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Based on the leakage of the coalbed methane (CBM) drilling engineering practice of Luan mining area in China, the author determines the safe drilling fluid density range for the stable borehole wall based on borehole wall collapse and fracture pressure. Such parameters as the drilling fluid hydraulic parameters (including pump pressure, pump power and displacement, nozzle diameter, bit pressure drop, bit hydraulic horsepower, circulation pressure drop, impact force and jet velocity) and drilling parameters (including weight-on-bit, drilling rotary speed, bit tooth wear) in each borehole section are optimized. Taking the lowest drilling cost as the controlling target, the drilling parameter optimization model is designed and solved by the genetic algorithm. Furthermore, a software named "CBM borehole wall stability parameter design and optimization" characterized by visualization and applicable for drilling formation condition, which can be used to design and optimize the borehole drilling technological parameters, is developed. This program includes such modules as drilling fluid density prediction, drilling technology design, database management, user management and help. The developed software is proven to solve the drilling leakage effectively in the No.67 borehole practice, which can help drilling engineers to optimize CBM drilling technological parameters safely and quickly.

Nowadays, more attention has been paid to the study of the properties and structure of contaminated clay, while there are few studies on the microstructure of contaminated clay, especially municipal waste contaminated clay. The purpose of this paper is to determine the threshold range by observing and processing the microscopic images of municipal waste contaminated clay with different seepage depths and concentrations. In this study, contaminated clay samples were taken at different depths in a column test and at different concentrations with CaCO3 and CH3COOH, imaged through scanning electron microscopy (SEM), and then subjected to binarization. Five steps for determining the pixel thresholds were discussed including: imaging contaminated clay through SEM, preprocessing and binarizing the SEM images with IPP, carrying out morphological processing by using PCAS and determining the pixel threshold value of the SEM images. The apparent porosity ratio of the contaminated clay and the fractal dimension of the clay particles were applied as the parameters for evaluation, the effective threshold range of the image pixel values of the municipal waste contaminated clay was determined to be 110–140. This method is validated by examining the relationship between the pixel threshold value and apparent porosity ratio, the variation in the porosity ratio and apparent porosity ratio with seepage depth, and the different compaction factors with seepage depth.