Study On Size Effect Of Seepage Characteristics Of Natural Rock Fracture Under Hydraulic Coupling

In the geothermal energy development project,understanding and mastering the underground dry-heat rock seepage rules plays a vital role in promoting the efficient use of geothermal energy.The fracture network is the main channel for fluid movement in the rock body.The single fracture surface is the basic element constituting the rock body network.The permeability and direction of the rock body are not only related to the development and cutting characteri stics of the fracture network,but also to the geometric characteristics of a single fracture(Such as crack aperture,direction,roughness and filling,etc.)are closely related.Therefore,the hydraulic conductivity obtained from the seepage experiment of the small-scale model is insufficient to represent the permeability characteristics of the engineering rock mass.To study rock hydraulics and reasonably predict the complex seepage state in the engineering rock mass,the size effect of the seepage characteristics of the single fracture surface must be used.Start with this basic subject.At present,there are still many deficiencies in the existing research on single-fracture seepage test and numerical simulation of size effect of permeability characteristics.For the single-fracture seepage test,most of them cannot achieve the goal of multi-scale visualization.Most of the fracture surfaces of simulation software are generated according to a certain distribution function,which cannot fully represent the geometric characteristics of natural rock fractures.The conclusions to be obtained need to be further demonstrated.In view of the importance of the rock fracture seepage problem and the current research deficiencies,this paper mainly studies the size ef fect of the natural rock fracture seepage characteristics under hydraulic coupling conditions.This study mainly relies on the independently developed MATLAB rock fracture hydraulic coupling simulation program for numerical simulation.At the same time,com bined with 3D scanning and 3D printing technology,a series of fracture seepage visualization experiments were carried out,which can directly observe and analyze the seepage phenomenon of the fracture surface.Through the visual experiment,the seepage flow and solute transport inside the fracture are described,and the equivalent hydraulic aperture is obtained,which verifies the reliability of the hydraulic coupl ing simulation program.Then,a handheld three-dimensional laser scanner was used to scan the fracture surface of the field rock engineering,and a three-dimensional rock fracture surface model was obtained.The basic mechanical properties of the rock were determined through uniaxial compressive testing,and the geometric characteristics of the fracture surface were evaluated based on the obtained fracture point cloud data.Finally,the MATLAB program was used to carry out numerical calculations for three types of natural rock cracks,and the three influencing factors of normal stress,crack roughness and crack aperture were discussed.This paper confirms that there is a size effect on the permeability of rock fractures.Its permeability is proportional to normal stress and fracture roughness,and inversely proportional to fracture aperture.Fracture aperture is the main controlling factor for permeability.In the same rock fracture,the smaller the fracture surface size stud ied,the more unstable the permeability is,and the larger the size,the closer to the permeability of the rock fracture.