Waterjetting 30a - Why cut slots in rock

Looking back over the period where we first started coming together to discuss high-pressure water jets, some 40-odd years ago I was reminded of the work of one of my then graduate students (and subsequent faculty member in Egypt) Dr. Ahmed El-Saie. He obtained his doctorate in 1977 and looking back on that work it is interesting to see how long it took for some of the ideas he worked on to come to fruition.

His dissertation focused on using a waterjet system (which I will discuss in a later post) as a way of cutting a slot around the edge of a tunnel before excavating the contained volume. Early in his dissertation, for example, he discussed the use of impact breakers as a method for improving the economics of tunnel driving over conventional drill and blast techniques. He felt that this would be particularly useful where the volume of overbreak around the tunnel beyond the desired size could be controlled by cutting a perimeter slot.

Apart from the benefits that come from mechanical excavation over blasting (workers don’t have to leave the working area during a blast, for example) other benefits can be shown by contrasting the damage to a block of Plexiglas when a detonator is fired in a small central hole in Plexiglas, with and without that perimeter cut.

Figure 1. Damage to a block of Plexiglas from a detonator fired in a central hole.

If, however, a relieving slot is first cut around the perimeter of the anticipated damage zone (we used the distance to some of the longest cracks) then a different result is obtained.

Figure 2. Effect when the experiment is repeated with a pre-cut slot around the perimeter.

As the photos clearly showed with the free outer surface the central core of material is broken out in pieces, there is a nice relatively flat front surface to the excavated hole, which lies at the back of the drilled hole. (This is a relatively important point in driving tunnels, since often the last third of the blast-hole length is not effectively broken out of the solid, and has to be re-drilled).

It is also important to notice that the cracks from the detonator explosion did not grow out beyond the edge of the slot, so that the tunnel wall would be stable and, because there would be no overbreak, the cost of tunnel support would be reduced considerably.

However, in the larger scale the depth that this slot would have to be cut is around 7-ft. This would require that the jets cut a slot wide enough for the nozzles to advance into the slot, and this required a considerably higher volume of rock to be removed by the waterjets.

Tests of such a device in a German coal mine used two different methods for cutting the slot. Initial trials at Rossenray Colliery in Germany used a waterjet assisted mechanical set of tools to to cut the slot to the desired width. The head, seen moving along the slot at the edge of the tunnel, had to make a number of passes to reach the depth needed.

Figure 3. Tunnel profiling in Germany using a combination of waterjets and metal tools to cut to the perimeter of a tunnel (after Bauman and Koppers)

Subsequent trials replaced the mechanical cutter with a set of waterjet nozzles alone, and this reduced even further the cutting forces required to make the slot (and would make the machine smaller and lighter as a result). Although the trial was successfully concluded the tool did not move into production, perhaps in part because of the change in the mining economy at that time.

To prevent the cracks from growing into the wall, however, a wide slot is not needed, and even a continuous crack around the edge of the hole can be effective. But how to control crack growth to a single direction from the borehole?

The answer came as part of the Master’s degree of another student, Steve McGroarty. If one drills a hole into a block of Plexiglas, and then notches the side, fills the hole with water, and fires an air rifle pellet into the hole, then the pressurized water will flow into the notches and cause the cracks to grow. These are a little difficult to see in the following picture, but the cracks grow at the bottom of the hole and from the edges of the v-cuts (made at the time with a saw).

Figure 4. Individual cracks growing out from 3 bored holes in plexiglas

The above test showed that energy could be focused into cracks if they could be properly aligned. (We could break off a corner of the block in a single piece, using a single notched hole). This work was then in the field by Steve in comparing results when he used explosives to drive a short tunnel underground.

In Steve’s case he drilled holes around the edge of the tunnel, and then notched some of these with a waterjet system. (Others were left un-notched to provide a comparison). The lance used had two jet nozzles and was fairly simple to insert, and the lance was run to the back of the hole, and the two opposing jets aligned to the proposed tunnel wall, raised to pressure and pulled back out of the hole, notching the walls. This is a fairly fast process, and used relatively little water.

The holes were then charged with a small amount of powder and fired just before the rest of the blasting round, which was distributed around the rest of the core rock, in order to break it into fragments.

Figure 5. Tunnel wall after the round had been cleared showing the clear break at the back of the holes, (the next round has been drilled along the edge) and parts of the drilled hole still evident in the wall of the tunnel. (after McGroarty)

The role of the waterjets was much smaller than if a complete slot had been cut, and this significantly reduced the cost financially, in energy and in time, and produced much the same desired result.

In later work we used the same notched borehole idea to break out large pieces of rock as we excavated the Omnimax Theater under the Gateway Arch in St. Louis, but that is a story for another day.

Next time I will discuss some of the ways that Dr. El-Saie used to cut the slot.

A.A. El-Saie “Investigation of Rock Slotting by High Pressure Water Jet for use in Tunneling”, Doctoral Dissertation, Mining Engineering, University of Missouri-Rolla, 1977.
Bauman L. and Koppers M. “State of Investigation on High Pressure Waterjet Assisted Road Profile Cutting Technology,” BHRA 6th ISJCT, paper G2, pp. 283 – 300, 1982).
S.J. McGroarty “An Evaluation of the Fracture Control Blasting Technique for Drift Round Blasts in Dolomitic Rock”, M.S. Thesis, Mining Engineering, University of Missouri-Rolla, 1984.

