Rapid Dissolution of Cinnabar in Crude Oils at Reservoir Temperatures Facilitated by Reduced Sulfur Ligands

Abstract

Mercury (Hg) is present in petrochemical samples, including crude oils, and the processing and use of petroleum products contribute to global Hg emissions. We present a refined theory on geochemical processes controlling Hg concentrations in crude oil by studying dissolution kinetics and solubility thermodynamics of cinnabar (α-HgS(s)) in different crude oils held at reservoir temperatures. In a black light crude oil, α-HgS(s) dissolved in an apparent zero-order reaction with a rate of 0.14–0.58 μmoles m–2 s–1 at 170–230 °C and an estimated activation energy of 43 kJ mol–1. For crude oil samples with a total sulfur concentration spanning 0.15–2.38% (w/w), the measured dissolution rate varied between 0.05 and 0.24 μmoles m–2 s–1 at 200 °C. Separate tests showed that thiols and, to a lesser extent, organic sulfides increased the solubility of α-HgS(s) in isooctane at room temperature compared to thiophenes, disulfides, and elemental sulfur. Long-term (14 days) α-HgS(s) solubility tests in a crude oil at 200 °C generated dissolved Hg concentrations in the 0.3% (w/w) range. The high α-HgS(s) dissolving capacity of the crude oils was more than 2 orders of magnitude greater than the highest reported Hg concentration in crude oils globally. On the basis of the kinetic and solubility data, it was further concluded that α-HgS(s) is not stable under typical petroleum reservoir conditions and would decompose to elemental mercury (Hg0). Our results suggest that source/reservoir temperature, abundance of reduced sulfur compounds in the crude oil, and dissolved Hg0 evasion processes are principal factors controlling the ultimate Hg concentration in a specific crude oil deposit.

Methods for Total and Speciation Analysis of Mercury in the Petroleum Industry

“…Produced gases are transferred to a gold amalgamator by a carrier gas that is then analyzed. ,, Other systems use the same principle without any amalgamation step. The sample is thermally decomposed in a graphite furnace (GF), and the gases are analyzed directly in the graphite furnace coupled with AAS. ,, However, without the fusion stage, the DLs are usually relatively high. In fact, Pontes et al wanted to compare their method, which did not have an amalgamation stage, with that of Liang et al who did have an amalgamation stage.…”

“…Several methods of sample preparation for the determination of total Hg may also be used (Table ). Sample injection into a carrier solvent was shown to be a practical method that does not require sample treatment and fast analysis. ,, This technique makes it possible to use very small quantities of samples, with a DL comparable to those of thermal desorption-AAS or AFS. , …”

“…Lambertsson et al recently developed a new method for determining Hg in liquid oil samples: ID-HS-GC-ICP-MS, which is based on the concentration of Hg in the head space (HS) of a sample vial combined with isotope dilution (ID), GC separation, and ICP-MS detection. An isotopic compound ( 198 HgCl 2 ) is added to the sample to accurately trace the Hg concentration in the samples.…”

“…This heavy metal is present in all petroleum fractions at levels varying by several orders of magnitude ranging from 0.01 ppb to 10 ppm . Its presence in hydrocarbon fluids might result from Hg present in organic matter prior to burial, mantle degassing of gaseous Hg 0 and scavenging by petroleum or mobilization of mineral Hg from surrounding rocks. , Hg is a toxic and volatile heavy metal. In its metallic form (Hg 0 ), it is generally found as a gas, although it might also occur as liquid drops when the gas phase is saturated.…”

“…The study of Hg speciation in liquid hydrocarbons is of interest because of the need for sufficient treatment procedures. Although Hg 0 might be the dominant form of Hg in deep reservoirs, the presence of oxidized Hg, particulate Hg and alkylated Hg species was reported. , For Hg concentration, various techniques are used to evaluate the relative abundance of these species. Section is dedicated to the description of these techniques.…”