This paper provides insights into the long-term production performance of a set of five HPHT wells drilled and completed with SG 2.0 cesium formate fluids across the entire sequence of gas-filled Brent Group reservoir formations of the Kvitebjørn field during the period 2004/5.
Three of the wells were completed in open hole with screens and the other two with cemented and perforated liners. All of the wells cleaned up naturally during production start-up. Well logs run in the first series of wells drilled and completed with cesium formate fluids provided enhanced reservoir information indicating improved reservoir quality which translated into a 33% increase in the Hydrocarbon Pore Factor (HCPF). After a year of production from these wells a new updated reservoir evaluation, factoring in the improved reservoir quality and more reservoir volume, increased the overall Kvitebjørn recoverable gas reserves by 50% to 79.97 GSm³ of gas and 30.13 million Sm³ of condensate. As a result of this evaluation the expected recoverable reserves for the first five production wells drilled in 2004/5 rose from 32.7 GSm3 to 48.3 GSm3 for gas and from 12.3 MSm3 to 14.8 MSm3 for condensate.
Apparent reductions in well productivity indices (PI) of 20-80% were measured in the first 5-7 years of production. The PI reductions could have resulted from a combination of scaling, condensate banking, reservoir compaction or even problems with the PI estimations. One of the wells with a liner completion was re-perforated twice (in 2012 and 2015) and another well with a screen completion was perforated through blocked screens in 2015, but the remaining three wells have continued to produce without intervention for 14 years. The cumulative gas production from the five wells up to October 2018 stood at 60.1 GSm3, which is already 24 % higher than the mean total recoverable reserves predicted in the 2006 reserves evaluation and almost double the figure anticipated in the original 2004 field development plan. The cumulative condensate production over the same time stood at 20.4 MSm3, which is 38 % higher than the mean total recoverable reserves predicted in the 2006 reserves evaluation and almost double the figure anticipated in the original 2004 field development plan.
In laboratory formation damage tests conducted at 150°C the oil permeability of a Kvitebjørn reservoir core plug flooded with a sample of the cesium formate drilling fluid recovered from the field after use in all five wells was reduced by 65% from 29 mD to 10.2 mD. This very valuable test may now allow a retrospective evaluation of the predictive value of that particular laboratory-based return permeability test method used to assess the formation compatibility of that formate fluid twelve years ago.