Use of coupled geomechanical forward modeling in prospect analysis and well planning

Abstract

There are many uncertainties in the exploration of frontier areas that may impact well planning: stress regimes, compaction or pore pressure profiles amongst others. Reducing these uncertainties demands the utilization of non-standard modelling tools instead of traditional methodologies. Here we present a study in a gravitationally driven fold and thrust belt, in which a coupled 2D geomechanical forward modelling was performed in an early stage of the exploratory project. The structure subject to this study is a submarine fold-and-thrust belt. The origin of the thin-skinned shallow thrusts in this belt is associated with the gravitational collapse of an aggradational wedge above a basal detachment causing shortening towards the basin center. The workflow applied starts with a structural restoration to pre-tectonic, deposition conditions, from where the total shortening experienced in the basin and the original thickness of the deformed sedimentary package are calculated. The evolutionary model is, then, built in the finite element analysis software ELFEN. The coupling between porous-fluid flow and plane strain deformation in this evolutionary model allows the estimation of the build-up and dissipation of tectonically-derived excess pore pressure. With this approach the current day stress field and formation pressures are obtained as a result of the structural evolution rather than deterministically. The key variables controlling the simulation (permeability, friction in the faults, normal compaction, basal friction) have been refined, corrected and cross-checked with the offset well observations in an iterative process so that the final model provides a reasonable framework in agreement with the deformation observed on seismic data and the available well log data. The model predicts a differentiated deformation pattern in the domain above the detachment conditioned by the lithological contrast, where the stiffer sandstone layer takes up most of the stress and deforms coherently with brittle behavior where there are pre-existing discontinuities. The low permeability associated with the shale formations induces a gradual increment in pore pressure during the tectonic shortening that leads to a pore pressure perturbation proportional to distance from the thrust front; consequently, most of the load originated by the tectonic compression is transferred to the pore fluid leading to the low effective stresses seen in some of the wells. The sandstone, however, can bleed off the extra pore pressure more efficiently acting as a fluid conduit and remains in magnitudes closer to hydrostatic as observed in the offset wells. We believe that this study demonstrates that despite the number of unknowns in the early exploration phase, forward geomechanical modeling can provide valuable insights into the evolution of key properties in the basin.

AAPG Datapages/Search and Discovery Article #90325 © 2018 AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin, Lisbon, Portugal, May 2-3, 2018