电热激励下页岩气在微纳孔隙中吸附-解吸及渗流的多尺度分析

Shale gas is a type of reservoir with very low permeability, micro/nano scale pore networks and large amount of adsorbed gases. It is difficult to recover from reservoir. Electrical heating technology has been used to recover shale oil and heavy oil reservoir. Based on the characterization of shale gas desorption, we proposed the technology enhancing desorption by electrical heating is an efficiency method to recovery a shale gas reservoir. And there are lacks of research focus on this method. The following questions are important to evaluate the efficiency of the method: What is the microscopic influence of electrothermal effect on shale gas adsorption/desorption and flow in micro/nano scale pores? Can electrical field and temperature field be produced in shale gas reservoir when electrical heating method is used? Can shale gas be desorpted and recovered from the reservoir? We introduce the molecular dynamics theory into study on gas adsorption on shale, which shows the potential applications in nanomaterials research, in order to show shale gas occurrence mechanism, adsoption/desorption mechanisms, the change of adsorption energy and the ratio of desorption in shale stimulated by electrical heating. And we use the molecular dynamics theory and lattice Boltzmann method to study the effects of electrical heating and interaction between gas-shale on flow area, slippage effect and flow velocity in micro/nano scale pore networks stimulated by electrical heating in microcosmic and mesoscopic scale, respectively. Based on these researches, we establish the mathematical model of recovery of shale gas reservoir with enhanced desorption by electrical heating method and numerically simulate the shale gas recovery process . We discuss the effects of electrodes place pattern, parameters of electrical heating technology and properties of shale gas reservoir on the distributions of electrical field, temperature field and gas flow in reservoir. The feasibility of shale gas recovery method enhanced by electrical heating stimulation is discussed. This research will provide theoretical guidance for shale gas development effectively.

页岩气藏具有渗透率低、孔隙尺度小及吸附气量大等特点，开采困难。根据页岩气吸附特点及电热页岩油、稠油开采技术作用机制，提出电热强化解吸开采页岩气藏技术。目前对此技术的研究还很少。电热作用如何从微观上影响页岩气在微纳孔隙中的吸附-解吸与渗流？电热技术能否在页岩气藏中形成有效的电场和温度场，从而使页岩气解吸、开采出来？对这些问题的解答有助于评价电热强化解吸开采页岩气技术的有效性。本课题拟探索将在纳米材料研究中展示应用潜力的分子动力学理论引入到页岩气吸附研究中，从微观上解释电热作用下页岩气吸附-解吸机制及对吸附能、解吸率的影响；利用分子动力学理论和格子Boltzmann方法，分别从微观和介观尺度上揭示电热、气-岩相互作用对孔隙渗流截面、滑移效应、渗流速度的影响机理；在上述基础上，建立电热开采页岩气藏数学模型，对电热强化解吸开采页岩气藏进行宏观模拟，讨论电极布置方式、电加热工艺参数、页岩气藏性质对电场、温度场及渗流场的影响，研究电热强化解吸开采页岩气藏的可行性，为页岩气的有效开发提供理论指导。

页岩气藏具有低孔隙度、低渗透率的特点，其赋存及流动规律与常规油气藏不同。页岩气藏的有效开发是解决目前国家能源供应的有效手段之一。本项目提出一种电热强化开采页岩气的新方法，并从微观到宏观不同尺度上研究了电热激励下页岩气在的吸附-解吸及渗流机理，讨论了电热强化解吸对页岩气产能的影响。开展了不同电场、温度条件下的吸附和渗流实验，以Langmuir等温吸附模型为基础，建立了考虑温度的页岩气吸附模型，吸附平衡常数随着温度的增加按玻尔兹曼规律递减。考虑吸附和扩散的影响，得到了修正的页岩气藏视渗透率模型。分别建立了岩石-气体、岩石-水-气体的分子动力学模型，在分子动力学模拟的基础上，模拟计算不同电场强度、频率、温度、压力等因素对页岩气在岩石表面的吸附能、吸附厚度及吸附量的影响，定量表征电场作用下页岩气的吸附-解吸规律。在分子动力学模拟的基础上，考虑吸附的影响，研究不同电场、温度场、压力条件下页岩气的渗流规律，确定了不同孔喉分布的页岩储层中，不同电场、温度和压力情况下页岩气的可流动性。利用格子Boltzmann非平衡分子输运理论定量描述页岩气在微纳米孔隙群中的流动特征。基于电流连续性方程、能量守恒方程、质量守恒方程，建立了电场、温度场、渗流场耦合数学模型，并进行了数值求解，模型考虑了盖底层的电能损失及热传导损失。将模型计算结果与室内电加热地层三维物理模拟实验结果进行了对比，验证了数学模型及算法的可靠性。讨论了直井、水平井电极加热时电能、温度、渗流场在地层中的分布，盖底层岩石电导率对电能分布、加热效果及其对页岩气解吸效率和产量的影响。结果表明，电热作用可以强化页岩气的解吸，增加页岩气产量。在含水条件下电场的场效应对页岩气吸附和渗流作用明显。另外，从微观尺度升级到宏观尺度还需要进一步进行研究。