comprises roughly 9% of the remainder, so that only 1% of the jet may be abrasive, and this is the component that does the cutting in harder materials.

There should be a different way of approaching this, and in the early 1980’s Mark Fairhurst, at the time a graduate student in the UK, came up with an answer, which was presented at the BHRA Conference held in Durham in 1986. The initial system was relatively simple, but demonstrated the principles of the approach which was initially known as the Direct Injection of Abrasive Jet (or DIAjet for short).

Figure 3. Initial flow circuit from which the DIAjet system evolved. (after Fairhurst-1**)

The concept of the DIAjet circuit is that the abrasive particles are first loaded into a pressure vessel, which is then closed. When the pump is turned on part of the water flow from the pump feeds into this vessel through two control valves. The first is at the top of the tank, while the second was directed to feed at the bottom of the Tank, making it easier to feed abrasive into the underlying ejector, which mixed it with the main water flow from the pump, and thence carried it to the nozzle. This approach has a number of advantages over that of the conventional mixing chamber. The immediately obvious one is that there is no air added to the system, and the energy imparted to the water by the pump is only shared with the abrasive particles, without the system losses that occur where air is added to the mixture.

As a result the abrasive particles acquire a higher percentage of the water energy, and achieve particle velocities that allow cutting at 3,500 psi and 5,000 psi, whereas otherwise with a conventional system the jets would be at pressures ten times this high (although we will get into some of the caveats to that statement as this segment of the series continues).

In the earliest version of the system (and in some stand-alone versions that developed later, as I will discuss later in the series) the abrasive was added by simply unscrewing the lid, adding the abrasive to the tank, and then resealing the lid. Part of the problem that this causes is that, if the feed is not properly controlled abrasive can be caught in the threads of the cap piece, and this will then gall the threads and rapidly wear out the connection.

BHR, who first developed the machine, overcame this problem initially by using a secondary circuit to feed the abrasive into the pressure vessel, and this could be arranged so that there were two pressure vessels (which rapidly transitioned into pressure cylinders modified from other applications) one of which could be charging, while the second was in use. The basic circuit then became:

Figure 4. Schematic flow for the first commercial DIAjet system (after Fairhurst-2***)

It is perhaps illustrative to show one of the modifications to the design that was made in Missouri, where we used a small pressure-washer pump to feed the water to the pressure vessels, while the abrasive storage (the hopper shown in figure 4) was made from the pressure tank used in high-pressure painting applications. Because the lid of that pressure vessel was not threaded it was quite easy to refill, and the two cylinders were operated alternately. The entire system was designed to fit into the bed of a pick-up truck.

Figure 5. A small portable cutting system based on the DIAjet system. The assembly is mounted on a metal platform, and includes a water reservoir so that it is largely self-contained, and simple to use.

This new way of adding the abrasive to the waterjet feed has been developed for a number of different applications, although, because of the problems that arose in operating valves which control flow that contains abrasive, there have been some problems that have persisted in finding circuit designs that can operate on a consistent basis for the steady cutting applications where long cutting times are needed. But this approach has a number of applications where the abrasive need only cut for a relatively short period of time, during which the valves can function effectively, and where the jets can perform a cutting operation that is difficult for other cutting applications to achieve. It is, for example, possible to use a DIAjet type of system (if controlled properly) to cut through a live explosive detonator, without causing the explosive to go off. But I will talk about some of these developments, and some of the other capabilities of the system in later pieces.

*Tabitz, Schmidtt, Parsy Abriak, and Thery “Effect of Air on accceleration process in AWJ entrainment system, 12th ISJCT, Rouen, 1994 p 47 - 58.
** Fairhurst, R.M., Abrasive Water Jet Cutting, MSc Thesis, Cranfield Institute of Technology, January, 1982.
***Fairhurst, R.M., Heron, R.A., and Saunders, D.H., "Diajet" -- A New Abrasive Waterjet Cutting Technique," 8th International Symposium on Jet Cutting Technology, Durham, UK, September, 1986, pp. 395 - 402.

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