A Novel Approach to Rejuvenate Brown Field by Validating Dynamic Model Response with Near-Wellbore Saturation Monitoring

Abstract

Oil India Limited's (OIL) operational areas, in Upper Assam-Arakan Basin, are located in a seismically active thrust fault zone (Bora et al., 2010). Multiple stacked layers, highly faulted anticlines and large number of compartments make the structural setting of these fields very complex. In terms of lithology, some of these reservoirs are low resistivity pays, leading to ambiguities in interpretation due to fresh water environment and complexities in evaluation of hydrocarbon-bearing and water-bearing sands (Koithara et al., 1973, Borah et al., 1998). Greater Nahorkatiya and Greater Jorajan, since the inception of commercial production in the 1950s, have been intensively studied to find prospective sweet spots, perforation intervals for new well locations and potential workover candidates. These forecasts, guided only by dynamic numerical model results, have had mixed results when implemented in the field. A validation of the dynamic model forecasts with near-wellbore saturation logs, can help to reduce uncertainty. This paper describes the success stories in field implementation of workovers, guided by dynamic reservoir model results and cross-validated with Pulse Neutron Tool (Roscoe et al., 1991, Schnorr, 1996) log recordings. The intricacy of delivering a precise dynamic reservoir model was managed by state-of-the-art seismic-to-simulation workflows, an integrated approach to improve the quality of the geological model and specific analytical techniques to fill in the data gaps. The calibrated model was analyzed for workover opportunities of zone transfer. In zones with high confidence, (i.e. high Hydrocarbon Pore Volume (HCPV), high porosity, permeability, etc., perforation intervals were recommended for hydrocarbon saturation monitoring to confirm the near-wellbore saturation predicted by the model. This workflow was followed in 8 wells which added immense value both technically and economically. The validation of model predictions with near-wellbore saturation was carried out in old wells which helped in making informed decision about tapping bypassed hydrocarbon pockets. It helped to avoid non-hydrocarbon bearing zones, which were removed from the existing workover plan. Moreover, it introduced confidence in the dynamic model which will be used in future for more aggressive economic development of the fields. This approach resulted in better understanding of the reservoir characteristics which led to identification of some potential reserves which could be characterized as "Reserve Growth".