煤层气井产出水微量元素地球化学特征、成因及开发意义以黔西织金区块为例

Geochemical characteristics, genesis, geological and development significance of trace elements in water produced from coalbed methane wells: A case study of Zhijin Block in western Guizhou Province

  • 摘要: 为深化煤系流体地球化学形成机理与煤层气富集高产机制,促进煤层气效益开发,基于黔西织金区块煤层气井产出水样品采集与微量元素地球化学测试,揭示产出水微量元素组成特征、差异分布与成因机制,探讨其地质意义以及对煤层气富集高产的指示意义。结果表明:织金区块煤层气井产出水较高的微量元素浓度是水与煤中无机矿物相互作用的结果,源于黔西煤中微量元素的赋存方式主要为无机态。微量元素溶出与沉淀为水−煤相互作用的2种机制,滞留条件下,易溶元素大量溶出,导致煤层水中微量元素浓度增加,以Li、Ga、Rb、Sr、Ba这5种特征微量元素为代表,溶出来源主要为黏土矿物。当水中微量元素浓度较高时,碱性水环境将促进可溶性元素的沉淀,反馈煤中微量元素变化。产出水微量元素浓度在不同含煤向斜之间差异显著,源于向斜间水动力条件的差异。产出水Li元素浓度与平均日产气量具有正相关性,特征微量元素浓度蛛网面积与平均日产水量具有负相关性,可指示地下水动力条件与产能潜力。珠藏向斜煤系水动力条件弱,产出水微量元素浓度高,加之构造改造弱,煤体结构相对完整,具有最佳的煤层气富集高产条件;阿弓向斜在保证排液能力的前提下也具有高产潜力,由以南东向缓倾翼为典型。研究成果从煤层气井产出水微量元素地球化学差异分布与成因机理的角度出发,为理解织金区块煤层气富集高产的区控性原因提供了新的思路,并为下一步开发部署提供了方向。

     

    Abstract: In order to deepen the formation mechanism of coal measure fluid geochemistry and the mechanism of coalbed methane (CBM) enrichment and high yield, and to facilitate CBM efficient development, based on the collection of water samples produced from CBM wells in the Zhijin block of western Guizhou and trace element geochemical testing, the composition characteristics, differential distribution and genesis mechanism of trace elements in the produced water were revealed, and their geological significance and indicative significance for CBM enrichment and high yield were explored. The results show that the high concentration of trace elements in the produced water from CBM wells in Zhijin block is the result of the interaction between water and inorganic minerals in coals, which is mainly due to the inorganic occurrence of trace elements in coals in western Guizhou. The dissolution and precipitation of trace elements are two mechanisms of water-coal interaction. Under retention conditions, easily soluble elements dissolve in large quantities, leading to an increase in the concentration of trace elements in coal seam water, represented by five characteristic trace elements: Li, Ga, Rb, Sr, and Ba. The main source of dissolution is clay minerals. When the concentration of trace elements in water is high, alkaline water environment will promote the precipitation of soluble elements and feedback the changes in trace elements in coal. The concentration of trace elements in the produced water varies significantly between different coal-bearing synclines, due to differences in hydrodynamic conditions between synclines. The concentration of Li element in the produced water is positively correlated with the average daily gas production, while the spider web area of characteristic trace element concentration is negatively correlated with the average daily water production, which can indicate the coal measure hydrodynamic conditions and CBM production potential. The coal measure hydrodynamic conditions of the Zhuzang syncline are weak, and the concentration of trace elements in the produced water is high. In addition, the tectonic reconstruction is weak, and the coal body structure is relatively complete, which jointly create the best conditions for CBM enrichment and high yield in the Zhucang syncline. The Agong syncline also has high CBM production potential while ensuring drainage capacity, typically represented by the gently inclined SE oriented limb. The research results provide new ideas for understanding the regional controlling mechanism for the CBM enrichment and high yield in the Zhijin block from the perspective of the geochemical differences and genesis mechanisms of trace elements in the produced water from CBM wells, and provide direction for the future CBM development.

     

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