Abstract

In this paper we investigate experimentally the effect of surfactants on the formation kinetics of methane hydrate. The compounds tested include: anionic, cationic and nonionic surfactants. Experimental results indicate that surfactants (at concentrations levels near their CMC) do not influence the thermodynamics, however, they have a strong influence on the kinetics of gas dissolution in the water phase as well as on the overall rate of hydrate formation. In addition, the formed hydrate particles in the presence of the various surfactants exhibit diverse agglomeration characteristics. This work is part of an overall effort to develop the technology required for the safe operation of offshore oil and gas transportation pipelines without inhibitors.

Introduction

Undersea oil and gas transportation pipelines in cold waters are often under thermodynamically suitable conditions for the formation of gas hydrates. Traditionally, the industry has designed pipelines and processing facilities to operate outside the hydrate formation pressure-temperature region. This is accomplished by the addition of inhibitors or by the dehydration of the gas/condensate fluids. However, this approach may be prohibitively expensive for offshore pipelines. We believe that a more economical approach is to allow the hydrates to form, and consider their transportation in the form of a slurry, where the formed particles neither deposit nor agglomerate. This paper addresses this problem by investigating experimentally the effect of surfactants on the formation kinetics as well as the physical characteristics of methane hydrate particles formed in the presence of surface active compounds.

Surfactants are substances that when they present even at very concentrations, have the ability to significantly alter the surface or interfacial free energies. Therefore, surfactants should have a direct effect on the hydrate formation process. The formed nuclei in the presence of surfactants should have a different critical radius and hence, a higher surface area per unit volume of the system. The later may have a significant effect on the global formation rate under certain regimes of operation.

Surfactants form colloidal-size clusters in solutions called micelles. The critical micelle concentration (CMC) is the minimum concentration of the dissolved surfactant above which the clusters (micelles) begin to appear in the solution. At CMC an abrupt change occurs in almost every physical property that depends on the size or number of particles in solution. This behavior is true for all types of surfactants - nonionic, anionic, cationic and zwitterionic - in aqueous solutions. The maximum reduction in surface tension occurs at CMC. As a result we decided to examine the effect of surfactants on hydrate formation kinetics when they are present near their CMC levels.

The compounds tested include: anionic and nonionic surfactants. Freezing point depression experiments with thirteen surfactants showed that they do not have any measurable influence at concentrations near their CMC.

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