注入N2 / CO2混合气提高煤层气采收率的数值研究及其地质意义,Energy Science & Engineering

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注入N2 / CO2混合气提高煤层气采收率的数值研究及其地质意义
Energy Science & Engineering ( IF 3.8 ) Pub Date : 2019-12-10 , DOI: 10.1002/ese3.571
Nan Fan ₁ , Jiren Wang ₂ , Cunbao Deng ₃ , Yongpeng Fan _{1,

4} , Yongliang Mu ₁ , Tingting Wang ₅

Affiliation

作为一种有效的碳利用技术，将N ₂ / CO ₂混合物注入煤层具有改善煤层气采收率的巨大潜力。考虑到纯CO ₂注入会降低井注入指数，纯N ₂注入导致甲烷快速回收的技术障碍，提出了一种注入恒定成分的富N ₂混合气的方法。在这项研究中，通过注入N ₂ / CO ₂提高煤层气（CBM）采收率的热-水力-机械（THM）耦合数值模型。建立混合物。该模型包括煤变形，竞争性吸附，三元气体渗透和传热的复杂相互作用。首先验证了THM耦合模型，然后将其用于研究在N ₂ / CO ₂增强的煤层气采收过程中混合气体浓度，储层渗透率，储层温度，CH ₄产量和N ₂ / CO ₂储量的演变。结果表明，随着注气压力的增加，煤层中混合气的位移半径和浓度增加。CH ₄的浓度随时间逐渐降低，早期下降快于后期。N ₂的扫掠流动加速了CH _4的解吸和运移，促进了生产井附近储层温度的降低。THM场内煤层中三元气体（CH ₄，CO ₂，N ₂）竞争性吸附，气压和地应力的综合作用导致了储层渗透率的演化。在甲烷自然耗竭阶段（250天以内），煤储层的渗透率先降低然后增加。随着N ₂ / CO ₂混合物的到达，渗透率急剧下降。从累积CH ₄的角度来看在生产过程中，最佳组成是最大程度地延长突破时间并最小化煤基质溶胀的协同效应。对于30％CO _2到70％N _2而言，CH ₄回收率达到71.76％，与自然消耗相比增加了16.67％。

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Numerical study on enhancing coalbed methane recovery by injecting N2/CO2 mixtures and its geological significance

As an effective carbon utilization technology, the injection of N₂/CO₂ mixtures into coal seams has significant potential for improving coalbed methane recovery. Considering the technical barrier that injection of pure CO₂ decreases the well injectivity index and pure N₂ injection leads to the rapid methane recovery, a method of injecting N₂‐enriched gas mixtures with a constant component is proposed. In this study, a thermo‐hydro‐mechanical (THM) coupling numerical model for enhanced coalbed methane (CBM) recovery by injecting N₂/CO₂ mixtures is established. This model includes complex interactions of coal deformation, competitive adsorption, ternary gas seepage, and heat transfer. The THM coupling model is first validated, and then applied to investigate the evolution of mixed gas concentrations, reservoir permeability, reservoir temperature, CH₄ production, and N₂/CO₂ storage during N₂/CO₂ enhanced CBM recovery. The results show that the displacement radius and concentration of the mixed gas in the coal seam increased with gas injection pressure increase. The concentration of CH₄ gradually decreased with time, and the early decline is faster than the later stage. The sweep of the N₂ flow accelerates CH₄ desorption and migration, promoting a reduction in reservoir temperature near the production well. Reservoir permeability evolution results from the combined effects of ternary gases (CH₄, CO₂, N₂) competitive adsorption, gas pressure, and geostress on the coal seam within the THM fields. At the methane natural depletion stage (within 250 days), the permeability of coal reservoir first decreases and then increases. With the arrival of the N₂/CO₂ mixture, the permeability decreases dramatically. From the perspective of cumulative CH₄ production, the optimal composition is dominated by the synergistic effect of maximizing breakthrough time and minimizing coal matrix swelling. For 30% CO₂‐70% N₂, the CH₄ recovery ratio reached 71.76%, representing an increase of 16.67% compared to natural depletion.

更新日期：2019-12-10

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