Hydraulic Fracture Evaluation And Outburst Elimination Prediction In Coalbed Methane Wells

The ground of CBM drainage before mining has become one of the most effective ways of gas disaster management for the high gas content and low permeability coal seam. Hydraulic fracturing is widely applied in coalbed methane Wells to increase production, and fracture morphology and distribution law of coal seam are main parameters to evaluate effects after fracturing. At the same time, through the hydraulic fracturing antireflection of CBM wells for pre-drainage, can the method achieve extraction design requirements, especially the requirements of outburst prevention,which also needs perfect outburst elimination predictive indexes to support.In view of this, combining the physical characteristics of coal reservoir and mechanism of hydraulic fracturing, the methods of microseismic monitoring technology and hydraulic fracturing forecasting model based on the filtration effect were applied to evaluate the fracture morphology and geometric size for CBM wells after hydraulic fracturing in the Tunliu well field and Wuyang well field. What's more, according to the different average daily extraction volume of single-well, a prediction model of outburst elimination time of CBM wells in fracturing zone has been established to quantitatively predict the pre-drainage time that can reach the outburst elimination indexes in fracturing zone, and research has achieved the results as follows:(1) In the study area 3# coal reservoir had characteristics of low saturation,low-permeability and relatively low gas content, and the coal seams are suitable for hydraulic fracturing. The stress state belongs to ?v>?H>?h and ?H>?v>?h, fracture morphology were mainly vertical fractures.(2)Microseismic monitoring results showed the main fracture extended along the main development direction of crack, which was in conformity with the fracturing mechanism. In the vertical cracks had been extended to bottom mudstone, and the control range of hydraulic fracturing was a ellipse.(3)Based on filtration effect hydraulic fracturing prediction model were applied to forecast the cracks of coalbed methane well. The length of cracks of coalbed methane well under hydraulic fracturing was an average of 170.59 m, and the relative errors were less than 10%. Evaluation results: 1 well meeted the design requirements;7 Wells reachedthe 80% of design fractures length, and 2 Wells were lower than the 80% of design length, which was largely in line with the design requirements.(4) The pre-drainage time of outburst elimination and daily average gas extraction present an exponential relationship of negative growth, accordingly, a prediction model of outburst elimination time in Single well fracturing zone is established to provide a guarantee for coal mine to arrange the gas pre-drainage plan reasonably and produce safely and effectively.