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Pore Pressure and Wellbore Stability Analysis of CB-ONN-2002/2 Block in Cambay Basin

Balaji Chennakrishnan
Exploration, Jubilant Energy, Noida, India

A study of pore pressure and wellbore stability was carried out in the wells drilled in CB-ONN-2002/2 block in Cambay basin. The first two wells drilled by JOGPL could not be successfully evaluated due to wellbore instability issues.

A multi pronged approach has been carried out to address the wellbore instability issue. Calibration of Pore Pressure and well bore stresses were done using resistivity, sonic, and density data. Image los and sonic logs were used to identify stress directions. The study revealed existence of shales with higher pore pressure in the Older Cambay shales, which are causing wellbore instability. The mud system was changed to SOBM from WBM to address Time dependant clay reaction.

As more wells were drilled new insights were thrown on the stress behavior of Cambay shales and Olpad Red claystone. It is understood that the problem was two fold.

1. Time dependant Clay reaction in WBM
2. Inadequate mud weight to balance shale pressures in Older Cambay Shales.

With the better understanding of pore pressures and wellbore stresses drilling efficiency has improved dramatically. The drilling time per well has decreased from 40 days to 27 days. This has further led to change in well design from 3CP to 2CP and reducing drilling time per well to 20 days.

This study will journey through drilling of six wells and highlight the dramatic improvement in drilling efficiency using Pore Pressure and wellbore stability analysis.

Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery