Three-dimensional geomechanical modeling of fault and fracture stability analysis in naturally fractured carbonate reservoir

Abstract

Fault and fracture stability/reactivation, reservoir compaction, and associated surface subsidence are observed in many oil and gas fields worldwide. A better understanding of the geomechanical parameters of reservoir formation and neighbouring lithology is therefore becoming highly important within the oil carbonate field development. Pore pressure, effective stresses, and geological structures as well as their evolution during an oil field life have a considerable impact on wellbore instability and fault and fracture behaviour. The main aim of this study is to determine the causes of faults and fractures instability in a natural fracture reservoir by using integrated wellbore stability (1D) analysis to 3D geomechanical study. In this research, different approaches were used to perform the 3D geomechanical model through integrated analysis of the drilling events, log and rock mechanics data. By 3D finite element, the principal stresses were calculated in two steps. Firstly, the gravity (overburden and underburden) and pore pressure were applied; the second step was involving the sideburden. This study indicates a 3D geomechanical modelling of the oil field as a gentle anticline in the Middle East area. Wellbore stability model (or 1D Geomechanical modelling) contains various stresses, pore pressure, and rock mechanics properties for offset wells were simulated by integrating a wide variety of good data from the field. These calibrated 1D geomechanical outputs were applied to model 3D geomechanical models and were further utilized for fracture and fault reactivation modelling. A 3D reservoir geomechanical modelling (or couple geomechanical modelling) was improved to utilize the geological static and reservoir dynamic models to estimate the changes in reservoir pore pressure and principle stresses in magnitude and orientation. Based on 3D geomechanical modelling, vertical and horizontal stresses have been evaluated for all faults and fractures. The tendency of the fault and fracture reactivation was determined in terms of minimum and maximum horizontal stresses. The simulation result indicated that the change of reservoir pressure during the initial phase of production since 1992 to 2054 has a significant impact on principal stresses in the field. On the other hand, the 3D map of minimum and maximum horizontal stresses on both sides of the main faults explain that faults are most stable compared to fractures in cap rock and reservoir sections. While high porous and permeable reservoir formation and impermeable cap rock (the combination of anhydrite, salt and shale) are experiencing normal to strike-slip stress fault regime, the strain and stress fluctuation due to oil production in more than 60 years’ simulation does not have a destructive impact (or activation) on different faults. But fracture behaviour changes from 2017 to 2054 due to pore pressure changes, the fracture instability in different directions was considerable and it must be considered in production optimization.