Abstract: This study aims to investigate the mechanism of multi-phase flow migration and the evolution rule of fluid pressure in the reservoir more scientifically and precisely, and to improve the accuracy of analytical calculation and analysis. The flow field was divided into three regions firstly and then inverted the saturation of each phase fluid in the mixed fluid flow region of two-phase fluid based on the conservation equation of seepage volume in the flow field. Subsequently, a generalised Darcy’s formula suited for the two-phase flow was obtained by directly introducing the total mobility into Darcy’s formula. Thus, a more accurate analytical model was derived for characterising the fluid pressure evolution in the reservoir. At last, this analytical model was applied to calculate a CO2 injection example, and the obtained results were compared with the explicit integral solution in existing references and the simulated results of TOUGH2/ECO2N. The compared results indicated that the reliability of the analytical model was verified and the superiority was reflected in the aspect of calculation accuracy. In addition, the calculation results showed that the full process of the actual fluid pressure evolution in the reservoir could be described well by the analytical model of this work, though it was obtained under the assumption of steady flow. The main reason is contributed to that the determination method of saturation in this model is more scientific and accurate. Hence, this model can be applied in practical engineering.

Key words: analytical model, two-phase flow, saturation, generalised Darcy’s law, CO2 geological storage