Basin Scale Geomechanical Modeling of Tight Carbonate Reservoir in Abu Dhabi: Implications on Pore Pressure Development

Abstract

Abstract The in-situ stress state in a rock mass is widely recognized as being of major importance in unconventional field. The inherent variation of stress distribution associated with geological environments is widely recognized as being a key factor in an unconventional field; hence, the need to understand from the basin-scale phase is critical. The three main ingredients needed to formulate a model for the mechanics of the porous sediments, are the concepts of bulk stress, pore pressure and compaction. Most available rock-stress-based numerical engines uses default laboratory properties for the set of lithology's in their library. However, understanding the mechanical properties of the lithologies and applying such to the model makes the result more representative of the local geology. This paper is an update on the previously published model (Mohamed et al., 2015), exploring the impact of rock stress and external geomechanical boundary (otherwise known as tectonic stress) on pore pressure development and rock failure. Middle Cretaceous formation (layer of interest ~ 150 m) was sub-divided into seven layers based on mechanical log signatures. Athy's law, formulated with effective stress combined with 3-D rock stress based on poro-elasticity, was used in the forward modeling simulator to improve the pore pressure prediction. Depositional events such as erosion and hiatus periods were also taken into account during simulation. The development of porosity, pore pressure, temperature, stress and related rock failure through time were simulated and calibrated to measured data. The main geomechanical properties, such as Young's Modulus, Poisson ratio, cohesion and friction angle depend on lithology and porosity. Appropriate relationship were established and ultimately used to calibrate the first–pass rock properties prediction from the numerical engine. Model porosity is dependent on burial depth, weight of the overburden sediment columns, and lithology properties. The mean stress was ultimately considered in deriving pore pressure results. Results reveal a dynamic representation of the three principal effective and total stresses within the studied formations. The maximum stress shows a NE-SW trend in the synclinal area and N-S trend on the anticlinal structures, the medium stress shows N-S trend on the synclinal structure with an E-W and NW-SE trend on Well A and Pseudo-well A wells respectively. The minimum stress reveals a NW-SE direction on the synclinal area and N-S trend on both flanks. The model also shows an improved pore pressure result ranging from 26.97 – 42.50 MPa (with 28.46 MPa average values within layers of interest) revealing ~ 1450 psi (10 MPa) difference when compared to the previous model where rock stress modeling was not performed.