Experimental Investigations Of Carbonate Fracture Dissolution Expansion By Groundwater

The evolution speed and magnitude of underground karstification within carbonate fractures are determined by the mechanism of groundwater-carbonates interactions.A deep study of carbonate fractures dissolution enlarging progress under water effect is most meaningful to the understanding of characters and patterns karst-dynamic system.This research established a series of carbonate fracture dissolution enlargement experiments under different temperatures,pressures conditions,where the limestone samples are from Shi huigou area,Qaidam basin.As a special type of karstification,this paper specifically studies the fracture dissolution experiment under the CO₂ geological storage temperature,pressure conditions.The saturated CO₂ solutions were pumped through the artificial fractured under different temperatures,fluxes,and CO₂ Partial pressures,while up/down-steam water head and outcome flux were monitored,but also Ca²⁺concentrations of exit solutions were tested.By analyzing the relationship between,the factors?such as,sample mineral composition,micro-structure,temperature,pressure,flux?and the experiment outcome data(including:aperture,outcome solution Ca²⁺concentrations,reaction order,reaction rate constant),more specific process and mechanism of karstification were revealed.In a karst dynamics system,matter and energy transactions within the three phases of CO₂-H₂O-CaCO₃ control the karstification growth behavior.When P_CO2?10^3.5 Pa,outcome solutions contained more concentrated Ca²⁺as the temperature gets higher;when P_CO2?10^4.5 Pa,along with the decreasing flux,Ca²⁺concentration at the exit which increased with the increase of temperature gradually changed to decrease with the increase of temperature.In addition,Ca²⁺concentration increased as not only P_CO2 increase flowing an exponential function,and but also flux decrease along a power function.After P_CO2O2 passed 10^4.5 Pa,factor of partial pressure got more dominating on exit solutions.In this paper,reaction order?n?and reaction rate constant?K_c?in the process of fracture dissolution under different conditions are solved.Both n and K_c are increasing as P_CO2 increases.However,there is a linear positive correlation between n and temperature,but an inverse one between K_c and temperature.Based on the relations of n and Kc against temperature,a fracture dissolution rate equation with different CO₂ partial pressure is presented,which can be used to calculate the fracture dissolution rate at any temperature,and the equation with high universality and reliabilityComparing with most of karstification types,CO₂ sequestration is an atypical one,which generates significant dissolution tubes on the fracture surface at a short time.After the experiment,aperture?B_h?of S₁,S₃ and S₄ were amplified to 3.4,1.4and 1.2 times,correspondingly,while that of S₂ got smaller at the early stage and larger after affected by the initial characters of the sample.Relations between?B_h?and time as be approximated as S-shape,logarithmic function,or polynomial functions.Average dissolution rate of S₁,S₂,S₃,S₄ are 2.50×10^-6,3.11×10^-6,2.70×10^-6?3.04×10^-6 mol/m²/s.The fracture surface dissolution pattern are closely related to the Pekley constant?Pe?and Darmkole number?Da?.When Pe is the same,the larger the Da,the more the fracture surface dissolution tends to be homogeneous.On the other hand,when the Da is the same,the greater the Pe,the more the fracture surface is in the channelizing dissolution pattern.Besides,the heterogeneity of fracture surface also has an important effect on dissolution pattern.Bioclastic aggregation,calcite joints,fracture surface micro-fractures and calcite vein filling location are more likely to occur priority dissolution.