致密油藏考虑毛细管力效应的相态理论及实验研究

The development of tight oil reservoirs has attracted more attentions all over the world. It is important to investigate the phase behavior of reservoir fluid. The effect of capillary pressure is noticeable due to the complexity of pore structures in the tight formations, leading to the significant difference in the phase equilibrium and fluid properties. In this study, the experiments of phase equilibrium in a nano-scale PVT cell were conducted using nanofluidic chips. The experimental data were used to modify the conventional capillary pressure calculation. Subsequently, the capillary pressure was introduced into the equation of state and a phase equilibrium algorithm considering effect of capillary pressure was proposed. Finally, based on the characteristics of tight oil reservoirs and multiphase flow, a numerical model was established to predict the well performance in tight oil reservoirs. The thermodynamic model considering capillary pressure effect was implemented and threshold pressure gradient and pressure sensitivity were included as well. This work provides a better understanding of phase behavior of fluid and performance of horizontal wells in the tight oil reservoirs, which is beneficial for the efficient development of the tight oil reservoirs.

致密油藏的开发已经得到国内外的广泛关注，流体的相态特征变化是致密油藏开发面临的重要难题之一。由于致密储层孔隙结构的复杂性，毛细管力效应显著，导致流体的相态平衡及物性计算与常规油气藏差异较大。因此，本项目首先开展纳米级流动通道内的流体相平衡实验研究，利用实验结果修正传统的毛细管力计算公式，研究不同纳米孔道内的流体相平衡过程；然后，结合油气状态方程，引入毛细管力项，建立考虑毛细管力效应的相平衡计算模型，分析毛细管力对流体相平衡过程的影响；最后，针对致密油藏储层流动特征，应用油气渗流理论，将考虑毛细管力效应影响的相平衡计算模型应用于水平井的产量预测中，同时考虑启动压力梯度及应力敏感因素，建立一套科学合理的考虑毛细管力对相平衡影响的致密油藏水平井产量预测模型。本项目的完成能够揭示致密油藏中流体相态变化特征规律，实现致密油藏产量的准确计算，对致密油藏的高效开发具有重要理论价值。

致密储层纳米级孔隙发育，孔隙中毛细管力效应显著，导致流体的相态平衡及物性计算与常规油气藏差异较大。因此，准确描述纳米孔隙中的流体相变规律和流动特征，实现致密油藏产量的合理预测就显得十分重要。本项目基于相平衡的热力学分析，首先开展了两组分、三组分以及多组分流体的相平衡理论与实验研究。然后，结合气液相平衡计算原理，引入毛细管力计算公式，建立了考虑毛细管力效应的流体相平衡计算模型，详细分析了毛细管力效应对体系露点、泡点等物性参数的变化，阐明了致密储层孔隙的毛细作用对流体相态以及物性参数的影响规律。最后，针对致密油藏储层流动特征，应用油气渗流理论，将考虑毛细管力效应的相平衡计算模型应用于致密油藏压裂水平井的产量预测中，建立了考虑毛细管力对相平衡影响的致密油藏水平井产量预测模型，实现了水平井产量的合理预测。研究结果表明：建立的相平衡计算模型能够较好地反映毛细管力效应对流体相态的影响。纳米孔隙中的毛细管力效应使流体的泡点和露点降低，两相共存区变小，且远离临界点区域，该效应更显著。此外，毛细管力效应使原油的粘度降低，体积系数升高，直接影响了原油在储层中的渗流过程。基于本研究建立的相平衡计算模型进行致密储层的压裂水平井产量预测，最终的油气产量升高，说明毛细管力效应有助于原油开采。该研究为致密油藏合理产量预测和高效开发提供重要的理论依据。