Part 3: Sulphate and Carbonate Scale Prediction in North Sea Oil and Gas Fields

Abstract

Abstract Water-borne scales of sulphate and carbonate are major production problem in North Sea oil fields. Scale formation results from two processes: reduction in mineral solubility in produced formation water due to drops in temperature and pressure during production, and chemical incompatibility between formation water and injected seawater. Temperature and pressure drops are chiefly responsible for carbonate scale precipitation. Chemical incompatibility between the reservoir formation water and the injected sea-water is the most common cause of sulphate scaling. Furthermore, the precipitation of one mineral scale may be aggravated or alleviated by competitive co-precipitation of other mineral(s) and specific ion effects. This chapter examines the susceptibility of North Sea formation waters to carbonate and sulphate scale precipitation. Seawater contains a few thousand milligrams per litre (c.2800 mg 1 -1 ) of sulphate anions (SO 2- 4 ), a small amount (c.420 mg 1 -1 ) of calcium ions (Ca 2+ ) and trace amounts of barium (Ba 2+ ) and strontium (Sr 2+ ) ions. In many offshore fields in the northern and central North Sea, the original formation water has trace SO 2- 4 but excessive Ba 2+ and Sr 2+ ions (up to 2000 - 3000 mg 1 -1 ) as well as Ca 2+ ions (up to tens of thousands mg l -1 ). In normal oil field conditions, these two types of brines are usually chemically stable when they are kept apart. However, as seawater is injected into the reservoir formation during a waterflooding process, it commingles with the formation water in various locations of both reservoir and production systems to deposit sulphate mineral scales of barite (BaS0 4 ), celestite (SrS0 4 )