In this paper, the theoretical state equation of gas foam is derived by considering the influence of gas solubility in formation water, salinity and pressure difference between inside and outside of bubbles caused by surface tension on the volume properties of gas foam. Combined with the analysis, the model is simplified. By selecting the appropriate density model for the gas phase and liquid phase of the bubble, the CO2 foam density was calculated by the CO2 foam state equation established in this paper and compared with the experimental values under different conditions.
Foam fluid has shown great application potential in drilling (Ma et al., 2005), oil and gas well stimulation, workover and EOR (Yin et al., 2010), and has gradually formed a series of technologies for foam fluid stimulation. The density of foam system is an indispensable parameter for construction design, so it is necessary to develop the theoretical state equation of foam system to estimate the density of foam system. At present, it is generally assumed that the liquid phase in the foam is incompressible and the gas phase is the actual gas to calculate the foam density. However, the compressibility of the gas phase dispersed into the liquid phase to form the foam is not the same as the compressibility of the gas-liquid two-phase alone (Li, 2009).
Ejofodomi et al. (2006) derived the theoretical state equation of N2 (insoluble gas) gas foam based on the three-point hypothesis. On this basis, the theoretical state equation of CO2 foam is derived by considering the dissolution of gas in liquid phase and neglecting the volume change of liquid phase caused by dissolved gas (Wang et al., 2010). The above equations can describe the PVT properties of foam to some extent, but there is a gap between the assumptions and the real foam system. Firstly, the compressibility of the liquid phase at high pressure needs to be considered, secondly, the influence of dissolved gas on the volume properties of the foaming agent solution, and thirdly, the actual foaming agent solution tends to have higher salinity, even some foamer solutions contain macromolecules as foam stabilizers or profile modifiers. These inorganic salts and organic compounds in foamer solutions have important effects on the volume properties of the liquid phase, the existence of foaming agent in liquid phase is the key to foaming system, and the effect of foaming agent on foaming system should be considered in the equation of foam state. Based on the above points, the foam state equation of dissolved gas is established in this paper. Taking CO2 foam as an example, the density model of CO2 foam is established.
