Well-scale experimental and numerical modeling studies of gas bullheading using fiber-optic DAS and DTS

Abstract

Bullheading is an effective well control technique to pump produced fluids back into the formation during drilling and completion operations to regain well control. However, existing bullheading procedures are often based on a trial-and-error approach due to the complexity of modeling counter-current gas flow mechanisms in well control simulators and the lack of empirical bullheading datasets in full-scale conditions. To address this gap, a 5163-ft-deep experimental wellbore was utilized to conduct bullheading tests using nitrogen and water to simulate a “gas below BOP” scenario. We present a novel application of fiber-optic distributed acoustic sensor (DAS) and distributed temperature sensor (DTS) in facilitating the measurement and monitoring of gas slugs in the wellbore for an improved understanding and modeling of the gas bullheading processes.Full scale tests were performed by injecting a nitrogen slug into the wellbore annulus which was bullheaded with water while taking returns via tubing. A detailed analysis of the bullheading data for two different gas slug volumes and bullheading rates is presented. DTS and DAS data were analyzed using a variety of time- and frequency-domain signal processing techniques to estimate displacement velocities and changing gas slug lengths during injection, subsequent expansion, and displacement in the annulus. Numerical simulations were also performed for an improved understanding of the bullheading processes using a drift flux model (DFM)-based transient multiphase flow simulator modified for counter-current flow situations. Different gas slip models, used in the DFM, were benchmarked for the best match with the DAS and DTS measurements obtained from the bullheading experiments. The data from downhole pressure gauges were also compared to the DAS and DTS measurements as well as the results from numerical simulations and satisfactory agreements was obtained.This study demonstrates a novel application of DAS and DTS in obtaining high-resolution spatial-temporal measurements for the facilitation of improved understanding and modeling of well-scale bullheading hydrodynamic behaviors. The evaluation, selection, and calibration of transient multiphase flow sub-models used in this study improved the understanding of the counter-current flow in a well and provided references for applying the DFM in simulating bullheading operations.