Comparison of WAG and Water Over Injection for Carbon Storage and Oil Recovery in a Heavy Oil Field
- Lorraine E. Sobers (University of West Indies)
- Document ID
- Society of Petroleum Engineers
- SPETT 2012 Energy Conference and Exhibition, 11-13 June, Port-of-Spain, Trinidad
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 4.1.4 Gas Processing, 6.5.1 Air Emissions, 5.7.2 Recovery Factors, 5.4.9 Miscible Methods, 5.1.1 Exploration, Development, Structural Geology, 5.2.2 Fluid Modeling, Equations of State, 5.2.1 Phase Behavior and PVT Measurements, 4.6.2 Liquified Natural Gas (LNG), 5.4.1 Waterflooding, 1.8.5 Phase Trapping, 5.4 Enhanced Recovery, 6.5.7 Climate Change, 5.4.2 Gas Injection Methods, 5.1 Reservoir Characterisation, 5.5 Reservoir Simulation, 5.3.2 Multiphase Flow, 4.6 Natural Gas, 5.2 Reservoir Fluid Dynamics, 2.4.3 Sand/Solids Control, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 4.3.4 Scale, 5.1.2 Faults and Fracture Characterisation, 1.2.3 Rock properties
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Compositional reservoir simulation studies show that CO2 enhanced oil recovery (CO2EOR) combined with carbon storage using water over gas injection improves production performance and carbon storage compared to WAG injection. The fluid description is based on matching the PVT-derived viscosity, total and relative volume of a heavy oil sample (density 893 kg/m3 [17 degrees API]). The reservoir description is based on an unconsolidated deltaic, sandstone deposit in the Gulf of Paria, offshore Trinidad. Water over gas injection is an injection strategy proposed by Stone and Jenkins to increase gas sweep efficiency. In this injection design water is injected in the upper portion of the reservoir and gas is injected in the bottom portion of the reservoir using either vertical or horizontal injectors. Increasing the sweep efficiency improves both oil recovery and carbon storage. The counter current flow of injected fluids uses gravity effects to reduce gas override and water underride typical of WAG injection strategies and continuous single fluid injection. The results show that the water over gas injection design results in a larger mixing zone that extends further into the reservoir. Although oil recovery was essentially the same, with water over gas injection, there was greater than 50% increase in carbon storage, 20% reduction in water cut and 85% reduction in producing GOR compared to WAG.
Global warming is the term used to describe the increasing average global temperatures which can lead to significant changes to the earth's climate (Houghton, 1990). Anthropogenic CO2 emissions have been identified as the main contributor to increasing atmospheric CO2 concentrations and global warming (Houghton, 1990). The United Nations' Intergovernmental Panel on Climate Change (IPCC) reports that the impact on human life, availability of resources and environmental effects will be catastrophic. Caribbean countries including Trinidad and Tobago will be particularly susceptible to more frequent and powerful hurricanes, rising sea level, flooding and widespread tropical diseases (Parry et al eds, 2007; Watson et al eds, 1997). Geologic storage of CO2 in hydrocarbon reservoirs, aquifers and coal seams has been proposed as an intermediate solution to reducing CO2 emissions. This paper focuses on carbon storage in a producing heavy oil reservoir.
Traditionally water alternating gas (WAG) injection has been used for CO2 enhanced oil recovery (CO2EOR). However this injection strategy was not developed or optimized for carbon storage. Sobers et al. (2010) concluded that manipulating injection parameters such as injection rates, size and number of WAG cycles are insufficient measures for permanently storing large volumes of CO2 to significantly reduce CO2 emissions. This paper compares four CO2 injection schemes in a moderately heavy oilfield (density 893 kg/m3 [API gravity 17]). The objective is to compare CO2 storage, oil recovery and performance of traditional methods, WAG and gas flood, to water over gas injection in an unconsolidated sand body. Water over gas injection is an injection strategy proposed by Stone (1982) where water is injected in the upper portion of the reservoir and gas is injected in the lower portion. This process is also known as modified simultaneous water alternating gas injection (SWAG).The horizontal and vertical water over gas injection schemes are shown schematically in Figure 1. In the vertical injection scheme the injector was completed over the entire reservoir interval where gas was injected in the lower 60 m and water in the upper 40 m of the formation. With the chosen dimensions of the reservoir, it is possible to maintain injectivity and remain below the reservoir fracture pressure for the duration of the flood.
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