Waterjetting 31d - thickening a waterjet to improve downstream pressure

There is a trick that one can learn while a teenager, which comes with the introduction “I so strong that I can blow a brick over!” Upon finding a suitable victim to impress, the brick is placed over a deflated balloon, which is then inflated, raising the brick which then, if suitably placed, topples onto its side – proving the strength of your lungs.

The critical part of the activity is to have the air that you blow be confined within the balloon, and equally exert pressure over the surface of the brick, so that a low pressure translates into a much more significant and powerful force. It is the confinement of the pressure that allows the build-up that moves the brick.

In most cases the use of high-pressure water as a cutting tool does not see much confinement of the water over the cutting process, with the water flowing into the cut, removing some material, and then flowing on out. Yet the water still has considerable energy as it leaves, and this means that the process is usually quite inefficient. How then can the contained energy in the jet be used in a secondary way to improve the removal efficiency of the process?

One answer to the question comes with the use of long-chain polymeric additives. These have recently seen a fair amount of publicity because of their use as the “slick water” components of the “slick-water-fracking” tools that have helped improve the production of oil and natural gas from long horizontal wells drilled into the hydrocarbon deposits in places such as the Bakken fields of North Dakota and the Barnett and Eagle Ford Shales in Texas. The long horizontal wells are separated into short intervals, within which the pressure within the well is raised until the surrounding rock cracks (fracks) with s series of cracks that extend out into the surrounding rock. The crack makes it easier for the hydrocarbons in the rock to escape and reach the well, improving the recovery to the point that the well can be economic to operate. The reason that the “slick water” is used is that the crack would normally close back up after the borehole pressure was lowered back down. In order to stop this happening the fluid in the well during the frack is made up with long-chain polymers and also contains grains of a sand or similar proppant. When the crack is formed the fluid in the well flows into the crack, carrying the sand with it, and this then holds the crack open when the pressure falls.

The polymer thickens the water which makes up most of the fracking fluid, so that it can carry more of the sand, and at the same time, the polymer reduces the friction of the water against the rock, so that it is easier for the water and sand to penetrate deeper into the cracks. Once the proppant particles catch against the sides of the crack they become held in place, while the fluid moves on and eventually returns back out of the well.

Back in the days when I was carrying out the research for my doctorate, I had used the long-chain polymer Polymerized ethylene oxide (Polyox) to reduce the friction in the delivery line from the pump to the nozzle. The increased cohesion of the jet (which I will cover in posts that follow this) meant that it would cut to a greater distance from the nozzle, with less decline in cutting power. However the increased cohesion of the jet had an additional benefit, which was noted by Chapman Young, as part of his development of tools for removing loose rock from around tunnels.

In a typical tunnel excavation, the miners drill a pattern of holes in the face of the tunnel, and then partially fill these with explosive, which is then set off in a controlled pattern of blasting. The central core of rock on the face is broken out by the explosive force, but some portion of the rock at the edge of the blast is only loosened from the solid, and still hangs in place. One of the more dangerous mining jobs (which I have done) is to take a long pry bar and insert this behind the loose pieces of rock around the opening, hoping to be able to wedge these loose, so that they no longer pose a risk to miners who then pass underneath.

Seeking to automate this process Dr Young and his colleagues tried using high-pressure waterjets to blast these lumps free from the wall. Subsequently investigators at Colorado School of Mines have shown that the jets give an improved cleaning of the wall, over other methods – but as normally applied they do not have the confined power to be able to get behind the block with sufficient confined force to be able to pry larger blocks free.

And this is where the balloon analogy comes in, because Dr Young realized that if he could increase the viscocity of the water in the stream sufficiently so that, for a short instance, it would be confined behind the block and could acquire some of the pressure from the following impacting jet, then enough pressure over a large enough area would provide the force needed to dislodge the block. He tried it, and it worked.

It is not, however, a simple process to carry out, since the jet path must be carefully aimed to ensure that there is enough confinement of the water behind the target block for pressure to be built up, and this requires that the jet contain a relatively high concentration of polymer. That in itself brings another problem, which can be anticipated by the “slick water” nickname. Where the water gets onto the floor the friction reduction properties mean that it makes it quite difficult to walk on the wetted rock. Now while that, in turn, opens up a new avenue for business (the chemical is sometimes referred to as Banana water in riot control) it makes it unpopular with those that have to work with it in the confines of a mining tunnel and so the technology has not caught on. But it does provide an introduction to the topic of different cutting fluids, which will be the next topic of discussion.