Building a 3D basin-scale geomechanical model of the subsurface, integrating different G&G disciplines, is of paramount importance for Carbon Capture and Storage (CCS) projects. In standard oil & gas workflows, geomechanical models are built around pre-existing reservoir models with oversimplified overburden and underburden, populated with averaged lithological and mechanical properties. Computational and geometrical constraints of classic pillar-based techniques result in a limited resolution, possibly insufficient when characterizing the entire rock volume of candidate sites for CCS projects.
In this regard, we have developed and applied to a real case an integrated G&G workflow for the generation of basin-scale complex structural models populated with geomechanical properties (Young Modulus, Poisson's Ratio, Density). These properties are derived from seismic partial-stack data by applying a Seismic Reservoir Characterization (SRC) workflow based on Extended Elastic Impedance (EEI) inversion and a multi attribute linear calibration. The resultant 3D model provides a comprehensive description of the structural framework and associated geomechanical properties of reservoir, overburden and underburden, covering a vertical thickness of about 4.5 km and several hundred km2 around an existing Field.
The main steps of the integrated workflow are:
(1) EEI inversion to derive impedance volumes with a geomechanical significance. Generation of seismic attributes describing the rock elastic properties (e.g. compressional and shear moduli, bulk density) that are relevant to lithology and fluid discrimination as well as to geomechanics, providing a full description of the overall series.
(2) Machine Learning approach to transform the impedances into geomechanical properties. Well logs and the EEI seismic attributes are used to train the ML algorithm and to establish a relationship that correlates the EEI seismic attributes with the target log property (i.e. geomechanical parameters) by means of linear regression. 3D dynamic properties are then converted to static and the density was validated from a gravimetric point of view.
(3) Structural model building and grid population. A preliminary high resolution tetrahedral structural model is simplified into an optimized unstructured mesh where subsurface geometries are preserved with higher resolution in the reservoir and gradually lower away from it. Seismically derived geomechanical properties are resampled and, in some case, extrapolated in the mesh.
A further step in the workflow integrates Geomechanics with the proprietary Basin Modelling tool in large regional models. Considering the structural evolution by decompaction and back stripping, basin modelling provides the evolution of pressure through time, which can be coupled with Geomechanics to get a consistent model.