This paper presents the results from recent laboratory studies on barium sulphate scale inhibition in high [Ba2+] produced waters and scale inhibitor thermal degradation/stability. In the barium sulphate inhibition experiments, the synergistic effect of different inhibitor molecules on scale inhibition is examined; static inhibition tests are compared with dynamic scale loop tests and difference between the two results is discussed. Results from a systematic study of scale inhibitor thermal stability and its effect on both barium sulphate and calcium carbonate inhibition efficiencies are described. The scale inhibitors which are thermally stable are identified and their thermal stability is explained with relation to the molecular structures.
High [Ba2+] offshore produced waters have presented a considerable challenge in providing effective prevention of barium sulphate scale precipitation by using a scale inhibitor. In some North Sea fields, such as Brae, Gyda, Miller, Njord and T-block fields. [Ba2+] in the formation waters ranges from 500 mg/L to 1500 mg/L. In such fields, barium sulphate scaling at both downhole and topside after seawater breakthrough, if a suitable/effective scale inhibitor is not applied, has a severe potential of damaging formation productivity and plugging well tubing, valves and flowlines, etc. In order to alleviate the scaling severity, some operators have installed seawater sulphate removal plant. Chemical manufacturers, in the meantime, have been striving to synthesise new inhibitor molecules.
In the other area of new developments. some high pressure and high temperature (HPHT) oil and gas fields in the North Sea, such as the ETAP fields and the Elgin and Franklin fields, are coming into production soon. The temperatures of these reservoirs are typically above 150 C. The very high temperature regime of the HPHT fields, in which a scale inhibitor will be applied, requires the inhibitor molecule to be thermally stable and maintain its scale inhibition efficiency. Two early papers had examined the effect of heat-aging on inhibitor barium sulphate inhibition performance, while no work concerning calcium carbonate scale inhibition has been reported. In the first paper, the thermal aging was at 154 C for a fairly short period of four days (oxygen not removed). The inhibitors included were two polyvinyl sulphonates, a polyacrylate, a sulphonated terpolymer, a phosphonated terpolymer and two amine phosphonates (penta and hexa). In the other paper, inhibitor solutions were degassed and nitrogen sparged before being sealed in heat-aging bombs. The most severe heat treatment was at 175 C for 21 days involving a polyvinyl sulphonate, a polyphosphinocarboxylic acid and amine phosphonate (penta). This laboratory study attempts to address these two topical issues, which are hoped to improve our understanding in chemical performances and aid the selection of appropriate scale inhibitors for such applications.
The barium sulphate scale inhibition tests were conducted by both static inhibition and dynamic flow loop. A very high [Ba2+] North Sea formation water, with more than 1400 mg/L barium ions, was used in a 50:50 mixture with seawater as the test brine.
In the thermal degradation tests, together 8 scale inhibitors were tested, including two amine phosphonates (penta and hexa), three carboxylic polymers (one polyacrylate, one PPCA and one terpolymer) and three sulphonated polymers (two copolymers and one terpolymer). The scale inhibitors at 1% concentration in synthetic seawater were aged at 180 C for 14 days. The heat-aged and untreated inhibitors then underwent both barium sulphate and calcium carbonate scale inhibition efficiency tests. The scale inhibition efficiencies with and without heat-treatment were then compared.
The next section of the paper describes the experimental procedures. The results are then presented and discussed. which are followed with our conclusions from these studies. P. 171