Simulation And Characterization Of Micro-nano Cracks In Hard Brittle Shale

Micro-nano cracks are common in hard brittle shale formation,if there is no effective plugging during the drilling,the liquid will drive straight along the strong diversion crack and quickly enter the deep formation,causing the wall to be unstable.In view of the existing evaluation of drilling fluid plugging capacity,the lower limit of crack width is about 10μm,which is obviously larger than that of microcrack width in hard brittle mud shale,which leads to the problem that the plugging evaluation method can not fit well with the actual situation,the quartz was chosen as the matrix material for the simulation of crack in this paper,the micro crack simulation was carried out on the interface of the semi cylindrical substrate by chemical etching and chemical platingThe characterization method of crack width is the basis for the preparation of micro-nano crack.The crack widths produced by chemical etching method were characterized by the permeability method,the resistance value method and the environmental scanning electron microscopy(SEM)method.The crack widths produced by chemical plating method were characterized by the permeability method and SEM method.The relative standard deviation of the value of the crack width measured by the permeability method and the SEM method is very small,it is proved that the method has good reproducibility.Study on stress sensitivity of initial cracks by permeability method,it is found that the stress sensitivity of quartz core is very weak,and the stress change has little effect on the crack width after pressurization,pressure relief treatment.The single factor analysis and orthogonal test were carried out by using permeability method,SEM method and resistivity method for the factors such as time,temperature and acid ratio of chemical etching.The results showed that:under the orthogonal experiment design,the preparation conditions of the minimum crack width are as follows:the temperature was 20min,5wt%HF:5%H2S04 = 1:2 reaction for 20min,the width characterized by permeability method was 1.317μm,and the width characterized by SEM method is 1.625μm.The factors influencing the chemical plating method were explored by permeability method and SEM method.The formula and condition for the preparation of the minimum crack under the orthogonal experimental design level are:the main salt solution is 4g/LAgN03,lOg/L NaOH,the reducing solution is:6g/L glucose,2g/L potassium sodium tartrate.When the reaction time is 10min,the crack width measured by the permeability method is 0.779μm,and the crack width deduced from SEM is 0.792μm.The feasibility of plugging simulation experiment was carried out by using the prepared crack.