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煤层气微波注热开采工艺数值模拟研究

Research on coalbed methane extraction technology based on in-situ microwave heating

  • 摘要: 微波加热作为一种加热速率快且易于控制的加热方式,可以快速提高煤层温度并改善煤体渗透性,进而提高煤层气的采收率。微波向地下煤层的传输与煤层的温度响应是开发新型煤层气开采工艺的关键。考虑温度对真实煤层介电常数的影响,建立了微波间歇加热整体煤层的电磁−热双向耦合数学模型。采用COMSOL Multiphysics模拟不同参数下微波间歇加热整个煤层的温度场分布,探究微波间歇加热时长,微波频率,输入功率,对煤层温度分布的影响,最后基于石油工程中定向钻井和连续油管技术,提出了一种可实现大范围煤层微波加热的煤层气增产开采工艺。结果表明:微波加热厚煤层时,采用短周期的间歇加热方式,加热4 d,静置2 d,加热效果更优,且有效减少能量浪费;频率对煤层平均温度与最大温度影响较小,与煤层穿透深度成反比,微波加热煤层时,260 MHz的微波综合平衡了穿透深度与温度场均匀性;微波功率对煤层温度与有效穿透深度影响显著,且呈正相关,在6 000 W时温度梯度最大为581.3 K/m,易于诱发煤层热破裂,提高煤层的渗透性,有效穿透深度也在6 000 W时达到最大;新型煤层气增产开采工艺解决了微波远距离传输的损耗问题与煤层气倒流微波传输管道的问题,利用连续油管的特性实现了煤层大范围的微波加热。

     

    Abstract: Microwave heating, as a fast and controllable heating method, can quickly increase the temperature and permeability of coal seams, and thus enhance the recovery rate of coalbed methane. The transmission of microwaves to underground coal seams and the temperature response of coal seams are the key to developing a new coalbed methane extraction technology. The influence of temperature on the real dielectric constant of coal seams is considered, and an electromagnetic-thermal bidirectional coupling mathematical model for microwave intermittent heating of the whole coal seam is established. Using COMSOL Multiphysics, the temperature field distribution of the whole coal seam under different parameters of microwave intermittent heating is simulated, and the effects of microwave intermittent heating duration, microwave frequency, and input power on the temperature distribution of coal seams are explored. Finally, based on the directional drilling and continuous tubing technology in petroleum engineering, a coalbed methane production enhancement extraction technology that can achieve a large range of coal seam microwave heating is proposed. The results show that: When microwave heating thick coal layers, employing a short-cycle intermittent heating method—four days of heating followed by two days of rest—results in superior heating effects and significantly reduces energy waste; The frequency has a minimal impact on the average and maximum temperatures of the coal layer but is inversely proportional to the penetration depth, when heating coal layers with microwaves, the microwave at 260 MHz balances penetration depth and temperature field uniformity; The microwave power has a significant and positive correlation with the temperature of the coal layer and the effective penetration depth, at 6 000 W, the maximum temperature gradient is 581.3 K/m, which easily induces thermal fracturing in the coal layer, enhancing its permeability. The effective penetration depth also reaches its maximum at 6 000 W; The novel coalbed methane enhancement and extraction process addresses the issues of microwave transmission loss over long distances and the backflow in microwave transmission pipelines for coalbed methane. By utilizing the characteristics of continuous tubing, extensive microwave heating of the coal layer is achieved.

     

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