Failure probability analysis of coal crushing induced by uncertainty of influential parameters under condition of in-situ reservoir

The uncertainties of some key influence factors on coal crushing, such as rock strength, pore pressure and magnitude and orientation of three principal stresses, can lead to the uncertainty of coal crushing and make it very difficult to predict coal crushing under the condition of in-situ reservoir. To account for the uncertainty involved in coal crushing, a deterministic prediction model of coal crushing under the condition of in-situ reservoir was established based on Hoek-Brown criterion. Through this model, key influence factors on coal crushing were selected as random variables and the corresponding probability density functions were determined by combining experiment data and Latin Hypercube method. Then, to analyze the uncertainty of coal crushing, the first- order second-moment method and the presented model were combined to address the failure probability involved in coal crushing analysis. Using the presented method, the failure probabilities of coal crushing were analyzed for WS5-5 well in Ningwu basin, China, and the relations between failure probability and the influence factors were furthermore discussed. The results show that the failure probabilities of WS5-5 CBM well vary from 0.6 to 1.0; moreover, for the coal seam section at depth of 784.3-785 m, the failure probabilities are equal to 1, which fit well with experiment results; the failure probability of coal crushing presents nonlinear growth relationships with the increase of principal stress difference and the decrease of uniaxial compressive strength.