The Effect of Stress on Water Jet Performance

Research on the use of high pressure water jet systems to date has mainly concentrated on laboratory simulation tests and field trials on the surface. The effects of field stress have only been simulated, therefore, by sample confinement in triaxial chambers. This paper contrasts such results with data obtained from a test site located 1000 ft below the surface in a stressed barrier pillar. Data is further supplied contrasting the water jet performance in this rock under stress with that achieved in the same area when the rock is destressed. A water jet slotting technique which was used for the destressing is described. Introduction Conventionally, the vast majority of holes that are drilled underground, particularly in hard rock mines, are drilled by means of pneumatic hammers. These drilling devices have conventionally achieved adequate performance in terms of productivity but are increasingly being cited due to the noise levels they generate, which lie far above the legal requirement of a 90 db level for 8 hour operation. The noise generated by pneumatic drills can be divided into three types (Ref. 1) that of the exhaust, that of the internal movement of the parts within the drill and that of the bit striking the rock. Of these, the first two can be muffled or reduced to an acceptable level. However, the noise generated by the bit impact of the rock is a function of the energy transmittal to the rock and if the noise is reduced by reducing the energy then so the performance of the drilling bit will also be reduced. 'This therefore, is an unsatisfactory so1ution and another method must be sought. There has been considerable interest in recent years in drilling holes less than 1 in. in diameter. This is particularly a problem in roof bolt drilling for resin bolt emplacement since the cost of resin makes large diameter holes expensive and it has been shown (Ref. 2) that if a small diameter rough hole is drilled that this will be equivalent in strength to a larger diameter smooth walled hole emplacement. Conventional drilling techniques however make it relatively uneconomic to drill holes with diameter less than 1 in. since the shaft in such cases becomes relative small and a high thrust along the shaft will therefore cause it to buckle, in turn deviating the hole from the straight line required. In order to solve both these problems, the application of high pressure water jets to drilling has come under investigation (Ref. 3,4). To the present time, the vast majority of this research has been carried out in the laboratory, with extremely promising results. However, it is necessary at some stage at the development of any process that its performance, not only in the laboratory but also in the field be evaluated. Laboratory Test Program In 1975, the University of Missouri-Rolla came under contract to the Energy Research and Development Administration to investigate the application of high pressure water jets for ultimate application in the development of geothermal resources. It was anticipated that the major effort in the research would be directed toward the drilling of hard rocks such as granite at hol.e diameters of the order of 6 in. However, in order to develop such a drill, the program was initiated working at hole diameters of 1 inch. and less and initially starting out drilling in sandstone and other sedimentary rocks. The reason for this approach was that the softer rock is much more responsive to water jet attack and therefore the effect of change in the control parameters is much more evident than would be the case were the granite used as the target material, where changes in the results would be on a much smaller scale and much less easier to discern. In the early test program, which has previously been described, (Ref. 5) it proved possible to achieve drilling rates of the order of 300 in./min and to achieve a hole of programmable roughness (Fig. 1) using 1 ft long samples of sandstone. The roughness of the wall can be controlled by varying the ratio of the advance rate of the machine to the rotational speed of the shaft. Since the water jet cuts beyond the nozzle diameter, the relative speed of the rotation will control the depth of the cut, while the ratio of the advance rate to rotational speed will control the incremental distance between each successive pass of the water jet across the rock surface. Because the water jets which are being used in this program are much smaller in diameter than those which have been used in the large scale fuel programs carried out by Gulf, Shell, and Exxon, (Ref. 6,7,8), one of the initial concerns of the program was to verify that these small diameter jets would still cut at depth. Not only is the rock pressurized at depth, but there will also be a large back pressure within the hole due to the weight of the overlying fluid column. A series of experiments was therefore carried out in which the effect of rock confinement on the hole diameter was established. These experiments were carried out at an advance rate of 40 in./min which level was standardized for all the subsequent tests. This figure was based on a discussion with Dr. Maurer who stated that