Abstract

Drilling with liquid-driven downhole hammers is a new competitive method for production of boreholes. The system requires large amounts of fluid during operation, which can be difficult and/or expensive to accomplish. One method to reduce the fluid consumption is to process used drilling fluid into a quality acceptable for re-use in the system. This will also make waste less voluminous and more easily handled. Extensive studies have been performed to find cost-effective suitable cleaning methods and corresponding compact mobile equipment. This paper describes the technical solutions and characteristics of the resulting prototype unit, together with a brief discussion about important design criteria.

Introduction

The technique of using water instead of air as the energy carrier to down-hole hammer tools has been known for many years. Technical difficulties associated with corrosion, cavitation and wear have however made it difficult and/or costly to apply these ideas in practice. This situation began to change in the early 1990's when the Swedish mining company LKAB1 started to use water driven down-hole hammers for production drilling of blast-holes. The use of the hammer-tool was coupled with continuous evaluation and improvements to the system, which today is a highly cost-effective and competitive drilling method. To date, more than 5-million meters of blast-holes have been drilled with the 4-inch model of the water driven hammer tool within the mining industry.

There are many advantages with this drilling method; the most important it's 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, which is not possible with air-driven tools. The working environment is improved since dust is eliminated and the air is free from oil residues. One disadvantage is that a large flow rate, of preferably high quality fluid, is required to drive the hammer tool. An ordinary 4-inch hammer-tool requires between 0.2-0.4 m3/min achieving a competitive rate of penetration. This means that drilling with the water driven down-hole hammer in environmentally sensitive areas and/or in locations with limited water supply requires a water recycling system to clean the used drilling fluid so that it can be re-used in the hammer-tool. The demands on the re-cycling systems are significant since particles in the fluid feed can significantly reduce the life of the tool and reduce efficiency. Whilst it is possible to use tungsten carbide as tool material, this increases the cost and is therefore normally only used in tools which use mud as the drilling fluid.

The work required to further develop and refine this drilling technique covers two main areas. (1) To develop efficient and reliable hammer-tools with long life and low manufacturing cost. (2) To find efficient low-cost mobile fluid cleaning systems. Both these areas must be addressed for the technique to become more widely used.

System design criteria

A fluid driven downhole hammer drilling system basically consists of a drill-rig with related components and a highpressure pump-package to pressurize the drilling fluid.

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