Playing It Forward – Modelling the Evolution of Deepwater Subsalt Fields on Passive Margins Using 2-D Finite Element Models

Abstract

Over the past decade, reservoir development has benefited from advanced 3D finite-element geomechanical models that aim to reproduce the present day state of stress as observed in drilling data. These allow prediction of highly variable stress states resulting from complex structural-stratigraphic architecture, salt bodies, and pore pressure depletion over field life. Most model inputs are relatively well-constrained, including structural framework mapped from seismic reflection data; mechanical properties measured in logs and core; 3D distribution of overburden pore pressure derived from seismic velocity transforms; and 4D reservoir pressures from reservoir simulators. Results are compared to measured field data, such as LOTs and minifrac. If there are mismatches, the input parameters can be adjusted and the model iterated until an acceptable solution is found. One key parameter that remains relatively unknown, however, is the initial regional stress, a boundary condition commonly applied as a ratio of vertical to horizontal stress (Ko). K0 is influenced by the structural history, varies spatially, and is difficult to measure in the field, therefore it is challenging to define as an input parameter. To place better constraints on the present day K0 for our field scale 3D models, we built a 2D forward model to characterize the complete evolutionary history of the structure and the change in regional boundary stress through time. To do this, we developed a basin-scale model that attempted to replicate regional restorations of the Gulf of Mexico Atwater fold belt, including the evolution from a Jurassic salt basin to a down-dip fold belt, as regional progradation drove deformation basinward. The model incorporated a flexural isostatic response to constrain the influence of basement relief on halostatic pressure and a fully coupled thermal solution field, including mantle heat flow into the base of the model and radiogenic heat production within the basement and overlying sediment wedge, to constrain the appropriate flow viscosity of salt. After 65 Myr of runtime, the final geometry resembles the present day structure. Longer run times and reduced frictional parameters allowed us to model allochthonous salt breakthrough and generate stresses comparable to 3D models in the area. This suggests we have developed the capability to start predicting stresses ahead of the drill bit, which will help to evaluate risk in future exploration prospects.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017