Powerful Synergy of Acetylenic Diol Surfactants with Kinetic Hydrate Inhibitor Polymers—Choosing the Correct Synergist Aqueous Solubility

Abstract

The performance of injected kinetic hydrate inhibitor (KHI) polymer solutions can be boosted considerably by judicious choice of the polymer solvent system. We report the excellent KHI synergism of the low-foaming acetylenic diol gemini surfactant 2,4,7,9-tetramethyl-5-decyne-4,7-diol (TMDD) with poly(N-vinyl caprolactam), N-vinyl caprolactam:N-vinyl pyrrolidone copolymer, and poly(N-isopropylmethacrylamide). Highpressure rocking cell tests, using the slow constant cooling method or the isothermal method, were carried out with a natural gas mixture giving structure II hydrates as the preferred thermodynamically stable phase. Poly(oxyethylene) derivatives of TMDD, which are far more water-soluble than TMDD, gave significantly lower synergetic KHI performance with the same polymers. It is conjectured that the low aqueous solubility of TMDD (1700 ppm at 20 °C) and its two isobutyl groups are key features contributing to the synergism. However, when decane was added to the system as a model liquid hydrocarbon phase, the synergetic performance decreases, probably due to partitioning of TMDD to the hydrocarbon phase. This highlights the need to choose synergist systems which are retained in the aqueous phase for optimal performance when condensate or oil is present in the produced fluids. Optimizing the structure and aqueous solubility of the synergist (solvent or otherwise) can be seen as complementary to the known principle of optimizing the structure and solubility of the KHI polymer.

Nonpolymeric Citramide-Based Kinetic Hydrate Inhibitors: Good Performance with Just Six Alkylamide Groups

“… 38 Also, 1 mL of decane was added to the cells, and the subcooling temperature was approximately 15.5 °C. 39 …”

Further Investigation of Solvent Synergists for Improved Performance of Poly(N-vinylcaprolactam)-Based Kinetic Hydrate Inhibitors

“…The decane has two main effects. First, it lowers the equilibrium temperature, in this case by about 0.3 °C . Second, it can become a solvent for the KHI formulation, in particular, iHexOl, in this study.…”

“…Some salinity is often present even in gas lines, so we wanted to explore the effect of added 0.5 wt % NaCl. Earlier work had suggested that sparingly soluble solvent synergists might lose their activity if they partition to the liquid hydrocarbon phase . Therefore, we were interested in confirming this claim using iHexOl, which has limited water solubility, and TBGE, which is very water-soluble and not expected to partition to the hydrocarbon phase.…”

“…First, it lowers the equilibrium temperature, in this case by about 0.3 °C. 46 Second, it can become a solvent for the KHI formulation, in particular, iHexOl, in this study. Removal of the iHexOl from the water phase to the decane phase appears to have a significant effect on the t o value but not on the t a value.…”

“…Earlier work had suggested that sparingly soluble solvent synergists might lose their activity if they partition to the liquid hydrocarbon phase. 46 Therefore, we were interested in confirming this claim using iHexOl, which has limited water solubility, and TBGE, which is very water-soluble and not expected to partition to the hydrocarbon phase. For a concentration of 5000 ppm of both PVCapBGE and iHexOl with 0.5 wt % NaCl, despite the lower liquid volume (which reduces the hydrate induction time), the synergetic effect improved compared to the test without NaCl.…”