Re-development of the Frøy Field: Selection of the Injection Water

Abstract

The Frøy Field, Norwegian North Sea, was closed down in March 2001 but there are plans to re-develop it. Previously, seawater (SW) injection caused persistent BaSO 4 scaling problems in the production wells, so this study was undertaken to determine whether there would be any benefit in using Utsira formation water (UW; low SO 4 ) as opposed to seawater as the injection water.Thermodynamic calculations indicated that whether SW or UW is injected, the principle scaling risks are from BaSO 4 and CaCO 3 deposition. Being more difficult to treat, the study focused on the BaSO 4 risk. ECLIPSE modelling was used to estimate FW:SW:UW ratios over time for each new well assuming (a) SW injection and (b) UW injection. The results indicated that in each case, initial peaks in BaSO 4 Saturation Ratio (SR BaSO4 ) would occur after ~1.5-2.5 years before declining. The peaks reflect co-production of seawater already present in the reservoir and formation water. The subsequent decline in SR BaSO4 reflects (a) decrease in the FW:SW ratio (SW injection case) and (b) dilution of produced Ba and SO 4 as the produced UW fraction increases (UW injection case).An evaluation of produced water analyses from Frøy and from analogue fields indicated that reservoir reactions will occur at Frøy whether SW or UW are injected but significant BaSO 4 would only occur in the presence of seawater. Two methods were used to quantify SR BaSO4 in the new production wells over time after accounting for BaSO 4 precipitation in the reservoir. In the first, the amount of BaSO 4 deposition in the reservoir was predicted from past produced water analyses and applied to the ECLIPSE results. The second involved the use of STARS to model the effect of reservoir reactions on the scaling potential. There were some differences in the results related to the amount of mixing predicted to be occurring in the reservoir, but contrary to initial expectations, both methods indicated that UW injection would not be beneficial.Based on these results, seawater has been selected as the injection water resulting in significant OPEX and CAPEX cost savings.

Predicted and Observed Evolution of Produced-Brine Compositions and Implications for Scale Management

“…One-Dimensional Reactive-Transport Model A 1D reactive-transport model (Parkhurst and Appelo 1999), as a hydrogeochemical simulator, is usually used for simulating a variety of reactions and processes in natural water or in batch and column laboratory experiments. However, recently, this 1D reactivetransport model has been applied to investigate the effect of scale formation on the produced-water composition during the mixing and movement of IW and FW from injector to producer (Fu et al 2012;Vazquez et al 2013).…”