Waterjetting 26c - Cutting tool shape

When it was first discovered that high-pressure waterjets could significantly improve the performance of mechanical cutting tools, whether in machining metal, or in cutting rock, it was anticipated that this would have a broad-ranging application. This has not been the case, and the reasons are varied, depending on the application, but quite often they relate to the way in which the mechanical tool was expected to work. The examples will, again, come from rock cutting, but also apply when cutting or machining other materials.

Figure 1. Three common types of picks used in cutting into stone for driving tunnels, or for cutting and mining coal.

The initial work of Mike Hood, in cutting quartzite, had used a relatively simple flat-faced bit that was dragged across the rock surface at a known depth of cut and directing s single, or pair of jets to cut along the line of contact between the rock and the carbide was relatively straightforward.

Figure 2. Locations of the jets for Mike Hood’s initial tests on improving jet performance. (Dr. Hood)

Getting the jets to cover the full zone of contact and rock crushing was critical to achieving the best results for the tests, and proved also effective when the cutting tools were tried in the field.

Figure 3. Relative normal forces on a cutting bit with change in the position of the jets assisting in the cutting of rock (Dr. Hood). Note that the machine stalled at 4 mm penetration without the jet assist (the black line).

The cutting picks that are more commonly used in softer rock, shown in figure 1, are not quite the same shape, nor have quite the same purpose. Early trials were with the forward attack pick, which through the early 1980’s was the most common design used.

Figure 4. Laboratory trials with a jet added to a forward attack pick

Rather than having a flat face, this pick has a wedge-shaped front face. This is so that, as the pick cuts into the rock, so the wedge shape pressing into that groove will put a high lateral load on the rock on either side of the cut, causing it to shear off the solid. Those chips can be seen to the front right of figure 4.

Where the jet cuts into and removes the crushed rock under the front of the bit, this allows the bit to make a deeper bite, and this, in turn, makes it more likely that the tool will make larger chips. This is not an unflawed benefit, particularly if the jetted slot now extends a little deeper than the tool.

Figure 5. Illustrating the wedging action of the tool in creating lateral chips beside the tool.

As the chips get larger so the force required to break them from the solid increases, and the actions do not occur symmetrically on each side of the tool. As a result the lateral loading on the tool becomes more significant, and because the process of chip forming and breakage is cyclic so there greater fluctuating forces make their way through the drive train back to the driving shaft and motors.

With most machine designs these fluctuating loads are, however, reduced in overall magnitude, because of the reduced forces needed to move the pick forward, without having to deal with the crushed material under the pick, which the water jet has removed, providing it is within about 1/10th of an inch of the cutting tool.

Achieving that positioning becomes a little more difficult with the transition to a radial pick, however there were additional problems with that intermediate design, particularly in harder rocks, where they wore out at rates as high as 7 picks per foot of advance. This led to the development of the point attack pick, as shown in figure 1.

This pick design has become the most popular for use in mining machines over the past 20-years. The round shape of the tool and shaft are designed so that, as the pick wears it will rotate in the holder, and this will spread the wear evenly around the tool, making it last longer – and in the case mentioned in the last paragraph a change to this design reduced pick costs to around 1 pick per foot of advance. But there are a couple of problems with adding waterjets to this tool.

Figure 6. Point attack tool geometry (Goktan and Gunes)

This geometry makes it very difficult to bring in a waterjet to hit the right point at the rock:tool contact, because of the double cone at the end of the tool. While the nozzle can be positioned so that it can direct a jet into the right point (for example by being at the point where α is in the figure) the problem arises with the size of the nozzle mounting block, and the small size of the jet, where a large number are being used to cover all the picks on the cutting head and total flow volume is limited. To fit the nozzle block means that it must be at a greater standoff distance from the point (perhaps four or five inches), while the small orifice size means that the effective range of the jet may be no more than an inch or two.

The change in pick design and the difficulty in adding waterjets to the new tool therefore led to a discontinuation of the trials of the combined system. This was unfortunate since the forward attack picks initially cut better than the point attack, but wore out more rapidly – hence the change. But with the addition of the waterjets the tool lifetime, and sharpness, was increased in some cases more than five-fold times, while the other benefits – such as the ability to use smaller machines to carry out similar performance – made capital investment less. But these events occurred at the wrong time, as the coal market was entering one of its down cycles as the developments were being made, and the technology was therefore not adopted.

I will conclude this small chapter next time, by addressing one of the answers to the problem of getting water to the point attack tool.

OGPSS - Future Bakken production and hydrofracking

Before there were refrigerators folks kept drinks cool by putting them into clay jars that had been soaked in water. The evaporation of the water from the clay cooled the container and its contents, which today includes wine bottles. On the other hand, for many years artisans have taken clay in a slightly different form, shaped it and baked it and provided the teacups which keep the liquid inside until we drink it.

