Numerical modeling of hydraulic fracture considering the pre-existing joints using PFC software

Document Type : Original Research

Authors
E. Attari
A. Golshani
Ministry of Science, Research and Technology
DOI: 10.22034/22.2.15
Abstract
Hydraulic fracturing is a new and widely used method for extracting reserves and energy resources in the depths of the earth. In the near future, due to increase in energy consumption on the one hand and depletion of energy reserves on the other, using of this method will become a necessity. One of the most important and effective parameters in this process is the pressure and how it is applied in order to create fractures and fracture progression in rock layers. Another important parameter is the interaction between pre-existing natural fractures with different angles and hydraulic fracture.

Due to the high costs of this process, the purpose of this study is to achieve the optimal state for the maximum progress of hydraulic fracture and the lowest amount of breakdown pressure at the same time. Numerical modeling was performed in two dimensions by Particle Flow Code ( PFC ) software from Itasca company on samples of Pocheon granite rocks with brittle behavior using distinct element method. PFC software uses circular disks in two dimensions to construct and make the sample using distinct element method. These particles are in contact with each other through bonds. In this program there are walls that interact with these disks to apply load on the sample. The flat joint model is used in order to create contacts between particles, and discrete fracture network is uded to to create pre-existing natural fractures for interacting with hydraulic fracture.

Given that in the distinct element method (PFC software) our sample consists of a large number of particles in two dimensions that the general characteristics of the sample are formed based on the interaction between these particles, so we need parameters as input data to our software, existing disks and the link between them, to finally obtain the specifications of the same laboratory sample after modeling. These specifications and input data are referred to as micro parameters, and the final specifications, which are the same as our mechanical parameters in the laboratory, are referred to as macro parameters.

To find the micro parameters of the sample we use the trial and error method. Here, our modeling is based on Brazilian and uniaxial compression experiments and … performed by Zhuang et al. laboratory investigations.

Due to the limitations of PFC software for fluid flow modeling and limitations for using of CFD relationships, pressure equals to fluid pressure can be used as a new solution. In this way, by modeling a number of walls that form a complete circle with overlapping each other, and by considering the servo control mechanism, we move them in the opposite direction of their normal vectors and off-center, creating a comprehensive pressure Which is actually same as the fluid pressure. From the obtained results, it was found that with increasing the loading rate, the sample reaches the breakdown pressure and breaks in less time, but the amount of breakdown pressure increases. Also, by increasing the natural fracture angle relative to the horizon (clockwise), the specimen breaks at a lower breakdown pressure. Finally, by increasing the natural fracture distance from the center of the sample, the effect of the presence of the joint in the sample decreases and the breakdown pressure approaches the seamless state.

Keywords

Subjects

[1] J. W. Tester et al., “The future of geothermal energy,” Massachusetts Inst. Technol., vol. 358, 2006.
[2] P. Valkó and M. J. Economides, Hydraulic fracture mechanics, vol. 28. Wiley Chichester, 1995.
[3] H. Fatahi, M. M. Hossain, and M. Sarmadivaleh, “Numerical and experimental investigation of the interaction of natural and propagated hydraulic fracture,” J. Nat. Gas Sci. Eng., vol. 37, pp. 409–424, 2017, doi: https://doi.org/10.1016/j.jngse.2016.11.054.
[4] J. Zhou, L. Zhang, Z. Pan, and Z. Han, “Numerical studies of interactions between hydraulic and natural fractures by Smooth Joint Model,” J. Nat. Gas Sci. Eng., vol. 46, pp. 592–602, 2017, doi: https://doi.org/10.1016/j.jngse.2017.07.030.
[5] Y. Zhang et al., “An experimental investigation into the characteristics of hydraulic fracturing and fracture permeability after hydraulic fracturing in granite,” Renew. Energy, vol. 140, pp. 615–624, 2019, doi: https://doi.org/10.1016/j.renene.2019.03.096.
[6] T. Ishida, Q. Chen, Y. Mizuta, and J.-C. Roegiers, “Influence of Fluid Viscosity on the Hydraulic Fracturing Mechanism ,” J. Energy Resour. Technol., vol. 126, no. 3, pp. 190–200, 2004, doi: 10.1115/1.1791651.
[7] L. Zhuang, K. Y. Kim, S. G. Jung, M. Diaz, and K.-B. Min, “Effect of water infiltration, injection rate and anisotropy on hydraulic fracturing behavior of granite,” Rock Mech. Rock Eng., vol. 52, no. 2, pp. 575–589, 2019.
[8] D. O. Potyondy, “The bonded-particle model as a tool for rock mechanics research and application: current trends and future directions,” Geosystem Eng., vol. 18, no. 1, pp. 1–28, 2015, doi: 10.1080/12269328.2014.998346.
[9] P. A. Cundall and O. D. L. Strack, “A discrete numerical model for granular assemblies,” Géotechnique, vol. 29, no. 1, pp. 47–65, 1979, doi: 10.1680/geot.1979.29.1.47.
[10] D. Potyondy, A flat-jointed bonded-particle material for hard rock, vol. 3. 2012.
[11] D. O. Potyondy, “A Flat-Jointed Bonded-Particle Model for Rock,” 2018, vol. All Days.
[12] P. Wang, M. Cai, and F. Ren, “Anisotropy and directionality of tensile behaviours of a jointed rock mass subjected to numerical Brazilian tests,” Tunn. Undergr. Sp. Technol., vol. 73, pp. 139–153, 2018.
[13] S. Kahraman, “Evaluation of simple methods for assessing the uniaxial compressive strength of rock,” Int. J. Rock Mech. Min. Sci., vol. 38, no. 7, pp. 981–994, 2001, doi: https://doi.org/10.1016/S1365-1609(01)00039-9.
[14] C. Fairhurst, “On the validity of the ‘Brazilian’test for brittle materials,” in International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 1964, vol. 1, no. 4, pp. 535–546.
Modares Civil Engineering journal
Volume 22, Issue 2
May and June 2022
Pages 241-260