Free-Fall Gravity Drainage in Fractured Matrix Blocks: Experimental and Network Modeling Simulation Findings and Observations

Abstract

<jats:title>Abstract</jats:title> <jats:p>Free Fall Gravity Drainage as an important recovery mechanism was investigated and analyzed experimentally and by numerical (network model) simulation.</jats:p> <jats:p>Sets of glass micromodels with real pattern of porous media and two numerical network model simulators were prepared in forms of fractured and non-fractured models to study and analyze the Free Fall Gravity Drainage recovery mechanism experimentally and by simulation. After validating the network model simulator, analysis of experimental and simulation results leads to some interesting findings and observations as follow:</jats:p> <jats:p>Based on network modeling simulation, direction of flow through fracture network shows that the oil in the middle horizontal fracture usually flows from one end of fracture to the other end, however in relatively small fracture apertures the oil from both vertical fractures at the ends of middle horizontal fracture flows into the horizontal fracture and then sinks downward to the lower matrix block. This means in relatively small fracture apertures, block to block interaction coefficient (α) reaches to 100%. Another point is that, it was observed from network model simulation data that this critical fracture aperture is a function of pore and throat sizes.</jats:p> <jats:p>Also, experimental and simulation results confirm that there is no break through of gas in free fall gravity drainage process. In addition a new mathematical approach (by means of flow potential and control volume concepts) to block to block interaction effects (Capillary Continuity and Reinfiltration phenomena) is presented to explain and interpret the mechanism of occurrence of these phenomena. This approach proves mathematically that decreasing the fracture aperture leads to increasing the fracture capillary pressure which in turn intensifies the feeding rate of liquid bridge (in the middle horizontal fracture) from upper block and lowers the discharging rate of liquid bridge from lower matrix block. Therefore, the liquid bridge can be more stable between the matrix blocks. As a result, fracture capillary pressure has positive effect on oil recovery from matrix blocks.</jats:p> <jats:p>Finally, a new approach for determination of the matrix block threshold heights at the end of free fall gravity drainage is presented and used to prove that matrix block threshold height inreases with increasing the matrix block height. Also, it will be proved that matrix block recovery factor increases with increasing the matrix block height although matrix block threshold height increases too.</jats:p>