Time-Lapse Seismic Modeling and Inversion of CO2 Saturation for Storage and Enhanced Oil Recovery
Authors: Meadows Mark A. 
Published: October 2008

Abstract
Injection of carbon dioxide (CO2) into subsurface aquifers and hydrocarbon reservoirs has become an important topic because of growing concerns related to global warming and energy security. In this study we describe a new method to predict and quantify the effects of CO2 injection in porous saline and oil/brine reservoirs using time-lapse seismic data. We demonstrate the method using data from the Sleipner CO2 storage site, Norwegian North Sea. We first perform a careful analysis of the properties of CO2/brine systems over the range of supercritical temperatures and pressures found at Sleipner, and use these properties to generate synthetic seismic data. We then measure time-lapse seismic attribute changes from field data and invert these changes, together with the synthetic modeled data, to generate maps of CO2 thickness. From this information, we estimate that 7.5% of the total CO2 injected over a five-year period reached the top of the Sleipner reservoir. Unlike many inversion methods, this model-based approach is flexible enough to invert for multiple reservoir properties and their uncertainties using data that may include seismic tuning and other potentially complicating effects.

Introduction
It is widely recognized that CO2 is an important agent in global warming, and much recent activity has focused on finding ways to reduce, eliminate, or sequester CO2 in order to minimize its impact on the environment. In addition, miscible CO2 floods have become increasingly important, both as an EOR (Enhanced Oil Recovery) method for recovering residual or bypassed oil, and as an effective mechanism for long-term storage of CO2. Clearly, an understanding of the effects of CO2 on rock/fluid systems, together with an ability to accurately map CO2 fronts during injection, are crucial for improving recovery rates, optimizing well patterns, locating bypassed oil, and minimizing the cost of injected CO2.
CO2 EOR and storage projects naturally lend themselves to time-lapse studies, but to date, few such studies have been performed for CO2 operations. Both time-lapse modeling and inversion require knowledge of the seismic velocities and densities, which are functions of the reservoir rock and fluid properties. At high pressures, CO2 is a supercritical fluid with complex, liquid-like properties that can vary abruptly within narrow pressure and temperature ranges, particularly near the critical point (31.1° C, 7.38 MPa). For example, CO2 compressibility drops dramatically near the critical point and can even be double-valued over a narrow temperature and pressure range, while the density is close to that of a fluid. In this paper we develop a method to invert for CO2 fluid properties from time-lapse seismic data, which takes into account the complicated behavior of CO2 mixtures as well as effects such as seismic thin-bed tuning. We test our method on data from the Sleipner Field, Norwegian North Sea, an industry-standard data set consisting of multiple vintages of 3D seismic data recorded over a shallow saline aquifer during CO2 injection.

Technical Approach
Our technical approach consists of three steps, namely, 1) modeling CO2 petrophysics under reservoir conditions, 2) using these petrophysical properties to model synthetic seismic data over a range of reservoir properties, and 3) making quantitative estimates of changes in reservoir properties, such as pore pressure and CO2 saturation, using the synthetic modeled data in a multi-parameter inversion of the time-lapse seismic field data.