Abstract

Solution gas drive from heavy oil reservoirs in the eastern Orinoco Belt, Venezuela presents foamy oil behavior and has shown a primary recovery factor of 8-12%. However, the cold production performance will get worse continuously during such a pressure depletion process, and steam injection sometimes is not an economically attractive follow-up candidate for improving the oil recovery further especially at the low oil price condition. Therefore, produced gas re-injection based processes in huff and puff mode, i.e., hydrocarbon-based cyclic solvent processes (CSP and ECSP) have been tested to evaluate their applicability to such reservoirs.

In this study, the oil sample used in the test is taken from Block M, Eastern Orinoco Belt. The viscosity and the API gravity of the stock tank oil of Block M are 14,488cp (at 54 °C) and 7.8 respectively. The produced reservoir gas is mainly a mixture of methane and CO2 (87:13 in molar ratio). Firstly, the heavy oil swelling test and unconventional PVT test are conducted. And then the behavior of primary depletion process and produced gas huff and puff (CSP) is investigated by conducting a series of sandpack tests. In addition, to further improve the performance of the produced gas CSP, three different enhanced cyclic solvent processes (ECSP), including produced gas and propane, produced gas and foaming agent, and produced gas plus oil viscosity reducer ECSP are tested.

The 3-cycle produced gas CSP test result shows that it can improve the oil recovery by 8.7% compared to primary depletion when the first CSP cycle is initiated at a certain pressure between the bubble point pressure and the pseudo bubble point pressure. The CSP process illustrates obvious restoration of the foamy oil drive mechanism which provides primary production, or so called secondary foamy oil drive mechanism. Above three different ECSP processes all improve the CSP performance with different degrees. The produced gas and propane ECSP (70% produced gas +30% propane), with produced gas slug injected before propane slug during each injection process, improves the oil recovery by 23.6% through three cycles compared to cold production due to viscosity reduction and solvent-gas-drive mechanisms. For 3-cycle produced gas and foaming agent ECSP, and produced gas plus oil viscosity reducer ECSP, the increased oil recovery are 15.5% and 11.2% separately.

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