Liu Yunfeng, Zhu Yangwen, Liao Haiying, Yu Hongmin, Fang Xin, Zhang Yao
Petroleum Exploration & Production Research Institute SINOPEC, Beijing 100083, China.
ACS Omega. 2023 Sep 13;8(38):35107-35120. doi: 10.1021/acsomega.3c04576. eCollection 2023 Sep 26.
Due to the small pore throat characteristics of tight oil reservoirs, their strong capillary pressure makes imbibition replacement an effective development method. Field data has indicated that only a little fracture fluid can flow back and that there is an enhancement in oil recovery with shut-in periods after volume fracturing. A large number of scholars have carried out core-scale experiments on imbibition characteristics, but there has been limited research on the quantitative characterization of the differential pressure and imbibition replacement during counter-current imbibition. At the same time, there was also controversy on the calculation method of the optimal soaking time. In this paper, a mathematical model of oil-water two-phase flow is first established. Then, a mathematical model representing differential pressure and imbibition replacement in tight reservoirs is derived with a diversion function. Based on the saturation equation, Corey relative-permeability curve, and function, the model is simplified to a mathematical model of spontaneous imbibition in the shut-in periods after volume fracturing. Second, based on the finite difference method, a dynamic solution method for the flow field considering the dynamic capillary force was established, and the variation law of the pressure field and the water saturation field during the soaking time was revealed. The simulation results show that imbibition characteristics are the core of flow field reconstruction, and the differential pressure action can ensure the advancing distance of the fracturing fluid; both of them are not a linear superposition on tight oil development but complement each other and promote each other. Third, according to the growth rate of the imbibition replacement between fractures and the matrix during the soaking time, the calculation method of optimal soaking time was established. Taking the development parameters of the volume fracturing development case in the Ordos Basin into account, a reasonable soaking time was optimized. Finally, we analyzed the optimal soaking time under different conditions, and a chart of optimal soaking time for different initializations was plotted. Such a chart has profound reference significance for engineers, and they can make quick and accurate decisions regarding development and adjustment.
由于致密油藏孔隙喉道细小，其强大的毛管压力使得渗吸置换成为一种有效的开发方式。现场数据表明，压裂液仅有少量能够返排，且体积压裂后关井期可提高原油采收率。众多学者针对渗吸特征开展了岩心尺度实验，但对于逆流渗吸过程中压差与渗吸置换的定量表征研究较少。同时，关于最佳浸泡时间的计算方法也存在争议。本文首先建立了油水两相流数学模型。然后，利用导流函数推导了表征致密油藏压差与渗吸置换的数学模型。基于饱和度方程、科里相对渗透率曲线及函数，将该模型简化为体积压裂后关井期自吸数学模型。其次，基于有限差分法，建立了考虑动态毛管力的流场动态求解方法，揭示了浸泡时间内压力场和含水饱和度场的变化规律。模拟结果表明，渗吸特征是流场重构的核心，压差作用可确保压裂液推进距离；二者在致密油开发中并非线性叠加，而是相互补充、相互促进。第三，根据浸泡时间内裂缝与基质间渗吸置换的增长率，建立了最佳浸泡时间计算方法。结合鄂尔多斯盆地体积压裂开发实例的开发参数，优化得到了合理的浸泡时间。最后，分析了不同条件下的最佳浸泡时间，绘制了不同初始条件下的最佳浸泡时间图。该图对工程师具有重要参考意义，可使其在开发调整决策时快速准确。
