Study On Flow Pattern Of Cave - Type Carbonate Reservoir

The characteristics of fractured vuggy carbonate reservoirs are greatly complex compared to conventional reservoirs due to its multiple flow units:matrix, fractures and vugs. Especially, the diameter of underground vugs could range from millimeters to hundred meters. Therefore, fractured vuggy carbonate reservoirs have severe macro heterogeneity. Consequently, fluid flow becomes rather complicated in this kind of reservoir. In order to exploit fractured vuggy carbonate reservoirs more scientifically, systematically and efficiently, it is significant to conduct a deep research on the flow model of fractured vuggy carbonate reservoirs.This paper analyzes the distinctive geologic characteristic of fractured vuggy carbonate reservoirs in the first place, and concludes that:(1) vug has large storage and high permeability;(2) fracture has relative small storage and high permeability;(3) matrix has little storage and extremely low permeability. Then, it designs and conducts a series preliminary verification experiments to simulate the gas flow in fractured vuggy carbonate reservoirs. After that, it innovatively establishes a new fracture-vug model for gas flow through naturally fractured vuggy carbonate reservoirs with complex fracture-vug connection regarding to its severe macro heterogeneity. Then it divides this model into four sub-categories:single fracture-single vug, fractures-vugs in series connection, fractures-vugs in parallel connection and fractures-vugs in complex connection. These models could analyze the patterns of flow rate changes in each fracture and pressure changes in each vug during pressure decline stage as well as pressure buildup stage. According to several computation examples of four-category models, it draws the curves of flow rate changes in each fracture and pressure changes in each vug and conduct a qualitative analysis on these curves.Afterwards, it shows a variety of pressure buildup well-test curves based on different computation examples with different model characteristics such as fracture aperture, fracture height, fracture length, vug volume and the connection relationship of fractures and vugs, and conduct a qualitative analysis on these curves. In addition to that, it tentatively deduces series formulae for bulk volume of vugs and characteristics of fractures according well-testing data.Besides, it also deduces a fracture-vug model for liquid flow through naturally fractured vuggy carbonate reservoirs model based on the properties of liquid, following a computation examples of model with3macroscopic vugs and3fractures in series connection. It manifests that new model could also be applied to liquid medium. Finally, it discusses several potential improvement approaches to this fracture-vug model on the respects of matrix transfusion, filling degree of vugs and properties of none-ideal underground fracture.