An improved visual investigation on gas–water flow characteristics and trapped gas formation mechanism of fracture–cavity carbonate gas reservoir

Abstract Water encroachment is a serious problem for developing an edge and bottom water reservoirs. The fracture–cavity gas reservoir has various reservoir media and strong heterogeneity, leading to the complex gas–water flow mechanism and trapped gas formation during water encroachment. Presently, no systematic study on the gas–water flow characteristics and trapped gas formation mechanism exists, especially through visual experiment. In this paper, the creation and testing of three types of visualization micromodels were described based on the CT scan images and microelectronic photolithography techniques. Subsequently, a 2D visual experiment was conducted initially to investigate the gas–water flow characteristics and the trapped gas formation mechanism intuitively. Then, ImageJ gray analysis method was utilized to study the distribution of trapped gas, saturation, and gas recovery quantitatively. Finally, several development suggestions were presented for different types of reservoirs based on the quantitative characterization results. Experimental results showed that the flow characteristics of fracture-, cavity-, and fracture–cavity-type micromodels are different during water encroachment, resulting from the large difference of microstructures. The trapped gas formed at dead ends or blind corners, circumfluence, and cutoff phenomenon, can be commonly found in three types of micromodels. However, the trapped gas formed at H-shaped channels, dumbbell-shaped channels, and microfractures are specific to certain micromodels. The main factors that influence the trapped gas formation are capillary force, hydrodynamic force, Jamin effect (the additional resistance effect of the bubble when it traverses narrow pores), and pore–throat connectivity. Quantitative results reflect that approximately 76% of the trapped gas in the cavity-type micromodel is formed by circumfluence and cutoff phenomenon, whereas approximately 67% of the trapped gas in the other two micromodels is formed at network of microfractures, dead ends, and blind corners. In addition, the ultimate gas recovery of cavity-type reservoirs increases while the displacement differential pressure increases; by contrast, the ultimate gas recovery of fracture- and fracture–cavity-type micromodels increases initially and then decreases with the increase in displacement differential pressure. The visual investigation presented not only improves our intuitive understanding of the gas–water flow mechanism in fracture–cavity carbonate reservoirs but also provides a novel image analysis method for quantitative characterization of visual experiments.