Experimental Study of Solute Transport in Fracture-matrix System Based on Self-potential
In order to finely delineate the process and characteristics of solute transport in the rock fractures,a sandbox(0.20 m×0.03 m×0.50 m,L×W×H)was made by plexiglass plates.There is a fracture in the center of the sandbox,and on both sides of the fracture is the matrix.The fracture and matrix in the sandbox form a fracture-matrix system.The solute transport experiments were carried out under different grain size matrix seepage velocities by self-potential,a nondestructive and fast monitoring means,to study the solute transport characteristics of the fissure-matrix system.The results are as fol-lows.①In the range of test flow rate 0.009 7～3.195 9 mm/s,the fracture permeability coefficient in the system is 5.5×10-2 m/s,and the difference between matrix permeability coefficient 2.4×10-3 m/s and 3.5×10-4 m/s steps is not signifi-cant,the flow conforms to Darcy's law.And with the continuous reduction of matrix particle size,the water flow changes from Darcy flow to non-Darcy flow when the difference between fracture permeability coefficient 5.5×10-2 m/s and matrix permeability coefficient 3.6×10-5 m/s is about 3 orders of magnitude.②Follow the solute transport direction under three different matrix conditions with different particle sizes,the peak SP of the fracture domain and matrix domain electrode measurement points decreased continuously and the peak arrival time increased one by one with the decreasing matrix per-meability coefficient.③With the increasing flow rate,the peak SP time of the slit domain and matrix domain electrode points appeared earlier and the peak SP time of the slit domain electrode measurement points at the same system flow rate was shorter compared with the matrix domain.④Flow velocity can be quantified by SP peak time and solute transport dis-tance.The calculated flow velocity value is slightly smaller than the measured flow velocity value of the system,and the correlation fitting coefficient R2 is greater than 0.99,among which the error coefficient calculated by using the self-potential value of the measurement point of the fracture domain electrode is controlled within 10%,which is higher than the calcula-tion accuracy of the flow rate in the matrix domain,which is highly consistent with the experimental phenomenon and pro-vides a reference for exploring the solute transport process in the fracture-system.
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