Hexa-1,3,5-hydrotriazines form the main product class used for chemical hydrogen sulfide (H2S) scavenging. The reaction mechanism between these triazine species and H2S is discussed in detail with the emphasis laid upon the reaction products and their fate. The paper goes on to then describe a novel method of oxidative dissolution of these reaction products, including a full analysis of the resultant species and a mechanistic postulation. The single reaction product of monoethanolamine (MEA)-triazine (1,3,5-tris(2-hydroxyethyl)hexahydro-s-triazine) has been repeatedly found by the authors to be initially monomeric 5-(2-hydroxethyl)-hexahydro-1,3,5-dithiazine which, when separated out of aqueous solution, invariably polymerizes to an insoluble, solid polymeric species. This solid product is referred to as amorphous polymeric 5-(2-hydroxyethyl)-hexahydro-1,3,5-dithiazine, abbreviated as apDTZ. The occurrence of this material in oil production systems causes heavy deposits or fouling in pipelines, valves, chokes, and turbine blades. Its removal by any means other than physical intervention is extremely challenging. The current work shows how the presence of a terminal hydroxyl functionality is critical in enabling the dithiazine to polymerize to form the apDTZ. This work goes onto dispel previous misconceptions in the industry and the literature regarding this process which is finally systematically addressed. Specifically, two very important issues are dealt with in this work which were previously unresolved in the literature. An explanation why the thiadiazine reaction product (first sulfur molecule substitution) from tris(2-hydroxyethyl) triazine (MEA-triazine) is never observed. Following upon the above explanation, why the dithiazine (second sulfur molecule substitution) in all cases never progress to the trithiane (third sulfur molecule substitution). This is probably the greater misconception in the industry and literature regarding triazine and H2S reactions. Despite the widespread occurrence of apDTZ in the oil and gas industry, there are very few studies of effective methods for its removal. This study presents such a process.
When hydrogen sulfide (H2S) gas is produced from oil reservoirs, it leads to a number of well-known problems relating to both metal corrosion and health and safety. Up to a certain concentration (∼1000s of ppm in the gas phase) the H2S can be removed by adding a chemical “scavenger” to the system, which reacts with H2S to form less harmful byproducts. A range of different chemical species have been applied as H2S scavengers in petroleum production systems, operating through either oxidative or nonoxidative mechanisms; the former group includes such species as alkaline sodium nitrite and hydrogen peroxide, and in the latter group triazine type scavengers have been very prominent. Although triazine based H2S scavengers have been hugely successful in practical and commercial terms, there have been associated issues with the related byproducts from the scavenging reaction (discussed in this work). An alternative non-triazine chemical range of H2S scavengers that has emerged in recent years has been based on hemiacetal chemistries, where a hemiacetal is produced by the reaction of an alcohol with an aldehyde. Where the aldehyde is formaldehyde, the resulting species are known as hemiformals. In this work, the synthetic chemical reaction pathway for hemiformals based on ethylene glycol and glycerol were studied. This work is novel in four major respects: (i) first, we examine hemiformal production via acid, base, and neutral catalysis systems; (ii) we perform detailed structural determinations of the oligomeric series thus produced (which are different) by analyzing the products by derivatized gas chromatography mass spectrometry and making subsequent structural assignments; (iii) we then assess the effectiveness of these species as H2S scavenging using an industry standard multiphase test methodology and make some correlation between their structure and performance; and (iv) finally, we studied the structures of product and byproducts of the scavenging reaction both before and after the scavenging reaction with H2S. This latter part of the study examines issues of byproduct structure early in the development of these newer products in order to address any problems that may be encountered “up front”. This greatly facilitates our application of using hemiformals as fully optimized non-triazine based scavengers by addressing byproduct chemical analysis of these relatively new H2S scavenger systems.
