Estimation of Fracture Geometry Parameters and Characterization of Rock Mass Structure for the Beishan Area,China

The accurate estimation of fracture geometry parameters and the characterization of rock mass structure are two important topics in the geological disposal system of high-level radioactive waste(HLW).The Beishan area,as the current preselected area for China’s HLW disposal,has three subareas considered to be the key survey area at the stage of site selection.In this paper,a comprehensive survey method conducted on the outcrop is developed to estimate fracture geometry parameters.Results show that fracture occurrence obeys a Fisher distribution,fracture trace length obeys a normal distribution,and the distribution of spacing obeys a negative exponential distribution.An evaluation index,Rock Mass Structure Rating(RMSR),is proposed to characterize rock mass structure for the three subareas.The results show that the Xinchang area is more suitable to act as China’s HLW disposal repository site.At the same time,the index can also be applied to characterize surface rock mass structure and rock mass integrity at the site selection phase of HLW disposal.

Effect of fractures on mechanical behavior of sand powder 3D printing rock analogue under triaxial compression

In practical engineering applications,rock mass are often found to be subjected to a triaxial stress state.Concurrently,defects like joints and fractures have a notable impact on the mechanical behavior of rock mass.Such defects are identified as crucial contributors to the failure and instability of the surrounding rock,subsequently impacting the engineering stability.The study aimed to investigate the impact of fracture geometry and confining pressure on the deformation,failure characteristics,and strength of specimens using sand powder 3D printing technology and conventional triaxial compression tests.The results indicate that the number of fractures present considerably influences the peak strength,axial peak strain and elastic modulus of the specimens.Confining pressure is an important factor affecting the failure pattern of the specimen,under which the specimen is more prone to shear failure,but the initiation,expansion and penetration processes of secondary cracks in different fracture specimens are different.This study confirmed the feasibility of using sand powder 3D printing specimens as soft rock analogs for triaxial compression research.The insights from this research are deemed essential for a deeper understanding of the mechanical behavior of fractured surrounding rocks when under triaxial stress state.

An optimal fracture geometry design method of fractured horizontal wells in heterogeneous tight gas reservoirs

In this work, the unified fracture design （UFD） is extended for the first time to the fractured horizontal wells in heterogeneous closed box-shaped tight gas reservoirs. Utilizing the direct boundary element method and influence function, the dimensionless fracture productivity index is obtained and expressed in the function of proppant volume and fracture geometry at the pseu- do-steady state. With the iterative method, the effectively propped permeability, kfe, is corrected using the i^-situ Reynolds number, NRe. The goal of this paper is to present a new UFD extension to design the proppant volume and the optimal fracture geometry. The results show that there exists an optimal proppant volume for a certain reservoir. The small aspect ratio （yJXe） and high permeability reservoirs need short and wide fractures to diminish the non-Darcy effect. On the contrary, long and narrow fractures are required for the large aspect ratio and low permeability reservoirs. A small proppant volame is prone to creating long fractures, while a relatively large proppant volume creates wide fractures. The new extension can be used to evaluate the previous fracture parameters and design the following fracture parameters of the fractured horizontal well in heterogeneous tight gas reservoirs, with the non-Darcy effect taken into account.

Permeability evaluation for artificial single rock fracture according to geometric aperture variation using electrical resistivity

A convenient approach was proposed by which to evaluate and monitor the permeability of a rock fracture by verifying the quantitative correlation between the electrical resistivity and permeability at laboratory scale.For this purpose,an electrical resistivity measurement system was applied to the laboratory experiments using artificial cells with the shape of a single rock fracture.Sixty experiments were conducted using rock fractures according to the geometry,aperture sizes,wavelengths,and roughness amplitudes.The overall negative relationship between the normalized electrical resistivity values and the aperture sizes directly linked with the permeability,was well fitted by the power-law function with a large determination coefficient(≈0.86).The effects of wavelength and roughness amplitude of the rock fracture on the electrical resistivity were also analyzed.Results showed that the electrical resistivity was slightly increased with decreasing wavelength and increasing roughness amplitude.An empirical model for evaluating the permeability of a rock fracture was proposed based on the experimental data.In the field,if the electrical resistivity of pore groundwater could be measured in advance,this empirical model could be applied effectively for simple,quick monitoring of the fracture permeability.Although uncertainty may be associated with the permeability estimation due to the limited control parameters considered in this research,this electrical resistivity approach could be helpful to monitor the rock permeability in deep underground facilities such as those used for radioactive waste repositories or forms of energy storage.