CO2 sequestration in mature oil and gas reservoirs is attractive due to the possible cost offsets from enhanced oil recovery (EOR). In this work we develop a 3D reservoir model and fluid flow simulation of the fractured Tensleep Formation using geomechanical constraints in advance of a proposed CO2-EOR and sequestration pilot at Teapot Dome Oil Field, WY. The objective is to model the migration of the injected CO2, and to obtain limits on rates and volumes to be injected, without compromising seal integrity. Additionally we want to investigate the effect of fractures on the permeability and mobility of CO2.
The pilot planned to inject 1 MMcfd of supercritical CO2 for six weeks. The results indicate that CO2 buoyancy and mobility could pose problems to EOR performance in this highly fractured reservoir. The injected CO2 will rapidly rise to the top layers, above the main producing interval, accumulating in the fractures. It takes almost a year to start saturating the fractures in the target interval and almost two years to penetrate into the matrix. Furthermore a well control strategy would be necessary to improve oil recovery without circulating CO2. Incremental oil production is predicted to be less than 10% or 30% if double the amount of CO2 is injected in twice the amount of time. Regarding storage capacity, the trap could hold up to 2 wells injecting 1 MMcfd each, for 15 years. This could sequester ~5x105 tonnes of CO2 equivalent to a small power plant emitting ~37,800 tonnes/year.
CO2 injection has been used as a commercial process for enhanced oil recovery (EOR) since the 1970s and is the second-most applied EOR process in the world (Jarrell et al., 2002). Traditionally, the goal has been to recover the maximum amount of oil from the reservoir while injecting the minimum amount of gas, because the cost of CO2 affects the profitability of the project. However, when the objective is to combine EOR and CO2 sequestration, different CO2 flooding designs will have to be implemented in order to increase the amount of CO2 left behind when production stops (Kovscek and Cakici, 2005).
A CO2-EOR and Sequestration pilot was proposed at Teapot Dome Oil Field, targeting the fractured Tensleep Fm. in a three-way closure trap against a bounding fault, termed the S1 fault (Figure 1). A comprehensive geomechanical analysis performed in the system (Chiaramonte et al., 2006 and 2008) found that the S1 fault does not appear to be at risk of reactivation and that the caprock integrity is not at risk from the buoyancy pressure of the maximum CO2 column height that the formation can hold. The presence of critically stressed minor faults and fractures in the reservoir was also established (Chiaramonte et al., 2011). If these minor faults are reactivated, they could enhance the permeability of the reservoir and create permeability anisotropy inside it.
In the present work, we develop a stochastic 3D reservoir model of the Tensleep Formation, as input for a fluid flow simulation, using the geomechanical constraints estimated by Chiaramonte et al., (2006 and 2008). Our objective is to model the migration of the injected CO2, as well as to obtain limits on the rates and volumes of CO2 that can be injected, without compromising seal integrity. Additionally we want to investigate the effect of fractures on the permeability and mobility of CO2.