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Abstract

Enhancing recovery of the remaining gas in ageing fields presents a commercial opportunity. As such, NAM took the decision to apply on-site generated N2 injection for the first time. The De Wijk gas field was selected based on size and geological setting, allowing the demonstration of N2 flooding of depleted gas in diverse reservoir types, as well as testing the efficiency of residual gas sweeping in the watered-out reservoir. De Wijk is one of the oldest onshore natural gas fields in The Netherlands, having produced c. 15 Bcm (billion cubic metres) of high calorific gas with a natural N2 content of 5–11% for more than 60 years, bringing it close to the end of field life. Enhanced gas recovery in this field would extend the field life and that of the Ten Arlo gas-processing system, thereby also lengthening tail-end production of other connected fields. In De Wijk, the existing 35 wells were used as dedicated injectors and producers. The production rates from the latter are used to help better understand reservoir flow behaviour. Integration of geological analysis and available production data helped to implement the N2 enhanced gas recovery technology in this mature field. Two processes of enhanced gas recovery were tested: a gas–gas displacement in the depleted gas leg (Nitrogen Assisted Depletion Drive: NADD); and gas displacing residual gas in the water leg (Nitrogen Enhanced Residual Gas: NERG). Observations after 1 year of N2 injection showed that planned v. actual performance of the NADD technology is comparable with an increase in production of 100 000–160 000 Nm3/day (normal cubic metre per day). NERG application restored a watered-out well; however, further investigation of this application is required. It is demonstrated that a detailed geological understanding of, for example, permeable Rogenstein oolite intervals and the lateral connectivity of internal high-permeability streaks in Volpriehausen sandstones is crucial to the successful application of enhanced gas recovery techniques in such ageing fields.

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