Experimental Measurement of Multiple Hydrate Structure Formation in Binary and Ternary Natural Gas Analogue Systems by Isochoric Equilibrium Methods

Abstract

Traditionally, it is commonly assumed that a single gas hydrate (or clathrate hydrate) structure/phase is formed in natural gas systemse.g., "s-II natural gas (NG) hydrates" or "s-I methane hydrates"based on that which is understood to be the most thermodynamically stable at incipient phase boundary conditions. This applies with respect to both studies of hydrates in natural sediments, and in the case of hydrocarbon production operations, including the testing of low-dosage hydrate inhibitors (LDHIs). Here, we present the results of experimental studies of simple propane (C 3 ), binary methane−propane (C 1 −C 3 ), and ternary methane−ethane−propane (C 1 −C 2 −C 3 ) hydrate systems, aimed at investigating phase behavior at higher subcoolings where significant water conversions to clathratesand associated gas fractionationmight be expected to result in the formation of more than one hydrate structure. Measurements were made by means of established, reliable, isochoric equilibrium step-heating/cooling approaches. In the single guest propane system, pressure−temperature (PT) conditions follow the gas hydrate phase boundary, in agreement with the univariant hydrate + gas + water (H + G + W) equilibrium. By contrast, binary and ternary mixes clearly show PT behavior consistent with the growth of at least two and four hydrate phases, respectively, with these forming sequentially on cooling, and dissociating in the corresponding reverse order upon heating. In-house hydrate model (HydraFLASH) predictions support experimental observations, pointing to processes being gas fractionation driven; the mixed C 3 −C 1 or C 3 −C 2 −C 1 s-II type hydrates formed first close to the phase boundary preferentially incorporating the most strongly clathrate cage (5 12 6 4 ) stabilizing propane, making the remaining gas leaner. Once C 3 is largely consumed, trends agree with less stable C 2 −C 1 structures (s-II followed by s-I) then growing (where ethane is present), with this, like C 3 uptake, making remaining free gas increasingly of almost pure C 1 . This ultimately drives systems toward the final simple, single guest C 1 s-I clathrate formation as the well-established univariant phase boundary for this structure is reached. As observed phase behavior arises because of gas fractionation, it can be expected to occur during hydrate formation in all mixed gas systems of relatively fixed composition (at constant volume or pressure). The findings highlight that care should be taken in the application of isochoric methods to multicomponent gas systems, given the potential for numerous (rather than just one) clear changes in heating curve slopes as different hydrate structures form/dissociate. In addition, results show that extending equilibrium steps well into the hydrate region can reveal important information on clathrate phase behavior with potentially significant implications for issues such as gas production from hydrates in sediments, hydrate technologies for gas capture/separation/storage in the energy industry, and flow assuranc...

Natural Pectin and Commercial Luvicap-Bio as Green Kinetic Hydrate Inhibitors: A Comparative Evaluation by Crystal Growth Inhibition Methods

“…While NGs are normally assumed to just form a single structure-II hydrate, various hydrate phases of different structures and guest gas composition may actually form, including C 2–4 stabilized s-II, mixed C 1 /C 2 rich s-II/s-I, and (nearly) pure C 1 (s-I and potential s-II) at higher subcoolings. , Therefore, at higher Δ T sub , in addition to the most stable s-II natural gas hydrate, other hydrate structures are likely to form, which could play a role in KHI performance, e.g., due to the variable ability of polymers to inhibit these.…”

“…17,36,55−57 Although methane is generally known as an s-I former, s-II methane hydrate can be formed as a second phase (stable or potentially metastable) depending on pressure and temperature conditions. Although methane molecules can theoretically enter both cavities of s-II (5 12 and 5 12 6 4 ), its molecular diameter has historically been considered too small to stabilize the larger 5 12 6 4 cages. In contrast, methane molecules better stabilize the smaller 5 12 6 2 large cavities of s-I.…”

“…Pure methane (99.995%) and a multicomponent natural gasboth supplied by BOCwere used to assess bio KHI performance. In the methane system, both s-I (most stable) and s-II may form, while a number of different hydrate structures can exist simultaneously in the multicompetent natural gas system, rather than just simple s-II hydrates. The specification of the chemical used in this work is listed in Table .…”

“…Water moleculeseither in the form of ice, liquid, or vaporand suitably sized guest molecules (e.g., methane/C 1 , ethane/C 2 , propane/C 3 , and CO 2 ) can form clathrate hydrates under suitable PT (pressure–temperature) conditions, generally at lower temperatures and higher pressures . Depending on the size of guest molecules and PT conditions, different hydrate structures form, including cubic s-I, s-II, and hexagonal s-H. − In multicomponent gas systems, various hydrate structures can coexist simultaneously depending on the gas composition and pressure/temperature conditions , and several hydrates with different compositions and structures can form at different subcooling temperatures…”

Heterogeneous and Coexisting Hydrate Phases─Formation under Natural and Laboratory Conditions

“…If there are no longer sufficient amounts of higher hydrocarbons for the formation of a structure II or structure H hydrate available in the feed gas phase, this may lead to the formation of a structure I CH 4 hydrate as a coexisting gas hydrate phase. This hypothesis of chemical fractionation of the feed gas phase was suggested as a possible explanation for the observation of the coexistence of hydrate phases in nature and laboratory experiments. ,, In contrast, the experimental work of Pan and Schicks showed that the depletion of the gas phase in structure-II-forming components does not necessarily result in the formation of a coexisting structure I hydrate phase . They were able to demonstrate in laboratory experiments that, even in a closed system with a limited amount of a complex feed gas mixture, a heterogeneous hydrate phase initially formed, which changed its composition over time.…”