Oil and Water Flow Rate Logging in Horizontal Wells Using Chemical Markers and a Pulsed-Neutron Tool
Authors: Roscoe Bradley A. , Lenn Chris 
Published: December 2007

Oil and Water Flow Rate Logging in Horizontal Wells Using Chemical Markers and a Pulsed-Neutron Tool.

A new approach to obtaining oil and water flow rates in producing horizontal wells has been developed using a pulsed-neutron tool. This approach uses separate measurements of oil and water velocities in combination with separate holdup measurements to obtain the flow rates.

The velocity measurement uses both water-soluble and oil-soluble chemical markers, each of which is insoluble in the other fluid phase for the measurement. 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 only in one phase, the velocity of each phase can be measured separately. This measurement has been made under both laboratory and field conditions 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.

The holdup measurement is performed using the inelastic data from a pulsed-neutron tool. With these data, it is possible to quantitatively obtain the holdup of all three phases by combining information from the inelastic near/far ratio with the near and far carbon/oxygen ratio. This approach to the holdup measurement has been demonstrated using a combination of laboratory data, Monte Carlo modeling, and field data. The results of this study have demonstrated that the RMS accuracy of this measurement is about 6% on each of the three phases.

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 can not 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 the second approach dealing with the measurement of individual oil and water velocities and their holdups.

Oil and Water Velocity Measurement

Several existing 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 allows 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 in 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 giving a measure of the water velocity. Unfortunately, this method does not address the oil velocity measurement.

P. 583