Fiber-Optic DAS and DTS for monitoring riser gas migration

Free gas in a marine drilling riser presents a hazardous situation as the gas can quickly expand to produce dangerous gas volumes at the surface. However, the conventional gas kick detection methods, that rely on surface measurements and data from point sensors or gauges, are often inadequate to predict the dynamic behavior of a given amount of gas entering the riser. This study presents comprehensive results from well-scale experiments that demonstrate novel insights into the real-time gas rise behavior across a 5163-ft-deep wellbore using distributed fiber-optic sensors. The experimental well simulates an offshore marine riser-like scenario with its larger than average annular space and fluid circulation capability at high pressures and rates. Thus, the experimental and numerical model results in this study provide useful insights on gas rise dynamics in a large annular space along long intervals, which are relevant for studying gas in marine risers.

Distributed acoustic sensor (DAS) and distributed temperature sensor (DTS) results from eight sets of well-scale tests are presented to investigate the effect of gas kick volumes (from 2 bbl to 15 bbl), circulation rates (from 0 to 200 GPM), and gas injection methods (through tubing or a ½-in. capillary injection line), on gas rise dynamics in the wellbore. Since slow-moving gas bubbles create small vibration and temperature effects, a variety of time- and frequency-domain signal processing techniques are developed to analyze the Fiber data were processed using frequency band energy (FBE), time-frequency scalograms, energy spectrums, frequency-wavenumber (FK) transform, and signal-to-noise ratio analysis. Gas velocities measured independently from DAS and DTS were validated using a numerical model, as well as with downhole pressure gauge data analysis, demonstrating good agreement for all eight trials. The numerical model presented in this study was validated with the downhole gauges and presents many useful insights for gas-in-riser conditions, such as gas arrival at the surface and rate of pressure build-up in closed wells.