Moduli variations in two-phase fluid rocks considering squirt flow and patchy saturation

Abstract

The complex pore structure and fluid distribution in underground rocks bring difficulties to rock physics modeling and reservoir interpretation. The existence of cracks and ellipsoidal pores will cause squirt flow during wave propagation, and the fluid may be present as patchy saturation. By considering the effects of both squirt flow and patchy saturation, a new rock physics model is proposed. The fluids in inclusions include a soft fluid such as gas and a stiff fluid such as water. The novelty is that an explicit expression of patch saturation with frequency is introduced. Thus, the proposed model is helpful to analyze the variations of moduli with frequency caused by squirt flow and patchy saturation simultaneously. By comparison, the proposed model matches well with existed boundary models and covers all possible behavior of the moduli. Then, variations in moduli with water saturation and frequency are analyzed in detail, as well as the impact of patchy saturation on the attenuation induced by the squirt flow. The analysis shows that a small amount of gas contained in inclusions can cause a large change in moduli, especially the bulk modulus. The effect of patchy saturation on the moduli increases with the increasing frequency and when the frequency is low, the effect only occurs at very high water saturation. Besides, at high water saturation, the squirt flow can cause a sharp change in bulk modulus. The proposed model is helpful for understanding wave propagation in partially saturated rock with cracks.