A rock physics simulator and its application for CO 2 sequestration process Ruiping Li 1 Kevin Dodds 2 A.F. Siggins 2 Milovan Urosevic 1 Key Words: rock physics, seismic response, CO 2 sequestration ABSTRACT Injection of CO 2 into underground saline formations, due to their large storage capacity, is probably the most promising approach for the reduction of CO 2 emissions into the atmosphere. CO 2 storage must be carefully planned and monitored to ensure that the CO 2 is safely retained in the formation for periods of at least thousands of years. Seismic methods, particularly for offshore reservoirs, are the primary tool for monitoring the injection process and distribution of CO 2 in the reservoir over time provided that reservoir properties are favourable. Seismic methods are equally essential for the characterisation of a potential trap, determining the reservoir properties, and estimating its capacity. Hence, an assessment of the change in seismic response to CO 2 storage needs to be carried out at a very early stage. This must be revisited at later stages, to assess potential changes in seismic response arising from changes in fluid properties or mineral composition that may arise from chemical interactions between the host rock and the CO 2 . Thus, carefully structured modelling of the seismic response changes caused by injection of CO 2 into a reservoir over time helps in the design of a long-term monitoring program. For that purpose we have developed a Graphical User Interface (GUI) driven rock physics simulator, designed to model both short and long-term 4D seismic responses to injected CO 2 . The application incorporates CO 2 phase changes, local pressure and temperature changes, chemical reactions and mineral precipitation. By incorporating anisotropic Gassmann equations into the simulator, the seismic response of faults and fractures reactivated by CO 2 can also be predicted. We show field examples (potential CO 2 sequestration sites offshore and onshore) where we have tested our rock physics simulator. 4D seismic responses are modelled to help design the monitoring program. INTRODUCTION Theories and empirical models developed in rock physics provide essential tools for the seismic analysis of porous media. They relate the micro-structure of a porous rock to its seismic response. Widely used theories and formulations have been discussed by numerous researchers in this field (Gassmann, 1951; Wyllie et al., 1958; Mavko et al., 1998). To make these models readily accessible to a wider geophysical community, we have developed a user-friendly GUI-driven rock physics simulator, for fluid substitution in generalised media. Specifically for CO 2 sequestration objectives, we also include computations of CO 2 and CH 4 properties under variable temperature and pressure regimes (Rowe and Chou, 1970; Span and Wagner, 1996; Angus and Reuck, 1976). Moreover, variation in CO 2 /CH 4 mixtures under variable pressure and temperature conditions is also incorporated into the calculations (Duan et al., 1992). Fluid-substitution modelling can then be performed to assess the 4D seismic response to CO 2 injection. During a long-term CO 2 sequestration process, the composition of fluid in the pore space may change, accompanied by chemical reactions, change in minerals composition, precipitation of new minerals, and often changes in reservoir pressure and porosity (Johnson et al., 2001). Such changes will produce different seismic responses over time, depending on the type of the host rock (lithology and mineral composition of the reservoir), its porosity, permeability, injection rate, pressure at the well, and the CO 2 state- of-phase. Our simulator transforms such changes into equivalent seismic responses. COMPUTATION OF ELASTIC PARAMETERS OF A RESERVOIR At low frequencies, Gassmann (1951) and Biot (1956) predicted the effective bulk modulus K saturated and shear modulus μ saturated of saturated porous rocks. The equations are as follows: (1) (2) Here, K grain and μ grain are the bulk modulus and shear modulus for grain minerals, while K dry , μ dry are for dry frame rocks. K fluid is the bulk modulus for the saturating fluid, and φ represents the porosity value. From equations (1) and (2), we need the values of K grain , μ grain , K dry , K fluid , μ dry , φ, to estimate the effective bulk and shear moduli of fluid-saturated porous rocks. Then we can compute the P- and S-wave velocities using: (3) (4) Here, ρ is the density for the fluid-saturated porous rocks. From well logs, core sample analysis, and a priori geological knowledge of the local basin conditions, porosity, density, pressure, temperature, permeability, mineral composition, P- and S- wave velocities may be estimated with various proposed fluids or their mixtures occupying the pore space. However, the evaluation of 1 Cooperative Reasearch Centre for Greenhouse Gas Technologies Department of Exploration Geophysics Curtin University of Technology GPO Box U1987, Perth, WA 6845, Australia Phone: Tel: (+61 8) 9266 7194 Facsimile: (+61 8) 9266 3407 Email: ruiping@geophy.curtin.edu.au 2 Cooperative Reasearch Centre for Greenhouse Gas Technologies CSIRO Petroleum Manuscript received 12 December, 2005. Revised manuscript received 21 December, 2005. 67 Exploration Geophysics (2006) Vol 37, No. 1 67 © 2006 ASEG/SEGJ/KSEG Exploration Geophysics (2006) 37, 67-72 Butsuri-Tansa (Vol. 59, No. 1) Mulli-Tamsa (Vol. 9, No. 1)