Numerical Simulation on Gas Invasion Behavior in Carbonate Fractured-Vuggy Formations

Carbonate fractured-vuggy reservoirs often develop numerous fractures and cavities, making gas kick incidents prone to occur during drilling operations. Due to complex flow spaces, characterizing gas invasion behavior accurately remains challenging. To address this issue, a gas-liquid two-phase flow model for fractured-vuggy formations is proposed to describe the flow behavior of formation fluids during gas invasion. This model incorporates multiple media (matrix, fractures, and cavities) and accounts for the compressibility of both rock and fluids. Based on homogenization theory, an equivalent permeability for cavities is introduced, leading to the establishment of a two-phase flow model for discrete media. The accuracy of the proposed model was validated by comparing its results with those from a Stokes–Darcy two-phase oil-water flow model applied in fractured-vuggy reservoirs. Furthermore, the effects of bottomhole pressure differential, fracture aperture, and matrix permeability on post-invasion water saturation, pressure distribution, and gas influx rate were analyzed. The findings show that the gas influx rate follows a specific trend: an initial slight decrease, followed by a gradual increase to a peak, and then a decline. An increase in the pressure differential from 0.5 MPa to 5 MPa results in an increase of 8.36 m3/h in the maximum gas influx. Likewise, increasing the fracture aperture from 0.1 mm to 10 mm leads to a growth of 8.54 m3/h in the maximum gas influx rate. Cumulative gas influx exhibits a power-law growth with increasing pressure differential and a linear increase with larger fracture apertures. Matrix permeability has a minor influence on gas influx rate and shows no significant functional correlation with cumulative gas influx. This study contributes to a deeper understanding of gas invasion mechanisms in fractured-vuggy formations and provides theoretical insights for the identification and control of gas kicks.