Features and prevention of gas hydrate blockage in test strings of deep-water gas wells

Abstract

Due to the uncertainties in formation mechanism of gas hydrate blockages in strings during the test of deep-water gas wells, inhibitors are either excessively consumed or inefficiently used when conventional prevention techniques are used. In this paper, a study was conducted on multi-phase flows, in terms of hydrate formation kinetics and hydrate particle migration and settlement kinetics. In this process, a model for quantitative prediction of hydrate blockage was built to predict when and where the blockage occurs in the strings and evaluate the severity of such blockage, in order to define the high-risk zones. Eventually, an innovative hydrate blockage prevention technique based on hydrate blockage free window (HBFW) was proposed to determine the optimal concentration and the flow rate of inhibitors. The study results are in the following four aspects. First, gas hydrates generated in the wellbore may deposit on the internal walls of strings. With the increase in thicknesses of such gas hydrate layers, the diameter of a pipe string decreases. Accumulation of gas hydrates generated around liquid film on pipe walls is the key contributor to the blockage in strings. Second, as the water depth increases or the gas production reduces, the HBFW turns to be narrower for production safety, and the time to initiate blockage is shorter. Third, application of hydrate inhibitors can effectively delay the occurrence of blockage and expand the window of safe production. Fourth, the innovative prevention technique can effectively reduce the volume and the flow rate of inhibitors (by 50% in the case study). The innovative technique effectively eliminates the problems related to the excessive consumption of inhibitors in the conventional methods and provides a valuable reference for the prevention of gas hydrates formation in deep-water gas well tests.