Problems in Hydrates: Mechanisms and Elimination Methods

Abstract

Abstract Unrestricted fluid flow of oil and gas streams is crucial to the petroleum industry. Unless preventative action is taken, gas hydrate plugs form under the high pressure, low temperature conditions inherent to offshore production. The oil and gas industry is facing increasing costs in inhibiting gas hydrate formation due to the development of offshore gas reservoirs. Recent international estimates of the cost of the conventional inhibitor, methanol, alone are in excess of $150 million/year. Gas hydrates are likely to form in subsea flowlines unless the water is removed down to the lowest dew point encountered, highly effective insulation is in place, or inhibitors are used. Since complete stripping of water from condensates and/or natural gas is prohibitively expensive, and effective insulation is beyond current economic limits, the most effective solution includes the use of hydrate inhibitors. This paper describes the state of the art of hydrate prevention, detailing hydrate structure, conditions and mechanisms of formation, and developing approaches - from the conventional to the cutting-edge - to hydrate inhibition. Its focus on low-dosage inhibitors, including a review of kinetic inhibitors and anti-agglomerants form, function, development, selection, modeling and applications, highlights gaps in current knowledge. Finally, a research agenda addressing both mitigation and deployment strategies is proposed. Introduction Since the 1930's when Hammerschmidt1 determined that the material plugging pipelines was gas hydrates, interest in gas hydrates has continued to increase. Hammerschmidt's discovery led to the regulation of the water content in natural gas pipelines2. In 1934, Hammerschmidt published a correlation summary of over one hundred hydrate formation data points. Unrestricted and problem-free flow of petroleum products during extraction, processing, and transportation is essential to the oil and gas industry. Whether heavy hydrocarbons such as crude oil, or low molecular weight hydrocarbons such as natural gas and natural gas liquids are the target end product, natural gas is almost always present in the fluid extracted during production. To varying degrees (most often low early in the life of a reservoir and high toward the end), the extracted oil and gas mixture also contains water. In the presence of water, and under a fixed range of pressure and temperature conditions, specific to each hydrocarbon mixture, hydrates of the light gases can form. Gas hydrates, which have a crystalline structure analogous to that of ice, form solid plugs and block the flow. Clearly, inhibition of hydrate formation is of utmost interest to industry. Hydrate formation is a substantial problem in deepwater production and underwater pipelines, which transport condensed phase hydrocarbons such as gas condensate or crude oil. In these situations, once plugs have formed, there are limited possibilities for removal2. Since the 1970's, the oil and gas industry has faced increasing costs associated with inhibition of gas hydrate formation, due to the development of offshore gas reservoirs. Recent international estimates of the cost of the conventional inhibitor, methanol, alone, are in excess of $150 million/year3. Gas hydrates are likely to form in subsea flowlines unless the water is removed down to the lowest dew point encountered, highly effective insulation is in place, or inhibitors are used. The first option is difficult when supersaturated condensates exist in the flowline even after the gas phase is stripped to saturation levels. Stripping condensate completely of water is prohibitively expensive and effective insulation is beyond current economic limits. Therefore, the most effective solution appears to be the use of inhibitors. Generically, there are two kinds of hydrate inhibitors: thermodynamic inhibitors, and the more recently identified low-dosage inhibitors. Thermodynamic inhibitors have been in use for a long time, and continue to be the industry standard. This kind of inhibitor works as an antifreeze by involving the water in a thermodynamically favourable relationship, so that it is not available for reaction with the gas.