Modelling rock fracture behaviour

Hydraulic fracturing or hydro-fracking is one of the most important techniques used to extract shale gas and coal bed methane (CBM). India currently has the fifth-largest proven coal reserves in the world, with about 2600 billion cubic metres of CBM and approximately 2000 trillion cubic metres of recoverable shale gas. It is therefore vital to know how the rocks will behave in the subsurface when subjected to fracture stress.

We know that rocks crack under stress, and have some idea why they do.

As a next step, if we could predict with some degree of accuracy when rock fracturing is likely to take place, it would be of great help to industries like petroleum and natural gas, mining, and earthquake engineering.

Figure 1: A macroscopic fracture from a smaller scale point of view

Bankim Mahanta, a research scholar with the IITB-Monash Research Academy, is therefore working on a project titled, ‘Investigation of fundamental mechanism on rock fracture by nanomechanics-based modeling techniques’, under the supervision of Prof T N Singh, Prof P G Ranjith, and Dr WenHui Duan.

How is small scale fracturing important for understanding rock behaviour?

Says Bankim, “The behaviour of micro-macro fractures plays a key role in rock mass strength. A proper understanding of how fractures originate, propagate, migrate, and terminate in the reservoir conditions is of vital importance from the standpoint of science and engineering—particularly for the study of energy and infrastructure.”

The IITB-Monash Research Academy is a collaboration between India and Australia that endeavours to strengthen scientific relationships between the two countries. Graduate research scholars here study for a dually-badged PhD from both IIT Bombay and Monash University, spending time at both institutions to enrich their research experience.

“Cracks propagate in rocks as a result of coalescence, branching, forking, and crossing,” explains Bankim. “By nature, fracture surfaces are irregular and rough because of the weak interface of rock grains which are the prime site for the beginning of a crack or fracture. External loads to the rocks lead to a magnification of the local stresses, as a result of which failure is induced because of an increment in the atomic forces, which leads to shearing and rupture of the chemical bonds, resulting in the failure of the rock. Significant research on this subject has so far been confined to the macroscopic scale; it is critical to understand what happens at microscopic levels as well.”

Bankim is therefore focusing on fracture behaviour from a micromechanical and nano-mechanical point of view under special boundary conditions like loading rate, strain rates and geological environmental parameters like temperature, pressure, and saturation. He has decided to pay particular attention to deformational behaviour of various reservoirs rocks like sandstone, shale and coal and parameters like fracture toughness [including modes like mode I (tensile), mode II (shear) and mixed mode (I-II)], Poisson’s ratio, and Young’s modulus.

Says, Prof Murali Sastry, CEO, IITB-Monash Research Academy, “The Academy is an exciting chapter in Indian-Australian relations that will see both countries creating binding links. This will enable us to tackle the research challenges that lie ahead and generate some long-lasting high impact outcomes for society. This research, for instance, is significant in a variety of areas including hydraulic fracture propagation, energy extraction and exploitation, in-situ stress determination, fragmentation of rock in mining using cutting and blasting actions, and slope stability assessment.”

Bankim Mahanta senses he is onto something big, and is banking on his rock-solid research skills to make a significant contribution in this field.

Research scholar: Bankim Mahanta, IITB-Monash Research Academy

Project title: Investigation of fundamental mechanism on rock fracture by nanomechanics based modeling techniques

Supervisors: Prof T N Singh, Prof P G Ranjith, and Dr WenHui Duan

Contact details: bankim42mahanta@gmail.com, bankimcm@iitb.ac.in

This story was written by Mr Krishna Warrier based on inputs from the research student, his supervisors and IITB-Monash Research Academy. Copyright IITB-Monash Research Academy.