Formation of Hydrates in Shut-down Pipelines in Offshore Conditions.

Abstract

Abstract Operation of oil and gas pipelines in deep-sea is sharply complicated by the formation of gas hydrates. The experience indicates that large gas hydrate plugs in gas and oil pipelines form most actively during the period of unforeseen long shutins. In static conditions three types of hydrate crystals form: surface-contact films and massive hydrates which form by sorption of gas and water molecules on the surfaces of growing crystals; bulk diffusional whisker-like which form both in the volume of gas, and in the bulk of liquid water through sorption of molecules on the growing crystal surface and by tunnel sorption of molecules at the base of the crystal; gel-like soft crystals which form in the bulk of liquid water at a deficiency of dissolved gas in water. Under the P-T conditions of hydrate formation there may be a simultaneous formation of crystals and decomposition of other crystals. Equilibrium content of gas dissolved in water which is in contact with a solid hydrate surface is lower than in water which is in contact with free gas. Introduction Gas hydrate is a metastable mineral. Its formation, stable existence and decomposition depend on pressure, temperature, and the compositions of water and gas. Gas hydrates are inclusion compounds, solid solutions, in which water is the solvent. Molecules of water tied together by hydrogen bonds form a space tilling lattice. Mobile molecules of gases, or volatile liquids are absorbed, or incorporated in the cavities of the water lattice. Gas hydrates are widespread in nature. They easily form in technological systems of production, transportation and processing of gases, Natural gas hydrates are a tremendous source of energy. Resources of hydrocarbons accumulated on earth in a hydrate state are estimated as 1.5.1016 m'. Several countries have already started to implement intensively their programs for development of gas hydrate deposits. An intensive growth of oil and gas production is accompanied by putting the fields in sub-Arctic regions and deep offshore into operation. At that the thermodynamic characteristic of the technological systems of hydrocarbon production and transportation almost always corresponds to the conditions of hydrate formation. There is a knowledge about numerous cases of large hydrate plugs formation in wells and gas pipelines. Analysis of the facts about the formation of large hydrate plugs in wells and oil or gas pipelines reveals that the most dangerous periods of time are the unplanned long shut-ins. Expenses for the prevention of hydrate formation are 10- 15'%.of the production cost. According to Savidge (I), on an average gas or gas-condensate field annual expenses for hydrate prevention are 5 to 15 million US $. Expenses for removal of a complete hydrate plug in a well or a gas or oil pipeline offshore are usually several millions of US dollars.