Integration of mini-frac stress results with core data to identify ductile behavior of overpressured sandstone reservoir in deep-water offshore Krishna-Godavari basin, India

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Insitu reservoir stress measurements are essential from geomechanics perspective to efficiently develop an oil and gas field. Wireline conveyed mini-frac tests gives accurate measurements of minimum horizontal stress. Wireline conveyed mini-frac test was performed in the clastic cretaceous syn-rift high pressure (9000 psi) gas bearing sandstone reservoir of an offshore deep-water well (water depth—1108 m) in Krishna-Godavari basin of East Coast off India in order to determine the insitu reservoir stresses. All the mini-frac tests attempted within the syn-rift reservoir were unsuccessful to achieve the formation breakdown pressure even with the applied pressures significantly higher than the vertical stress. This paper discusses the post-job investigation of all these unsuccessful mini-frac tests within the reservoir. Distinct large plastic deformation behaviors were identified in all these test. This investigation in conjunction with rock mechanics data derived from core samples suggest that strong relationship exist between pore fluid pressure and rock mechanical properties. High pore fluid pressure within the reservoir has made sandstone rocks mechanically weak and ductile in nature. The distinct large plastic deformation behaviors are the manifestation of this reservoir ductility. Awareness and evaluation of rock ductility in high overpressured reservoir condition is necessary during planning stages of mini-frac stress testing job. Additionally, paper shows that the rock mechanical properties plays significant role on rock deformation mode within reservoir with pressure depletion during production phase. Paper demonstrates the effective use of rock mechanical properties to predict the brittle–ductile (B–D) transition value in Q–P space (differential stress vs. mean effective stress), which will help in forecasting the reservoir deformation mode during field production stage. Furthermore, the paper highlights the importance of performing critical laboratory tests to aid significant value in field development program from geomechanics perspective.