Geomechanical modelling of subsurface formations in Palouch Oil Field, Melut Basin (South Sudan)

Abstract

The geomechanical issues which are related to mechanical properties of rocks and stress regimes in the area are one of the major causes of non-productive time (NPT) in oil and gas operations. In Palouch field area, this NPT in some cases can be attributed to the borehole instability resulting from unexpected or unknown mechanical responses of subsurface Formations to petroleum operations. In order to understand the mechanical behavior of subsurface formations during drilling, this work has first estimated the mechanical elastic properties of subsurface Formations and stress magnitude and direction around the borehole in Palouch filed area using wireline log data and secondly, built the 1D MEM from these computed parameters and stress field. The 1D MEM was used to analyze the wellbore environments and after calibration of this MEM with drilling data, the safe mud- weight window was predicted for future drilling. Thirdly, a simple 3D MEM was built from seismic data and this was populated with well derived properties with the aim of extending the application of the results to reservoir production. However, this requires production data more especially pressure data from production logging which was not available. Finally, all the results and limitations were discussed and the recommendations were made base on the findings. The computed mechanical stratigraphy and elastic properties indicate that, the subsurface Formations in this field exhibit various mechanical behavior which can be attributed to their composition and degree of compaction. The elastic moduli show inverse relations with Poisson’s ratio and increase with depth. The magnitude of stresses in Palouch field area generally increase with depth whereby the SHmax is the greatest principal (δ1), Sv is the intermediate principal stress (δ2) and the SHmin is the least principal stress (δ3) suggesting that, this field has experienced a strike-slip faulting regime. The δ1 and δ3 are oriented NW-SE and NE-SW respectively (nearly perpendicular to each other). The 1D MEM have shown some zone of instability in the wellbore caused by either mud-weight being above or below the safe window which was predicted by the 1D MEM to be 8.9 – 11.9ppg. The 3D MEM indicated that, the slip tendency along the fault plane is high near the surface and the far-field stress increases with depth. The results of this work can be significantly used to manage wellbore stability and minimize NPT (i.e. to reduce drilling risks and costs) in future drilling in Palouch field area if further study is done based on these findings. To further extend the MEM to be used for reservoir production optimizations, robust data including seismic data and production data must be used to build 3D or 4D MEM whereby reservoir simulation can be performed.