Rate transient analysis for coupling Darcy flow and free flow in bead-string fracture-caved carbonate reservoirs

Abstract

Fluid flow dynamics in fracture-caved carbonate reservoirs are usually simulated using the traditional continuum flow theory. With the presence of large karst caves, fluid flow in these reservoirs may not always obey the Darcy law. Actually it follows the pipe flow, free flow or Darcy law-coupled free flow. The continuum flow theory is therefore not able to accurately capture the underlying flow dynamics within the large karst caves. Large caves in fracture-caved reservoir are widely interconnected with a series of tectonic fractures, which resembles a string of beads and is thus defined as a multi-cave multi-fracture reservoir unit. By simplifying the configuration of actual fracture-caved carbonate reservoir as a multi-cave multi-fracture series connection domain, this paper develops a novel mathematical model to accurately describe the complicated fluid behaviors coupling Darcy flow and free flow, among of which the fluid flow in fractures still obeys the Darcy law while that in large caves is assumed to free flow. A semi-analytical solution of the proposed model is then derived in order to plot the dimensionless typical curve for rate transient analysis (RTA) in fracture-caved reservoir. Factors affecting the characteristics of the RTA typical curve are further investigated. Finally, case study in a bead-string fracture-caved reservoir unit is performed to estimate the reservoir and well properties using the proposed model. The deep understandings can provide good insights for reasonable exploitation of this type of fracture-caved carbonate reservoirs.