Abstract

This paper details a program to test saturation logging in a Middle East carbonate reservoir that recently began a secondary recovery project. The ability to measure oil and water saturations in the target flood zones is critical for optimizing recovery and finding unswept zones. Water saturation is typically determined using a pulsed neutron capture logging tool (PNC). The PNC differentiates between the neutron capture cross-section of high salinity water and hydrocarbon. In the case of this waterflood, seawater is injected, with a salinity of 25,000 ppm. This seawater has a similar neutron capture cross-section to the oil in the formation, making PNC tools unsuitable for determining water saturation in break through zones.

A test program was developed to determine the reservoir fluid saturation using carbon-oxygen (C/O) logging techniques. The new generation through- tubing C/O tools with higher measurement resolution enables detection of low salinity seawater. The test program consisted of a log-off between three logging service companies providing C/O tools. The C/O derived saturations were compared to the open hole log saturations and to PNC saturations in two wells in the waterflood pilot area - a comparison that was possible since the injected fluid in the pilot was high salinity formation water.

Most of the wells in the waterflood area are completed with dual tubing strings. The saturation logging target interval is generally in the short string of the dual completion, making wireline surveillance difficult. The test program also evaluated the ability of the contractors to log in a dual environment. This paper details the results of the C/O test logging program and provides recommendations on future saturation logging requirements for this type of reservoir.

Introduction

The Sabiriyah field was first brought on production in the 1950's and produced intermittently on primary recovery up through the Iraqi invasion of Kuwait. Many of the wells in the field were sabotaged and were redrilled or recompleted. The Mauddud carbonate is the shallowest horizon under development in the field. Beginning in late 2000, a secondary project was initiated in the Mauddud using inverted 9 spot patterns of approximately 1000 acres. Twelve patterns cover the crest of the field for the first phase of secondary development (Figure 1).

The Mauddud is a limestone reservoir with a gross thickness of about 365 feet. The reservoir is divided up into stratigraphic layers 1 through 11, with the main target of the waterflood consisting of layers 2 through 7. Porosities in the pay intervals vary from 8% to 30% with low (<10%) initial water saturation. Matrix permeability ranges from 10 to 80 mD, with high permeability streaks measuring 5 to 10 times matrix. Fractures and vuggy zones are distributed in thin layers in the Mauddud but primarily in the uppermost C zone. Virtually all of the water in the crestal area of the field is immobile. Formation water salinity is approximately 230,000 ppm.

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