uIntroduction

uFigure captions

u3D structure

uSmall-scale fractures

uConclusion

uReferences

uIntroduction

uFigure captions

u3D structure

uSmall-scale fractures

uConclusion

uReferences

uIntroduction

uFigure captions

u3D structure

uSmall-scale fractures

uConclusion

uReferences

uIntroduction

uFigure captions

u3D structure

uSmall-scale fractures

uConclusion

uReferences

uIntroduction

uFigure captions

u3D structure

uSmall-scale fractures

uConclusion

uReferences

3D Structure Verification 

Complex geological structure such as those observed in contractional settings, where thrusting and folding are known to be challenging, as they often are poorly imaged, are known to be potentially good hydrocarbon traps. In such context, it is essential to check the consistency of the subsurface structural interpretations. Many geometric and kinematic techniques used by structural geologists have been developed in that respect. Measures of gaps and overlaps between the restored parts of a model give qualitative values to test the strength of the geological interpretation. In order to physically verify complex structural models, we have developed 2D and 3D geomechanical methods that honor the full complement of physical laws that govern rock deformation. Recent studies (Maerten et al., 2000; Maerten and Maerten, 2006) demonstrate how such methods can be applied to improve the subsurface-interpretation understanding and obtain more realistic reservoir models.  

The first example (Figure 1) illustrates how 3D forward modeling with Poly3D can be used to mechanically check the consistency of a faulted reservoir model. The basic methodology consists of calculating the large-scale deformation of the area using the available reservoir structure data, such as the observed fault surfaces, the rock properties, and the tectonic load. Then, the computed deformed structure caused by the slip on the major interpreted faults is compared to the observed structure of the top reservoir. Here, the main trends (i.e., subsidence and uplift zone shapes) are reproduced; this gives us confidence that the 3D structural model is geomechanically consistent.   

In the second example (Figure 2), we investigate the geometric consistency of the so-called SEG-EAGE 3D overthrust model (Aminzadeh et al., 1997), which has been largely used to test seismic velocity models. Here we use Dynel3D to mechanically restore the 3D volume of the overthrust model. The workflow consists of building a 3D solid geological model with appropriate rock properties, from fault, horizon, and boundary surfaces. Then, kinematic boundary conditions are set in order to mechanically restore the model by flattening the uppermost layer and running the FEM solver. Analysis of the resulting model shows large fault cut-off mismatches along the central part of the thrusts associated with anomalous strain concentrations. Therefore, the SEG-EAGE 3D overthrust model seems to be geomechanically inconsistent. This could have been anticipated knowing that it is a complex non-cylindrical structure that has been created from a series of synthetic, but balanced, 2D cross sections.

Small-scale Fracture Characterization 

Structural heterogeneities, such as faults and joints, are known to be capable of significantly altering the flow of hydrocarbons, either during the migration from the source to the reservoir rock or during production of the reservoir. Therefore, understanding and quantifying the spatial and temporal development of these features as well as their properties (e.g., geometry, throw, aperture, permeability, etc.) can have great economical impact on the recovery of natural reserves. However, despite the tremendous detail now available from 3D seismic reflection techniques, many of these features cannot be detected at the current resolution of the seismic reflection data. In order to more realistically model the spatial and temporal development of structural heterogeneities and to address these economical issues, we have developed geomechanical methods that take into account mechanical concepts and the fundamental physical laws that govern fracture development. Recent studies (Maerten, 1999; Bourne et al., 2000; Maerten et al., 2006) have shown that adding a geomechanical rationale to stochastical techniques improves their predictive capability and leads to more realistic faulted and fractured reservoir models.     

The basic methodology consists of calculating the stress distribution at the time of fracturing using the available reservoir structure data such as faults, fractures and folds, the rock properties and the tectonic setting that can be characterized by stress or strain magnitude and orientation. Then, the calculated stress fields, perturbed by the main structures, combined with rock failure criteria are used to model fracture networks (i.e. orientation, location, and spatial density) as illustrated in Figure 3. Applications to both outcrops and reservoirs demonstrate how geomechanics can provide a high degree of predictability of natural fracture networks. The 3D BEM implemented within Poly3D has been successfully applied to model subseismic faults (Maerten, 1999; Maerten et al., 2006) in Northern North Sea highly faulted reservoirs, as well as undetected joints in naturally fractured carbonate reservoirs (Bourne et al., 2000).

Conclusion 

Rigorous analysis based on well established geomechanical principles can play a key role to further reduce uncertainties, refine seismic interpretations, and to understand fracture characteristics in order to improved reservoir characterization.

References 

(For a more complete reference list, please go to www.igeoss.com and follow the link R&D/Publications).

Aminzadeh, F., Brac, J., and Kunz, T., 1997, 3-D Salt and Overthrust Models: Society of Exploration Geophysicists 3-D Modeling Series, No. 1.

Bourne, S. J., and Willemse, E.J.M., 2001, Elastic stress control on the pattern of tensile fracturing around a small fault network at Nash Point, UK: Journal of Structural Geology, v. 23, p. 1753-1770.

Maerten, L., Pollard, D.D., and Karpuz, R., 2000, How to constrain 3-D fault continuity and linkage using reflection seismic data: A geomechanical approach: AAPG Bulletin, v. 84, p. 1311-1324.

Maerten, L., Gillespie, P., and Daniel, J.-M., 2006, 3-D geomechanical modeling for constraint of subseismic fault simulation: AAPG Bulletin, v. 90, p. 1337-1358.

Maerten, L., and Maerten, F., 2006, Chronologique modeling of faulted and fractured reservoirs using geomechanically based restoration: Technique and industry applications: AAPG Bulletin, v. 90, p. 1201-1226.

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