不同煤阶煤储层吸附/解吸特征差异及其对产能的影响

Differences in the adsorption/desorption characteristics of coal reservoirs with different coal ranks and their effects on the reservoir productivity

  • 摘要: 吸附/解吸参数是煤层气储量及产能潜力评估的核心,而解吸过程规律与煤层气井产能动态密切相关。为研究不同煤阶煤储层吸附/解吸特征及其对产能的影响,基于沁水盆地南部樊庄、安泽区块和鄂尔多斯盆地东缘保德、柳林区块的49件煤心样品的煤岩煤质、含气量及等温吸附测试数据,结合试井及排采资料,揭示了煤岩煤质特征对等温吸附特征的影响,划分并对比了煤层气解吸阶段特征差异,建立了煤吸附/解吸特征参数与煤阶之间的数学关系,基于20口典型煤层气井探讨了解吸特征参数对气井产能的影响,并对不同煤阶煤储层煤层气勘探开发策略给出针对性建议。结果表明:(1) 从山西保德到晋城樊庄区块,随着煤阶的增高,煤储层解吸过程中的转折压力、启动压力和敏感压力均逐渐增大,低效、缓慢、快速和敏感解吸阶段范围逐渐向高压区间方向偏移;高阶煤对应的有效解吸阶段区间宽度明显要大于低阶煤,对煤层气开发更有利。(2) 高阶煤与中低阶煤相比,煤层气井的解吸指数更高,达到稳产的时间更短且产气潜力更高,但较低的渗透率是制约其产能的关键。(3) 综合考虑资源丰度、解吸能力和渗透率,高阶煤优势在于解吸指数较高,而中低阶煤优势在于渗透性较好;高阶煤煤层气高产的关键在于大规模高效压裂技术,中低阶煤则在于煤层气资源富集甜点区的精准优选技术。

     

    Abstract: Adsorption/desorption parameters are critical to the evaluation of the reserves and productivity potential of coalbed methane (CBM). Moreover, the law governing the desorption process is closely related to the productivity of CBM wells. This study aims to investigate the adsorption/desorption characteristics of coal reservoirs with different coal ranks and their effects on the productivity of the reservoirs. Based on the data of the coal macerals, coal quality, gas content, and isothermal adsorption experiments of 49 coal core samples from the Fanzhuang and Anze blocks in the southern Qinshui Basin and the Baode and Liulin blocks in the eastern Ordos Basin, as well the data on well tests and production data of these blocks, this study revealed the effects of coal macerals and coal quality on isothermal adsorption characteristics, classified the CBM desorption stages and compared their characteristics, and built the mathematical relationship between coal adsorption/desorption characteristic parameters and the coal rank. Moreover, based on the data on 20 typical CBM wells, this study explored the influence of desorption parameters on the productivity of CBM wells and put forward some specific suggestions on CBM exploration and extraction strategies of coal reservoirs with different coal ranks. The results of this study are as follows: (1) With an increase in the coal rank from the Baode block in Shanxi to the Fanzhuang block in Jincheng, the starting, turning, and sensitive pressures gradually increased during the desorption of coal reservoirs, and the ranges of inefficient, slow, quick, and sensitive desorption stages gradually shifted toward the high-pressure interval. High-rank coals had significantly wider intervals of the effective desorption stages than low-rank coals, creating more favorable conditions for CBM development; (2) Compared with the medium- and low-rank coals, the high-rank coals have higher CBM desorption index, shorter time required for stable production, and higher gas production potential. However, low permeability is a critical factor restricting the productivity of high-rank coals; (3) In combination with resource abundance, desorption capacity, and permeability, high-rank coals have a high desorption index, while medium- and low-rank coals have high permeability. Accordingly, the key to high CBM productivity is to employ the large-scale and efficient fracturing technology for high-rank coals and is the precise selection of the optimal sweet spots of CBM enrichment for medium- and low-rank coals.

     

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