Rate transient analysis of fractured-caved carbonate reservoirs under different cave connecting modes

Abstract

In fractured-caved carbonate oil reservoirs with well-developed dissolved caves, most of the caves are generally connected by large fractures to form separate and independent fractured-caved systems. The oil well basically intersects some fractures, and fractures connect one or two large-scale dissolved caves in series or parallel model to form an independent fractured-caved unit. Moreover, due to the large size of the dissolved cave in the fractured-caved unit, it no longer follows Darcy's law, but shows features belongs to pipe flow, free flow or a seepage-pipe flow. In this case, the conventional equivalent multiple porous media seepage mathematical model is inaccurate to characterize this kind of fractured-caved reservoir units. This study simplifies the structure of the large bead-shaped fractured-caved reservoir and shows how the well-cave and cave-cave units are connected through fractures. Results show that the connected fracture flow exsits in the periphery of the large cave dissolved caves in the area far from wells. Thus either a linear series or a parallel mathematical model is applied to simulate the connection of the two dissolved caves. The fluid flow in fractures is still considered as seepage flow, while in large dissolved caves it is considered as free flow. Therefore, the dual-cave dual-fracture series mathematical model and dual-cave single-fracture parallel mathematical model considering seepage-free flow coupling are established. Laplace transform is then used to mathematically solve the model, and Stehfest numerical inversion is conducted to draw the typical rate transient analysis curves. Finally, sensitivity analysis is applied to discuss the relationship between controling factors of the flow and actual reservoirs. Based on the rate transient analysis type curves, geometric parameters such as radius of cave, permeability and length of the large-scale fracture and dynamic reserves can be obtained. Finally, a field case in a bead-string fractured-caved reservoir unit of Tarim basin is implemented to verify the effectiveness of the proposed models and methods. The parameters obtained by type curve match support exploitation of fractured-caved reservoirs. • The large bead-shaped fractured-caved reservoirs are simplified based on reservoir static carving. • The fluid flow is considered as free flow in large dissolved caves. • The dual-cave dual-fracture series and dual-cave single-fracture parallel mathematical models are established and solved. • Sensitivity analysis is applied to discuss the relationship of the reservoir parameters. • The specific processes of type curves matching have been given in detail.