Fractured vuggy carbonate reservoirs usually develop several large-scale caves which are the main reservoir space of oil-gas. The volume and position of caves are important parameters for production guidance, but there are no effective well test methods of calculating the cave volume of multi-cave reservoirs at present. This paper establishes a multi fracture-vug composite model for fractured vuggy carbonate reservoirs, divides reservoirs into various fracture-vug seepage zones according to the cave distribution, establishes a mathematical model through the coupling of seepage zones, and then obtains analytical solutions of the mathematical model using dimensionless and Laplace transformation. By plotting the log-log type curves of well test, the paper analyzes the flow characteristics of the model. It is found that the pressure derivative of inter-porosity flow segment from caves to fractures is presented as a V-shaped concave, and the pressure and pressure derivative curves at the fracture flow segment take on the parallel lines with the slope of 0.5. A parameter sensitivity analysis indicates: the bigger the cave is, the steeper the V-shaped concave of the pressure derivative curve goes; the farther the distance between cave and wellbore is, the more hysteretic the V-shaped concave becomes. Finally, the paper demonstrates the field example analysis and application by fitting measured pressure data to calculate the volume and distance of caves.
Keywords Fractured vuggy reservoirs; Well test method; Transient pressure analysis