碎软低渗高突煤层井下长钻孔整体水力压裂增透工程实践

Practice of permeability enhancement through overall hydraulic fracturing of long hole in outburst-prone soft crushed coal seam with low permeability

针对阳泉矿区碎软低渗高突煤层开展了井下长钻孔整体水力压裂增透技术的工程试验研究,工程实现了井下一次性整体压裂煤孔段长度达307 m,单孔注入水量达1 510 m3,最大注水压力达26.09 MPa。效果检测表明钻孔压裂影响半径最大达58 m,压裂后煤层透气性系数提高了2.67倍,百米钻孔瓦斯流量衰减系数降低了55%,230 d内钻孔日抽采纯甲烷1 395~2 810 m^3,平均2 173 m^3,钻孔累计抽采纯甲烷50.86×10~4m^3,抽采瓦斯浓度为49.38%~83.70%,平均64.31%。分析认为:水力压裂能改善煤层裂隙和孔隙的连通性、降低煤层有效应力、提高煤层渗透率,注水能促进煤层瓦斯从吸附态向游离态转化,是煤层压裂后钻孔高效抽采瓦斯的关键,依据填砂堵缝压裂技术原理提出了碎软低渗煤层长钻孔整体水力压裂煤层裂隙开启、扩展和延伸机制。工程试验成果及认识可为井下长钻孔整体水力压裂增透高效抽采瓦斯提供借鉴。

Aim to the crushed and soft coal seam with high outburst danger in Yangquan mining area,the paper put forward a technology of whole hydraulic fracturing and permeability enhancement using long hole. The engi-neering practice test realized that the length of the coal segment of the hole underground for the whole fracturing was up to 307 m,the injected water volume of single hole was 1 510 m^3,and the maximum injection pressure was up to26.09 MPa.The penetration effect of fracturing was investigated and the results were the influencing range of borehole fracturing was up to 58 m,the permeability coefficient of coal seam increased by 2.67 times,the gas flow attenuation coefficient per 100 meters of hole decreased by 55%. During a 230 days period,the extraction of pure methane was from 1 395 to 2 810 m^3/d with 2 173 m3/d in average and 50.86×10^4m^3 in cumulative volume,the concentration of extracted gas was from 49.38% to 83.70 with 64.31% in average.It was analyzed that hydraulic fracturing improved theconnectivity of coal seam fracture and porosity,reduced the effective stress and increased the permeability of coal seam,promoted the coal seam gas from the adsorption state to free state transformation. So the above factors were the key to the effective extraction of gas after fracturing coal seam. According to the principle of sand-filling and plug-up seam fracturing technology,the mechanism of opening and extension of the whole hydraulic fracturing seam with hydraulic fracturing in soft and low permeability coal seam was presented.The achievements and knowledge of the engineering practice provides a practical experience and technical support for the whole hydraulic fracturing and highly efficient gas drainage.