the maximum advance rate that can be achieved in field drilling is of the order of 200 ft/hr. Logistical problems of handling the drill pipe make increased advance beyond this level relatively impractical. In the initial. test program, rock was prepared in samples of 1 ft long, 6 in. in diameter and confined in a triaxial chamber. Confining pressure up to 6,000 psi and back pressures up to 3,000 psi were imposed on the rock specimens. The confining pressure was applied through a rubber jacket which was fitted over the specimen and the triaxial cell was conventionally pressurized. The back pressure was established by gating the flow of the spent fluid from the cavity out of the cell through a valve and pressure gage which could be so adjusted to give the required back pressure. The system was stabilized prior to the advance of the drill into the rock and measurements were only taken in the lower half of the hole assuming that the initial section was effected by the end conditions. The results of these tests show that there was an onset (Table 1) effect at a load of approximately 500 psi which effect was discernable whether the load was applied as a confining pressure on the rock or as a back pressure within the bore hole. The initial assumption was that this pressure was due to lowering the pressure drop across the nozzle and that this effect was causing the reduction in jet cutting ability. A second experiment was therefore carried out in which the pressure drop across the bit was varied by changing the jet pressure but at no back pressure and at no confining pressure although the samples of Berea sandstone were submerged. The results indicated (Table 2) that the jet pressure reduction did not create as large effect as did the confining pressure within the bore hole and it must therefore be assumed that the effect which was discerned is a function of some change in the rock properties. Experiments were carried out in an extended test series in marble, limestone, and granite. And it was noted in, for example, the granite the effect of increased back pressure was much reduced over that for the sandstone whereas, in the sandstone, an application of 500 psi back pressure reduced the hole diameter 44%, in the granite the reduction was only 12%, from 1 in. to .82 in. It is postulated that the advance rate is controlled by a structural property of the material which is strongly effected by confining pressure and the likely candidate would be the permeability of the rock structure. Field Test Program The results of the laboratory experiments have shown that there is an effect on water jet drilling performance due to imposed stresses on the rock or in the borehole. The small scale of the test program and the possibility of an effect due to the small size of the samples indicated that these results should be treated with some degree of caution. The University therefore, signed a cooperative agreement with the St. Joe Minerals Corp. in the summer of 1977 in order to pursue this program further, by using a water jet to drill a series of holes in a barrier pillar 1,000 ft down in the St. Joe Lead Mine at Indian Creek. The rock in which this test program is being carried out is predominately a sandstone, coarse grained in nature in which lead is found. Preliminary investigation with hand samples brought to the laboratory indicated that drilling rates equivalent to that of the Berea sandstone, could be achieved in this rock. The water jet system was accordingly taken underground (in the form of a high pressure pump driven by a 150 hp motor and capable of achieving either 25 gal/min at 10,000 psi flow or 15 gal/ min at 18,000 psi flow). The pump unit was attached to a sled and the drilling system was fitted to the drill sash on one arm of a Gardner-Denver* drill jumbo (Fig. 2). In the initial configuration based on the laboratory tests, it was anticipated that holes could be drilled successfully at 10,000 psi. The high pressure water was supplied through high pressure hose to drilling steel comprising of a 9/16” OD high pressure steel pipe to the end of which a nozzle was affixed. The connection between the hose and the tubing was through a free floating high pressure rotating coupling (Fig. 3) . Drive to the motor was established through a pair of hydraulic motors, one to provide rotational speed and the other to provide advance. The use of hydrau1ic motors, a modification of the equipment from the original compressed air drive, was found necessary in order to achieve the acquired control on the advance rate and rotational speed which could not easily be achieved with the compressed air.Once the system had been fabricated a secondary control was also attached to the system behind the coupling and comprised a micro-switch which when contacted by the coupling would temporarily halt the advance rate on the steel. The intention of this device was so that, if the drill bit came in contact with the rock, it would move the steel and the coupling back against a spring triggering the micro-switch and halting the advancement until the time as the water jet cut out the obstacle. The spring would then push the steel forward and the advance rate would be resumed. While this