Oil and Water Velocity Logging in Horizontal Wells Using Chemical Markers
Authors: Roscoe Bradley A. , Lenn Chris 
Published: December 2007

Abstract
A new method of measuring both the oil and/or water velocities in producing horizontal wells has been developed. This approach uses both water-soluble and oil-soluble chemical markers, each of which is insoluble in the other fluid phase. The markers are injected into the borehole by a logging tool at one location and detected by a pulsed-neutron tool at a second location. The transit time between injection and detection of the marker gives a measurement of the fluid velocity. Since the markers are soluble in only one phase, the velocity of each phase can be measured separately. This measurement has been made under laboratory conditions (flow loop) to measure velocities from 10 to 500 ft/min at horizontal and several degrees deviation from horizontal. The results of these tests show good linearity and repeatability of the measurement.
An experimental downhole logging tool has been built in order to perform the oil and water velocity measurements in real wells. This tool has logged several horizontal wells in combination with other production logging tools. Logs from several of these wells will be shown to validate the measurement.

Introduction
As horizontal wells have become more prevalent, the ability to reliably evaluate the production performance of these wells has become increasingly important. Existing production logging techniques, such as spinners, that have been successfully used in vertical wells cannot always be applied to horizontal wells with full confidence due to the segregated flow in the borehole. For this reason, new techniques must be developed to evaluate oil and water flow rates in horizontal wells.
To determine the flow rates of the oil and water phases in a horizontal well, one must either 1) measure the individual oil and water flow rates directly, or 2) measure the individual oil and water velocities in addition to their holdups. (It should be noted, that for most production logging applications in horizontal wells, measuring only the holdup or only the velocity of the production fluids is usually insufficient to determine the source of production problems.) This paper will address part of the second approach, the measurement of individual oil and water velocities. Once determined, these velocities can be combined with holdup information, obtained from several possible approaches to obtain oil and water flow rates.

Background
Several currently available technologies make it possible to measure water velocity in horizontal wells. The oldest of these uses a radioactive tracer such as Iodine-131 with an 8-day half-life. The iodine is placed in a water-soluble form. This material is injected into the borehole and then measured as it passes a gamma-ray detector. The time between injection and detection enables the calculation of the flow velocity of the water. This method can also be applied with some success to oil velocity measurements by placing the iodine into an oil-soluble form. The limitation of this approach is that the oil-soluble form is usually an emulsion that can exhibit some unique problems due to the nature of emulsions.
With the increased restrictions and risks associated with the use of radioactive tracers in the borehole, it is desirable to have a method of performing these velocity measurements without using radioactive tracers. This is one of the reasons that the WFL Water Flow Log was developed. This approach relies on the activation of oxygen in the water using a 14-MeV neutron generator and measures the transit time of the activated oxygen in the borehole, thus giving a measure of the water velocity. This technique has been successfully applied to flow in the borehole and behind casing or tubing. Unfortunately, this method does not address the oil velocity measurement.
A new approach has been developed that is capable of independently measuring the oil and water velocities in horizontal wells without the use of radioactive tracers. The PVL Phase Velocity Log uses a chemical marker for its measurement and is therefore a much safer approach to these velocity measurements. P. 943