Production Logging Low Flow Rate Wells with High Water Cut

Abstract

Abstract Production logging low flow rate wells is difficult because mechanical spinners have a small dynamic range in slow moving fluids. Low flow rates in horizontal wells means the fluid holdups in the stratified flow are very sensitive to the wellbore inclination, and the high water cut means a small proportion of the flowing liquid will be oil. At what point do these compounding affects limit the ability of current technology to measure low oil flows? This paper explores this question by analyzing production logs from wells with water cuts as high as 99%. The horizontal well examples show that stratified flow regimes, as expected from flow loop publications, vary hugely with changing inclination. The ability to measure low oil rates and small holdups in this stratified flow is determined by the physical distance of the production logging sensors from the top of the hole, the geometric size of the sensors, and the measurement resolution of the sensor at high water cut. These specifications are tested by the challenging logging objectives for the wells described in this paper. The logging data also illustrates other dynamic fluid behavior that can reveal oil and water entry; for example a burst of bubbles created by local turbulence. These practical examples from North Oman show what can be achieved in low flow rate wells with high water cut with today's technology. Included also is an explanation of why these logs were acquired. In some cases it was to diagnose non-productive intervals for planning a water shutoff, while in others it was to learn more about fault-water channeling and reservoir behavior. Introduction For over a decade operators have developed thin hydrocarbon accumulations with horizontal wells for maximum reservoir contact to enhance recovery. These wells have introduced new intervention challenges that have been addressed by the development of new technologies such as coiled tubing logging and wireline tractors. As these fields age, their water cuts have increased due to water encroachment mechanisms like coning, flow through natural and hydraulic fractures and short-circuiting between injectors and producers. The requirement to measure oil inflow in high water cut horizontal wells has resulted in an evolution in the nature and arrangement of production logging sensors. The objective of production logging is to measure the hold-up and velocity of each phase along the horizontal drain, which is especially challenging when the water cut is high. From these measurements and the total flow area the oil and water inflow profiles can be calculated. Density measuring gradiomanometers and full bore spinners are inadequate for characterizing separate phases in stratified flow. One approach used is a bulk tracer or radioactive marker technique that is only sensitive to one of the phases, another is multiple phase-sensitive sensors; each with a high geometric precision and a geometric arrangement that allows at least one probe to identify the presence of the lightest phase. This paper describes what has been achieved with logging tools and interpretation techniques that utilize the latter approach.