Integrated study of a fractured granitic basement reservoir with connectivity analysis and identification of sweet spots: Cauvery Basin, India

Abstract

Basement exploration in India has seen increased interest after the recent discovery of a field in the Cauvery Basin in southeastern India, with an average individual well production of 700 b/d from a fractured basement reservoir. The field is presently under development, with several development well locations identified for drilling. Optimized development of a fractured basement reservoir requires identification of areas with a permeable fracture network. To meet this objective, we adopted a comprehensive integrated workflow involving the use of common reflection angle migrated seismic data, fracture modeling, a 1D mechanical earth model (MEM), identification of critically stressed fractures in 3D space, fracture permeability/connectivity analysis, and sweet spot identification. The workflow yielded a robust discrete fracture network model based on 3D directional fracture intensity, a 1D MEM that gave regional stress gradients (pore pressure, overburden, Shmin, and SHmax), and rock strength and elastic properties. In addition, we generated a critically stressed 3D fracture model and performed sequential stratal surface restoration for predictive strain modeling that was calibrated at wells. Our fracture permeability and connectivity analysis showed that existing hydrocarbon-producing wells are located within areas that have a fracture cluster/swarm with associated good fracture connectivity. A 3D basement facies model constructed by integrating well data and a poststack inversion impedance volume showed that major flow zones occur in weathered basement associated with low impedance. This model, in combination with fracture intensity data, provides good indication of the location of basement sweet spots in the Cauvery Basin. The understanding gained on the controls of occurrence of basement fractures explains why some wells in the field are producers and others are dry. This led to greater confidence in optimizing the locations of previously proposed new development wells.