Towards successful drilling operations through geomechanical modelling and wellbore stability analysis: a case study on the Ouan Kasa reservoir in the Ghadames Basin, Libya

The Ouan Kasa shaly sand reservoir in the Ghadames Basin of Libya presents significant challenges to drilling operations, particularly due to wellbore instability. The absence of prior geomechanical studies in this area raises concerns about the risks associated with drilling future wells. This study aims to construct one-dimensional mechanical Earth models (1D MEMs) to evaluate formation stability and define an optimal mud-weight window, thereby improving drilling efficiency and reducing operational risks. Data from two wells were analysed, including gamma-ray, sonic and bulk density logs, along with formation micro-imager (FMI) logs. Rock mechanical properties were derived using empirical correlations, the shear-wave velocity was estimated using the Greenberg–Castagna relationship and pore pressure was calculated using Eaton's method, calibrated against modular dynamic tester (MDT) data. Horizontal stresses were estimated using the poroelastic horizontal strain model, while stress orientations were inferred from FMI analysis. Results indicate that the Ouan Kasa Formation has a reduced mechanical stability due to its high shale content and ductile nature. A recommended mud-weight range of 11.2–14.5 ppg was identified to mitigate shear failure and ensure borehole integrity. In addition, the Devonian system is characterized by a normal faulting stress regime ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>σ</mml:mi> </mml:math> v &gt; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>σ</mml:mi> </mml:math> H &gt; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>σ</mml:mi> </mml:math> h ), with the maximum horizontal stress orientated NW–SE (135°) and the minimum stress orientated NE–SW. This study provides the first integrated geomechanical evaluation of the Ouan Kasa reservoir and offers valuable insights for drilling optimization and the safe development of future wells in the area.