Field-Test Results of a Three-Phase Holdup Measurement in Horizontal Wells Using a Pulsed-Neutron Source
Authors: Roscoe Bradley A. 
Published: December 2007

A new method of obtaining three-phase holdup (oil/water/gas) information in horizontal wells has been field tested. The method uses the data available from a pulsed-neutron tool. This measurement uses the inelastic near/far countrate ratio in addition to the near and far carbon/oxygen ratios. The inelastic near/far countrate ratio response is dominated by gas in the borehole, while the near and far carbon/oxygen ratios are used to separate the water and oil contributions of the borehole and formation. By using a linear programming approach, this information is combined to produce quantitative three-phase holdup answers. Calibration of the measurement is achieved by the combined use of laboratory measurements and Monte Carlo modeling predictions.

In order to obtain independent verification of the three-phase holdup answer, a local probe tool was run in combination with the pulsed-neutron tool. The local probe tool is sensitive to two phases (water and hydrocarbon) and hence can be used for a partial verification of results. Field-test results show good agreement between the two independent measurements under various conditions of gas, oil, and water holdup in the horizontal borehole.

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.

The TPHL Three-Phase Holdup Log method described in this paper is a way of obtaining three-phase holdup (oil/water/gas) in horizontal wells using the data available from a pulsed-neutron tool. For this measurement, the RST Reservoir Saturation Tool is used since it has several measurements which are differentially sensitive to the various fluids in the borehole. This tool was originally designed to measure the oil saturation of the formation without depending on formation water salinity. This was accomplished by using a carbon/oxygen (C/O) measurement. A large part of converting a C/O measurement into oil saturation is accounting for the effect of the borehole on the measurement. To properly determine the formation oil saturation, the borehole composition must be known reasonably well. In general, a 5% error on the borehole composition can cause a 15-s.u. error in the formation oil saturation. For this reason, the RST tool was designed with two detectors to compensate for the borehole effect.

To determine the three-phase holdup in a well, the RST tool uses the inelastic near/far countrate ratio in addition to the near and far carbon/oxygen ratios. The inelastic near/far countrate ratio response is dominated by gas in the borehole, while the near and far carbon/oxygen ratios are used to separate the water and oil contributions of the borehole and formation. When this information is properly combined, a quantitative measurement of the three-phase holdup can be obtained.

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