Abstract
Mangala field is one of the largest onshore oil fields in India. The Fatehgarh reservoirs consist of multi-Darcy permeability sandstones with low connate water saturations and moderate reservoir temperatures (62°>C). The reservoirs contain waxy and moderately viscous crude (9 – 22 cP) resulting in adverse mobility ratio between oil and water. The field was originally developed using an inverted 9-spot, hot waterflood. Given the viscous nature of the oil, even before development of the field was initiated, it was envisaged that sweep improvement methods would have good potential to enhance overall recovery from the field. After detailed laboratory studies and a successful pilot, a full-field polymer flood has been implemented. Experience from the pilot highlighted the need for maintaining polymer solution integrity and ensuring good propagation of the flood; acquisition and analysis of reservoir surveillance data are crucial in order to implement a successful polymer flood.
Initially, lab viscosity measurements of actual samples from the full-field project's injection lines conducted in aerobic conditions showed significantly lower values than what would be expected based on measurements made on polymer solutions in synthetic brine. Readings from an online viscometer, however, showed somewhat higher viscosities. Use of a stabilizer chemical during sample collection helped protect the sample from aerobic degredation, and the viscosity measurements more closely matched the online viscometer. The data re-emphasized the need to prevent oxygen from entering the system, and modificationsto the polymer preparation plant, Central Polymer Facility (CPF), to minimize oxygen ingress showed marked improvement in viscosity readings. This paper will detail the procedures followed to minimize the impact of oxygen (air) while taking samples and measuring viscosity.
A detailed well and reservoir surveillance program was implemented at the start of polymer injection. Surveillance activities includedspinner surveys, fall-off tests, bottom-hole pressure measurements, and saturation monitoring. The paper also presents an analytical approach for the estimation of in-situ viscosity with fall-off surveys. These results are compared with modeled viscosity from simulation. All polymer injector wells were pre-produced for extended periods prior to start of polymer injection; previous experience showed that this improved injection (reduced skin) and conformance. The saturation monitoring survey and fall-off tests suggest good sweep and even re-saturation of oil in some sands. Integration of high resolution seismic data between injector-producer pairs and surveillance data helped the operator to take appropriate action on some under-performing producers which later showed significant improvement in performance.
Initial oil production response from producers which are near to polymer injectors is encouraging, with significant increases in oil rate and decreases in water-cut. Samples collected from some of the producers show polymer breakthrough in minor quantities. This paper presents the results of the initial phase of implementing the full-field polymer flood in Mangala.