Two different forms of the same basic geological material, with two different behaviors and uses. Why bring this up? Well there is a growing series of articles which continue to laud the volumes of oil and natural gas that the world can expect from the artificial fracturing of the layers of shale in which these hydrocarbons have been trapped for the past few million years. It has been suggested that there is no difference between this “unconventional” oil and the “conventional” oil that has been produced over the past century to power the global economy. And yet, despite the scientific detail which some of these critics discuss other issues, they seem unable to grasp the relatively simple geologic and temporal facts that make the reserves in such locations as the Marcellus Shale of Pennsylvania and the Bakken of North Dakota both unconventional and temporally transient. Let me therefore try again to explain why, despite the fact that the oil itself may be relatively similar, the recovery and economics of that oil are quite different from those involved in extracting conventional deposits.

But, before getting to that, let’s first look at the current situation in North Dakota, using the information from the Department of Mineral Resources (DMR). According to the January Director’s Cut the rig count in the state has varied from 188 in October, through 186 in November, and 184 in December, to 181 at the time of the report. Why is this number important? Well, as I will explain in more detail later, the decline rate of an individual well in the region is very high, and thus the industry has to continue to drill wells at a rapid rate, just to replace the decline. (This is the “Red Queen” scenario that Rune Likvern has explained so well.) The DMR recognize this by showing the effect of several different scenarios as the number of rigs changes.

For example they project that 170 rigs will be able to drill around 2,000 wells a year. At that level, and with some assumptions about the productivity of individual wells that I am not going to address here, but which Rune discussed. I would, however, suggest that it is irrational to expect that new wells will continue to sustain existing first year levels as the wells move away from formation sweet spots. Yet, accepting their assumptions for now, DMR project that the 170 rigs will generate the following production from the state:

Figure 1. Achieved and projected North Dakota production when 170 rigs are used to continue to develop the field into the foreseeable future. (ND DMR).

The DMR plot also assumes that the wells are developed and brought into production in a timely manner. In October the state produced an average of 749 kbd of oil, which was through mid-January the current peak level of production. Currently it is estimated to cost $2 million to frack a well, and in January there were 410 wells waiting on that service.

In order to reach a higher level of production (and bear in mind that OPEC has been projecting significant further increases in production to make their anticipated supply and demand levels balance) the DMR looked at estimates of production if there were 225-250 rigs, and contrasted that with what would happen if the rig count fell almost immediately to 60.

Figure 2. North Dakota oil production with either 225-250 rigs, or with 60. (ND DMR)

Note that at 60 rigs the state production goes into an immediate decline. Somewhere in between those two extremes lies the likely future, but with the Director noting a December price of $77.09 that future may be at the lower, rather than higher end of the scale. (Though in January it popped back up to $87.25).

To illustrate the sensitivity of these numbers consider that if the rig count fell from 170 to 100, then production would decline to 800 kbd but would still fall into decline in 2020, while at 200 rigs the production would rise to a peak of 1 mbd, although the peak interval might only be four years from the 2,400 new wells added each year.

The ferocity of the decline rates of these wells is part of the reason that they are called unconventional, since they do not behave in the same manner as a conventional well, nor can they be developed in a similar way.

To return to the geology of the deposits (and shale is a consolidated clay) the middle Bakken formation is made up of a combination of layers of shale, sandstone, siltstone and limestone. These are, in general, rocks that have a very low permeability, and that property was explained in more detail in an earlier post. Simplistically it is a measure of how easy it is for fluid to flow through the rock, and for most of the Bakken rock it is not easy at all. If it were then there would be no need to put in the crack paths that the oil uses to reach the well. Let me repeat a figure from that post:

Figure 3. Block of sandstone with a crack in it (shown by the arrows).

I have been on a site where my hosts (a federal agency) had injected fluid that they were hoping would penetrate a layer of ground so that it would form an impermeable barrier. It had not, even though the ground was relatively easy for the fluid to penetrate. Instead it had all flowed into a crack no bigger than the one shown in the picture above, and the attempt was a failure.

Put that into reverse where you are trying to pull fluid out of the ground. There are two places where the fluid (oil or gas) is located, in the natural cracks and joints of the rock – which the hydrofrack is designed to cut across. And in the much lower permeability of the blocks of rock that are edged by these fractures, bedding planes and joints.

Figure 4. Representation of a horizontal well drilled in the Marcellus, shown against the natural fracture pattern (Source AAPG )

Over the millennia the oil/gas has migrated to those bedding planes and natural joints and fractures in the rock. When the well is first put in place it is that fluid that is more easily available to flow through the intersecting crack pattern to the well. But as those interstices empty out it is much more difficult to move the oil from the rock surrounding the natural cracks into that crack and thence to the well.

Most illustrations of hydraulic fracturing show a network of artificially induced cracks getting more numerous as they move away from the well. That, actually, is not the way it normally happens. The majority of the cracks that open are already there, and these are much easier to develop – as my unfortunate hosts learned – that it is to try and generate a multiplicity of new fractures, as I have previously explained here and here. The production, to go back to my initial metaphor, begins to move, over that first year of production, and dramatic fall in yield, from relying on the permeability of the wine cooler part of the rock, to that of the teacup.