Combined Influence of Hydrate Structure and Corrosion Inhibitor on KHI Performance in Natural Gas Condensate Systems

Kinetic hydrate inhibitors (KHIs) are now well established as an effective technology for gas hydrate control in the oil & gas industry. They can offer significant CAPEX/OPEX advantages when compared with traditional thermodynamic inhibitors (e.g. methanol, glycols), in addition to helping significantly reduce the carbon footprint of conventional production operations. Historically, KHIs were tested by means of nucleation induction/hold time measurements, although in recent years the focus has turned to more robust crystal growth inhibition (CGI) approaches, which evaluate chemistries under worst-case scenario ‘hydrate present’ conditions. 
It has been appreciated for some time that hydrate structure is important to KHI performance, with leaner s-I forming natural gases (NGs, mainly methane, with limited heavier hydrate formers) typically presenting more of a challenge than richer (in ethane, propane, butane) s-II dominated mixtures. CGI studies have facilitated a better understanding of the processes involved, and, for example, point to KHI ‘failure’ in s-II NGs often apparently being the result of an inability to inhibit s-I at higher subcoolings.
Here, we present the results of detailed CGI laboratory studies of three commercial KHIs (termed A, B & C) for a real gas-condensate (GC) production system. Products were initially investigated in detail using a generic s-II/s-I forming binary 98% CH4 / 2% C3H8 natural gas analogue, before subsequently being tested with a multicomponent/multi-structure forming natural gas (NG) system at field target dosages. All experiments were undertaken with real field condensate, and in the presence (both gases) / absence (binary gas) of incumbent corrosion inhibitor (CI), to assess the effect of this on performance.
In support of hydrate structure playing a key role, a high consistency in failure subcoolings relative to s I phase boundaries was found between the different products / gas systems, implying an ultimate inability to inhibit this structure rather than the more stable s-II. Results of detailed binary gas system studies support such a hypothesis. Here, KHI A was found to completely inhibit s-II (CH4-C3H8) growth, with failure clearly resulting from s-I hydrate formation. As a direct result of this, upon heating to generate CGI method residual hydrate fractions, PT conditions had to be kept close to the s-I boundary (as opposed to s-II), lest all hydrates / ‘history’ be lost, and performance on re-cooling be misleading high / poorly reproducible. In effect, KHI-A resulted in (s-II) ‘hydrate history elimination’, or more specifically prevented this developing, which is unusual for KHIs, and has significant implications for both field use and laboratory testing. The presence of CI appeared to stop such behaviour, seemingly by encouraging the growth of s-II hydrates and the preservation of these on heating, making residual hydrates/history retention control simpler. Consistent with a notable CI-hydrate interaction, the corrosion inhibitor had an overall positive effect on KHI-A performance.
KHIs B and C were not subject to the same ‘history elimination’ issue as A, while the inhibition performance of both products was likewise largely unaffected by corrosion inhibitor, suggesting a general generic KHI compatibility for this CI chemistry. With respect to hydrate structure, KHIs A & C highest performance CIR regions appeared most closely related to the primary s-I boundary for the test gas used, although this is not the case for KHI-B, which showed markedly greater efficiency in the natural gas system. For the NG case, a larger number of hydrate phases/structures may form, and it may be that KHIs A & C are more susceptible to these.