As part of a multi-disciplinary investigation to optimize a tight reservoir development in the Sultanate of Oman, a comprehensive geomechanical characterization was performed and its results used as input for 3D non-planar hydraulic fracturing simulations. The simulation results led to better understanding of the reservoir response during hydraulic fracturing stimulation and thereby improved the decision making process for future field development. The focus of this paper is to highlight the geomechanical aspects of the analysis which explained several of the difficulties encountered during stimulation.
Geomechanical models were constructed covering the target sandstone and overlying clay-rich formation for ten horizontal and vertical wells by integrating diverse data including openhole logs, core rock mechanical tests, stress-induced failure interpretations from image logs, and stress measurements from mini-frac data. The geomechanical models were further supported by the results of available temperature, tracer and production logs. 3D geomechanical models were created by capturing the lateral and vertical variations of rock and geomechanical properties from these 1D models away from the wellbores, guided by the variations in seismic attributes using a co-simulation method. 3D modeling revealed a number of stress barriers supported by location of microseismic events in the target reservoir.
The geomechanical setting of the target formation is found to be complex with significant variations laterally and vertically. The West area of the field was found to have relatively lower stress compared to the Main area. Also, the Middle and Lower intervals of the target formation were shown to have considerably higher horizontal stresses (strike-slip/reverse faulting regime) compared to the Upper interval (normal/strike-slip faulting regime). The high stresses in Middle and Lower sections have the negative consequence of reducing the fraccability of these intervals as they require high breakdown pressures. In some cases, where breakdown was achieved, the resulting horizontal hydraulic fracture yields disappointing production results due to its inability to connect the reservoir vertically. Another important lesson learnt from geomechanical characterization in this field was the role of high angle bedding in truncating the vertical growth of hydraulic fractures. This understanding can further help to optimize the location of perforation intervals in stimulation designs of future development wells in this field.
Geomechanical characterization of this reservoir demonstrated considerable lateral and vertical heterogeneity that could only be captured by very detailed integration of well-based and seismic scale data. In addition, the effects of the in situ stresses on high angle beddings demonstrated the importance of these features on geometry and efficiency of created hydraulic fractures. From the 1D to 3D geomechanical modeling we show that characterizing formation heterogeneity, in situ stress variability, and bedding structures is critical to the creation of any predictive hydraulic fracturing model in this field.