Abu Dhabi land has a giant gas field consisted of layered carbonate reservoirs. The long term plan of the field has been to produce the reservoirs with the safest maximum depletion. A detailed geomechanical study was undertaken to identify changing field stresses and to understand the possible reservoir rock collapse mechanisms.
The foundation for any 3D geomechanical modeling is 1D Mechanical Earth Model that includes elastic and strength properties, overburden stress, pore pressure and magnitude and direction of horizontal stresses. The input data for 1D modeling is openhole logs (density and compressional and shear sonic logs). Image data, caliper logs, pore pressure and closure and breakdown pressure measurements are necessary to calibrate the models. To improve quality and reliability of the 1D MEMs, Abu Dhabi Company for Onshore Oil Operations (ADCO) requested lab measurements to calibrate elastic and strength rock properties and decided on pore pressure and stress measurements in one of the upcoming wells. Wireline Formation Tester (WFT) technique was selected to provide pore pressure, as well as closure pressure to calibrate magnitude of minimum horizontal stress directly and breakdown pressure to calibrate magnitude of maximum horizontal stress indirectly. Acquired compressional and shear sonic logs allowed building continues properties, pore pressure and stress profiles.
3D geomechanical modeling was conducted for the interested reservoirs in the Bab field. The main objective of this study was to investigate possibility of formation rock collapse, particularly within severely depleted areas, and associated well collapse and completion integrity damage. This study also aimed at assessing potential geomechanics-related risks due to early depletion (cap rock integrity and fault sealing capacity).
Figure 1 shows the general workflow of this study. Firstly, 1D Mechanical Earth Models (MEMs) were constructed using available wireline logs. The measured data was used to calibrate the calculated elastic and strength rock properties, pore pressure and horizontal stress profiles. Drilling reports were analyzed to extract wellbore instability-related events to calibrate the 1D MEMs further. To better capture variation of elastic and strength properties across the reservoir, 1D MEMs were constructed for several wells. Then, a 3D MEM was built based on the static geological model and dynamic reservoir model. 1D MEMs were used to populate the mechanical properties for 3D modeling carried out for the duration of pre-production until present. The results were calibrated against the production data. The calibrated 3D model was then used to perform two-way coupled modeling for future rock behavior in the cap rock and reservoir depletion mechanism.