A new paper from Japanese earth scientists has proposed a method for calculating rock strength using drilling data alone
Understanding the characteristics and behaviours of rock mass is vital to drilling and well operation. One of the key metrics in informing this understanding is in-situ rock strength, which allows earth scientists to evaluate slip tendency, faults, stress states and other characteristics.
Typically, this information can be assessed by taking core samples which can be tested and evaluated in the laboratory. However, in addition to being a costly and time-consuming process, the quality of data depends heavily on the quality of the core samples taken. Samples only allow for estimations of in-situ strength obtained either through experiment or seismic correlation, and remain inaccurate owing to the limited amount of rock samples and the uncertain conditions below the seafloor such as pressure and temperature.
In response, scientists at the Japan Agency for Marine Earth Science and Technology (JAMSTEC) have devised a new method, which they say allows for direct and continuous measurement of relative in situ rock strength using only drilling performance data. Using information recorded while drilling – specifically drill string rotational torque, bit depth and string rotational speed – they have constructed a conversion equation which allows the inference of rock strength, a metric dubbed equivalent strength (EST).
Using drilling metrics to inform studies of formations was first proposed in the mid-1960s, and used known parameters like weight on bit (WOB) and string rotational torque to optimise drilling in various types of rock. Recent proposals have even included smart bits capable of evaluating rock strength while drilling. However, in the case of deepwater drilling, rough seas and downhole friction can often affect the data – and any measurements must be recorded by an on-bottom measurement-while-drilling (MWD) system, or else the information cannot be used.
By contrast, JAMSTEC’s method uses just three key pieces of information – torque, bit depth and pipe rotation speed – all of which are measured from the surface of the rig during drilling. In essence, the EST value is calculated based on the stress supplied to the formation, with the equation assuming a simplified bit shape.
Unlike in the above methods, torque losses from resistance and friction are cleaned up in the final EST equation, with lost torque subtracted from torque measured during data processing to reveal distinct formation properties. The team’s paper, “Continuous depth profile of the rock strength in the Nankai accretionary prism based on drilling performance parameters” was recently published in Scientific Reports.
Led by Dr Yohei Hamada at Agency’s Fault Mechanics Research Group, the method was used to study rock in Japan’s Nankai Trough. Using data recorded from drilling operations in the Nankai Trough Seismogenic Zone Experiment (NanTro SEIZE) by the deepwater drilling vessel Chikyu, it helped confirm that the shallow accretionary prism in the formation had been strengthened by horizontal compression owing to plate subduction.
Their results show that rock strength in the area of study was larger than expected from the pressure condition, inferring that sediments above the Nankai megathrust zone – the cause of the 1944 Tonankai Earthquake – have been strengthened by horizontal compression as a result of the plate conversion.
Strength and numbers
EST calculations provide a more accurate reading of in-situ conditions over a much larger scale than core sampling. Speaking to InnovOil via email, Dr Hamada also explained that “basic physical properties can be calculated by the EST. It is known that some physical properties (porosity, permeability, internal friction and so on) correlate with strength,” meaning EST data can be used to provide information on stress state, and pore pressure and temperature – something laboratory tests do not offer.
Hamada is confident that additional information could be used to verify results: “This estimation will be accurate if we can use logging data or rock information other than strength. We plan to investigate the correspondent relationship between physical properties and EST based on laboratory drilling experiments.”
In addition to geotechnical applications, the method could also be used to inform future hydrocarbon exploration. The JAMSTEC team has already submitted a paper which looks at applications of EST for gas hydrate drilling, and Hamada added: “The logging technique is commonly used as an indicator of penetration of gas and oil. These resources or their reservoirs can show characteristic strength contrast comparing to surrounding formations. Thus it will be possible to estimate the existence/distribution of resources, as with borehole logging.”
In their paper, the researchers acknowledge some limitations to the method: namely, that in-situ stress and pore pressure conditions cannot be measured continuously and are thus unable to be verified exactly. As such, they say, EST “should be treated as only an ‘equivalent strength’, which can be thought of as the relative in-situ rock strength over a wide depth scale,” rather than an exact value.
However, that does not mean its implications should be discounted. The method is especially promising as an aid in drilling projects where core sampling is impractical or extremely challenging, such as in hydrothermal areas.
For now, the team will concentrate on refining and improving the accuracy of EST measurement. Hamada added that further studies would be necessary, including drilling experiments in the lab, in order to increase the accuracy of the conversion.
“The formula we proposed is quite simplified in order to focus on correlation between drilling parameters and rock strength. We plan to modify the formula reflecting more realistic bit shape,” he said. “Also, we will perform laboratory drilling experiment to clarify how the bit shape affects the drilling parameters and EST.”
However, the prospects for such a tool are very promising, and not just in terrestrial applications. Hamada reiterated that because the method did not require any physical sample retrieval, future deployments could see it reveal the strength profile of “any drillable area”, including active tectonic margins, upper mantle – “even the crust of other planets.”