A Production Logging Measurement of Distributed Local Phase Holdup

Abstract

Abstract This paper describes early UKCS experience of a new production logging sensor, the Digital Entry and Fluid Imager, or DEFT tool. Deviated and horizontal wells can cause complex flow regimes to arise that render interpretation of conventional production logging sensors difficult or sometimes impossible. The DEFT tool is a new type of production logging tool which facilitates the understanding of the nature of wellbore flow and improves confidence in interpretation. The tool makes four independent measurements of the phase holdup, distributed in differing quadrants of the pipe cross section. Results to date show that the DEFT tool gives a direct count of the dispersed phase bubbles. Each probe can give an indication of the flow type, the absolute velocity of the dispersed phase, and the bubble flow direction. As the four measurements are distributed at different locations within the pipe cross section, an image of pipe flow may be interpreted. Early results also show how the DEFT tool can be used to define the location of oil entries and to pin-point circulatory flow. The paper briefly describes the tool principles. Interpretation is illustrated with example logs made during its field introduction. Introduction Production logging is principally concerned with the measurement of the well-bore inflow of each flowing fluid phase. A family of sensors are commonly used by the industry to monitor and diagnose producing and injector wells. The principal sensors are spinners to measure fluid velocity, and differential pressure sensors to obtain phase holdup. Spinner flowmeters respond to the fluids within which they are immersed. When two phases move at the same velocity, interpretation is straightforward. When however mixing is poor and fluid phase segregation occurs, it is not clear to what degree the spinner responds to the movement of either phase. The spinner may indicate downflow in situations that are clearly impossible. Scrimgeour identified one aspect of this problem relatively early in the life of the North Sea producing fields. Hill has studied the behaviour of production logging tools in multiphase flow. For an authoritative treatment of the subject the reader is referred to his work. It may therefore be concluded that phase segregation renders conventional production logs difficult to interpret. This is often associated with low rates, deviated wells and large pipe sizes. With the increasing use of horizontal and extended reach wells this problem can be expected to become more common. Differential pressure sensors are not suitable for use in highly deviated wells. As the well deviation increases, the vertical separation between the sensor measure points is reduced and the measurement looses resolution. As fluid flows into the well, holdups change, fluid velocities increase and the flow regime present within the producing section of the well may change. A well may be in laminar, single phase flow at the lowermost producing horizon, which may become bubbly flow, slug, churn or annular mist flow dependant upon the nature of the inflow. The flow may therefore exist in several different regimes within the completion Although the flow regime has an important influence upon spinner and differential pressure sensors, the flow regime cannot often be determined by direct measurement. A normal production log interpretation procedure is to determine flow regime by comparison of the measured pressure gradient with a theoretical prediction (3). This procedure includes uncertainties as the friction pressure losses are proportional to the continuous phase fluid velocity and pipe wall roughness. Both these parameters are not necessarily known with any precision. P. 183