Organophosphorus-based scale inhibitors (SIs) have been widely used in the petroleum industry for several decades. Among them, aminomethylenephosphonates (possessing at least one −NH+–CH2–PO3H– moiety) have shown outstanding inhibition efficiency against carbonate and sulfate oilfield scales. However, one of the main drawbacks of aminomethylenephosphonate-based SIs with multiple phosphonate (−PO3H2) groups is their poor tolerance against high-calcium brines. In this work, the calcium tolerance of aminomethylenephosphonates was improved by reducing the number phosphonate groups in the inhibitor backbone to less than three and introducing a variable-length non-polar alkyl side chain while maintaining acceptable inhibitory efficiency levels. Hence, we synthesized a series of variable-length alkyl chain-based amino-di(methylenphosphonate) [(H2O3P–CH2)2–N–(X)] inhibitors, (X = methyl, ethyl, propyl, butyl, hexyl, octyl, and dodecyl). In addition, we also studied the role of the alkyl side-chain length in the diphosphonate structure backbone on the scale inhibitory activity. All newly synthesized aminomethylenediphosphonates were evaluated as SIs for calcium carbonate (calcite) and barium sulfate (barite) in brines based on the Heidrun oilfield using a high-pressure dynamic tube blocking rig at 100 °C and 80 bar. Furthermore, we investigated the calcium compatibility of all aminomethylenediphosphonate SIs at several levels of calcium stresses. The new aminoalkyldiphosphonate inhibitors possessing shorter alkyl chain lengths (X = methyl, ethyl, propyl, butyl, and hexyl) gave better calcite scale inhibition performance and outstanding calcium compatibility of up to 1000 ppm of Ca2+ compared to the commercial benchmark ATMP SI [amino-tris(methylenephosphonate)] and longer alkyl-chain diphosphonates, that is, octylamine-N,N-di(methylenephosphonate) (ODMP, C8-D) and dodecylamine-N,N-di(methylenephosphonate) (DDMP, C12-D). Moreover, we tested the thermal stability (at 130 °C for one week) of the best-performing additive (based on calcite inhibition and calcium tolerance performances), and it was found that methylamine-N,N-dimethylenephosphonate (MDMP, C1-D) was thermally stable under these harsh conditions without any loss of its inhibition performance. The Ca-C8-D “complex” was deliberately synthesized and structurally characterized. It appears that the major factor for its poor Ca tolerance is the tight packing of the octyl side-chain groups.
Oilfield scaling is one of the most prevalent water-based oilfield production problems. Chemical inhibitors are often used to combat and prevent scale deposition. The phosphonate functional group is commonly used in many scale inhibitors. A series of bolaamphiphilic alkyldiamine tetrakis(methylenephosphonic acid)s have been synthesized from 1,X-diamines, where X in the polymethylene spacer groups has an even number of 2–12 carbon atoms. These inhibitors were coined CX-T (X is the number of C atoms between the N atoms on the inhibitor backbone, and T stands for tetraphosphonates). All tetraphosphonates showed excellent compatibility with calcium ions at 100 ppm or less, i.e., normal scale inhibitor dosage levels. At higher dosages, the calcium concentration and length of the alkylene spacer chain between the nitrogen atoms also determine the compatibility. Crystal structures were determined for the calcium “complexes” with the tetraphosphonates C2-T, C4-T, C6-T, and C8-T. All tetraphosphonates were investigated for calcite and barite scale inhibition under dynamic conditions using oilfield produced waters based on the Heidrun field water composition, offshore Norway. For calcite scaling, tetraphosphonates with ethylene (2 carbons) and hexamethylene (6 carbons) spacer groups gave the best inhibition performance. For barite scaling, the best inhibition was observed for the tetraphosphonates with ethylene and tetramethylene spacer groups, which relates to the ability of both disphosphonate moieties to interact with a single barium ion.
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