煤层气井产出水离子及同位素演化特征研究

Study on evolution characteristics of water ions and isotopes produced by coalbed methane wells

为揭示高煤阶煤层气井产出水中的地球化学特征,探究煤储层产出水在时间和空间上的演化规律,对柿庄南区块山西组17口排采时间超过5年以上的生产井进行水化学检测,通过对水样的阴阳离子浓度测试和水样的氢氧同位素、溶解无机碳同位素测试,发现此次的测试结果同该区块2011-2014年间的水化学数据有明显差异、结果显示水样中SO4^2-的浓度和TDS含量较往年偏小,而δD的测试值大于之前的数据,通过对比不同季节的样品测试数据,得出HCO3^-和Cl^-的浓度随季节变化最明显;Ca^2+、Mg^2+浓度具突变性;SO4^2-离子浓度随季节变化小。研究结果表明:HCO3^-和Cl^-浓度受季节性降水影响大,Ca^2+、Mg^2+浓度受地质因素影响大,SO4^2-离子浓度主要受脱硫酸作用影响;通过水样同位素分析表明δ13CDIC与δ18O呈正相关性,与δD呈负相关性,与SO4^2-呈现反比例变化趋势;δD随排采年份增加呈现偏重的趋势,并逐渐向大气降水线上方偏移,表明随排采年份的增加,煤储层排水范围由裂缝向远处扩大,促使煤层中的原始孔隙水不断被排出,从而导致δD呈现逐渐偏重的趋势;经分析表明δ13CDIC可以用于检测煤储层的局部封闭性。最后依据上述研究结果建立了体现煤层气井产出水在时间和空间上的变化规律的地球化学反应模型图。

In order to reveal the geochemical characteristics of the produced water from high rank coalbed methane wells and to explore the evolution law of the produced water from coal reservoirs in time and space,water chemistry tests were carried out for the drainage from the 17 production wells in Shanxi Formation in the study area with recovery time of more than 5 years.After having tested the concentrations of anion and cation,hydrogen and oxygen isotopes,and dissolved inorganic carbon isotopes in the water sample,we found significant difference between this study and the water chemistry data for the same study area from 2011 to 2014. The results showed that the SO4^2-concentration and TDS content in the water sample were lower than those in previous reports,and the δD test value was higher than older data. By comparing the test data from different seasons,it is concluded that the concentration of HCO3^-and Cl^- is the most susceptible to seasonal change;the concentration of Ca^2+and Mg^2+can undergo abrupt spike;and the impact on concentration of SO4^2-from seasonal changes is the least significant. Further study also reveals that the concentration of HCO3^-and Cl-is also affected by seasonal precipitation to larger degree,the concentration of Ca^2+and Mg^2+is mostly affected by geological factors,and the concentration of SO4^2-is mainly controlled by desulfurization. The isotope analysis of water samples reveals a positive correlation between δ13CDIC and δ18O,and a negative correlation between δ13CDIC and δD,It is also found that δ13CDIC is disproportional to the concentration of SO4^2-. The δD tends to intensify as the discharge year increases,and gradually shifts to the upper level of the atmospheric precipitation line,indicating that as the discharge year increases,the drainage range from coal reservoirs would expand from cracks to distant places,resulting in the continuous discharge of original pore water from the coal seam which causes the gradual intensification of δD along with time. Analysis also indicate that δ13CDIC can be useful for assessing the local sealing of coal reservoirs. Finally,this work led to the establishment of a geochemical reaction model illustrating how the produced water in the coalbed methane well evolves along with time and space.