By Tariq Siddiqui
CO2 Enhanced Oil Recovery (EOR) is well researched subsurface technology that is tested and proven in the field. When engineered well, it provides the additional post water-flood oil recovery (10-20 %) and helps in reducing the carbon footprint. The other subsurface technology to store CO2 is geological sequestration.
This article is a primer for executives and practitioners; it provides an overview of the subsurface consideration, before you embark on engineering the project. Upstream EP Advisors LLC can help you with CO2 EOR: CO2 Injection Variants
Generally, CO2 is not first contact miscible but achieves miscibility after several contacts. Due to this reason and its high cost, CO2 is injected in combination with other fluids. There are several variants of CO2 injection used in the field:
Continuous CO2 injection
CO2 injection followed by gas
CO2 injection followed by water
Water-Alternating-Gas (WAG) injection
CO2 Solvent injection
CO2 /Heat Injection followed by water
Geoscience/Reservoir Consideration
There are many factors that govern the CO2 injection, the important ones are:
Immiscible vs. Miscible Injection:
Miscible CO2 injection is more suitable in light to medium crudes,
Immiscible displacement (generally not used) may work on heavier crudes
Secondary vs. Tertiary Application:
CO2 can be used as a secondary and tertiary recover (after water-floding)
Horizontal vs. Vertical Displacement:
Reservoirs that have high dip or relief (vertically), the gravity segregation mechanism will help the vertical displacement of CO2 followed by the drive-gas that will achieve mobility control. In the low dip or relief reservoirs (horizontal), WAG displacement of CO2 will better achieve mobility control.
Mechanisms for Increased Recovery
Following reservoir factors will help improve CO2 recovery:
Miscibility Effects: Reduction in interfacial tension and residual oil saturation
Crude Oil Swelling: Increases the stock-tank oil (increase in FVF)
Reduction of Crude Oil Viscosity: increases the oil production rates
Increase in Injectivity: Improves the amount of gas injected
Internal Solution gas drive: improves the overall recovery
Improve relative permeability effects: will result in additional oil recovery
Reservoir Screening
Following are some geological and reservoir engineering consideration that can help in a successful design of CO2 injection.
RESERVOIR CHARACTERISTICS
Ensure, significant data is available or collected for the geological characterization of reservoir. Following factors may impair the performance of the CO2 injection & reduce the oil recovery:
Low Net-to-Gross (NTG) reservoirs are not suited for CO2 injection (low net pay)
Highly heterogeneous reservoirs cause premature CO2 breakthrough
Highly stratified reservoirs will cause injection fingering and must be avoided
Naturally fractured and highly faulted reservoirs will impair the performance
Volumetric Sweep: Miscible CO2 flood can theoretically recover 100% of remaining oil, natural heterogeneity will limit the volumetric sweep to ~ 60%
Poor permeability and porosity development will reduce storage and injection
Thin pay zones underlain by thick large aquifers.
Shale barriers; especially in vertical displacement are very detrimental
Baffles in the reservoirs will reduce the sweep efficiency
Vuggy Carbonates; especially if they highly vugular and fissured
Very deep reservoirs high injection pressure requirement may preclude deep zones
FLUID CHARACTERISTICS`
Oil In Place before starting CO2 EOR should be adequate to make economic sense post waterflood oil saturations (Soil) should be no less than 40%
Oil Gravity: Best API gravity range: 25 < API Gravity < 30 (Max. range 15-55 API)
Reservoir Temperatures: Best results are achieved when Temperature < 200 F
Compressibility: Avoid highly volatile & condensate reservoirs on your first trial
Initial Water Saturation: Initial Sw < 50%; Avoid tight/silty reservoirs (Sw > 50%)
Formation Water Salinity: < 200,000 ppm or else miscibility would be difficult
Data Review/Collection
Beside reviewing existing performance data and collecting standard reservoir data (Seismic, logs, cores, SCAL, fluid, pressure, temperature etc.), additional specific data is needed to evaluate the performance of CO2 flood:
Minimum Miscibility Pressure (MMP) test
Swelling test for Crude oil
Viscosity reduction test
Asphaltine precipitation test
Properties of carbonated water