Fracture characterization and fluid flow simulation with geomechanical constraints for a CO2–EOR and sequestration project Teapot Dome Oil Field, Wyoming, USA

Abstract Mature oil and gas reservoirs are attractive targets for geological sequestration of CO2 because of their potential storage capacities and the possible cost offsets from enhanced oil recovery (EOR). In this work, we analyze the fracture system of the Tensleep Formation to develop a geomechanically-constrained 3D reservoir fluid flow simulation at Teapot Dome Oil Field, WY, USA. Teapot Dome is the site of a proposed CO2-EOR and sequestration pilot project. The objective of this work is to model the migration of the injected CO2 in the fracture reservoir, as well as to obtain limits on the rates and volumes of CO2 that can be injected, without compromising seal integrity. Furthermore we want to establish the framework to design injection experiments that will provide insight into the fracture network of the reservoir, in particular of fracture permeability and connectivity. Teapot Dome is an elongated asymmetrical, basement-cored anticline with a north-northeast axis. The Tensleep Fm. in this area is highly fractured, and consists of an intercalation of eolian-dune sandstones and inter-dune deposits. The dune sandstones are permeable and porous intervals with different levels of cementation that affects their porosity, permeability, and fracture intensity. The inter-dune deposits consist of thin sabkha carbonates, minor evaporates, and thin but widespread extensive beds of very low-permeability dolomicrites. The average permeability is 30 mD, ranging from 10–100 mD. The average reservoir thickness is 50 ft. The caprock for the Tensleep Fm. consists of the Opeche Shale member, and the anhydrite of the Minnekhata member. The reservoir has strong aquifer drive. In the area under study, the Tensleep Fm. has its structural crest at 1675 m. It presents a 2-way closure trap against a NE-SW fault to the north and possibly the main thrust to the west. The CO2-EOR and sequestration project will consist of the injection of 1 million cubic feet of supercritical CO2 for six weeks. A previous geomechanical analysis suggested that the trapping faults do not appear to be at risk of reactivation and it was estimated that caprock integrity is not a risk by the buoyancy pressure of the maximum CO2 column height that the formation can hold. However, in the present study we established the presence of critically stressed minor faults and fractures in the reservoir and caprock, which if reactivated, could not only enhance the permeability of the reservoir, but potentially compromise the top seal capacity. The results of the preliminary fluid flow simulations indicate that the injected CO2 will rapidly rise to the top layers, above the main producing interval, and will accumulate in the fractures, where almost none will get into the matrix.