The high cost and the financial risk imposed on both oil and mineral companies by drilling means efficiency is paramount. There is a continual drive to improve the design of drill bits to maximise their ability to cut a wide variety of rocks at considerable depths with maximum productivity and minimum fault or delay.

Recent research, led by Australia’s Commonwealth Scientific and Industrial Research Organisation’s (CSIRO) petroleum resources division (now part of the newly formed CSIRO earth science and resource engineering division – CESRE), has focused on the complex yet crucial relationship between drill bit and rock.

Demand for data

An improved understanding of exactly what happens at the so-called "bit-rock interface" will, the scientists believe, allow for better bit design and recommendations for drilling control parameters to increase the efficiency of drilling, tunnelling and mining operations.

According to mechanical engineer and CSIRO research scientist Dr Luiz Franca, "as the borehole starts at the bit-rock interface, a better understanding of the processes at work at this interface can not only drive innovations in bit design and drilling systems, but also provide robust drilling data interpretation tools.

“There is a continual drive to improve the design of drill bits to maximise their ability to cut a wide variety of rocks.”

"Considering the bit-rock interface laws for impregnated diamond bits, for example, there is a crucial lack of fundamental information on the operation of these bits despite their use over the past 50 years. Indeed, the basic mechanism of cutting is not totally understood and the need for a description of the cutting process and the enhancement of performance prediction models remains," he says.

"Also, a lot has been invested in downhole sensors and data transmission systems but there is a common belief in the industry that very little is available in terms of data interpretation tools. Drilling data contains information about the "wear state" of the bit and about the mechanical properties of the rock that is being excavated.

"If these sets of information could be reliably extracted, interpretation of the drilling data would lead to an optimisation of the drilling process or simply to logs of rock properties to be used in the planning of other wells or for other purposes, such as well-bore stability.

"In particular, the possibility of a real-time optimisation of field operations relies on the ability to interpret drilling data, using the bit-rock interface laws, in terms of the rock strength properties and the bit’s wear state," Franca says.

Making the cut

The objective of Franca’s research programme is to better understand the drilling response of different bits as they operate in different conditions and rock formations. A phenomenological approach has been used to investigate the bit-rock interface laws in this case – "phenomenology" – meaning without attempting to explain the observations in terms of more fundamental quantities that cannot be measured or observed at the scale of the experiments. Using this approach, quantitative information from drilling data related to rock and bit properties as well as the drilling efficiency can be investigated in terms of lumped parameters.

The research programme has both a theoretical and an experimental component. The theoretical research focuses on developing phenomenological laws of the interface process and to build an understanding of the physical mechanisms for different drill bits.

The experimental work aims to obtain accurate data on the bit-rock interface laws by conducting experiments under rate of penetration and rotary speed imposed (kinematically controlled) so as to eliminate any artefacts induced by machine vibration.

CESRE’s drilling mechanics group has access to state-of-the-art research facilities and laboratories, which allows its scientists to conduct specialised research and service for the drilling industry.

The group’s equipment includes:

  • Kinematically controlled drilling rigs (imposed depth of cut and cutting speed)
  • Cutting devices for testing polycrystalline diamond compact cutter and impregnated diamond segments over a large range of cutting speeds
  • Rock cutting devices for testing under confining pressure (up to 70MPa)
  • Topography measuring devices for impregnated diamond segments.

Efficiency by design

Franca says the drilling action of drag bits is characterised by a cutting or scratching process. "In the case of the roller-cone bits, we observe a combination of cutting and indentation processes; in impregnated diamonds bits, on the other hand, the drilling action is dominated by the wear process of both the diamonds and the bonding matrix.

"Although different processes govern the drilling action of each bit type, we have observed that the interface laws rely on a combination of two distinct processes; ‘pure’ cutting and frictional contact, and the difference between the drilling actions of different dill bits are only in the range of the model parameters," he says.

“An improved understanding of exactly what happens at the so-called ‘bit-rock interface’ will allow for better bit design.”

"This means that the drilling efficiency can be investigated in terms of only two parameters, the specific energy and the friction coefficient, independently of the bit type (drag bits, rollercone and impregnated diamonds bits)."

Franca and his team are carrying out laboratory tests to quantify the influence on the specific energy of various parameters such as drilling method, cutter geometry (size and shape), depth of cut, cutting speed and mud pressure.

"Based on this data, we can provide recommendation for the drilling control parameters as well as for bit design," Franca says.

A number of bit design and manufacturing companies, the service company Schlumberger and the Brazilian oil company Petrobras have already shown interest in Franca’s results.

This article first appeared in the CSIRO’s Earthmatter magazine issue 10. For more information, click here.