Physico-mathematical modelling of mechanical processes of rock fracturing at the micro- and nano-scales

Reliable forecast of fracture propagation during hydraulic fracturing operations in complex reservoirs rocks is a complicated task. It is tightly coupled to studying their mechanical parameters, microstructure at various scales and elastic strength characteristics of rocks. The objective of this work is to investigate and evaluate the mechanical parameters and boundary conditions of the studied intervals at microscale that need to be created in unconventional rock reservoirs to obtain an extensive network of non-main fractures. This allows to increase the efficiency of reservoir stimulation of unconventional hydrocarbon fields and maximize production from non-connected previously pores. To achieve the denoted goal, the authors propose the method which contains the following workflow: building a dataset containing petrophysical, geomechanical and mineral data, preparation and initialization of 2D and 3D microscale digital rock models and numerical simulations of their stress-strain states and fracture propagation in them. In this work, authors conduct a set of experimental investigations of mechanical parameters of rock samples, CT before and after the formation of fractures, QEMSCAN and mineral composition of rocks. Next step was the multimodal segmentation and registration of 2D QEMSCAN and 3D X-ray micro-CT data to develop a workflow for constructing 3D mineral digital rock models. Finally, a grid was built on a 3D digital model of segmented rock and loaded into a mechanical simulator where the rock matrix was assigned the appropriate mechanical properties. As a result of numerical simulations, stress-strain state for different loading conditions were obtained and the conditions under which the highest fracture formation occurs were chosen. The example of using the proposed workflow is based on the results of the study Russian most promising unconventional tight gas formation with pore space up to tens of nanometers.

1. Elgmati M. et al., Submicron-pore characterization of shale gas plays, SPE-144050-MS, 2011.

2. Chugunov S.S., Cheremisin A.N., Digital core model of the Bazhenov formation for hydrodynamic and geomechanical calculations (In Russ.), EAGE Geomodel’. 2015, DOI: 10.3997/2214-4609.201413999.

3. Holt R.M., Li L., Larsen I., Digital rock mechanics: A discrete way of approaching failure, Proceedings of 51st U.S. Rock Mechanics/Geomechanics Symposium, San Francisco, California, USA, 2017/8/28, 2017.

4. Tan X., Konietzky H., Chen W., Numerical simulation of heterogeneous rock using discrete element model based on digital image processing, Rock Mechanics and Rock Engineering, 2016, V. 49, no. 12, pp. 4957-4964, https://doi.org/10.1007/s00603-016-1030-0.

5. Agalakov S.E., Bakuev O.V., New objects of hydrocarbon prospecting in over-Cenomanian deposits of the Western Siberia (In Russ.), Geologiya nefti i gaza, 1992, no. 11, pp. 25–28, URL: http://geolib.ru/OilGasGeo/1992/11/content.html.

6. Bondarev V.L. et al., Unconventional gases in the north of Western Siberia (In Russ.), Geologiya, geofizika i razrabotka neftyanykh i gazovykh mestorozhdeniy, 2008, no. 10, pp. 4–17, URL: http://www.npcgeo.ru/files/page/page26-file-netradgazy.pdf

7. Nezhdanov A.A., Ogibenin V.V., Skrylev S.A., The structure and prospects of gas content of the Senonian deposits of the north of Western Siberia (In Russ.), Gazovaya promyshlennost’, 2012, V. 676, pp. 32–37, URL: http://gasoilpress.ru/gij/gij_detailed_work.php?GIJ_ELEMENT_ID=51714&WORK_ELEMENT_ID=51735.

8. Kazak A. et al., Integration of large-area SEM imaging and automated mineralogy-petrography data for justified decision on nano-scale pore-space characterization sites, as a part of multiscale digital rock modeling workflow, Proceedings of SPE/AAPG/SEG Unconventional Resources Technology Conference, 24-26 July 2017, Austin, Texas, USA, https://doi.org/10.15530/URTEC-2017-2697437.

9. ASTM D2936-08. Standard test method for direct tensile strength of intact rock core specimens, 2008.

10. ASTM D3967-05. Standard test method for splitting tensile strength of intact rock core specimens, 2005.

11. Nachev V. et al., Development of an integrated model of rock fracturing at nano/microscale, Proceedings of Skoltech & MIT Conference «Shaping the Future: Big Data, Biomedicine and Frontier Technologies», 25-26 April 2017.