干酪根对页岩油相态行为演化的作用机理

The dissolution and liberation of hydrocarbons in kerogen will change the composition of shale oil that is equilibrating with korogen, and thereby imposes a significant impact on the phase behavior of shale oil. Quantitative assessment of such impact is of fundamental importance for understanding how the kerogen expels hydrocarbons in the source rock, how the shale oil/gas mixtures are residing inside the shale reservoir, and how the shale oil/gas mixtures migrate in the shale reservoir. At present, few researches have been conducted to address this issue, albeit its imperativeness. On the basis of both experimental measurements and theoretical modeling, this research will be focused on elucidating the influence of temperature/pressure variation on the phase behavior of shale oil/gas mixtures, and aimed at addressing two issues. First, by considering the density of the phase that is in contact with kerogen, we will develop a modified solubility model for calculating the solubility of hydrocarbons in kerogen. The unknown parameters in the solubility model, i.e., the binary interaction parameters and characteristic parameters of kerogen, will be optimized by tuning them to match the measured solubility data for individual components in kerogen. Second, we will establish two types of three-phase flash calculation models that can deal with gas-liquid-kerogen equilibrium and liquid-gas-kerogen equilibrium, respectively. The difference between these two types of equilibria is in that, as for the gas-liquid-kerogen equilibrium, kerogen remains in contact with the liquid phase only, while, as for the liquid-gas-kerogen equilibrium, kerogen remains in contact with the gas phase only. Based on the density-dependent solubility model and Peng-Robinson equation of state, two three-phase flash models dedicated for the gas-liquid-kerogen system and liquid-gas-kerogen system are developed. Finally, we apply the newly developed three-phase flash calculation models to the shale oil contained in the Triassic Yanchang #7 formation of the Ordos basin. We quantitatively examine how the dissolution of hydrocarbon in kerogen will affect the bubble/dew points of shale oil, as well as the shifting of the two-phase envelope of shale oil as temperature/pressure vary during the production of shale oil.

干酪根对烃类的溶解和解析而导致的页岩油相态行为的改变与演化是页岩生排烃作用、页岩油气赋存状态、页岩油气流动特征等研究中的重要基础问题；而目前对该问题的认识和研究还处于探索阶段。因此本项目将基于实验分析与理论推导相结合的研究思路，围绕温压变化过程中页岩油的相特征及其行为开展研究，着重解决两个方面的问题：一是从接触相（与干酪根直接接触的相）的密度函数入手，通过实验数据与理论模型之间的迭代拟合来确定各烃组分与不同类型干酪根间的交互作用系数、不同类型干酪根的属性参数，从而构建页岩油在干酪根中的溶解平衡模型；二是基于建立的溶解平衡模型和二元立方状态方程，分别以干酪根的接触相（气相或液相）为关联耦合相建立两套气-液-固相平衡闪蒸模型。最后通过鄂尔多斯盆地中生界三叠系延长组长7页岩油在干酪根影响下的相态特征参数变化规律、温压变化过程中瞬时相图的包络形态变化及相图位置迁移研究，对研究成果加以应用与验证。

据EIA 预测，全球页岩油资源量储备至少4385 亿吨，因此页岩油有望成为未来常规石油资源的接替。为了支撑在未来页岩油的勘探开发，目前亟需解决其油气赋存状态、分布规律、流动机理等方面的基本问题。本项目主要针对页岩油藏中干酪根的溶胀现象以及干酪根中的有机微孔隙导致的页岩油相态特征及行为的变化进行了研究。首先，我们通过结合Flory-Rehner和Regular Solution 理论建立了描述干酪根溶胀行为的数学模型，并利用这一模型预测了不同干酪根在不同溶液中的溶胀行为。随后，我们通过结合Young-Laplace公式和Peng-Robinson状态方程开发了微孔隙中的油气水三相平衡闪蒸模型和考虑毛管力影响的油气两相VT闪蒸算法，并研究了不同的润湿性对对相平衡的影响。在此基础上，通过结合Peng-Robinson状态方程和Tan和Piri改进的Young-Laplace公式，我们建立了更加准确的微孔隙中的油气两相平衡模型，并针对六种常见的烃类和两种常见的二元烃类混合物系统建立了参数λ的相关性函数。研究结果表明，干酪根的溶胀比随着溶液的溶解度参数增大而增大，并在溶液的溶解度参数与干酪根的溶解度参数相等时达到最大值。干酪根对芳香烃具有很强的选择性溶解特性。计算结果显示，当干酪根中的硫含量增加时，干酪根对烃类的溶解将会减弱。另外，热成熟度较低的二型干酪根的溶胀比例相对热成熟度较高的二型干酪根更高。研究还发现毛管力会显著影响微孔隙中的相平衡；微孔隙的半径越小，毛管力对油气水三相平衡的影响越明显；从模型计算的相图中可以看出，在等温条件下，油气水三相平衡的边界受到微孔隙中的毛管力影响会向低压区域移动，而在等压条件下则会向高温区域移动。在油气两相VT闪蒸计算中，毛管力会导致气液两相的平衡压力和质量密度减小；同时，毛管力也会导致较轻组分的摩尔分数增加。在毛管力的影响下，泡点会在更大的摩尔浓度出现。而通过与实验数据对比可以看出，应用改进的Young-Laplace公式能够大幅提高微孔隙中的油气两相平衡模型的准确性。在改进的Young-Laplace公式中，λ的值随孔径的增大而增大，随烃类碳数的增加而减小。