Three-Phase Holdup Determination in Horizontal Wells Using a Pulsed-Neutron Source
Authors: Roscoe Bradley A. 
Published: December 2007

Abstract
In horizontal wells, it can be very difficult to interpret conventional production logging tools due to the fluid segregation in the borehole. This is a even more of a problem when there are more than two phases present in the borehole, i.e., oil, water, and gas. A pulsed-neutron tool measures many parameters which are differentially sensitive to all three possible borehole phases. Therefore, it is possible to combine the information available from a pulsed-neutron tool to determine the 3-phase holdup in horizontal wells.
One of the major difficulties in evaluating the response of a tool to 3-phase holdup is obtaining good data under realistic downhole conditions, i.e., realistic gas densities. Laboratory measurements cannot readily be made under these conditions; therefore, modeling techniques must be used to evaluate and characterize tool response. To validate a computer model, laboratory data are needed for benchmarking; therefore, for this study, over 400 laboratory formation measurements were performed using air to simulate gas. These formation conditions were also modeled using Monte Carlo techniques. The agreement between measured and modeled data proved to be good enough that modeling can be used to confidently predict the tool response with air or realistic gas.
Once the ability to predict tool response under realistic downhole conditions exists, it is possible to combine information from a pulsed-neutron tool to quantitatively determine the holdup of all three phases. This is accomplished by combining the inelastic near/far ratio with the near and far carbon/oxygen (C/O) 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 cannot always be applied to horizontal wells with full confidence because of 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, water, and gas holdups. Once determined, these holdups can be combined with velocity information, obtained from several possible approaches to obtain oil and water flow rates.

Background
Pulsed-neutron tools have previously been used to qualitatively determine the 3-phase holdup in horizontal wells. This approach uses the borehole sigma and the inelastic near/far ratio for this determination. The method is considered qualitative since tool calibration information is not available for the ratio measurement or the sigma of the gas.
Recent work reported by Peeters et. al. has attempted to quantify the pulsed-neutron measurement for holdup in horizontal wells. Their approach utilizes three measurements from a single pulsed-neutron tool centered in the borehole: C/O windows ratio, borehole sigma, and capture near/far ratio. The measurements are combined through a linear response matrix to produce the desired holdup measurements. The coefficients for the matrix are determined by regression of modeled or measured tool responses to known conditions.
A more quantitative approach has been employed with the RST Reservoir Saturation Tool.
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