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Effective use of water in a system for water driven hammer drilling
G?ran Tuomas,
Division of Renewable Energy, Lule? University of Technology, SE-97187, Lule?, Sweden
Received 28 November 2002;? revised 10 August 2003;? accepted 18 August 2003.?; Available online 14 October 2003.
Abstract
Drilling with water driven down-the-hole (DTH) hammers is a recently developed method for competitive production of boreholes. In order to prevent large amounts of water being used during operation, the drilling fluid is here directly processed into a quality acceptable for reuse. The effectiveness is evaluated in well drilling with a mobile prototype water cleaning and pressurising unit. Especially the presence of abrasive particles in the fluid can drastically reduce tool life and make the method inefficient. The vital significance of this relation has called for detailed studies and a process simulation model for determining particle concentration and size distribution has been developed. This paper describes the model and how it is applied. Simulation results of different system configurations are also presented.
Author Keywords: Drilling; DTH; Hammer; Down-the-hole; Particle; Flow; Water; Simulation
Article Outline
1. Introduction
2. Prototype system description
2.1. General description
2.2. Fluid cleaning system
3. System process model
3.1. Hammer tool
3.2. Mixing tank and drilling fluid tank
3.3. Lamella thickener
3.4. Hydro-cyclones
4. Resulting calculations
4.1. Results
5. Conclusion
Acknowledgements
Appendix A. Field data
References
1. Introduction
The technique of using water instead of air as an energy carrier to DTH-hammer tools has been known for years. However, technical difficulties associated with corrosion, cavitation and wear have made it difficult and/or costly to put these ideas into practice. This situation began to change in the early 1990s when the Swedish mining company LKAB started to use water driven DTH-hammers for production drilling of blast-holes. The use of the hammer-tool also meant continuous evaluation and improvements of the system, which today is a highly cost-effective and competitive drilling method. Today, more than 5-million meters of blast-holes have been drilled with the water driven hammer tool within the Swedish mining industry.
There are many advantages with this method; the most important are its cost-effectiveness and competitive performance. The technique offers high penetration rates and low energy consumption as well as the possibility to drill to virtually any depth ([Tuomas and Nordell, 2000]). The working environment is improved since dust is eliminated and the air is free from oil residues. However, one disadvantage is that a large flow rate of preferably high quality water is required to drive the hammer tool. For instance, an ordinary 4-inch hammer-tool requires between 0.2 and 0.4 m3/min to achieve a competitive rate of penetration. This means that the water should be recycled when this drilling method is used in locations with limited water access and/or when waste disposal is difficult to accomplish (Fig. 1).
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Fig. 1. Principle flow in a drilling system with re-circulation.
The concentration of particles in the drilling water depends mainly on the actual water flow rate, penetration rate, and the density of the drilled rock. Mass concentrations (w/w) between 4 and 12% are common for rock drilling with an ordinary 4-inch hammer. This corresponds to approximately 13–27 kg/min particle flow, which means that high-capacity cleaning equipment has to be used. The particle size distribution varies with a certain number of factors. Rock characteristics, drill bit design and impact energy, are some of them. An important limiting factor during vertical or inclined drilling is the speed of the flushing water, since this must be larger than the particles settling speed. Otherwise the particles will settle in the borehole and will be re-crushed by the drill bit until the size is small enough to follow the flow. Particles generated during typical 4-inch well drilling are usually smaller than 1 mm with mass median sizes (d50) at approximately 0.1 mm.
For the technique to be successful, the fluid cleaning system must be correctly designed and implemented since fluid quality directly affects component life. Abrasive particles and/or aggressive chemical substances in the feed water significantly reduce tool life, especially when ordinary tools made of hardened steel are used. It is, however, possible to use tungsten carbide as tool material, but this increases the cost and this material is, therefore, normally only used in mud driven tools. For this reason, knowledge of how different water related parameters affect the life of a given tool or material is of vital importance when designing cost effective systems.
Interesting data have been obtained from practical use of these tools, especially within the mining industry where automated drill-rigs produced millions of meters of 4-inch blast holes. Results from water-analysis and data of the corresponding tool-life, show that time between repairs corresponds to approximately 1500 drilling meters in hard rock when the feed water contains maximum 0.02% w/w solids. The mean penetration rate during these drillings was 0.9 m/min, which gives a total of approximately 6 million piston blows between repairs, since the piston blow frequency is about 60 Hz. Other experiments have shown that the life was drastically reduced by large amounts of solids in the feed water. For example, life less than 100 drill-meters have been measured when the feed water contained about 0.5% w/w solids (?deryd 2001).
To evaluate the possibilities of this system, a complete mobile prototype service unit for use with low-cost clear-water hammers has been constructed ([Tuomas, 2001]). The unit includes all components required for efficient drilling, i.e. systems for both pressurising drilling fluid and particle-fluid separation (by a lamella thickener and a hydro-cyclone unit) to enable recycling. The prototype unit is presently undergoing initial operational tests in order to establish the relation between tool life and particle content in the drilling fluid. System characteristics for the prototype were estimated by simulations with a process model, implemented within the Matlab Simulink math package. Particle size distributions, concentrations, and flows are resolved at strategic locations, which make the model suitable as a tool for optimisation and development of next generation systems.
This paper describes the process in the prototype system and how it is modelled, and discusses the simulated results for different system configurations. In addition, field test data of the lamella thickeners cleaning capacity are also presented.
