NUMERICAL SIMULATION OF OVERPRESSURE DURING SEDIMENTATION

The evolution of pore pressure (including overpressure) during sedimentation is an important part of analyses determining whether an increase in pore pressure results in rock failure. Conditions for rock failure can be quantified using 2D and 3D finite element stress/displacement. Standard procedures include pre-stressing to account for gravitational equilibrium of the simulated rock. However, the dynamic evolution of pore pressure during sedimentation, termed pre-pressuring here, is often not considered in numerical modeling approaches. The pore pressure is either added as a static value (i.e. as an initial condition) or coupled to a fluid flow simulator. However, these methods cannot determine the detailed evolution of the combined poro-elastic processes during sedimentation. As sediment loading increases, porosity and permeability in the subsequent sedimentary layers are changed and overpressure may result as a result of compaction disequilibrium.

This study uses 3D porous-elastic-plastic finite element analysis to simulate the evolution of pore pressure during sedimentation. Based on continuous sedimentary loading and the resulting overburden load the porosity in the model is continuously updated (as are the elastic properties as a function of porosity). The evolving porosity distribution is also coupled to the resulting permeability: as porosity and permeability decrease, overpressure is more likely to develop. The model tests variations in sedimentation rates and initial permeability distributions. Initial results show that overpressure does not develop for rock layers with a permeability m² when sedimentation rate is lower than 500mm/year (a very high value rarely seen in nature). For layers with permeabilities as low as m², over pressure will occur for sedimentation rates in the range of 5mm/year. The results also show that a combination of low permeability and high sedimentation rates results in tensile horizontal stresses and fractures are induced hydraulically.