ABSTRACT

Recently, coalbed methane (CBM) has become one of the major unconventional gas resources in the world. In 2017, the US CBM proved reserves were estimated at 336 Billion Cubic Meter (BCM) while the production was about 27 BCM. Coal formations possess unique characteristics that necessitate accurate characterization and fundamental understanding of the prevailing mechanisms. For instance, gas sorption processes in CBM require a comprehensive characterization integrating sorption factors such as the pressure, temperature, moisture content, ash content, and gas adsorption capacities. In addition, the uncanny gas diffusion behavior in CBM formations poses a challenge for accurate gas diffusion quantification accounting for the unusual varying nature of the gas diffusion coefficient with pressure and temperature. Besides, the stress-sensitivity nature of the CBM permeability still represents an active research area where experimental measurements are needed to develop relationships for the modeling studies. Additionally, CBM reserve estimates are highly dependent on the in-place gas contents. Therefore, one needs to consider not only the adsorbed gas content, but also the free and soluble gas contents – particularly in lower rank CBM reservoirs. Consequently, it is challenging to predict the fluid behavior in CBM formations given the uniqueness and complexity of the governing mechanisms. In this review, we are providing both an overview of the CBM’s fundamental and the recent research studies that revolve around the major aspects of CBM’s characterization and modeling.

摘要

最近，煤层气（CBM）已成为世界上主要的非常规天然气资源之一。2017年，美国的煤层气探明储量估计为3360亿立方米（BCM），而产量约为27 BCM。煤层具有独特的特征，因此必须对特征进行精确的表征并需要对流行的机理有基本的了解。例如，煤层气中的气体吸附过程需要综合表征吸附因子的综合特征，例如压力，温度，水分，灰分和气体吸附能力。另外，考虑到气体扩散系数随压力和温度的异常变化，CBM地层中异常的气体扩散行为给准确的气体扩散定量提出了挑战。除了，煤层气渗透率的应力敏感性特性仍然代表着一个活跃的研究领域，在该领域中需要进行实验测量以建立模型研究的关系。此外，煤层气储量估算高度依赖于现场天然气含量。因此，不仅要考虑吸附气体的含量，还要考虑游离气体和可溶性气体的含量，尤其是在较低等级的煤层气储层中。因此，鉴于控制机制的独特性和复杂性，预测煤层气地层中的流体行为具有挑战性。在这篇综述中，我们既提供了煤层气基础知识的概述，又提供了围绕煤层气表征和建模主要方面的最新研究。