Identifying the Potential in Complex Basement Reservoir: Advance Application of Borehole Images and other Openhole Logs, A Case Study From Western Offshore, India

Pattanaik, Sambit¹; Parashar, Sarvagya¹; Sikdar, Koushik¹; Basu, Indrajit¹; Nangia, Viraj¹; Agrawal, Ashwini²
¹Schlumberger, Navi Mumbai, India.
²ONGC, Mumbai, India.

Exploring unconventional reservoirs for hydrocarbons has been of key interest for major oil companies across the globe for some time now, and basement remains one the most prolific candidates. Mapping the reservoir property in basement is always a big challenge due to drastic vertical and lateral variations in porosity and permeability. However, recent advances in measurement and evaluation techniques have provided an advantage for low-porosity, low-permeability basement exploration. This study focuses on one such methodology for characterization of these reservoirs using static and dynamic well data.

Formation resistivity micro-imager data along with acoustic and nuclear magnetic resonance datasets were used to characterize rock texture and fracture network. Validation of the open fracture zones on image has been done in light of conductivity data from images and Stoneley fracture analysis from acoustic logs. Borehole images were further utilized to derive fracture density and also to estimate aperture and porosity, which were calibrated with the borehole-sonic-derived results to estimate the gross voidage in the basement rock. However, in fractured basement rock, the average porosity is usually very low; hence, fracture transmittivity or permeability plays a critical role in estimating the fluid movement in a basement rock. Two methodologies were used to derive the permeability in the basement rock. The first methodology was based on the advanced rock heterogeneity analysis, which was performed to get the raw permeability indicator (RPI) based on conductivity values on the image log. The RPI was further scaled using a data function to read permeability. Similarly, second permeability determination method was used utilizing fracture aperture and fracture density of open fractures in the heterogeneous basement. The estimated permeability results from these two approaches were further validated with a dynamic formation tester data.

The derived permeability, along with the dynamic dataset (MDT), further validated the presence of possible sweet zones within the reservoir. In one of the wells, the permeability results added vital clues for explaining the non-productivity of the basement section. This methodology proved to be of immense help in characterizing the heterogeneous basement reservoir, and thus, it can be further used to optimize the production.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012