A Study on Gas Migration in Stagnant Non-Newtonian Fluids
Authors: Santos Otto L.A. , Azar J.J. 
Published: December 2007

A comprehensive experimental work has been conducted to investigate the gas migration phenomenon in stagnant non-Newtonian fluids inside inclined annuli. This condition is of interest in oil well control operations in which the wellbore should remain closed and with no circulation. Important features of an experimental apparatus specially designed and built for this research are presented. Experimental data collected from the apparatus are presented and discussed. A semi-empirical correlation for estimating gas migration rates is proposed. A comparison between correlation predictions and experimental results is presented. Also presented is a numerical example of the application of the proposed correlation.

Introduction

When drilling for oil or natural gas, a situation called blowout should be prevented. They may result in casualties, heavy losses of drilling equipment and hydrocarbon reserves, and pollution to the environment. Blowouts happen when the rig crew fails to control a kick. A kick is defined as the gas flow from the formation into the wellbore It can occur when the hydrostatic pressure of the drilling fluid in the well becomes less than that of the gas inside a permeable formation. Thus, to minimize the risk of blowouts, kicks should be avoided, consequently, the rig crew should be trained to prevent a kick. However, if it occurs, the drilling personnel should detect it as early as possible, close the well promptly to stop the gas flow and circulate the produced gas out of the well. This circulation consists in injecting the drilling fluid through the drill string to displace the gas to the surface through an armulus formed by the wellbore and the drill string.

The prediction of the pressure behavior before and during this gas removal process is needed for a proper design and execution of a well control operation. An adequate anticipation of this behavior requires a good understanding of gas migration and two-phase flow processes. To attain a better comprehension of these phenomena. The University of Tulsa has conducted a comprehensive study on these topics. Its first phase comprised the investigation of the gas migration phenomenon in closed wells where the drilling fluid cannot be circulated. In these situations, the gas migrates upward through the stagnant drilling fluid. This paper focuses these situations. Thus, its main objective is to present and discuss experimental data of gas migration through stagnant non-Newtonian fluids in vertical, inclined and horizontal annuli.

The importance of this present work is evident in well control situations where the drilling fluid does not circulate. These cases include the shut-in period after the well closure following a kick event or during the implementation of the volumetric well control method. Also, the gas migration rate in static conditions is an important parameter for the estimation of the gas in-situ velocity when the circulation is present.

The experimental data shown in the paper have been collected during several tests conducted in a flow loop specially built for this work. The important features of this apparatus and the experimental procedure used in the tests are disclosed. Although the flow loop can generate any size or kind of bubble, this paper only considers large bubbles similar to Taylor bubbles that occur in pipes.

The variables considered in the experimental tests were the bubble size, the inclination angle of the flow loop, and mud systems that include bentonite muds, polymer muds and fresh water.