Abstract
Karst cavities and caves are often present along fractures in limestone reservoirs and are of significance for oil and gas exploration. Understanding the formation and evolution of caves in fractured carbonate rocks will enhance oil and gas exploration and development. Herein, a reactive transport model was established considering both the matrix and fractures. Different factors affecting the dissolution along fractures were considered in the simulation of matrix–fracture carbonate rocks, including the magnitude and characteristic length of the matrix porosity heterogeneity, intersecting fractures, and complex fracture network. The results show that a strong heterogeneity of the matrix porosity significantly affects the cave formation along the fracture and the existence of fractures increases the heterogeneity due to the high permeability as well as the dissolution area. The characteristic length of the matrix porosity heterogeneity affects the cave location and shape. The larger permeability of intersecting fractures or the matrix greatly increases the cave size, leading to the formation of large, connected cave areas. A complex fracture network leads to more developed karst dissolution caves. The topology of the fracture network and preferential flow dominate the distribution of caves and alleviate the effect of the matrix heterogeneity.
Highlights
Carbonate rocks often contain karst cavities and caves, which form large aquifers as well as huge oil and gas reservoirs [1,2]
In Case 2 with a small heterogeneity, dissolution is affected by the distribution of the porosity and the dissolution extension on both sides of the horizontal fracture is asymmetrical
Resultsspace show changes that the magnitude of the matrix tion fractures to larger caves with greater size variation. These results show that the heterogeneity has a significant impact on the formation and shape of the cavities. magnitude of the matrix heterogeneity has a significant impact on the formation and shape of the cavities
Summary
Carbonate rocks often contain karst cavities and caves, which form large aquifers as well as huge oil and gas reservoirs [1,2]. Large fracture–cave carbonate reservoirs have been reported worldwide [3,4,5]. Fractured caves can connect with each other vertically or horizontally through a string/fracture, and several individual bead–string units form a string–bead complex [12,13]. This pattern has been reported as “beads-on-a-string” cave pattern in the Bahamas and Marianas [14].
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