微纳孔隙间页岩气藏的相平衡与赋存特性研究

Shale gas is a kind of important unconventional natural gas resources with huge reserves, the characteristic of the gas reservoir has an great research value. For the shale gas occurrence characteristic hasn’t yet been studied systematically, the project proposes that a systematic study is taken for the shale gas occurrence state and accumulation regularity in the micro- and nano-pore from the view of phase equilibria. The core research content of the project is the thermodynamic properties, their internal relation and equilibrium mechanism of the adsorption\free\dissolution phases in shale gas reservoirs. Combining with Statistical Associating Fluid Theory(SAFT) and Cubic Plus Association(CPA) equations of state, the adsorption-free-dissolution phase equilibria model will be bulit. Based on the research of phase equilibria in gas reservoirs, the occurrence characteristic of shale gas will be further studied, and the influences of mineral characteristics, temperature and pressure will be investigated for the gas occurrence state and accumulation regularity. The key scientific problems is how to describe the thermodynamic properties of the adsorpted shale gas in the pore. For this, the pore influence for the molecular adsorption heat and chemical potential will be researched by analyzing the internal molecular force, intensively. The chemical potential calculation equation of adsorpted molecule will be researched on the basis of the statistical thermodynamic approach and Density Functional Theory(DFT). Through the research, it is hopeful for achieving the occurrence characteristic and accumulation regularity of shale gas in the micro- and nano-pore, which will provide a beneficial theoretical basis for the research of shale gas accumulation mechanism, production mechanism and exploitation technology.

页岩气是一种储量巨大的非常规油气资源，其气藏特性具有重要的研究价值。针对页岩气赋存特性缺乏系统研究的现状，本项目提出从页岩气藏吸附态、游离态和溶解态分子间相平衡角度研究气体的赋存状态和富集规律。核心研究内容是微纳孔隙中页岩气藏各相的热力学状态以及各相间的内在联系与平衡机理，结合SAFT和CPA状态方程构建页岩气吸附态-游离态-溶解态分子的相平衡模型。进而根据相平衡的研究分析页岩气的赋存特性，并考察矿藏特性和温度压力条件对页岩气赋存状态和富集规律的影响。关键科学问题是微纳孔隙对吸附态分子热力学性质的影响机理和计算模型。拟采用研究方案是通过从分子间作用力层面分析孔壁表面对分子吸附热和化学势的影响，根据统计热力学和密度泛函理论构建化学势计算模型。本项目研究有利于系统掌握微纳孔隙中页岩气赋存特性和富集规律，为研究页岩气的成藏机理、产气机制与开采技术提供有益的理论基础。

页岩气是一种储量巨大的非常规油气资源，其气藏特性具有重要的研究价值。本项目针对页岩气赋存特性缺乏系统研究的现状，从页岩气藏吸附态、游离态和溶解态分子间相平衡角度研究气体的赋存状态和富集规律，重点研究微纳孔隙中页岩气藏各相的热力学状态以及各相间的内在联系与平衡机理，分别选用PR方程、SAFT和CPA状态方程，建立了汽相平衡模型。由于吸附相的热力学性质受微纳孔隙壁面的影响，本课题从分子间作用力层面分析孔壁表面对分子吸附热和化学势的影响，从Lennard-Jones(12/6)势能模型导出圆柱体内分子所受壁面的影响而产生的外势，进而基于吸附实验数据，回归分析吸附层厚度的变化规律，在以上基础上，计算吸附相的热力学参数，进而构建微纳孔隙内页岩气的相平衡模型，特别研究了吸附相-游离气相间的规律。利用该模型研究了矿藏特性和温度压力条件对页岩气赋存状态和富集的影响规律。通过研究可得：.第一，根据密度泛函理论与圆柱体势能模型，可知页岩气在微纳孔隙中是一个密度不等的体系，离壁面越近（大于分子直径），则页岩气密度越大，越靠近孔隙中心，页岩气密度越接近游离气密度；.第二，吸附相厚度与孔隙半径成正比，当孔隙半径大于50nm后。吸附相厚度基本不随孔隙尺寸变化而变化，在一般赋存条件下，5nm的孔隙半径，吸附相厚度约为0.8nm。.第三，孔隙半径越小，吸附相占的页岩气比例越大，压力越低，吸附相分子占比越大，同时随着压力升高，页岩气会在微纳孔隙内产生毛细凝聚；.第四，基于密度泛函理论与圆柱体势能模型，可以构建吸附相的热力学参数的计算模型，进而利用吸附相-游离气的相平衡关系，研究页岩气的吸附规律，其关联结果与实验数据能较好的吻合。.此外，本课题还研究了页岩表面粗糙度对页岩气吸附的影响等一系列科学问题。因此本课题研究了微纳孔隙中页岩气赋存特性和富集规律，为研究页岩气的成藏机理、产气机制与开采技术提供了有益的理论基础。