An approach to combined rock physics and seismic modelling of fluid substitution effects Tor Arne Johansen, 1 *A Ê smund Drottning, 1{ Isabelle Lecomte 2 andHa Êvar Gjùystdal 2 1 Institute of Solid Earth Physics, University of Bergen, Alle Âgt41,5007Bergen,and 2 NORSAR, PO Box 51, 2027 Kjeller, Norway Received May 2000, revision accepted May 2001 ABSTRACT Theaimofseismicreservoirmonitoringistomapthespatialandtemporaldistribu- tions and contact interfaces of various hydrocarbon fluids and water within a reservoir rock. During the production of hydrocarbons, the fluids produced are generally displaced by an injection fluid. We discuss possible seismic effects which may occur when the pore volume contains two or more fluids. In particular, we investigate the effect of immiscible pore fluids, i.e. when the pore fluids occupy different parts of the pore volume. The modelling of seismic velocities is performed using a differential effective- medium theory in which the various pore fluids are allowed to occupy the pore space in different ways. The P-wave velocity is seen to depend strongly on the bulk modulus of the pore fluids in the most compliant (low aspect ratio) pores. Various scenarios of the microscopic fluid distribution across a gas±oil contact (GOC) zone have been designed, and the corresponding seismic properties modelled. Such GOC transition zones generally give diffuse reflection regions instead of the typical distinct GOC interface. Hence, such transition zones generally should be modelled by finite-difference or finite-element techniques. We have combined rock physics modelling and seismic modelling to simulate the seismic responses of some gas±oil zones, applying various fluid-distribution models. The seismic responses may vary both in the reflection time, amplitude and phase characteristics. Our results indicate that when performing a reservoir monitoring experiment, erroneous conclusions about a GOC movement may be drawn if the microscopic fluid-distribution effects are neglected. INTRODUCTION A major challenge in seismic exploration is the mapping of type, location and extent of hydrocarbon fluids. In seismic reservoir monitoring, the aim is to follow the temporal variations of the various spatial fluid distributions during the production phase. In this respect, it is of particular importancetomapthefluid-contactinterfacesandhowthey change geometrically with time. Such time-lapse studies are generallyperformedon3Dseismicdataacquiredatdifferent times.RecentstudiesofthedrainageofNorthSeareservoirs have been reported by Watts et al. (1996), Johnston, McKenny and Burkhart (1997) and Landrù etal. (1999). In this process it is crucial to know how the reservoir and its fluid properties are linked to seismic velocities and dens- ities, and thereby the seismic data. In the literature, there exists a variety of rock-physics models aimed at connecting ß 2002 European Association of Geoscientists & Engineers 119 Geophysical Prospecting, 2002, 50, 119±137 *E-mail: torarne.johansen@ifjf.uib.no {Present address: UniGEO a.s., Thormùhlensgate 55, 5008 Bergen, Norway.