Abstract

The Mangala, Bhagyam and Aishwariya oilfields, located in Rajasthan, Barmer basin India, have been producing since 2009, 2012 & 2013, respectively. The fields have undergone extensive reservoir surveillance activity to optimize hydrocarbon recovery. The fields are under water flooding and polymer flooding where cased hole pulsed neutron log (PNL) in Sigma and Carbon/Oxygen ratio modes have been effective tools to monitor saturation variation over time. The fields comprise high-quality clastic reservoirs with excellent porosity (20–26%), permeability (200 md to 20 D), and low irreducible water saturation (~5- 10%). The slightly oil-wet nature of the rock and fresh formation water (~8,000 to 20000 ppm TDS) support time-lapse monitoring using C/O logging. PNL logs, with a shallow DOI (~6– 7 in.), provide reliable saturation in unperforated zones but fail to identify dominant waterproducing intervals, showing 70–90% water saturation across all perforated intervals. To address this problem, cased hole resistivity with a significantly greater DOI around 84–384 inches was piloted. This paper presents case studies for cased hole resistivity applications in these fields, highlighting both its potential and limitations. While it showed encouraging correlation with open hole resistivity in unperforated/unproduced intervals, its readings above the free water level (FWL) ranging from 30–40 ohmm in all sand units, and in some cases dropping to 4–5 ohmm (water zone resistivity) posed challenges for water shut-off decisionmaking. Cased hole resistivity offers deeper resistivity insights but requires integration with other tools for effective water cut management in mature reservoirs.