低渗透性煤层井下低压液态CO_2促抽瓦斯工程实践

A case study of methane drainage promotion by injecting liquid carbon dioxide into low permeability coal seam

针对韩城矿区3#煤层渗透率较低,瓦斯抽采难度大、抽采效率低等难题,在理论分析液态CO_2低温相变增渗和驱替置换煤层瓦斯机理的基础上,提出了低压(2~3 MPa)液态CO_2顺层钻孔压注增透技术,研发了适用于煤矿井下的低压液态CO_2压注系统和工艺,确定了压注过程的关键参数。桑树坪2号井3#煤层工业性试验结果表明:整个CO_2压注过程大致可分为相态平衡建立、注液和保压3个阶段;液态CO_2在3#煤层中的渗流扩散半径超过18 m,此半径范围内瓦斯抽采浓度整体得到提高,抽采活跃期至少一个月。距压注孔6,12和18 m的抽采孔单孔瓦斯浓度平均值分别达41.66%,35.43%和24.14%,且随距压注孔间隔的增大,单孔瓦斯抽采浓度呈逐渐减小趋势,与钻孔内CO_2浓度变化趋势一致。整个抽采活跃期内,支管平均瓦斯抽采浓度和纯量分别达48.49%和1.42 m3/min,明显高于水力割缝后的瓦斯抽采效果,压注后32~40 d,衰减至29.37%和0.88 m3/min,仍高于原始煤层瓦斯抽采浓度。

：In light of the problems of poor permeability,difficult gas extraction and low extraction efficiency of No.3 coal seam in Hancheng Mining Area,the theoretical analysis was carried out to investigate the mechanism on liquid CO 2 promoting methane drainage.The phase transformation and methane displacement of liquid CO 2 played an important role in promoting methane extraction.Thus,a novel methane control technology namely liquid CO 2 injection has been proposed,and a set of equipment and technology of liquid CO 2 injection were developed.Moreover,the key parameters of the liquid CO 2 injection was determined by the field practice.The field extraction test indicated that the entire process of CO 2 injection can be roughly divided into 3 stages：phase equilibrium,liquid injection and pressure maintenance.The seepage diffusion radius of CO 2 in No.3 coal seam reaches 18m and the active period of gas drainage lasts at least one month.The average gas concentration of single-hole with 6,12 and 18 m away from the injection hole was 41.66%,35.43% and 24.14%,respectively.And with the increase of the distance between the extraction and injection hole,the single-hole gas drainage concentration gradually decreased,which was consistent with the variation trend of CO 2 concentration in the boreholes.During the whole active extraction period,the average gas drainage concentration and the amount of methane extraction of the branch pipe reached 48.49% and 1.42 m 3 /min,respectively,which are obviously higher than that of hydraulic fracturing.Whereas,after the CO 2 injection for 32～40 days,the average gas concentration and the amount of single borehole decreased to 29.37% and 0.88m 3 /min,which is still higher than gas drainage concentration in the original coal seam.