2. Prototype system description
2.1. General description
The process in the prototype system is described in Fig. 2. A plunger pump (P4) pressurises water, which is used for driving the hammer tool and for flushing the borehole. Particle-contaminated water is returned for cleaning before re-use. The cleaning process is based on a lamella thickener with a flocculation system and a hydro-cyclone unit. The equipment was built into a single container (Fig. 3) for ease of transport and handling. The complete system is described in more details by [Tuomas, 2001]. Table 1 summarises some important specifics of the system.
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Fig. 2. Schematic flow-chart describing the prototype process.
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Fig. 3. Prototype system unit.
Table 1. System specification
2.2. Fluid cleaning system
The prototype cleaning system uses gravity sedimentation for primary separation of particles from drilling water. The lamella thickener is of cross-flow type, leading to a horizontal flow between inclined lamellas (Fig. 4). Particles settle onto the lamella and slide towards the centre of the unit and eventually reach the bottom of the tank. A horizontal conveyor transports the sediment towards the end of the settling unit, where another inclined conveyor removes the waste out of the system. This second conveyor also serves to dewater the waste in order to achieve low water consumption. The settling unit is equipped with a pump for sediment removal if the conveyors are insufficient. Efficiency of sedimentation processes can be significantly improved by adding a flocculent to the incoming slurry flow. These substances gather individual fine and colloidal particles into clumps (flocks) that settle out more easily.
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Fig. 4. The prototype lamella thickener (T3 in Fig. 2). The unit is of cross-flow type and equipped with screw conveyors for grit discharge.
In addition, particle-fluid separation can be achieved with hydro-cyclones. The idea is to use the hydro-cyclones as an alternative to flocculation. The hydro-cyclone unit has a d50 cut-point below 5 μm (particles with density 2750 kg/m3 in water). It is designed for a 0.3 m3/min flow and consists of sixty 10-mm hydro-cyclones.
3. System process model
A numerical model for simulation of particle flows in the prototype system has been developed. Mathematical expressions for significant components are derived, and the whole model is implemented within the Matlab Simulink? math package. Results of main interest are the time dependent particle size distribution functions Φ(s,t) and corresponding volume flow rate functions, q(t), at different locations in the system. Fig. 5 shows the principle flow scheme and mathematical descriptions of the different blocks are presented in the following sections.
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Fig. 5. Principle flow scheme of the process model.
3.1. Hammer tool
The hammer tool block in the model adds particles to the system. This is mathematically described as:
Φout(s,t)=Φin(s,t)+Φh(s)
(1)
where Φin(s,t) and Φout(s,t) represent the particle size distributions in the fluid entering and leaving the hammer tool. Φh(s) represent the particles that are generated during drilling. The Φ-functions also represents the volume concentration of particles in the corresponding slurry according to equation:
(2)
where qsolids is the volumetric flow rate of solids and q is the flow rate of slurry. Φh(s) in Eq. (1) is calculated as:
(3)
where v and A represent the penetration rate and borehole cross-area, respectively. Φc(s) is a time independent function which represents the shape of the particle size distribution curve generated by the hammer tool. The curve used in this study (Fig. 6) comes from laboratory analysis of a drill water sample, taken during typical rock drilling on 100 m depth with a 4-inch hammer tool. The shape of Φc depends on various parameters, such as the actual borehole depth, borehole orientation, flow rate, mineral type and drill bit design as well. The shape of the curve is, however, assumed constant in this model.
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Fig. 6. Particle size distribution in a drill water sample, taken during drilling with a 4-inch water driven DTH-hammer tool at approximately 100-m depth. The curve is used to represent function Φc in the described process model.
The slurry flow rate, qout, from the hammer block is assumed equal to the incoming flow rate, qin.
3.2. Mixing tank and drilling fluid tank
In a tank containing a substance with concentration c, the changed particle concentration by time is described by a differential equation:
(4)
where q is the flow, c is the concentration at n number of intake- and outlet ports in the tank, V is the volume, which may vary with time. After inserting Eq. (2) into Eq. (4), the equation for a tank with n number of intakes is derived as:
(5)
where Φin is the particle size distribution in the fluid entering the tank, qin is the corresponding fluid flow rate to the tank and Φ is the particle size distribution in the tank. The model assumes that both the mixing- and drilling fluid tanks are initially filled-up with clear water. The initial condition to Eq. (5) is, therefore, Φ(s,0)=0. The volume in the drilling fluid tank will steadily decrease during drilling. The reason is that the separation processes in the lamella thickener and hydro cyclones consume fluid during operation. Opening a water intake at a low fluid level, and closing it when the tank is filled solves this problem. The model is designed to work in a similar way. One of the intake flows, qin in Eq. (5), is changed from zero to a user defined positive value when the tank level V has reached the low limit, and goes back to zero when the upper limit is reached.
3.3. Lamella thickener
The lamella thickener (Fig. 4) is designed for a horizontal flow of slurry between inclined lamellas. Particles settle against the lamella and slide towards the centre of the unit and eventually reach the bottom of the tank. Fig. 7 shows some principle particle trajectories between two lamellas during steady flow conditions.
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Fig. 7. Principle outline of two particle paths in a horizontal lamella thickener. Ideally, all particles larger than d2 (belonging to path 2) will go to the underflow. Particles with sizes d
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