Geomechanics to Solve Structure Related Issues in Petroleum Reservoirs
Laurent Maerten IGEOSS, Montpellier, France
Numerical models of rock deformation based on continuum mechanics can provide significant means for the interpretation and characterization of geologic structures in the context of hydrocarbon exploration and production and thus significantly improve decision making and reduce production risks. With that respect IGEOSS has been developing, in collaboration with Stanford University and industry sponsors such as BG Group, BHP Billiton, ChevronTexaco, ConocoPhillips, ENI, ExxonMobil, RepsolYPF, Shell and Total, two geomechanical tools, Poly3D and Dynel2D/3D, and associated industry applications. Both tools use innovative iterative techniques developed by IGEOSS research team and take advantage of the latest computer technologies available today such as the dual core processor allowing for fast computation while preserving the model structural complexity. 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, which gives us confidence that the 3D structural model is geomechanically consistent.
Poly3D is a fast numerical tool based on the boundary element method (BEM). It uses surfaces with friction as 3D discontinuities (i.e. faults, joints, salt domes, bedding interfaces or cavities) and a heterogeneous elastic medium that does not need to be discretized as with the finite element method (FEM). Discontinuity surfaces are made of triangular elements, which are particularly well suited to model complex surfaces such as a curving fault with irregular tip-line. Poly3D computes the deformation and perturbed stress field around 3D discontinuities for one or more tectonic events. The main industry applications are the characterization of undetected fractures in reservoirs, drilling location decision, fracture reactivation during depletion and drilling, well bore stability, reservoir geomechanics and verification of 3D structural interpretations.
Dynel2D/3D is a numerical tool based on FEM, which honors the full complement of physical laws that govern geological deformation. It is primarily used for structural restoration (Maerten and Maerten, 2006), where physical laws and linear elastic theory replace kinematic and geometric constraints used by the existing methods. Dynel2D/3D computes the deformation and perturbed stress field from the restoration of complex geological structures through time. These complex structures include faults, joints, folds, bedding slip, and inhomogeneous mechanical properties. The main industry applications are the verification of 2D or 3D structural interpretations, the characterization of undetected fractures in reservoirs, and the basin evolution through time.
The first example (fig. 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, which gives us confidence that the 3D structural model is geomechanically consistent.
In the second example (fig. 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 upper most 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.
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 with 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.
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 fig. 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).
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.
(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 Model: Society of Exploration Geophysicists.
Bourne, S. J., 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., Karpuz, R., 2000, How to constrain 3-D fault continuity and linkage using reflection seismic data: a geomechanical approach: American Association of Petroleum Geologist, v. 84, p. 1311-1324.
Maerten, L., Gillespie, P., Daniel, J.-M., 2006, 3-D geomechanical modeling for constraint of subseismic fault simulation: American Association of Petroleum Geologists, 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: American Association of Petroleum Geologist Bulletin, v. 90, p. 1201-1226.