Sulfate minerals commonly form in the oilfield environment due to secondary recovery operations. Two essential tools for identification of these minerals are Fourier transform infrared (FT-IR) and Raman spectroscopies. In this paper, a comprehensive database of the FT-IR and Raman spectra of these minerals has been composed. The symmetry of the sulfate tetrahedra in the various sulfate molecules has been assigned from observation of the vibrational degeneracy. Force constant calculations show that cation mass and polarizability are the dominant influences upon sulfate symmetry. Studies of the vibrational peak positions have demonstrated a direct relationship between properties of the associated sulfate cation and the positions of both the infrared and Raman v1 and v3 vibrations. The major influences were found to be cation electronegativity, cation mass, and cation ionic radius. These observations were confirmed from force constant calculations. It has been concluded that both FT-IR and Raman spectroscopies are valid techniques for identification of oilfield scale minerals. Raman spectroscopy is particularly useful as it can be applied to aqueous systems and the in situ measurements of scale formation.
Oxazolidine-based products were originally used for their biocide properties, but they have become more widely applied recently for scavenging H 2 S. Oxazolidine H 2 S scavengers offer an oil-soluble alternative to hexahydrotriazine water-based scavengers and may mitigate some of their drawbacks. While the same reactants are used for oxazolidine (an aldehyde and primary amine) as are used for hexahydrotriazines, the synthesis conditions are strictly anhydrous and involve the elimination of water to yield the 5-membered ring heterocyclic species. However, we find that their structural identity is more complex than might originally be suspected. The synthesis can be extended beyond the 2-carbon N,O spacer yielding a 5-membered heterocycle to include the 3carbon N,O spacer, which leads to a 6-membered 1,3-oxazinane species. Oxazolidine-based scavengers are readily converted into the corresponding hexahydrotriazine and very likely owe a large degree of their scavenger activity to this hydrolytic conversion. Due to this conversion, oxazolidines may not be true "nontriazine" alternatives, but they are still very useful H 2 S scavengers, and both their chemistry and H 2 S scavenging efficacy deserve further study. This paper contributes to three important aspects of the study of oxazolidine H 2 S scavengers. First, this work presents several new results on the chemistry of the two bisoxazolidine H 2 S scavengers, 3,3′-methylenebis[5-methyloxazolidine] (MBO) and 3,3′-methylene-bisoxazolidine (unsubstituted-MBO or US-MBO). Our study focuses on the structure of the scavengers themselves as well as on their reaction with H 2 S mechanistically, including the reaction product characterization. Second, a benchmarking reference for the scavenging performance of MBO and US-MBO is presented using a range of standard industry techniques for H 2 S scavenger assessment. Third, we go some way toward explaining why the most commercial of the bisoxazolidines is in fact the monoisopropanolamine (MIPA)-derived MBO. This latter result is surprising given the relative availability and cost of monoethanolamine (MEA).
As the oil and gas industry strives to replace ageing, environmentally undesirable scale inhibitors there is an ever increasing use of polymeric inhibitors. Incorporation of phosphorus functionality into a polymer backbone has been shown to improve inhibition efficiency, enhance adsorption characteristics and allow the polymer concentration to be analysed by elemental phosphorus. It is known that some phosphorus tagged polymers can be problematic to analyse in oil field brines as they typically have a low phosphorus content which is difficult to determine from the background.The development of novel phosphorus functionalised polymeric scale inhibitors was previously described (SPE 130733). This paper follows the development of the inhibitor class. Utilising extensive laboratory testing the interactive nature of the scale inhibitors and reservoir lithology was studied. These novel phosphorus functionalised inhibitors were compared to a number of other available P-containing polymers.Following successful development, one of the phosphorus functionalised polymeric inhibitors was subject to sequential fieldtrial in a harsh BaSO 4 scaling, highly naturally fractured North Sea carbonate reservoir. The phosphorus functionalised inhibitor provided improved performance compared to the incumbent product. Following the successful deployment in a carbonate reservoir the novel inhibitor class was also deployed in a number of North Sea sandstone reservoirs and one field wide case study is reported with details on the innovative management and implementation strategy presented. Focus has been given to the application of these products and a detailed analysis of field proven benefits given. This paper concludes with a detailed comparison of these Phosphorus functionalized scale inhibitors with the incumbent products they have replaced in the case histories described. The benefits that these novel, innovative products have given to the operator are described along with a technical synopsis of incremental performance benefits.
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