Conditions under which anhydrite precipitation can occur in oil reservoirs as a result of seawater injection

摘要

Recently, precipitation of sulphate minerals in oil reservoirs as a result of mixing ofinjected seawater and formation water has been the subject of extensive investigation.Of particular interest to scale management is that this process causes a reduction inproduced water sulphate mineral scaling potential leading to significant reductions inscale mitigation costs against those originally anticipated. Most interest has focusedon BaSO4 because it is a more common scaling mineral but deposition of CaSO_4(anhydrite) in the reservoir has the greatest potential to reduce sulphate mineralscaling potentials where Ca-rich formation waters are present.To better understand the conditions under which the latter process will occur,predictions of sulphate mineral deposition in the reservoirs of 81 fields in the UKNorth Sea (UKNS) and Norwegian Continental Shelf (NCS) have been undertaken.In each case, it has been assumed that the reservoirs are under seawater flood andsulphate mineral deposition occurs only as a result of heating of injected seawater andmixing of seawater and formation water, the mechanisms by which sulphate mineralsare most likely to precipitate.These calculations have shown that deposition of CaSO_4 is likely to be a relativelycommon process in UKNS and NCS reservoirs, and in almost 1 in 4 fields it is likelyto cause a significant reduction in sulphate mineral scaling potential. The amount ofdeposition of CaSO_4 in the reservoir is mainly dependent on the Ca content of theformation water and the reservoir temperature. The minimum temperature observedwhere significant deposition has been predicted (and therefore significant SO4 loss) is105oC. Generally, significant deposition also occurs where formation water is Ca-rich(>~6,000 mg/l Ca) but as temperature also plays a role, there are exceptions (i.e. insome cases, below ~133oC, Ca concentrations need to be greater and vice-versa). Carichformation water usually has elevated salinity (>70,000 mg/l Cl). SignificantCaSO_4 deposition and SO4 removal is predicted to occur in deep reservoirs with Carichformation water and in particular those located in the Skagerrak Formation(Triassic), Pentland (M. Jurassic), Ula/Gyda Formation (U. Jurassic), and FulmarFormation (U. Jurassic) of the Central Graben overlying the Zechstein Formation,although they can be located elsewhere if geological conditions are favourable.Based on the results of this study, the anticipated effect of sulphate mineral depositionon produced water compositions during the well life cycle is described with particularreference to reservoirs that are either more, or less, heterogeneous. Tools availablefor quantitative prediction of produced water compositions when sulphate mineraldeposition occurs in the reservoir are also discussed.The results of this study may aid identification of those fields under developmentwhere precipitation of CaSO_4 in the reservoir should be considered during scalemanagement planning. They may also help explain observed trends in producedwater scaling ion concentrations and sulphate mineral scaling potentials where CaSO_4deposition is occurring in the reservoir.