Depressurization Under Tertiary Conditions in Near-Wellbore Region: Experiments, Visualization and Radial Flow Simulation

摘要

Depressurization can be a very interesting process for recovering hydrocarbons from waterflooded oil reservoirs with high gas-oil ratio. Most of the published results are related to depletion experiments under secondary conditions (virgin reservoir). Data are more scarce under tertiary conditions after waterflooding (Ligthelm et al., 1997; Grattoni et al., 1998; Naylor et al., 2000). This paper treats the issue of the depressurization under tertiary conditions in the near-wellbore region. Practically, this has been achieved by combining experimental results, obtained from both core and a transparent micromodel, with radial flow depletion simulations that are representative of the conditions prevailing in the near-wellbore region. A validated methodology previously presented (Egermann and Vizika, 2000) has been used to design the experiments on core (the core under tertiary conditions was continuously flushed with water at a fixed rate, while the pressure at the outlet was decreased to reproduce a drawdown). The saturation profiles, the pressures and the fluid productions as a function of time were recorded in the two experiments that are presented. From the core experiments, critical gas saturation (S-(gc) and relative permeability (k_r) have been determined using a specific simulation code that takes into account nucleation, diffusion and mobilization processes related to the appearance of the gas phase from solution. Constant depletion rate experiments were also conducted on a transparent micromodel to study the process of connection of the gas phase under tertiary conditions. The specificity of the near-wellbore region (high dP/dt and radial flow) was then investigated by conducting a numerical experiment with radial geometry. It is shown that gas relative permeabilities obtained under tertiary conditions are lower than the ones obtained under secondary conditions. This behavior is attributed to the double connection process that is needed for the gas to get mobilized: connection of the disconnected oil phase and connection of the gas phase itself. This phenomenon is also confirmed by the visualization experiments conducted on the micromodel. The numerical experiment representative of the near-wellbore conditions with a radial geometry demonstrates that S_(gc) is a function of the distance to the wellbore, which can have a large impact on reservoir simulation results.