Experimental Approach To Limestone Fracture Dissolution Under Turbulent Groundwater Condition

Groundwater's dissolution on limestone fracture possesses an important role on the scale and pattern of storage space within Limestone Geo-reservoir.A deep Study on dynamic mechanism of limestone fracture dissolution and expansion under the action of groundwater can assist and promote further understanding of underground Karstification.The three pieces of limestone samples in this thesis are obtained at ZK5-2 drill hole 260m-390m from surface in Shihuigou Area,which is in front of the Oulongbuluk Mountain located in northeast margin of the Qaidam basin,China.Based on Diffusion Boundary Layer?DBL?film model,considering the compatibility of the experimental apparatus,a flux?Q?-concentration?C?equation was derived from the diffusion rate formula:F=K_c(C_eq-C)ⁿ.According to the single variable method,N₂-CO₂ mixture gas under different CO₂ partial pressures(10^2.5Pa,10^3.5Pa,10^4.5Pa)were dissolved in de-ionized water.The carbon acid solutions following experiment sets design,were pumped through the artificial dissolution tube.The outlet was captured and concentration of Ca²⁺was tested by ICP-OES.Through the results,30m L/min is determined as flow state boundary with Reynolds Number around 1700.Smaller currents lower than the boundary were laminar flow,while the larger ones ware turbulent flow.Using the laminar flow data,the actual diameters of the three artificial tubal passage,all of which are about 0.6mm,are calculated by means of Hagen-Poiseuille law.The diameter variation of each experiment set was within 2.3%,with no unified increasing trace.Therefore,it can be assumed that the enlargement of the tube diameter caused by dissolution in the experimental process can be neglected.ICP test result show an extremely low saturation of Ca²⁺,indicating that reaction stayed far away from chemical balance.Reaction order?n?and dissolution rate constant?K_c?were derived from test result by optimal curve fitting of Q-C plots.Furthermore,dissolution rate?F?changing behavior with Ca²⁺concentration under given conditions and the dissolution patterns of limestone pipelines under turbulent flow pattern was investigated.Calculated by diffusion rate formula,turbulent flow of the experiments released8*10^-5 to 30*10^-5 mg Ca²⁺per square-centimeter per second,which were 1.2-1.5times of laminar flow.Ca²⁺was released faster with higher P_CO2 or solubility.However,higher CO₂ partial pressure or solubility,caused a decrease of dissolution rate constant?K_c?,with the support of DBL thickness analysis.Damkohler number?Da?and Peclet number?Pe?in the experiment is calculated and analyzed.It is considered that dissolution rate of the turbulent flow were limited by molecular diffusion.The dissolved mass transport is dominated by the advective transport.