system did work effectively, one of the problems with the original set-up was that a considerable volume of water was being placed in the hole (Fig. 4) and,as the hole diameters got smaller particularly with the large drill bits, this water could not easily pass the bit and therefore tended to pressurize the cavity ahead of the nozzle. This in turn provided sufficient resistance to the advance of the bit that the micro-switch was triggered and the device ultimately had to be removed for this reason. The tests began at an operating pressure of 10,000 psi through the nozzle. Various different nozzle geometries were tested and it was found that a number of conclusions could be drawn from these early tests. In the laboratory there had been a range of nozzle angles tested in order to determine the most effective angle for water jet drilling. The results of these tests had indicated that the larger the angle of the reaming jet to the advance of the drill, the more effective the bit. However; this did not prove to be a valid criterion in the operation of the system underground. A major reason for this is that as the jet angle is increased so the hole diameter correspondingly increased but, concurrently, the thrust component of the jets perpendicular to the axis of the hole also increased. Since the bit does not normally contact the rock, there is no bracing support to hold the head in position and the drill steel, as it advanced into the hole, therefore became unstable. and tended to precess within the hole. The results of this, since the hole was larger in diameter, was that the bit became caught in one of the rough spots on the hole profile with the jet directed perpendicular to the obstruction and the rig would sieze up. On the other hand it was noted that where the jet was directed with a very small angle forward, that the reaming jet would rather cut to the central core of the hole than to the perimeter and the edge of the bit would bind up. In the laboratory a 15 degree included angle was sufficient to have the jet cut to the periphery rather than to the central core, however in the field this value had to be increased to between 20 degrees and 25degrees before a satisfactory clearance was achieved. In the laboratory testing, the samples had comprised only sandstone and lead. In the field it was found the sand deposit also contained marcasite, which is considerably harder than either the sandstone or the lead and this material, while it could be cut by the water was not cut nearly as effectively as was the sandstone at 10,000. In consequence, the jets would differentially drill the hole. The result of this was that, as the hole depth increased, so the drill would deviate around these hard inclusions and since they occur on all sides of the bore hole the final result was that the flexible steel would bend in a number of consecutive directions and become jammed in the hole. The only satisfactory solution to this problem which has been found is to increase the jet pressure. In the search for a nozzle which would effectively cut a large diameter hole it was concluded that a rapid and inexpensive method of nozzle manufacture could advantageously be tried and therefore preliminary experimentation involved the use of steel nozzles of the ball nose variety (Fig. 5) which were tested to determine the best nozzle geometry. It was accepted that the performance of these nozzles would be reduced over that of better quality nozzles. A discharge coefficient of approximately 0.7 was measured for these nozzles relative to the 0..95 of better quality nozzles. However, since the water jets were cutting only a distance of 1/2 in. or less, from the orifice, it was concluded that lack of performance would not critically affect the drilling rate providing that the change in discharge coefficient could be adequately compensated for. One feature of this type of nozzle is that the nozzle orifice is located on the leading surface of the drill. This is in contrast with the conventional method of drill nozzle location in which the nozzle is recessed within a holder and the orifice is not in close contact with the rock. A problem with this particular design was quickly discerned in that the close proximity of the orifice surface to the rock meant that the face of the nozzle was scoured by the rebound of the water from the target surface carrying with it particles of sand. A very effective sandblasting system developed and very rapidly removed the surface of the drilling bit (Fig. 5). It was established however using the normal nozzle holders that if this distance could be offset from the face by 0.25 in. that erosion was satisfactorily minimized. It was also found that placing a carbide insert across the face of the bit to help achieve this stand-off distance was effective although there was some abrasion of the carbide. This was not considered to be a problem. Drilling rates in the field have been much reduced over those found in the laboratory. It has also been noted that in areas of the pillar where the rock is stressed, not only is the hole diameter very rapidly reduced from the 3 in. or more which the jet drills at the entrance of the hole down to approximately 1 in. in the