Published by the DTI Oil & Gas Directorate for the reservoir engineering and IOR community in the UK.
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Plasma-Channel Drilling for Use in the Oil and Gas Industry

Scott MacGregor (s.macgregor@eee.strath.ac.uk), Professor - Centre for Electrical Power Engineering, Department Electronic and Electrical Engineering in the Institute for Energy and Environment at the University of Strathclyde discusses the results of an EPSRC funded project to investigate the viability of using plasma-channel drilling in the oil industry.

Economic and environment pressures on the Oil and Gas Industry have focused the industry’s attention on new and innovative technologies that will reduce the cost of improving recovery from developed reservoirs and reducing the cost of tapping new reserves. The Oil and Gas Industry Task Force has estimated that as much as 1.3 billion boe could be found through the application of new recovery technology in existing wells. Also, if this technology can be applied to new field exploitation, there is a potential 4.3 billion boe in the North Sea that could be developed in the next few years.

Plasma channel drilling (PCD) technology is an example of this new technology. PCD is a novel electrically driven, non-rotary drilling method that differs from other electrical methods. It does not rely on rock melting or shockwave reduction, but forces a short lived plasma discharge to form inside the rock ahead of the drill. This can be achieved because of the differences in electrical characteristics between the liquid and the rock. Under impulse conditions, the liquid acts as a superior electrical insulator, encouraging the development of electrical discharges through the bulk of the rock. For impulse voltages with rise times less than approximately 1ms, the rock will electrically fail before the liquid and a conducting channel will be created through the rock. Joule heating of the gas in this channel causes it to expand extremely rapidly. The pressure wave caused by the expanding column exceeds the tensile strength of the rock and therefore the surrounding rock fractures and fragments. The plasma channel drill has a radially symmetric electrode design with a central disk-shaped electrode at high voltage and an outer tubular electrode at earth potential. This configuration creates an annular discharge gap, and the plasma channel can form between the electrodes at any position in this gap (Figure 1).  If the plasma discharge is formed many times a second, a highly effective drilling press results. There is no requirement for drill head rotation as the plasma discharge position will automatically change as regions of rock are removed.

Penetration rates of progress and specific energies were measured in the laboratory for a number of different designs of drill electrodes, all with the general form of Figure 1. The minimum specific energy achieved in the experiments was 400 J/cm³.  The maximum rate of progress obtained was 12 cm/min. It compares reasonably with figures obtained for rotary drilling in similar material.  However, it is thought to be too early in the development of this technology to directly compare the rates of progress, which are lower than that of a rotary drill.

Figure 2 shows examples of holes drilled through water saturated red sandstone. Most of the development work was undertaken with a maximum drill diameter of 5 cm and a discharge gap of 8-9 mm. This allowed the magnitude of the impulse voltage applied to the drill to be less than 40kV. The photographs in Figure 2 clearly show that the holes cut using a plasma-channel drill are well defined with smooth bores. Experience has indicated that the plasma, which only exists for a few microseconds, does not cause any thermal damage to the surrounding formation, and therefore the drilling process itself does not damage the formation, although this has not yet been assessed quantitatively. The maximum hole depth during the laboratory trials was approximately 50 cm due to the limited size of the rock samples that could be handled. A three minute long video clip of a typical laboratory trial can be found at the web address www.eee.strath.ac.uk/InFocus/pcd.htm. In the video, the large ruler scale is in inches. The pulse repetition rate was 20 pulses/sec and the average system power during the experiment was only 1800 W. 

The plasma channel method could be used to create small diameter holes (5-10 cm) in a way that is potentially more cost effective than conventional rotary drilling. Reductions in cost could be achieved through a reduction in the personnel and equipment necessary for drilling operations, and the fact that plasma channel drilling could be developed specifically for deployment via coiled tubing or even wireline. Only a few kW of average power is required compared to the several hundreds of kW necessary for rotary drilling. Potential reductions in the environmental impact of the drilling process may be achieved through the use of non-toxic drilling fluids such as sea water and brine, and a reduction in the size of the drill cuttings compared to the size generated by rotary drilling. These much smaller cuttings can be more readily disposed of without the need for further processing.

Figure 1. Diagram of the plasma channel drilling process

Figure 2. Photograph of typical holes in red sandstone.

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