CO2 gas fracturing: A novel reservoir stimulation technology in low permeability gassy coal seams

Abstract

Coal mine methane (CMM) is one of the major safety hazards in China’s gassy coal mines. To prevent the gas related mining hazards, effective gas drainage is a pivot technology to reduce the gas content in coal seams. For low permeability coal seams, permeability enhancement is one of the top challenges for enhancing gas drainage efficiency. In the study, a novel methane drainage technology for coal permeability enhancement, namely CO2 gas fracturing (CO2-Frac), has been proposed and investigated in Luan coal mine field. CO2-Frac is achieved by multiple stage directional CO2 gas fracturing with a newly renovated CARDOX system. Through optimization of the CO2-Frac boreholes parallel to face cleats, a complex and high permeability fracture network was generated in CO2-Frac borehole and hence the improvement of methane gas drainage effectiveness. Mine-scale tests of CO2-Frac have been conducted in 5 coal mines in Luan mine field from December 2011 to December 2015. Methane concentrations and flow rates from both CO2-Frac boreholes and adjacent boreholes without CO2-Frac were continuously monitored for evaluating the stimulation efficiency. It was observed that permeability and methane drainage effectiveness in the CO2-Frac boreholes was significantly improved: (1) gas permeability increases up to 20–50 folds; (2) extracted methane concentration increases ∼10 folds ranging from 50 to 90%, compared the concentration of 5–30% in normal boreholes without CO2-Frac; (3) methane rate in a single CO2-Frac hole varies from 140 to 280m3/d with an increase of 10–20 folds, compared the rate of 14–20m3/d in normal boreholes; (4) methane content of the coal seam in drainage area reduced from original 11.7m3/t to 6.5m3/t after 210days of drainage; (5) upgraded methane from CO2-Frac boreholes can be better utilized in air ventilation methane (AVM) power plant. CO2-Frac is confirmed as the best technology to drainage methane in this low permeability coal seams in Luan mine field, and it is not only beneficial on coal mine gas control and safety management but the CMM economic utilization by upgrading drained methane gas concentration.