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Abstract 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.
Introduction Fracture-cavity carbonate reservoirs are widely distributed in the Tarim basin, China, generally with depth higher than 6000m. 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. 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. 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.
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