stressed zone, but also that the hole no longer remains circular. The normal bedding of the rock is essentially horizontal and where the jets cut through weak layers of the rock under such conditions, then the hole becomes oval aligned along this plane of weakness. However, in areas where stress is evident the hole becomes oval in a vertical direction. This can be anticipated since one would anticipate that the rock at the top and the bottom of the opening being drilled is under some tensile component of load while that on the horizontal diameter is under increased confining pressure and this appears to be evident in the hole result which is achieved (Fig. 6) . Where the pressure of the jet is increased to 16,000 psi, then advance rates of over 90 in./min have been achieved underground and the hole is satisfactorily straight. However at 10,000 psi the water jet does not give an adequately straight hole and drilling rates of no more than 72 in./min could be achieved in contrast with the 300 in./min achieved in the laboratory. This reduction in drilling rate with rock confining pressure must be anticipated in the movement of water jets from the laboratory to the field. Further, the water jet pressure should be so adjusted that it is capable of cutting through not only material which is anticipated to be present in the hole but also any harder inclusions which might also occur. A pressure of 16,000 psi has proved adequate to drill not only through the sandstone and lead but also through the marcasite in this instance. One other factor which must be bore in mind is the flow rate through the bit should be optimized relative to the hole size being drilled. Conventionally, it has been a point of belief that the optimum conditions for water jet cutting were once, the critical pressure of the rock had been achieved, that any major increase in horsepower to the jet should be through an increase in jet diameter rather than through an increase in pressure. However, in the water jet drilling in depth allowance must be made for getting the water out from ahead of the bit and if a sufficiently large hole is not being drilled then the addition of more water to the bit head will be counter productive since it will tend to pressurize the cavity head of the bit and reduce bit performance. A further measure of the effect of the confining stress on jet cutting ability should also be mentioned. It was originally intended that the rock in situ be destressed by cutting a slot around the test area so that drilling in a destressed block could be contrasted with that of the surrounding stressed material (Fig. 7). It was intended that the water jet slot through this rock using a linear traverse of the drilling bit. It was noted that where the water jets cut through the sandstone, where the rock was destressed, that a cut depth of approximately 8 in. was achieved on a single pass. However, where the water jet was cutting through a stressed area of the rock a depth of only some 1 1/2 in. was achieved on a single pass.. Because of the instability of the drill steel system as it was advanced beyond the end of the drill sash it was decided not to use this system for the destressing and instead a series of consecutive holes has been drilled to a depth of 6 ft in order to establish the destressed area. During the drilling of these holes it has been demonstrated that providing some care is taken with the operation of the drill that the water jet drill can drill through the pre-existing hole and continue the alignment of the new bore hole passing through the hole previously drilled. During the course of these trials noise levels were monitored in order to determine the effect of the water jet drill. It has been noticed that one yard from the hole a noise level of 85 db was monitored during the operation of the drill. While this level is increased during the time that the bit stings in to approximately 95 db, it is nevertheless one advantage to this system that a quiet rock drill has been developed. Conclusions The continuing program herein described has shown that water jets can be used effectively for drilling rock underground. However, it is also demonstrated that the results obtained in the laboratory cannot be directly translated into performance underground since the effect of rock confining stress is to reduce the performance of the water jet drill. This performance loss to a degree can be compensated by increasing the pressure of a system over that used in the laboratory trials to an equivalent level commensurate with the increased confining stress. However, the pressure should also be increased to take into effect the additional likelihood of a higher strength rock existing within the natural formation beyond that of the rock which is being tested in the laboratory. The flow rate and nozzle geometries should be designed to cope with the need to get water away from the bit once the jet has made its cut and the bit nozzle orifices should be recessed within the bit to protect them from the abrasive nature of the initial flow from the cutting surface. *The company identified herein is for the purpose of identification only and no product endorsement should be read into this reference.