Potential of triaxial stress investigations in rocks using time-of-flight neutrons at EPSILON / JINR Dubna

In geodynamics as well as in reservoir geomechanics, the knowledge of the triaxial state of stress in a rock volume is essential to describe and understand processes such as mountain building, plate motion and especially seismicity, natural as well as induced. Induced seismicity in the vicinity of gas production reservoirs frequently is ascribed to production of natural gas on one hand. Production induced seismicity is observed in gas fields in The Netherlands, Germany and France. However, only a limited number of gas fields shows seismicity, and only after several years of production. On the other hand, wastewater injection has induced earthquakes in Mid-Western US. Furthermore, injection induced seismicity was observed around injection wells of geothermal projects, especially if the water is injected into critically stressed faults. In both cases, production and injection, the pore pressure within the reservoir causes changes in effective stress. If the shear stresses in the reservoir reaches the frictional strength of faults, which are optimally oriented, these faults can be reactivated. To address and study the processes leading to induced seismicity during gas production or water injection the evolution of stress with pore pressure and time has to be studied in-situ. Standard laboratory experiments on large-scale specimens can handle the vertical and horizontal stresses as well as pore pressure, but it is not possible to obtain results in strains and stresses within the sample. We developed a new triaxial cell for the neutron time-of-flight strain diffractometer EPSILON at the pulsed neutron source IBR-2 at JINR Dubna. This new innovative equipment enables to investigate the reaction of polycrystalline material under reservoir stress and pore pressure conditions. The tool has been installed end of 2017 and first diffraction pattern provide new insight into the coupling of pore pressure and stresses.