Quantitative evaluation of the contamination degree of the drilling fluid for fractures of different scale in coal

During coalbed methane well drilling, the drilling fluid contaminates and blocks the pores and fractures of coal seam. In order to find out the difference of pollution degree of drilling fluid coal and the contribution rate of the pores and fractures of different scale to permeability, the coal of Changping mine was taken as the research object, the scanning electron microscopy(SEM) and the Image Pro Plus(IPP) processing technology were used, the parameters such as the average length, the average width, the total area and the total perimeter of the fractures of different scales in coal before and after contamination were quantitatively characterized. Based on the fractal theory, the mathematical model of the permeability of the fractures of different scales before and after contamination was established, the change of the permeability before and after contamination was evaluated quantitatively, and was compared with the results of the experimentally tested permeability to verify the feasibility of the calculation method. The results show that after contamination, the average length and the width of fractures of different sizes were reduced, and the decreasing degree was: millimeter scale > nanometer scale > micron scale. The degree of contamination was different in different scales, millimeter scale > micron scale > nanometer scale. The contribution rate of different scales to permeability was different, and the millimeter fracture was the preferential flow channel of fracturing fluid, contributed the most to the permeability, accounting for 46%-72%; the contribution of micron level was the second, accounting for 27%-53%; the contribution of nano scale to permeability was negligible, generally less than 2%. When there are many millimeter and micron scale fractures in coal, and the curvature and width heterogeneity are strong, there may be large-scale to small-scale transformation and small-scale fractures increase. Different injection conditions (such as injection pressure, injection time) and drilling fluid types need to be further studied.