Fracture-cavity carbonate reservoirs have abundant reserves of crude oil in place, but they are composed of matrix, fractures and cavities of various sizes, and characterized by strong heterogeneity. Fracture-cavity carbonate reservoirs in China are typically discovered with depth higher than 5000 m at high temperature, high pressure and strong stress. The deformation of fracture and cavity exerts a great impact on fluid flow, while the traditional continuum mechanic theory is inefficient to cope with the strong fluid-solid coupling effect occurred in this type of reservoir. This paper proposes an embedded discrete fracture-cavity model considering fluid flow and solid deformation in order to perform transient production analysis during depletion-drive stage of fracture-cavity carbonate reservoirs. Based on the double logarithmic characteristics of typical production history curve, oil production of fracture-cavity carbonate reservoirs can be divided into four stages: fluid flow in fracture, transition from fracture to cavity, response to cavity and boundary control. Bottom water only affects the fluid flow when entering the boundary-control stage, which can greatly inhibit the sharp decline of oil production history. By accurately conducting transient production analysis, true reservoir properties in actual fracture-cavity unit can be obtained by numerical inversion of production history. The proposed methodology provides a theoretical support for understanding reservoir fluid flow in fracture-cavity carbonate reservoirs.
Fracture-cavity carbonate reservoirs are widely distributed in the Tarim basin, China, generally with depth higher than 6000m[1]. Various reservoir spaces exist simultaneously including matrix, fracture and cavities of different sizes in this type of reservoir. Strongly affected the high in-situ stress, severe heterogeneity, various reservoir spaces and complicated oil-water relationships, depletion-drive development is mainly enforced. The natural decline of oil production is larger than 25%, and there exists a great difficulty to further improve production performance[2-5]. The traditional continuous theory is no longer suitable, and great efforts have been made to clarify to underlying mechanism of fluid exchange and its effect on actual oil production[6,7]. To resolve these issues, an embedded discrete fracture-cavity model considering the fluid flow and solid deformation is proposed in this paper, and used to carry out transient production analysis to investigate the underlying fluid flow during depletion-drive development. Using the proposed methodology, the reservoir properties of actual fracture-cavity carbonate reservoir are further estimated.
