Impact of Stress Perturbations nearby Salt Bodies on Fault Stability: Examples from Reservoir Geomechanics

B. Orlic, J.D. van Wees, and F. Mulders
TNO Geological Survey of the Netherlands, Utrecht, The Netherlands

     Geomechanical modeling is nowadays regularly used in the field of reservoir geomechanics to assess ground deformations related to oil and gas extraction, such as subsidence, or to assess the potential of existing faults for re-activation. Current practice of geomechanical modelling at the reservoir scale requires the use of a workflow for integrated geomechanical modeling. Such a workflow comprises three modeling steps: (i) geological modeling, in which a geological modeling package is used to create a “static” geological model of the hydrocarbon field; (ii) multi-phase fluid flow modelling, in which a reservoir simulator is used for flow simulations; and (iii) stress and deformation modeling, in which a finite element simulator is used for prediction of stress changes in the subsurface and the associated deformation. The degree of coupling between the reservoir simulator ant the stress simulator may be different: from uncoupled simulations, to iteratively coupled and fully coupled (in order of increasing complexity and decreasing frequency of use). Important characteristics of geomechanical modelling at the reservoir scale are that: (i) deformation and strain on the initial ‘static’ geological model are usually small, and (ii) the time scales considered are short, as they are related to the lifetime of a hydrocarbon field (tens of years).
     The experience gained in modeling the stress and deformation at the reservoir scale shows that the state of stress is determined by geometrical complexity and irregularity of the subsurface, constitutive behavior of geo-materials and the far-field tectonic stress. Simple assumptions about the stress distribution in the subsurface, used in many modeling packages, are often not adequate. As an illustration, we show several examples of the stress field close to salt bodies that is perturbed with reference to the far field stress.
     Salt diapirs and salt layers are present in many hydrocarbon basins in the world. Drilling wells close to salt diapirs has resulted in some instances in well failures and huge financial losses. Geomechanical considerations of this problem revealed that the far-field stress is perturbed close to the interfaces between the salt and non-salt formations. In the salt, the state of stress is isotropic, as salt can not sustain deviatoric stresses. This is in contrary with the stress state in the surrounding rocks that can support deviatoric stresses. Current understanding of stress perturbation nearby salt bodies is mostly qualitative; however, full geomechanical simulations have been recently performed to quantify the stress perturbations for four idealized deepwater Gulf of Mexico geometries including a spherical salt body, a horizontal salt sheet, a columnar salt diaper, and a columnar salt diapir with an overlying tongue (SPE 84554).
     Salts are excellent top seals of many hydrocarbon reservoirs. Stress perturbation close to the interfaces of salt and the underlying reservoir may affect the stability of nearby faults and, consequently, their sealing potential. In several synthetic cases with idealized geometries, and in two case studies of Dutch gas fields, we analyzed stress perturbations close to the salt-reservoir interface and their impact on the stability of nearby faults.
     The cases studied revealed high horizontal stresses that salt exerts on the rigid reservoir, i.e. fault, block. This reduces normal stresses on the fault and makes it easier to re-activate. In an extensional tectonic stress regime, the fault may even open up over geological time as verified by salt inflow observed on fault zones in the hydrocarbon fields considered in our study.
     Another important observation from the calculated cases can be made about the relaxation of the horizontal stresses above the crestal part of a hydrocarbon field, above the contact of an overlying non-salt layer and an underlying salt layer. Such an observation has been made previously for an idealized spherical salt body and is also valid for anticlinal and domal structural traps, as we concluded from two case studies.

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands