Reports: AC848385-AC8: New Approaches to 3D Structural Restorations Using Mechanical Constraints for Improved Petroleum Trap and Reservoir Characterizations

John H. Shaw , Harvard University

We investigated a new approach of performing 3D structural restorations to develop better methods for characterizing petroleum traps and reservoirs. These new methods have been developed by the Nancy School of Geology, France, in collaboration with ourselves and several industry sponsors. They use finite element techniques and consider mechanical rock properties in calculating 3D restoration displacement and strain fields. Thus, they offer promise for better defining the geometry and evolution of petroleum traps, as well as constraining patterns of natural fractures and other strain properties in reservoirs that influence petroleum production.

The goal of geologic restorations is to remove slip on faults and unfold sedimentary strata in a manner that obeys balancing constraints. Traditionally, these restorations have been performed using geometric approaches that are inherently two-dimensional, and do not consider rock strengths. In contrast, the geomechanical restoration techniques that we employ are capable of performing fully three-dimensional restorations that are governed by rock properties. The methods employ elastic constitutive laws that are simple approximations of the naturally complex deformation processes in geologic structures. Thus, we benchmarked these new methods by restoring a series of mechanical forward models developed with the discrete element method (DEM). These DEM models generate complex structures similar to natural systems, yet we know the full displacement, strain, and stress fields for these mechanical models. Applying the restoration methods to these forward models gave us an ability to assess quantitatively how well the restoration technique performs in describing complex deformations under specific sets of boundary conditions. Our results indicate that the geomechanical restoration approaches are capable of recovering three-dimensional displacement fields for faulted and folded structures (Figure 1). In particular, these restorations perform best in cases where fault geometries and displacements governed displacement fields. Restoration strains were also shown to closely approximate strain patterns in the forward models. The restoration generally defined the regions of high strains and their average magnitudes for a wide range of structural models. This gives us confidence that the geomechanical restoration approaches, despite their inherent limitations, can be applied successfully to restore structures in three dimensions and to map the distribution strains within them.

Figure 1. Comparison of strains from forward DEM model with those calculated by the restoration method. The DEM forward model (upper left) was used to generate a mesh (upper right) that was restored using the new method. Strains in the forward model (lower left) are localized along the fold limb, similar to the strain pattern calculated by the restoration (lower right). Strain magnitudes are locally higher in the DEM model due to breaking of particle bonds, a process that cannot be explicitly represented in the finite element-based restorations.

The second phase of our efforts focused on applying these restoration techniques to natural petroleum traps, and on comparing strain patterns derived from the restoration with geophysical attributes and geologic observations that help to constrain reservoir properties such as fracture density. In these efforts, we have collaborated with a number of industry sponsors, and our project areas include fields in California, China, the Caspian Sea, and the Arabian Peninsula.

We have successfully restored different types of geologic structures, including detachment, fault-bend, and fault-propagation folds in contractional settings, extensional rollovers in rift environments, and restraining bends in strike-slip systems. Restoration of these structures are constrained by patterns of deformed syntectonic strata, as well as various constraints on fault slip such as cutoffs and piercing points (e.g., channel offsets). We performed sequential restorations by successively flattening these syntectonic strata, and calculating 3D displacements fields and strain patterns at each step. A primary goal of our research effort is to establish how well these calculated strain fields represent the natural strains that occur in the strata represented in the models. We generally focus our efforts on comparing components of the calculated strain fields in the reservoir sequences with well and seismic data that independently constrain deformation fabrics, such as natural fractures. In a number of cases, we find that values of dilatation and elongation derived from the restorations generally correlate most strongly with natural fracture densities and orientations observed in well data. In addition, we find that cross correlation of restorations strains with geophysical attributes such as seismic coherency yield significant predictive power in defining areas of the reservoirs with numerous natural fractures. These correlations are further examined by comparing strain and seismic attributes with dynamic production data, helping to quantify the role of natural fractures in determining effective reservoir permeabilities.

In summary, we find that the geomechanical restoration techniques perform well in describing the evolution of complex geologic structures in a variety of tectonics settings. They are generally superior to geometric and kinematic techniques in addressing inherently 3D aspects of deformation, and in describing natural structures where strength contrasts play major roles in governing deformations styles (e.g., detachment folds, shear fault-bend folds). Pitfalls can result from the simplistic constitutive laws and boundary conditions, and these are best avoided by using geologic constraints to govern the restorations, such as realistic fault geometries, cutoffs, and piercing points. Deformed syntectonic strata also offer important guides to evaluating sequential restorations. With these guides in place, we find that geomechanical restorations are able to define effectively patterns of strain in hydrocarbon reservoirs. Through comparison with seismic attributes and well bore observations, these modeled strain help to forecast natural fractures and other properties that affect reservoir performance.

Figure 2. Perspective view of the calculated restoration strain field for an anticlinal trap. Dilation (volume change) values are shown on the tops of each of the stratigraphic tops represented in the model, and generally vary from ±1.5% (-0.015 to 0.015 in the scale). We compare the dilatation and other strain values at the reservoir level with data from production wells and seismic attributes that define fracture distributions. In this example, we established a robust correlation between high value of negative dilatation (volume expansion in a forward geologic sense) and fracture enhanced permeability in wells.

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