Modeling Multiphase Flow in Naturally Fractured Vuggy Petroleum Reservoirs

A multicontinuum conceptual model is presented and implemented into a three-dimensional, three-phase reservoir simulator, using a generalized multicontinuum modeling approach. The conceptual model, proposed for investigating multiphase flow and displacement through naturally fractured vuggy carbonate reservoirs, is based on observation and analysis of geological data, as well as on core examples from the carbonate Tahe Oil Field in China. In this conceptual model, naturally fractured vuggy rock is considered to be a triple-continuum medium, consisting of (1) highly permeable and well-connected large-scale fractures; (2) low or impermeable rock matrix; and (3) various-sized vugs or cavities. The base matrix system may contain many small or isolated cavities (of centimeters or millimeters in diameter), and large cavities (or vugs) ranging from centimeters to meters in diameter. Vugs may be (1) directly connected to large fractures, (2) indirectly connected to large fractures through small fractures or microfractures, or (3) isolated from large fractures by rock matrix. Similar to the conventional doubleporosity concept, the fracture continuum is responsible for the occurrence of global flow, whereas vuggy and matrix continua (mainly providing large-storage space of source/sink) are locally connected to each other as well as interacting with globally connected fractures. In the numerical implementation, a control-volume, integral finite-difference method is used for spatial discretization, and the resulting discrete nonlinear equations for the three-phase fluids, coupled with each continuum, are solved fully implicitly by Newton iteration. The numerical scheme, verified by comparing its results against those of available analytical solutions, is used to simulate wateroil flow through the fractured vuggy reservoirs of Tahe. Introduction Naturally fractured reservoirs existing throughout the world represent a significant amount of the world oil and gas reserves. Since the 1960s, studies of flow and transport in fractured rock have received increasing attention, and significant progress has been made in numerical modeling of flow and transport processes in fractured reservoirs. Research efforts, driven by the increasing need to develop petroleum and geothermal reservoirs (as well as to resolve subsurface contamination problems), have developed many numerical modeling approaches and techniques 21, 11, 12, . In the past decade, the petroleum industry has faced a growing demand for oil and natural gas, while at the same time few new oil reserves have been found worldwide. The efficient development of naturally fractured reservoirs has become a top priority of the oil industry. Because of the known low oil recovery rates (in general) from naturally fractured reservoirs, interest in enhancing oil and gas recovery from such reservoirs has grown, with more investigations conducted for multiphase flow and transport phenomena in fractured reservoirs 15, 3, . Since the 1970s, in parallel to the development in the oil industry, environmental concerns over subsurface contamination have motivated many studies of fluid, chemical, and heat transport in variably saturated fractured formations. Moreover, suitability evaluations for underground geological storage of high-level radioactive waste in fractured rock have generated renewed interest in investigations of multiphase and radionuclide transport in a fractured geological system 22, . Even though significant progress has been made towards the understanding and modeling of flow and transport processes in fractured rock since the 1960s 21, 11, , most of those studies have focused on naturally fractured reservoirs, without including cavities. Recently, driven by the need to develop underground natural resources, and by environmental concerns, characterizing vuggy fractured rock has currently received attention, because many naturally fractured vuggy reservoirs have been found worldwide and can significantly contribute to reserves of oil and gas. Significant interest is being generated in investigating vuggy fractured reservoirs 19, 14, 10, . Mathematical approaches to modeling flow through fractured reservoirs in general rely on continuum approaches and involve developing conceptual models, incorporating the geometrical information of a given fracture-matrix system, setting up mass and energy conservation equations for fracture-matrix domains, and then solving discrete nonlinear algebraic equations. The commonly used mathematical methods for modeling flow through fractured rock include: (1) an explicit discrete-fracture and matrix model, (2) the dualSPE 102356 Modeling Multiphase Flow in Naturally Fractured Vuggy Petroleum Reservoirs Z. Kang, Inst. of Petroleum Exploration and Development of Sinopec; Y.S. Wu, SPE, Lawrence Berkeley Natl. Laboratory; and J. Li, Y. Wu, J. Zhang, and G. Wang, Inst. of Petroleum Exploration and Development of Sinopec