One of the flow assurance challenges facing the deepwater oil and gas field development and production is mineral scale control. In a deepwater production system, barium sulfate scale deposition may arise from two causes, namely: a) commingling of injected seawater with connate formation water, and b) temperature cooling from well bottomhole to wellhead and then in a long subsea flowline. The first phenomenon has already been well understood in the offshore oil industry. However, there is very little knowledge in barium sulfate scaling and inhibition mechanisms in a deepwater subsea system, where the fluid temperature can be cooled to a few degrees Celsius. This lack of knowledge is further hampered by the lack of barium sulfate solubility data at the appropriate temperatures.
In this work, barium sulfate solubilities were measured in BaSO4-NaCl-H2O system at 5°C, in which sodium chloride concentrations ranged from 0 to 5 molal in the solutions. This new solubility data is presented in this paper in comparison with the published solubility data for 25°C and 95°C. Significant reduction in barium sulfate solubility from 25°C to 5°C (and even more from 95°C) was noted, which is more pronounced at higher solution ionic strength (NaCl molality). This paper then presents new results from a series of laboratory experiments that investigate the effect of a wide range of temperatures from 95°C to 5°C on barium sulfate scale precipitation and inhibitor efficacy. The inhibitor effectiveness is studied both dependent and independent of barium sulfate scaling tendency change. Both static jar tests and dynamic scale tube-blocking experiments were carried out. It is found that the apparent reduction or loss of inhibitor effectiveness at colder temperatures in a given brine, as seen in the static tests, is mostly due to increased barium sulfate scaling tendency as temperature decreases, while the inhibitor function may have changed little from one temperature to the other. The other interesting finding is that, although the supersaturation of barium sulfate increases considerably as temperature lowers, the kinetic rate of scale precipitation also slows down at colder temperatures. As a result, barium sulfate supersaturation is not the only driving force for it to precipitate, but temperature plays a role as well. This conflict between thermodynamic and kinetic effects of temperature cooling on barium sulfate scaling results in clearly different results between static scale inhibition tests and dynamic tube-blocking tests. That is, in a given brine, a scale inhibitor apparently performs better or equally at a lower temperature than at higher temperature in a flowing system, while apparently performs worse at lower temperatures in a static solution. Also, as far as barium sulfate inhibition is concerned, the two inhibitor chemistries included in this study did not correspond to temperature change or different test methods in exactly the same manner.
In the deepwater development and production of oil and gas fields, flow assurance both assumes a critical importance and involves technological challenges1,2. Mineral scale control is one of the flow assurance issues. One distinct feature in the deepwater that is relevant to scale formation and inhibition is the cold temperature environment in the subsea systems, where the seabed temperature is typically about 4°C3, where the produced fluids may get cooled to the seabed temperature in a long subsea flowline.
In order to fill the gap in barium sulfate solubility data in the literature that is relevant to deepwater subsea temperatures, barium sulfate solubilities were measured in BaSO4-NaCl-H2O system at 5°C, in which sodium chloride concentrations ranged from 0 to 5 molal.
The author et al. previously investigated barium sulfate scale formation and inhibition as a function of temperature, ranging from 95°C to 23°C4–5. It is found that, for a given brine, barium sulfate scaling tendency increases significantly as temperature decreases, and scale inhibitor effectiveness (expressed as %BaSO4 Inhibition) reduces correspondingly.