3D Structural restoration and Geomechanical Forward Modeling in a visco-plastic medium to natural fracture prediction in a Malay producing field, offshore Malaysia

Abstract

In order to ensure a successful drilling campaign, it was required to obtain knowledge of the possible location of natural fractures generated by different tectonic events affecting the Malay Basin through geological time to a high level of certainty. Acquiring this information would assist in the optimization of well trajectories to avoid these naturally fractured regions which would likely result in severe wellbore instability problems. As described by Petronas (1999&2007) the Malay Basin underwent an extensional phase during the Oligocene period with deposition of lacustrine source rocks. The Miocene was a time of fluvio-deltaic source and reservoir deposition and a compressional phase starting Early Miocene resulted in structural inversion. A major unconformity underlined the peak of the tectonic inversion in relation with the closure of Proto-South China Sea and the opening of South China Sea further to the North. In the Malay Basin, the geological history led to a complex structure trapping which requires further works and then an innovative approach based on the geomechanics was proposed to predict the locations containing natural fractures. The objective of the study was to model the heterogeneous paleo stress and strain states by the construction of a geomechanical model in this field and to relate the predicted strain and stresses to fracture characteristics. The first part of the study was to quality control the interpretation of the 3D structure of the area, which had been constructed from 3D seismic interpreted surfaces and faults. 1D Mechanical Earth Models computed from well data provided the required elastic parameters for the 3D structural restoration. The technique used to perform the restoration was based on the Finite Element Method, particularly suited for modeling complex geological structures. The second part of the study consisted in the computation of the paleo stress state via geomechanical forward modeling and relating the resulting plastic strain attributes with fracture density. In this step, the strains calculated from the displacements derived from the structural restoration were applied at the boundaries at each geological time-step in order to calculate the internal stress condition and plastic strain distribution accumulated through geological time.