Improving Sweep Efficiency by Zonal Isolation Using High Expansion Ratio Inflatable Plugs: A Case Study

Abstract

Cairn Oil and Gas operates the M field in the Rajasthan block along with its JV partners. The M field is the largest onshore hydrocarbon find in India in the last two decades. The field contains medium gravity viscous crude (10-20cp) in high permeability sands. Application of EOR technique is considered pivotal in sustaining the plateau production rate and maximizing the ultimate recovery from this field. Field M is currently under polymer flooding with wells completed in a 5-spot pattern. The high viscosity crude in this field, with an unfavourable mobility-ratio with water, mandated the need to switch from water to polymer flooding. Though good sweep improvement was observed in the majority of the patterns, a few pattern producers didn't respond to polymer flood as expected. They exhibited poor sweep efficiency which resulted in bypassed oil and early water/polymer breakthrough. The poor sweep efficiency adversely affects the project economics by reducing the Expected Ultimate Recovery (EUR) and increasing the opex associated with produced water handling. This paper outlines how the installation of "high expansion ratio inflatable plugs" in the pattern producers, improved sweep efficiency. A detailed analysis of pattern flood was undertaken to identify producer wells which were shut-in, due to high water-cut and production handling constraints. Saturation logs were carried out to locate the poorly swept sand packages. Temporary isolation was required to accommodate plans for future ASP flooding. Both mechanical and chemical isolation methods were explored. The mechanical method was selected because of its higher success rate in achieving a perfect isolation compared to chemical methods. An additional concern with chemical isolation treatments was the inherent risk of permanently damaging the formation. The existing completion design required the isolation plug to pass through a minimum ID of 2.3" and set in a 7" production casing. For this reason, high expansion ratio retrievable inflatable plugs were proposed. The waxy nature of the reservoir fluid mandated the use of Coil Tubing (CT) as the plugs required a clean fluid environment for expansion. The jobs were successfully conducted and the zones were selectively isolated in two producer wells. The isolation resulted in a fourfold increase in the instantaneous oil production of these wells with an associated drop in produced water cut. Production logs in the associated pattern injectors showed increased injection in the zones which previously had been poorly swept. This demonstrated how the treatments improved the selective drainage of the poorly swept sands by allowing preferential movement of flood front in these sands. The intervention helped the operator selectively produce from the poorly swept zones without the requirement of a major workover operation. Based on the success of the pilot project, the technique will be deployed in all applicable 7" cased wells. Its use in 9-5/8" cased wells is being investigated. Learning and information shared in this paper will be useful to anyone interested in using high expansion ratio plugs for wellbore isolation treatments.