Solid substitution: Modeling elastic properties of porous and cracked rocks saturated with liquid and solid Octodecane

Abstract

© 2018 SEG. Quantifying the impact of pore fill on elastic properties of porous rocks, especially cracked rocks, is of ongoing interets in geophysics. Different from fluids, the finite rigidity of high viscoelastic or solid pore fill prevents the pressure equalibriation within the pore space due to the coexistence of stiff and soft pores. Indeed, the variation of the shear modulus of the pore fill can produce larger change in the effective elastic properties than one may expect from the change of the shear modulus of inclusions. Our analysis shows that the pressure dependency of elastic properties of cracked rocks is controlled by the squirt flow between stiff, compliant, and so-called intermediate pores (with an aspect ratio larger than that of compliant pores but much less than that of stiff pores). In this paper, we present a triple porosity recipe for solid substitution. Compared to ultrasonic measurements on an octodecane saturated sandstone, the model predictions reveal a reasonable fit for both bulk and shear modulus with the aspect ratio of stiff pores set to 0.22, which is much smaller than the previously assumed value of 1. 3D micro-CT image analysis shows that the typical aspect ratio of stiff pores is close to 0.4. However this value may be affected by the way volumes of individual pores are extracted from the images.