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Title: Heavy Oil Recovery by Gas Assisted Gravity Drainage in Naturally Fractured Reservoirs
Author: Cabral e Silva, Rui Rodrigo
Advisor: Maini, Brij
Keywords: Engineering--Chemical;Engineering--Petroleum
Abstract: Most of the consolidated rock reservoirs are to some extent naturally fractured, but when the fractures do not affect the fluid flow, they are not taken in account and the reservoirs can be modelled as conventional reservoirs. However, in some reservoirs the fractures play a large role in the fluid flow. In such cases, the fracture properties must be considered in modeling the flow processes and the reservoir must be modelled as a naturally fractured reservoir (NFR). The exploitation activity related to the NFR, is often limited to the primary recovery. Only recently, the industry has started to implement Enhanced Oil Recovery (EOR) techniques in such reservoirs. With advancements in the data acquisition technologies, it is now possible to determine the fracture direction, length, thickness, morphology and angle. Such information is crucial for the implementation of EOR processes in NFRs. The laboratory modelling of EOR processes in NFRs is also challenging and the models used in the past were limited in terms of the number and the direction of the fractures. This work has developed a physical model that can mimic various multi-fractured systems and work at high pressures and elevated temperature. This model was used to investigate the performance of the gas injection under different conditions of injection rate and operating pressure. The operating pressure covered the range over which the gas moves from being immiscible with the oil to achieving first contact miscibility with the oil. In terms of recovery, the results were good in all tests and the oil recovery factor achieved ranged from 37 % to 43%, showing that even in a multi fractured system the oil can be recovered by gravity drainage and solvent extraction. The increase in pressure from immiscible conditions to first contact miscible condition resulted only in a modest increase in recovery. The effect of gas injection rate on recovery performance was significant for immiscible runs but at pressures higher than the minimum miscibility pressure the recovery factor versus pore volumes injected curve was not significantly affected by the injection rate. This analysis of recovery performance was complemented by using a numerical simulation model for investigating the effects of parameters that could not be changed easily in the experiment model. A sensitivity analysis was performed to see which parameters affect the recovery more strongly. It showed that the recovery is very sensitive to matrix capillary pressure. Other parameters that affect the performance significantly include matrix wettability and fracture spacing. These results showed that the simulator can provide good production forecasts, but accurate representation of all physical phenomena, such as asphaltene precipitation and matrix-fracture interaction, would require fine grid simulation and a huge amount of numerical work.
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