Shining light on shale: geomechanics and 4D fracture characterization

The successful development of a shale gas industry in the UK can be aided by improved understanding of the fracture and physical properties of these rocks. The key petrophysical properties that determine how fractures initiate and grow are determined on the scale of flaws in the rock. Laboratory measurements of these properties are essential to successful modelling studies of hydraulic fracture, and hence optimisation of production. In this research a range of innovative technologies will be used including, for the first time, 3D observation and quantification of hydrofracture growth with time inside the rock by means of synchrotron X-ray tomography coupled with location of crack growth events using acoustic emissions. This will show how hydrofractures initiate and develop, the role of pre-existing flaws in their initiation, the amount of fluid pressure required to initiate them, and how their form might be modified to optimize gas production.
Interpretation of the results depends on parallel measurements of the strength, friction and resistance to fracture, and also permeability to fluids, because rock strength depends on the extent to which fluids can penetrate their pores. These properties are also affected by the mineralogy, organic content and microstructure of the shale. Therefore we will test several shales with different such characteristics. These properties also determine, in ways yet to be understood, the stability of boreholes. Permeability of shales to gas and liquids is also known to depend on value of the fluid pressure in the pores for a given depth of burial, thus it changes as gas pressure is drawn down by production. Knowledge of this pressure sensitivity is essential for the successful interpretation of well tests, from which gas in place and likely reservoir yield must be estimated. Thus this research will produce results that can impact on a range of aspects of shale gas exploitation.

Beneficiaries and how they will benefit

The principal commercial beneficiaries are the oil and gas sector, but also possibly the geotechnical sector. This should be through improved understanding of the initiation and growth of hydrofractures, which should impact upon more realistic modelling of crack growth, and of the interpretation of seismic detection of propagating hydrofractures. Through the incorporation of rock strength, fracture toughness and permeability data into understanding the hydrofracture process it may prove possible to enhance the complexity of crack networks, with improved yield. Determination of the stress sensitivity of permeability should lead to improved interpretation of well test data, improved methods for evaluation of shale gas resources from well tests, improvements to modelling of hydrofracs and possible impact on hydrofrac procedures. These results should be able to impact upon industry practice after about 3 years, if effectively disseminated.

Benefits to the burgeoning UK shale gas industry also mean benefits to UK economy as a whole in terms of job creation, enhanced energy security and reduced dependence on foreign gas imports. As has been demonstrated in the United States, more competitive and secure local energy supplies translates into incentives for development of other businesses.

The wider public are expected to benefit as a result of our dissemination of results (website) and further development of the outreach activities in which we already engage (radio and TV presentations, public lectures), encouraging better understanding of the industry. It is already clear that the wider public have a thirst for knowledge on shale gas exploitation and how it may affect their lives, and they naturally turn to academia for informed and unbiased opinion.

The two PDRA staff working on the project will develop practical and theoretical skills in mechanical testing and X-ray imaging that will transfer into their future careers and employment in several areas of geoscience, geotechnics and materials science.