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Abstract

This paper describes a workflow that fully utilizes the post-stack seismic attributes to derive reliable geologic and fracture models that are validated with multiple blind wells and reservoir simulation. The first step in the workflow is to run post-stack seismic processes, which includes volumetric curvature, post-stack inversion and spectral imaging. The second step consists of using the various post-stack seismic attributes to derive 3D geologic and fracture models. The third step is to use the derived models in a reservoir simulator to verify the validity of the models.

This workflow was applied to the Tensleep reservoir at Teapot Dome in Wyoming. A large number of post-stack seismic attributes were generated in time and then depth converted within a 3D geocellular grid. These seismic attributes were used as input in REFRACT™, Prism Seismic fracture modeling software, to create geologic and fracture models. An effective permeability was estimated by using a linear combination of the scaled fracture density and the matrix permeability. Two reservoirs unknowns were estimated by history matching in a black oil simulator: the strength of the aquifer and the scaling factor used to convert fracture density to fracture permeability. Water cut was matched at all the wells, confirming the reliability and accuracy of the derived geologic and fracture models and the usefulness of the workflow. With the derived dynamic model, a compositional simulator was used to test various CO₂ injection rates and their effects on the breakthrough time.

Introduction

Injecting CO₂ in hydrocarbon reservoirs is an established EOR (enhanced oil recovery) process in the oil and gas industry. CO₂ injection can usually extend an oilfield’s production and increase the ultimate oil recovery. Proper planning of a CO₂ injection project requires an accurate characterization and simulation of the reservoir, with the goal of optimizing the oil recovery. The presence of faults and fractures in the reservoir complicates this process. Fractures can enhance reservoir permeability, and thus allow larger volumes of CO₂ to be injected. However, these fractures could also create early CO₂ breakthroughs in existing wells, or leakage through fractures and non-sealing faults to other formations or even to the surface. The reservoir characterization and simulation must accommodate these complications in order to ensure a successful result. This paper describes a robust characterization and simulation workflow for a fractured reservoir considered for CO₂ injection. This workflow has been successfully applied to various oil and gas fields,^1-8 and is illustrated in this paper using the Tensleep reservoir at Teapot Dome, Wyoming.