Effects of caprock sealing capacities on coalbed methane preservation: Experimental investigation and case study

Abstract

Caprocks play an important role in the trapping of coalbed methane (CBM) reservoirs. To study the sealing capacities of caprocks, five samples with different lithologies of Neogene clayrock, Paleogene redbeds, Permian sandstone, Permian mudstone and Permian siltstone were collected and tested using experimental methods of microstructure observation, pore structure measurement and diffusion properties determination. Results indicate that with denser structures, lower porosities, much more developed micropores/transition pores and higher pore/throat ratios, mudstone and siltstone have the more ideal sealing capacities for CBM preservation when comparing to other kinds of caprocks; the methane diffusion coefficients of mudstone/siltstone are about 6 times higher than sandstone and almost 90 times higher than clayrock/redbeds. To further estimate the CBM escape through caprocks, a one-dimensional CBM diffusion model is derived. Modeling calculation result demonstrates that under the same thickness, the CBM sealing abilities of mudstone/siltstone are almost 100 times higher than those of clayrock/redbeds, and nearly 17 times higher than sandstone, which indicates that the coal seam below caprocks like clayrock, redbeds or sandstone may suffer stronger CBM diffusion effect than that below mudstone or siltstone. Such conclusion is verified by the case study from III3 District, Xutuan Colliery, where the coal seam capped by Paleogene redbeds has a much lower CBM content than that capped by the Permian strata like mudstone, siltstone and sandstone.

摘要

盖层的封盖能力对煤层气藏的保存起着至关重要的作用。为研究盖层的封盖能力, 本文采集了 五种不同岩性的盖层岩样(包括: 新近系黏土岩、古近系红层、二叠系砂岩、二叠系泥岩和二叠系粉 砂岩), 综合采用显微结构观测、孔隙结构测量和扩散性能测定相结合的实验方法开展相关研究。研 究结果表明: 泥岩、粉砂岩由于其所具有的结构致密、孔隙率低、孔径分布以微孔和小孔为主以及孔 喉比高等特征, 是煤层气保存较为理想的盖层; 甲烷在泥岩和粉砂岩中的扩散系数约为砂岩的6倍、 粘土岩和红层的90倍。为进一步对煤层气通过盖层的逸散情况进行评价, 本文构建了一维的煤层气 扩散模型。建模分析结果显示: 在相同盖层厚度条件下, 泥岩、粉砂岩对煤层气的封盖能力是黏土岩 和红层的近100倍、砂岩的近17倍。因此当煤层的盖层由黏土岩、红层或砂岩组成时, 其下部煤层 所遭受的瓦斯逸散作用将更为强烈。许疃煤矿ΙΙΙ3采区的开采实践验证了上述结论, 许疃煤矿的开采 实践表明: 与二叠系岩层(泥岩、砂岩、粉砂岩)相比古近系红层下方煤层的煤层气含量出现显著降低。