Numerical study of the gas-fluid displacement invasion behavior in a fractured carbonate reservoir

• Gas-fluid displacement (GFD) invasion is not widely known in drilling engineering.
• GFD invasion occurs frequently during drilling fractured carbonate gas reservoirs.
• 3D models were established to simulate GFD invasion.
• Effects of several factors on GFD invasion were discussed.

Gas-fluid displacement (GFD) invasion occurs frequently when drilling in fractured carbonate gas reservoirs. GFD invasion is a type of gas cut that is difficult to discover and is not widely known. In this study, the GFD invasion behavior was simulated based on the volume of fluid (VOF) method using a multidimensional two-phase flow model under different conditions and Fluent 6.3 software. The causes and process of GFD invasion were also explored. The effects of fracture width, drilling fluid viscosity and pressure difference were studied. The results show that the gas pressure increases because drilling fluid invades the fracture. Finally, GFD invasion occurs when the gas pressure is greater than the downhole pressure. A fracture width between 0.2 mm and 0.5 mm causes GFD invasion to obviously occur, and as the width increases, GFD invasion occurs more easily. Increasing the drilling fluid viscosity can postpone the GFD invasion to some degree. A small positive pressure difference between the wellbore and the fracture causes a more obvious GFD invasion. These results are also helpful for preventing and correcting GFD invasion.