Previous Talks

Wednesday, February 11th, 2009

Mudstone geophysical properties for pressure prediction

Dan Ebrom (StatoilHydro)

Abstract

Mudstones are nearly ubiquitous in sedimentary basins yet remain perhaps the most poorly understood lithology from a geophysical perspective. In particular, density and neutron logging measurements of mudstone porosity conflate the mechanically distinct porosities of immoveable (bound) water and moveable (pore space) water. Immoveable water is mechanically part of the crystalline structure of the mudstone and contributes to both the bulk modulus and shear modulus of the rock. Moveable water in the pore spaces contributes to the bulk modulus of the saturated rock, but not to the shear modulus. Earlier work modeling mudstones has utilized the Hashin-Shtrikman lower and upper bounds (Holt et al., 2004). Holt was able to produce P-wave and S-wave velocities from his model that replicated his shale laboratory measurements. Holt's mathematical model assumes spherically concentric distribution of elastic materials for constituent particles, which is not consistent with tabular clay minerals which can be described as platelets. I propose an alternative mathematical model described by Reuss (1929) which assumes flat plates. This is more consistent with the repeated unit cell geometries that characterize individual clay platelets. Total porosities from the literature (Velde, 1996) are used as bounding constraints on the model, and the model separates the moveable and immoveable porosities based upon observations of clay compaction behavior. P-wave and S-wave velocities from this new model compare favorably with field and laboratory measurements (Hamilton, 1972, and Gregory, 1976). Better modeling of mudstones should ultimately lead to fewer surprises in practical pressure prediction as the mudstone-effective stress relationship becomes better understood.

Speaker Biography

Dan Ebrom is a geophysicist with StatoilHydro’s Global Exploration group in Houston. He has previously worked for BP (2000-2008), Texaco Research (1996-2000), and the University of Houston (1990-1996). With Phil Heppard and Marty Albertin, Dan was a co-winner of the BP’s 2005 Helios Award for Innovation, for a method of predicting subsalt pressure using mode converted reflections. Dan’s current research obsession is how the micro-structure of clay-rich sediments determines their elastic properties.