RED gets ready

Resonance enhanced drilling could help speed up drilling time and efficiency. Elaine Maslin reports.

Professor Marian Wiercigroch with the vertical RED full scale experimental rig.  Images from the University of Aberdeen.

After nearly two decades research, work looking at how high-frequency impacts effect the dynamic fracturing of rocks, in a way which could significantly boost drilling efficiency, is close to completing its latest trials – with the hope the next step will be field trials.

Resonance Enhanced Drilling (RED) has been developed at the University of Aberdeen-based on research by applied dynamics Professor Marian Wiercigroch, based on theoretical mechanics.

The technique, which uses high frequency, to create resonances and to generate a controllable zone at the bit, improving significantly rate of penetration and reducing bit wear, as well as stress on the bit, has its roots in theoretical dynamics and fracture mechanics. Wiercigroch has been keen to prove and calibrate the theories through experimental analysis on a purpose-built large-scale test rigs (see the photograph and schematic of the vertical and horizontal RED experimental rigs).

The result, he says, is a flexible technique, able to be optimized according to rock type, which could improve on average drilling rates by at least 40%. In case of hard rock drilling, the improvement can be as large as 250%.

“There is nothing similar out there,” says Wiercigroch, who is director of the Centre for Applied Dynamics Research within the University of Aberdeen’s School of Engineering. “It is excellent for hard rocks and could reduce the cost of drilling significantly which is so important in the current situation.”

Classic drilling technology uses axial static force (weight on bit) and rotation, to shear off layers of the formation being drilled. RED adds high frequency oscillatory loading to the rotary drill bit. In other words, with RED controllable high-frequency axial vibration is added, creating resonance conditions which dynamically fracture the rock. The frequency, typically around 150-800Hz depending on the application, with 1mm maximum amplitude, is created using a transducer, and can be adjusted according to the drilled formation.

To protect instrumentation behind the drill bit, transducer and exciter unit, a vibration isolation unit has been included in the assembly, and has proved effective, Wiercigroch says.

Early small-scale experimental studies were launched in about 2000, using the piezoelectric transducer, with results indicating a large improvement in penetration rates, a steadily propagating crack zone, and good borehole stability. In 2006, drilling tests were carried out on basalt, pink granite and sand stone, with drilling rates improved by 20X compared to conventional drilling techniques, Wiercigroch says.

To further understand and refine the results, more theoretical studies were performed, and then bifurcation analysis, confirming the mathematical models. This meant the model could be refined to operate at “sweet spots,” – the best frequencies and amplitudes.

The work also looked at how different porosity and permeability rocks would impact the stress and strain values. A joint study with Brunel University has also looked at fracture processes at the borehole.

A schematic of the horizontal RED full scale experimental rig (the total length is circa 8m and the rig allows for comprehensive testing programs with a large flow rates of drilling fluids).

Testing has been performed on a suite of gradually larger and larger test rigs, including a 3.4m-high rig, culminating in the full-scale, horizontal test-rig, which the university is using today, using industry-standard polycrystalline diamond compact (PDC) and roller-cone drill bits. Both rigs work by rotating the sample, rather than the rig and the most recent experiments have shown drilling time rate improvements of a factor of four and five.

Finally, the mathematics have been simplified to enable faster predictions, which means the drilling can be controlled downhole in real time, Wiercigroch says. This does mean that the current technology for communication down hole to topside, when the unit is controlled downhole, is sufficient [See “DSATS” story on page 24]. Work is also ongoing to develop the drilling head to optimize pressure pulse signals to the surface to 50 pulses per second.

“With this technology you would not have to change the drill-bit so many times because the static weight is less,” he says. “This means less non-productive time. Drilling could be few times faster – and the borehole stability could be better. You can drill with less force on the formation because the force required to break the rock is obtained from a dynamic contribution. This means you can redesign the drill-string so it doesn’t have to be as heavy and you can drill much more easily horizontal wells.” The dynamic stress is also propagated in the direction of the drill bit, not perpendicular to it, maintaining good borehole stability.

The vertical RED full scale experimental rig (the height is around 4m and the rock sample is turned instead of drill-string).

RED is not designed for soft formations, but, as it uses classical drag type drill-bits, this can be accommodated for, says Wiercigroch, by turning off the RED module. The key is to determine and maintain the resonance in the borehole for varying conditions through monitoring feedback responses.

The RED research program has come a long way since it first started in 1998, with funding from the Centre for Petroleum and Marine Technology, and has since had backing from the Royal Society, Royal Academy of Engineering and the Engineering and Physical Sciences Research Council as well as from industry including BP, BG Group, NOV, QinetiQ and others.

This year a 60-month, £4.6 million research and development program funded by ITI Energy, focused on upscaling the RED technology and bringing it closer to the commercial world (now part of Scottish Enterprise), will be completed. The program has been a team effort involving in total six academics, 12 research fellows six PhD students and two technicians.

“We are close to the stage where the RED technology can be tested by industry and I hope this will happen soon,” says Wiercigroch. He adds: ”RED has a diverse range of application and as well as use in oil and gas drillinghe the technology can be used in geothermal drilling, as well as in mining and even in dentistry.”