Un-planned Shut-in and Deepwater Gas Hydrate Prevention

Abstract

Abstract The main objectives of this paper are to (i) investigate key factors affecting gas hydrate formation and (ii) recommend methods for preventing gas hydrate in deepwater flowing and shut-in gas wells. In this paper, key factors affecting gas hydrate formation in deepwater gas wells are discussed. Critical times when gas and water may contact each other at temperatures below the gas hydrate temperature inside the tubing during flow and shut-in periods are examined. For flowing wells, since gas hydrate formation can easily be detected by monitoring the gas rate, pressure and temperature at the wellhead, gas hydrate prevention can be planned and implemented properly if there is a need. For shut-in wells, since there is no real time data available to determine if gas hydrate formation is taking place, preparations for gas hydrate prevention should be made available at all times, especially for wells which may encounter unplanned shut-ins during the operations. In order to assist in illustrating the need for gas hydrate prevention under different circumstances, examples using hypothetical data to represent different field or well cases are presented. Also included in this paper is an example which shows results from a gas hydrate study for a deepwater shadow gas well. Introduction Gas hydrate prevention is one of the big challenges[1-6] when developing a deepwater gas field or testing a deepwater gas well in a cold seabed environment. For a deepwater gas well, when gas flows at a high rate during normal operations, no gas hydrate will form because of the warm reservoir fluid flowing through the wellbore. However, during the well start-up, restart, shut-in, or when a well is flowing at low rate, gas hydrate may form due to the low wellbore temperature if both gas and water exist in the wellbore. As the formation of gas hydrate depends on wellbore conditions and fluids inside the wellbore, it is important to understand the wellbore fluid behavior and heat transfer among fluids and all mediums inside and outside the wellbore such that the possibilities for gas hydrate formation can be predicted and its prevention can be planned. Effect of Water on Gas Hydrate Formation As gas hydrate is mainly caused by the existence of water inside the wellbore, prior to discussing the gas hydrate formation, possible water sources which may contribute to water production while producing a gas well should be identified:Water from water zone:When a gas well is being produced, water may come directly from the perforated zone that contains water or indirectly from the water zone through channeling behind casing due to poor cement bonding.Condensed water from natural gas:Unless the produced gas from the reservoir is completely dry, it is inevitable that some water vapor in the natural gas will condense out when the reservoir gas flows through tubing with temperatures that have declined from their initial values.