A Barium Sulphate Scaling Post Analysis on Dunbar

Abstract

Due to the commingled production of reservoir and injected sea waters, the Dunbar Asset has had to face Barium sulphates scaling problems. In 2001, a post analysis was made over the entire past production history. This study was carried out by a team consisting of well productivity engineers, reservoir engineers and a production chemist from both Headquarters and the Affiliate. The analysis was directed at two different objectives:Fulfil an operational need: A water based pre-emptive treatment applied in 1997 on well D02 caused a substantial reduction in well productivity. This adverse effect led to major uncertainty in the planning and design of new treatments. The Affiliate wanted a complete analysis of this problem in order to come up with new procedures.Learn from Dunbar experience for future developments: Would it be possible, through a production history analysis, to look in detail at the scaling phenomena in order to pinpoint facts or identify engineering tools that could be of use in new development projects? And, as a feedback for future similar projects, could we assess what advantages a nano-filtration plant might have offered? Both the scaling process and the treatment impact were analysed. This was made easier by the fact we had available data from six production logging operations on a key well. In terms of modelling, Reveal model was used to assess physico- chemical related behaviours to be further exported into the Reservoir model. The results of this modelling were matched against actual production history. The paper will focus on the impact on well productivity of the scaling process as it was highlighted through the exercise. As a matter of fact, a relative manageable effect was observed after the first layer water breakthrough. The effect is much more severe after the second layer water breakthrough, which can create actual bridges into the tubing and consequently affect drastically the flow and can trap reserves from lowermost layers. As this method allowed playing back the history, it opens tracks for planning and dealing with the perforation strategy. In other words, it places the scaling prevention concern right in the middle of the reservoir engineers' type of expertise. Apart from technical consideration, the paper will also present quick economical issues. All scale related operating costs and production slowdowns have been compiled and weighted against the Capex and operating costs that would have represented a nano-filtration plant installed from the beginning. This consideration on economics is only meant as a reference point should we need to design developments in similar contexts, and still needs to be examined for any given project. Introduction Dunbar is an oil and gas field located in the UK sector of the North Sea. Its two main panels have been developed by water injection, with 9 producers and 4 injectors drilled during the first phase of the development. During that same period, 7 more wells were drilled to produce other panels under natural depletion. The reservoir pressure was initially 560 bars, to be maintained around 400 bar where water has been injected. The oil is very volatile oil (Bo=3v/v) The Injection water is pumped from Alwyn main platform The maximum WHIP at Dunbar is 270 bar at a maximum rate of 80000 b/d