Factors Affecting Bubble-Rise Velocity Of Gas Kicks

Abstract

An experimental study determining important factors affecting the velocity of large bubbles rising through the drilling fluid in the annulus found that bubble-rise velocity is greatly affected by the inside and outside diameters of the annulus. Previous bubble-velocity measurements were made only through extended liquids and liquids in circular tubes. The experiment, using laboratory models and a 6,000-ft well, also determined other parameters affecting bubble velocity. Introduction As the petroleum industry moves farther offshore to explore for additional reserves, the cost of drilling rapidly increases. In many areas, the total operational cost of an offshore rig is as much as $80,000/D. This extremely high cost, as well as the problem of offshore pollution, has underlined the necessity for good well-control procedures. procedures. A blowout occurs when the hydrostatic pressure exerted by the mud column is less than the formation pore pressure, thus allowing an influx of formation fluid into pressure, thus allowing an influx of formation fluid into the wellbore. The influx of gas into the wellbore is commonly called a "gas kick." Normally, a gas kick is removed by circulating the well using an adjustable surface choke. The choke setting must be varied so that the bottom-hole pressure is held constant at a value slightly above the formation pressure. The pressure recorded at the surface choke plotted against the cumulative amount of fluid circulated is an annular backpressure curve. A typical backpressure curve is shown in Fig. 1. The maximum pressure encountered often occurs when the gas reaches the surface. Alternative well-control procedures and equipment for unusual conditions are evaluated often during the wellplanning phase using computed annular pressure profiles. Also, some operators have on-site computers capable of performing the backpressure-profile calculation after a performing the backpressure-profile calculation after a kick is taken, but before circulation of the well begins. In some cases, when the anticipated maximum pressure is too great, the operator may want to deviate from the normal operating procedure. Thus, determining accurate backpressure curves can be quite important. Records and Everett, Schurmann and Bell, Moore, LeBlanc and Lewis, and Rehm have presented procedures for predicting annular backpressure curves. In all these procedures, predicting the annular backpressure curve is accomplished by assuming that the gas remains as a continuous slug occupying the whole cross-sectional area of the annulus and that the gas moves at the same velocity as the mud. The first computerized mathematical model of a gas kick was developed by LeBlanc and Lewis. In addition to a sensitivity study of several kick parameters, they compared the computed annular backpressure profile with an observed well-control operation. As seen in Fig. 1, LeBlanc and Lewis found that there was considerable disagreement between the model annular backpressure profile and the surface casing pressure profile recorded profile and the surface casing pressure profile recorded for the well-control operation." They attributed the disagreement to gas-slip velocity and friction losses in the annulus. Records later improved the LeBlanc and Lewis model to include the annular friction losses. The purpose of this study was to determine experimentally the factors affecting the bubble-rise velocity of a gas kick in an annulus. A knowledge of the mechanisms involved in the rise of gas bubbles through drilling fluids in the annulus or in the open hole is required before a gas-slip correlation can be included in a mathematical backpressure model. JPT P. 571