Gas Holdup Imaging Identifies Complex Gas-Liquid Flow Regimes and Introduces a New Velocity Measurement in a Long, Cased Hole, Horizontal, Production Well

Abstract

Gas holdup probes run in a producing horizontal well on the Norwegian continental shelf are used to identify downhole flow regimes and compute the velocity of gas in intermittent flow. The observed flow regimes are compared with the predictions of a number of gas-liquid flow correlations. The theory and quality of velocity correlations are discussed. Introduction A number of horizontal gas-liquid flow models exist and are routinely used to predict the down hole flow regime that will be encountered under normal production and logging operations. As these models tend to be derived from, or validated by, surface observations at relatively low pressures and low gas densities it would be reassuring to test or validate these models under real down-hole conditions. The identification of correct flow regime is important for the production log interpretation, as it dictates which slip model to use. In particular the ability to distinguish dispersed bubble flow from the other flow regimes is important as slip effects are believed to be negligible in dispersed bubble flow. Similarly the stratified and intermittent flow regimes may have quite different slip velocities. Traditional global measurements of nuclear density or a single dielectric sensor cannot directly see the downhole flow regime. Only the recent introduction of distributed gas holdup probes in production logging toolstrings permits the downhole flow regime to be imaged and determined. This technique was applied to a North Sea well producing saturated oil and gas from a 1400 m horizontal. The well is completed with a 7" cemented and perforated liner. In this example, due to the happy conjunction of an undulating well path and a steadily increasing flow rate, the production logging sensors can see stratified flow, intermittent flow, and bubble flow. This, almost textbook, collection of flow regimes is tested against a number of models to see how well the models agree with this particular reality. As the local probes are not only distributed radially around the borehole, but also separated axially, the delay between holdup disturbances (in intermittent flow regimes) as they travel from one tool to another can be converted into a velocity. This has been performed and a gas velocity computed. Gas Local Holdup Probes In gas local holdup probes, fiber optic principles are adapted to measure gas and liquid downhole. Light is injected into a sapphire probe and a photodiode measures the reflected signal. The optical contrast in refractive index between gas and liquid varies the reflected signal and allows a local holdup and bubble rate to be measured. The bubble rate is defined as the number of transitions between liquid and gas recorded per second by a single probe. As the sensing tip of the probe is only 0.1mm in diameter the probes provide a quantitative measurement in all but the highest velocity gas wells (where bubble size can approach the dimensions of the sensing tip).