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吸附热对煤-气相互作用的数值模拟研究

Numerical simulation research on heat of adsorption on coal-gas interaction

  • 摘要: 煤层气开发或二氧化碳地质封存均涉及气-固界面吸附-解吸效应。已有大量研究证实气体分子在固体表面吸附或解吸过程中会释放出热量或吸收热量,进而导致气-固系统温度上升或下降。但是,常用的研究方法是在温度-压力平衡条件下评价煤吸附气体能力。而煤储层产气或注气过程是非平衡态过程,会产生大量的吸附热改变煤体温度,这与绝大多数研究中的恒温假设条件矛盾。为了研究吸附热对煤-气相互作用的影响,在固体变形-气体流动-吸附/解吸耦合关系中引入吸附热,将煤-气系统的储热项划分为裂隙系统与基质系统,吸附热表达成放热功率的形式;并以固体应变作为耦合项,考虑了煤与环境热交换过程,建立考虑吸附热的“变形-渗流-扩散”耦合计算模型。模拟的煤表面温度能较好拟合文献实验数据。此外,二氧化碳封存模拟结果表明恒压注气时吸附热导致的注气效率的降低约为16.8%;恒速注气时,预计最终6.2 MPa可完成的注气速率保持工作,在吸附热的影响下需要将注气压力提高至7.2 MPa,为煤层气开采时煤层温度演化提供参考。

     

    Abstract: Both coalbed methane development and carbon dioxide geological storage involve adsorption-desorption effects at gas-solid interface. A large number of studies have confirmed that gas molecules release heat or absorb heat in the process of adsorption or desorption on the solid surface,which leads to the temperature rise or fall of the gas-solid system. However,the commonly used research method is to evaluate the gas adsorption capacity of coal under the condition of temperature-pressure equilibrium. However,the process of gas production or gas injection in coal reservoir is an unbalanced process,which will produce a large amount of adsorption heat to change the temperature of coal body,which is contradictory to the assumption of constant temperature in most studies. In order to study the influence of adsorption heat on coal-gas interaction,the adsorption heat was introduced into the coupling relationship of solid deformation-gas flow-adsorption/desorption. The adsorption deformation of coal was defined as a temperature function,and the heat storage term of coal-gas system was divided into fracture system and matrix system. Taking solid strain as the coupling term and considering the heat exchange process between coal and the environment,a deformation-seepage-diffusion coupling calculation model considering adsorption heat was established. The simulated coal surface temperature can fit well with the experimental data in literature. In addition,according to the simulation results of carbon dioxide storage,the decrease of gas injection efficiency caused by adsorption heat is about 16.8%. Under the influence of adsorption heat,the gas injection pressure should be increased to 7.2 MPa,which provides a reference for the evolution of coal seam temperature during coalbed methane exploitation.

     

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