Abstract

The use of a percussion hammer can provide high penetration rates through hard rocks when air drilling.The work of this paper was partly sponsored by the US Department of Energy to develop hammer drilling techniques for drilling deep hard rocks, using real drilling muds.Unfortunately, development is complicated because understanding rock breakage and cuttings removal under the dynamic loading conditions of hammer drilling is difficult.Historically, empirical relations have been used to relate hammer blow energy and blow rate with drilling.

This paper presents results of carefully measured single cutter impact—as well as high rate and ‘static’—rock indentation tests under high borehole pressure conditions, using real drilling muds.The results include cutter force-displacement and crater volumes for both first stress wave (initial impact) and long time (quasi static) rock indentations.From this, specific energy for rock breakage (i.e. the energy to excavate a unit volume of rock) can be calculated.

The specific energy values measured in these single cutter impact tests show the rock destruction efficiency that can be achieved by impact loading.They are compared with specific energies measured during full scale hammer and rotary drilling experiments, under relatively similar conditions.

For a given rock type, impact stress, cutter configuration, wellbore pressure, and mud type are shown to be critical parameters.A key observation is that most of the rock breakage occurs during the first stress-wave, and for higher impact stresses above the dynamic confined indentation strength, relatively little additional rock breakage seems to occur.

Introduction

Drilling is required for exploration and exploitation of oil, gas, and other energy resources. Furthermore, with the depletion of shallow energy resources, the cost of drilling is becoming increasingly greater as deeper and harder rock formations are penetrated.Under these conditions, the instantaneous rate of penetration often controls a significant portion of the total well cost; therefore increasing rate of penetration becomes highly desirable.

Various tools and techniques have been investigated in the past to drill efficiently in such conditions [for example Maurer, W.C., 1980; Rao, U.M., 1980, and Mishra, B, 1998]. Percussion or hammer drilling often gives efficient, high penetration rates under air drilling conditions.Consequently hammer drilling is one of the drilling tools being investigated for use in difficult drilling conditions, including deep and hard formations. Tibbitts et.at. (2002) has shown laboratory hammer drilling results under simulated deep drilling conditions, and Deutch et al. (1990) showed ultra deep drilling results (15,000–30,000 ft) for crystalline igneous rocks.

Although air hammer (or percussion) drilling is used extensively in the mining industry, and much research has improved understanding and perfected the application, for deep oil and gas wells, high borehole mud pressure creates a different and complicated environment.As shown extensively by TerraTek and others, under high borehole pressures the rock strengthens and behaves in an apparent ductile manner (for exmple Robinson, L.H., 1958, Green, S.J., et. al. 1972, and Maurer, W.C., 1980).Also, rocks behave differently at high strain rates (Green, S.J. et. al. 1968, Green, S.J. et. al. 1974, Green, S.J. et. al. 1982)—although strain rate effects are very complicated due to pore fluid effects. Additionally, chip hold down occurs (Garnier and Van Lingen, 1959) and cuttings removal becomes more difficult (for example Van Lingen, 1962). Therefore, in order to optimize deep-well hammer drilling, quantifying rock deformation and breakage under high pressure impact loading is essential.

This work extends the understanding of hammer drilling by carefully measuring single-cutter impact rock breakage, under high borehole drilling mud pressures. Under these conditions the time of loading is 130–150 micro-seconds.Three rock types were considered.

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