A field application of methane drainage by surface vertical well fracturing in complicated geologic structure area: a case study

The pre-drainage elimination outburst technology by the surface vertical well fracturing (PDEO) is a method to guarantee the safety of coal mining and promote the coalbed methane development, however, its application effect in complicated geologic structure area is ambiguous. Taking the field test in the Xinjing coal mine as an example, the effects of outburst elimination and gas production by PDEO are examined, the control effects of geological structure on the gas drainage are clarified. Results show that the hydraulic fracturing comprehensive curve can reflect the gas production effect, and the gas production of CBM wells with different fracturing curve types follows this sequence: stationary type > descending type > ascending type > fluctuating type. The cumulative gas production shows an S-shape evolution trend with time and can be described with the Logistic equation. The maximum gas content and gas desorption index of monitoring points in the drainage affected area are averagely reduced by 12.83% and 7.30% than those beyond the drainage affected area, confirming the effectiveness of PDEO technology. The CBM wells located near the closed reverse fault structural belt and the coal seam erosion zones have high gas production, while the gas production of CBM wells near the axis of the syncline with water accumulation decreases to a certain extent. It is difficult to obtain the ideal drainage effect for CBM wells near water-rich abnormal area. This study will provide the engineering experiencefor the surface CBM extraction and offer some references for the prevention and control of coal and gas outburst in complicated geologic structure area.