When Will Low-Sulphate Seawater No Longer Be Required on the Tiffany Field?

Abstract

Abstract Injection of sulphate rich seawater into reservoirs with formation brines rich in calcium, barium or strontium may result in the precipitation of sulphate scales. One technique for managing mineral scales in fields where scale inhibitor squeeze treatments may prove very difficult or ineffective is injection of low sulphate seawater. The CNR operated Tiffany field in the North Sea is one of the oilfields that has been swept with low sulphate seawater for the longest period (> 10 years) and from the start of water injection, and consequently yields useful information on brine mixing and brine-rock interactions during low sulphate seawater sweep. This paper presents the evolution of individual well brine chemistry data, backed up by reservoir simulation and reactive transport flow modelling, which demonstrates the effect that low sulphate seawater injection has had on the produced brine chemistry. The main impact is that scale inhibitor squeezes have only been required for carbonate scales. The modelling has been extended to predict future individual well brine compositions to identify potential barium and sulphate concentrations to end of field life, and hence identify any remaining potential barium sulphate scaling risk. However, the predictive modelling has also been used to study the sensitivity to timing of a switch from desulphated to full sulphate seawater injection towards the end of field life, to address the question of when will low sulphate seawater no longer be required on the Tiffany field? This study has significant implications for other basins around the world where desulphation projects are about to commence (Brazil and Angola). Introduction Oilfield scale deposition costs the petroleum industry millions of dollars each year and, if not managed appropriately may cause significant production decline. It may simply be defined as the deposition of insoluble inorganic minerals that are formed as a result of precipitation of solids from brines present in the reservoir and production flow system. This deposition of scale can cause significant production problems such as blocking perforations, casing, production tubing, valves, pumps and down-hole completion equipment, thereby damaging equipment and preventing fluid flow.1–3 Scale can form wherever there are changes in the physical conditions starting from injection wells through the flow cycle to the surface facilities, and is formed by three principle mechanisms.1, 2Decrease in pressure or increase in temperature of brine which leads to precipitation of carbonate scales (eg CaCO3).Mixing of two incompatible fluids, formation water (rich in barium, calcium or strontium) mixing with seawater (rich in sulphate), leading to precipitation of sulphate scales (eg BaSO4, CaSO4, or SrSO4).Brine evaporation resulting in salt precipitation (eg NaCl). The most common mineral scales formed from produced waters are calcium carbonate (CaCO3) and barium sulphate (BaSO4). Barium sulphate is the most difficult problem to solve because it has an extremely low solubility in water and also because it has a high thermodynamic stability and hardness once formed.3 Many fields require water flooding from day one of field production to maintain the pressure within the reservoir above the bubble point pressure and thereby prevent gas evolution in the porous medium, to sweep the hydrocarbons towards the production wells, and to supplement the reservoir energy required to lift fluids in the production wells. The most economical water flooding option available in offshore fields is usually to inject seawater into the formation. This may cause barium sulphate scale to form when sulphate from injected seawater mixes with barium from the formation connate or aquifer water. Barium sulphate can cause serious damage, such as pore blockage if it occurs near the well bore, constriction in the tubing or damaging safety valves and surface facilities. Radium may also be incorporated in the scale, making it a radioactive health hazard.