Systematic Molecular-Size Variants in Diphosphonate Inhibitors for Oilfield Scale Management

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