Abstract
Carbonate reservoirs are mainly fractured-caved reservoirs with very well-developed dissolved pores, fractures, and caves. They have strong heterogeneity with various types of reservoir pore spaces. Using seismic inversion and reservoir static characterization, the result shows that the fractured-caved carbonate rocks in China are mainly caves with poor connectivity and complex oil-water distribution. Large-scale dissolved caves are mostly discrete and isolated, while the fractures are complex and various. The fracture features are observed either as a single large fractures or as a local fracture network. The characteristics of fluid flow in fracture-caved reservoirs vary as a result of the different combinations of fractures and caves. Currently, the static characterization technology of fractured-caved reservoirs is influenced by the limited resolution of seismic data, leading to large interpretation errors. In contrast, the dynamic method is a more reliable and effective method to determine reservoir parameters. However, traditional seepage equations cannot accurately characterize the flow pattern of fractured-caved carbonate reservoirs. In the case of a single large-scale dissolved fractured-caved reservoir, oil wells are usually connected to large caves through large fractures or directly drilled into large dissolved caves. In this study, the large-scale dissolved caved reservoir is simplified into two cases: (1) a single-cave and single-fracture series model composed of a single-cave and a single-fracture and (2) a composite model of dissolved caves and surrounding fracture networks. Note that the flow in a large cave is considered as free flow due to its large scale. The flow in a large fracture connected to the cave is considered as flow through porous media, and the flow in the reservoir surrounding the fracture network is considered as multiple-porosity model seepage flow. The corresponding seepage-free flow coupling mathematical model of different fractured-caved reservoirs has been established on this basis. We also obtained the rate transient analysis type curves of the oil well, conducted sensitivity analysis of each parameter, constructed the corresponding rate transient analysis curves, analyzed sensitivities of each parameter, and finally designed a dynamic evaluation method of well and reservoir parameters for different types of fractured-caved carbonate reservoirs. This study extensively applies this method in the Halahatang Oilfield of China and evaluates parameters such as reservoir reserves and physical properties to provide rational guidance for developing fractured-caved carbonate reservoirs.
Highlights
The fractured-caved reservoir is a vital type of carbonate reservoirs, characterized by strong heterogeneity, various types, complex connectivity, complex fluid flow mechanisms, and complicated gas-water contacts [1,2,3]
Fractures have significant permeability and are the main seepage channels. Their storage capacity is lower than that of large dissolved caves, so the fracture is generally considered to be a unit with low storage and high seepage and is the primary fluid flow channel [6,7,8,9,10]
In cases where large-scale dissolved caves have been developed in carbonate reservoirs, the caves are connected to the wellbore through large fractures, or vertical wells are drilled directly on large caves; the capacity and permeability of the matrix cannot be taken into consideration in the reservoir and the reservoir can be described mathematically utilizing a simplified discrete model with seepage-free flow coupling
Summary
The fractured-caved reservoir is a vital type of carbonate reservoirs, characterized by strong heterogeneity, various types, complex connectivity, complex fluid flow mechanisms, and complicated gas-water contacts [1,2,3]. In cases where large-scale dissolved caves have been developed in carbonate reservoirs, the caves are connected to the wellbore through large fractures, or vertical wells are drilled directly on large caves; the capacity and permeability of the matrix cannot be taken into consideration in the reservoir and the reservoir can be described mathematically utilizing a simplified discrete model with seepage-free flow coupling. When large-scale caves are developed in carbonate reservoirs and small-scale dissolved pores and fracture systems are evenly distributed around the caves, the permeability and storage capacity of the matrix pores and fracture network around the caves must be taken into consideration In this case, the simplified large-scale cave and multiple-porosity composite model can be used for reservoir characterization. The dimensionless partial differential equation describing the composite model composed of large-scale caves and surrounding formation is as follows (considering improved matrix that develops small-scale caves):
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