• Esteban, Lionel
• Kiewiet, Leigh
• Lebedev, Maxim
• Clennell, Michael
• Mueller, Tobias
• Mikhaltsevitch, Vassily
• Correia Lopes, Sofia
• Qi, Qiaomu
• Burgar, Iko
• Liu, Jie
• Dautriat, Jeremie
• Gurevich, Boris
• Shulakova, Valeriya
• Freij-Ayoub, Reem
• Dillinger, Antoine
• Maney, Bruce
• Pervukhina, Marina
• Sarout, Joel

Abstract

We describe a workflow that combines petrographic, petrophysical, geomechanical and rock physics data to predict the 4D seismic response of a carbonate reservoir. Fifty standard 38mm dia x 60 mm long core plugs were provided from eight lithofacies in an Albian age carbonate reservoir in offshore Brazil. A disc 6-8 mm thick was cut from each plug end, polished on both sides and used for imaging and electrical-dielectric measurements. A mini-core from selected discs was used for x-ray micro-CT. Electron microscopy,helium porosity/permeability and nuclear magnetic resonance (NMR)were used to identify the lithotypes and textural characteristics of grains and pores in the rock. Medical x-ray CT was used to assess the degree of heterogeneity of each plug, screen for vugs, fractures or core damage that could affect rock properties.Eight peloidal packstone samples from the top reservoir were used for mechanical studies, and subjected to UCS, isotropic compaction, single stage triaxial tests or zero later strain stress-path tests to define the Mohr-Coulomb failure envelope and pore collapse pressure for this lithofacies both brine saturated and dry. Samples representative of the peloidal packstones, grainstones, and oncolitic floatstones were subjected to a range of stress-path rock physics measurements, again either dry or brine saturated to define the stress sensitivity. Saturation sensitivity of Vp for 5 samples was assessed during brine flooding experiments monitored by x-ray CT. Low frequency seismic response (1-100 Hz) was measured on 5 samples under dry and brine saturated conditions. Mechanical and rock physics data show that the majority of lithofacies are strongly water weakening and seismic velocities reduced substantially in the presence of brine to values much below that predicted by Gassmann substitution. In 4 of the low frequency samples, the shear modulus became indeterminate after brine saturation. Digital rock models were useful to assess permeability and mechanical upscaling laws, but estimates of velocities from digital rock models, even when calibrated with nano-indentation, were inaccurate owing to unresolved microstructure, and water weakening. Complex pore structures led to poor discrimination of pore size distributions from NMR, but dielectric models based on the Hanai-Bruggeman theory could be used to assess pore aspect ratio. High frequency electrical measurements correlate strongly with both porosity and with seismic velocities.

Topics

• pore
• grain
• theory
• experiment
• permeability
• electron microscopy
• isotropic
• porosity
• Nuclear Magnetic Resonance spectroscopy