3D Natural Fracture Prediction Using Integrated Method of Structural Restoration and Geomechanical Forward Modelling: Case Study in South Sumatra Basin, Indonesia

Abstract

Naturally fractured reservoir has important role in oil and gas development in onshore South Sumatra Basin Indonesia. There are several fields in Indonesia have hydrocarbon (oil or gas) potential in this type of reservoir. One of the fields is Northeast Betara operated by PetroChina International Jabung Ltd. The Northeast Betara structure consists of a basement high with tertiary burial that has been inverted due to compressive stresses during Late Miocene – Pliocene. The company plans to develop Lower Talang Akar Formation (LTAF) conglomeratic sand reservoir of Northeast Betara field which is believed to be naturally fractured. Well-B was drilled targeting conglomerate and fractured basement reservoir in the field. Unfortunately, even the Well-B intersected more fractures, the hydrocarbon test result was under expectation compared to previous Well-A 5 Km away, which encountered similar reservoir but shows better production test. Due to productivity discrepancy, this study is conducted to answer this issue by predicting natural fracture distribution across the field. An integrated structural restoration and geomechanical forward modelling is carried out thoroughly in order to better target the next well intersecting productive fractures. Structural restoration with finite element method provides layer geometries from initial deposition to the present day enabling explicit coupling with Stress Simulation engine at each geological time. Forward modelling could then be achieved by applying strain boundary conditions at the base of the model using known differential vertical displacement from one geological time to the next and lateral strain at the vertical boundaries of the model. Geomechanical forward modelling (GFM) simulates the evolution of structures of a geomechanical model from deposition up to present day and captures the geomechanical details with geological time. The main result of the study is plastic shear strains across the field, which subsequently converted into fracture density with orientation and inclination and can delineate the location of productive fractures. The fracture planes are defined by orientation and inclination matched over >85% of the observed fractures in the wells. Simulation results suggest that most fractures in the location of Well-A in the field are critically stressed and therefore expected to have better hydrocarbon production potential. This paper showcases approach of advanced geomechanical technique to predict 3D natural fracture distribution using existing data. The result will be used as reference to determine further development strategy.