Experimental and numerical simulation studies on the imbibition front of different rocks in coal-measure gas composite reservoir

Abstract

Spontaneous imbibition (SI) is an important mechanism during the development of coal-measure gas composite reservoirs. However, due to the highly heterogeneity of composite reservoirs, there are differences in the imbibition laws of different lithological reservoirs. In this study, a numerical simulation model of the imbibition mass (IM) and imbibition front (IF) of the composite reservoir was established, and the influencing factors of the SI as well as the change law of the IF of the composite reservoir were studied. The experimental results showed that the IM is mainly controlled by porosity and the influence of wettability is significantly weaker than that of porosity. The time to imbibition mass inflection point (Ti) ranged from 20 to 45 h, IM ranged from 0.2314 to 0.5430 g. The inflection point of the IF occurs at 0.5 Ti, ranged from 1.32 to 2.21 cm, and the displacement increment of the IF shows the characteristic of first becoming larger and then slowly decreasing. Before the inflection point, the displacement curve of the IF is in the form of a power function, and the displacement increment gradually decreases from more than 110% to less than 40%; after the inflection point, the displacement curve of the IF is close to a straight line, and the displacement increment does not change much. The variation of the direction of action of the capillary force and the capillary diameter resulted in a displacement of the IF in the decelerating development phase that was approximately 2 times greater than in the slow stabilisation phase for all samples, and this quantitative relationship was generalizable across all samples. Results show that our model provides a powerful tool to revealing water-gas flow behaviors and for modeling two-phase flow in coal-measure gas reservoirs.