Delta Flow: An Accurate, Reliable System for Detecting Kicks and Loss of Circulation During Drilling

Abstract

Summary A system to monitor drilling-fluid flow rate has been developed that detects kicks and lost returns in floating, fixedplatform, and land-based drilling operations. The system uses flowmeters that monitor the flow rates of drilling fluids entering the borehole through the standpipe and leaving the well through the return flowline. These readings are processed in a computer-based, data-acquisition system to form a filtered delta-flow signal that identifies the occurrence of downhole fluid gains or losses. The system is designed to trip an alarm when a gain or loss exceeds 25 gal/min [1.6 dm3/s]. even in a floating drilling environment. This sensitivity will generally keep gains or losses to less than 5 bbl [0.8 m3]. Introduction In the mid-1970's, we embarked on a major research effort to develop the technology necessary to drill in water depths up to 6,000 ft [1,830 m]. One area of concern was well monitoring and control. Because early detection is the key to keeping an influx into the wellbore as small as possible, previous investigators undertook a study to determine which of the parameters associated with the drilling process could be measured by surface or seafloor in-strumentation to provide the most sensitive indication of change downhole. These previous investigations indicated that the output flow rate was the parameter best suited to detect abnormal well conditions in the shortest possible time. The approach implemented was the delta-flow system (Fig. 1), which uses magnetic flowmeters as sensors in the pump output line and in the return flowline. Magnetic flowmeters were selected because of their ability to monitor reliably and accurately the flow of a dirty, cuttings-laden, variable-viscosity fluid (e.g., drilling mud), provided that the fluid is conductive. As such. the monitoring system is designed to be used with water-based mud systems. The delta-flow approach was selected so. that changes in the steady-state input flow rate would not require the continual resetting of the alarm levels that is currently required with sensors that monitor only the return flow rate. This system was extensively tested during the drilling of three land wells. It was found to be very reliable and had the sensitivity desired for detection of abnormal well conditions. Sensitivities of 25 gal/min [ 1.6 dm3/s] were maintained throughout the land-based tests, with false alarms occurring only when large changes were made in the base flow rate, such as when connections were made and the pumps were turned on or off. Offshore exploration wells, however, are frequently drilled from a floating vessel, and the stroking action of the slip joint as the vessel heaves causes large fluctuations in the output flow rate. In floating drilling operations, the delta-flow signal was expected to vary with a magnitude many times greater than the +25-gal/min [ 1.6-dm3/s] alarm sensitivity used during the land-based tests. Therefore, the purpose of the final phase of this project, reported here, was to adapt the delta-flow system to the floating environment while the sensitivity achieved during the land tests was maintained.