氧化作用下煤样渗透率与润湿性对有机质变化的联合响应

Joint response of coal permeability and wettability to changes in organic matter induced by oxidation treatment

  • 摘要: 氧化作用可改善煤层渗透率,但部分煤样氧化后亲水性增强,对煤层气长期产出不利,相近条件下氧化后渗透率显著增大且亲水性增幅小或减弱的“优质”煤层显然具有更好的改性效果。氧化作用下有机质变化为渗透率及润湿性的变化提供部分驱动力,当前可见煤样氧化后官能团呈同向变化但改性效果相反的报道。为探究氧化作用下煤层渗透率与润湿性对有机质变化的联合响应,对取自我国煤层气开发热点地区的8个煤矿样品开展了氧化改造前后渗透率、孔隙结构、润湿性及大分子结构测试。结果表明:煤样氧化改造后不同有效应力条件下渗透率多呈增大趋势,初始渗透率(无应力条件下,k0)与润湿角(θ)平均变化幅度分别为65.8%及−40.9%,但二者变化幅度未见明显相关性,k0θ变化幅度比值的绝对值(\left| \dfracC_k_0C_\theta \right| )介于0.2~4.2。氧化剂进入煤样后,有机质被溶解形成新孔裂隙,导致总孔隙率及渗透率增大,k0增幅随孔隙率增大呈现出增大趋势。部分脂肪及羟基等被氧化形成极性不同的含氧官能团,是导致改性效果呈现出差异的根本原因。\left| \dfracC_k_0C_\theta \right| 与脂肪、羧酸变化幅度及羧酸与脂肪变化幅度比值的绝对值呈负相关,基于煤样有机质变化可初步评估氧化改性效果,当脂肪减幅超过20%且羧酸与脂肪变化幅度比值的绝对值小于0.5时,\left| \dfracC_k_0C_\theta \right| 超过2,指示相较于渗透率的明显改善,亲水性变化相对较小。

     

    Abstract: Oxidation treatment can enhance the permeability of coal, but several coal samples are accompanied by increased hydrophilicity after oxidation, showing negative effects on long-term coalbed methane (CBM) production. The “high-quality” coal with significantly high permeability enhancement and decreased/smaller increased hydrophilicity after oxidation apparently obtains higher benefits under similar conditions. Changes in organic matter induced by oxidation treatment provide part of the driving force for the variations in permeability and wettability, but the coals with similar changes in functional groups and opposite oxidation effects are reported in current studies. To determine the joint response of coal permeability and wettability to changes in organic matter after oxidation, permeability/pore and molecular structures/wettability experiments are conducted on eight samples collected from mines in CBM exploitation hotspots in China. The results show that the permeability under varying effective stress increases after oxidation, and the average change in the initial permeability (stress-free conditions, k0) and contact angle (θ) is 65.8% and −40.9%, respectively. However, the change in k0 poorly correlates with the change in θ, and the absolute value of the ratio of the change in k0 and θ (\left| \dfracC_k_0C_\theta \right| ) ranges from 0.2 to 4.2. When the oxidants are injected into coal, new pores and fractures are formed during organic matter dissolution, directly leading to permeability and total porosity enhancement., and the change in k0 increases with the change in porosity. Moreover, part of the fat and hydroxyl are oxidized to form oxygen-containing functional groups with different polarities, which is the fundamental reason for the differences in the oxidation effects. \left| \dfracC_k_0C_\theta \right| is negatively correlated with changes in fat, carboxylic acid, and the absolute value of the ratio of the changes in carboxylic acid to fat. The oxidation effects can be preliminary assessed based on the change in organic matter, and when the fat reduction exceeds 20% and the absolute value of the ratio of carboxylic acid to fat is less than 0.5, \left| \dfracC_k_0C_\theta \right| is greater than 2, indicating that the hydrophilicity change is relatively mild compared with the marked permeability enhancement.

     

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