Coiled tubing operation is popular in the development of oil and gas. However, during the operation in winter, especially at night, the temperature of gas and formation fluid drops while flowing to the surface along wellbore. Gas hydrates may be formed at a certain pressure, blocking flowback channel. Thus, the coiled tubing may be stuck, resulting in safety risks in severe cases. In this study, the causes, hazards, diagnosis methods and preventive measures of ice blockage were analyzed based on the causes of gas hydrates, as well as the field operations. Results show that, (1) ice blockage can be formed by factors including temperature and pressure, fluid saturation, sand particles or drill cuttings in wellbore, and sudden changes in flow conditions; it is most likely to occur at the positions like wellhead, blowout preventer, blowout preventer box, and lubricator; (2) ice blockage may lead to downhole tool sticking, stress corrosion, pipe string damage, well control equipment failure and so on; (3) ice blockage is common in two types of coiled tubing operation: one is the process that liquid is pumped into wellbore and circulated out of wellhead, such as drilling, grinding, and flushing, the other is the process that formation liquid and gas flow together to the surface along wellbore, such as velocity string operation; in operation, ice blockage can be diagnosed by observing the abnormal changes of technical parameters, so that effective measures can be taken in time; and (4) the prevention and control measures include physical methods such as steam heating, thermal insulation with industrial explosion-proof electric blanket or electric heating belt, and pumping hot water, and chemical methods such as adding chemical inhibitors (e.g. ethylene glycol, desulfurizer, and CaCl2); in practice, one or more of these measures is/are usually used, meanwhile, specific control techniques are proposed depending on coiled tubing operations. The above research results were applied on site. The abnormality was observed in 4 wells during the coiled tubing operation, diagnosed as ice blockage, accordingly, proper measures were taken for blockage removal. The successful application proves that the research is effective and can provide support for safe operation.
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