SEISMIC RESPONSE OF A PERMEABLE DUAL POROSITY LAYER

Abstract

Fractures interconnected to each other and with the intergranualar-pores(equant pores) contribute most part of the permeability in a dual porosity rock. On passing P- wave through such a rock,the partial exchange of fluid takes place between pores and the fractures. The fluid flows from the fracture to the pores during the compression phase because the fracture is more compliant than the pore. During the phase ofexpansion, some fluid collected in pores is sucked back into the fractures. Such tendency of fluid-exchange makes the rock comparatively more compliant than the isolated fractures and the pores. More permeable rocks are more extra compliant to it. The ellect of velocity decrease and compliance increase would be pure elastic, if rocks were having infinite permeability. In rocks having finite permeability flow of such fluid absorbs some energy from the propagating p-wave to cause inelastic attenuation. The fluid flow stops if permeability is tending to zero, hence the inelastic attenuation of the fluid flow tends to zero also. Therefore, the inelastic attenuation reaches its maximum in some intermed iate permeability. The efkct of P-wave energy loss and the compliance of dual porosity rocks depend on frequency. If frequency is very high then the fluid will not get sufficient time to move in between fractures and pores of the rock. These two effects also depend on matrix permeability beside to the fractures faces. This concept was given by Schoenberg G urev ic h( 1999) and Wapcnaar(2004). In the present work we take the model of single permeable layer which is surrounded by impermeable rocks. We compute source pulse of Ricker wave(incident P-wave).spectruni of source pulse, reflected wave, reflection coefficient, wave velocity in the layer. We also study the inelastic and elastic effects of flow Of fluid due to propagation of P-wave on the seismic signature ofa single reservoir permeable layer.