Abstract

The application of modern carbon/oxygen (C/O) technology for quantitative saturation monitoring in barefoot completions has been limited so far. To improve understanding and test the feasibility of this application in such completions, this paper presents results from simultaneous field testing of the latest generation carbon/oxygen (C/O) technology in two barefoot completed wells. The example wells show the comparison of C/O saturation results from three service companies against the results from resistivity and pulsed neutron capture data. It also discusses the improvements made in barefoot completions to C/O tool characterization and data acquisition parameters. Promising and quantitatively robust saturation predictions were observed from C/O flowing data whereas shut in data was greatly affected by borehole fluid re-invasion effects. Finally, as an outcome of the field test results, an additional example is given which shows the typical added value of salinity-independent C/O measurements in a mixed-salinity environment.

Introduction

In the giant oil fields of Saudi Arabia, a regular assessment of reservoir sweep efficiency by means of accurate measurements of remaining oil saturation is crucial. The application of different saturation monitoring technologies is greatly dependent on formation water salinity. Varying salinity results from an active waterflood program using fresh injection fluids from non-reservoir sources. Generally, if formation water salinity is high and known, saturation monitoring is carried out with pulsed neutron capture tools or resistivity devices which both rely on knowledge of formation water salinity. In case of fresh or unknown water salinity, these methods become unreliable and pulsed neutron spectroscopy (C/O) logging is most commonly used. Since the amount of oxygen in water is essentially independent of salinity, the method has a distinct advantage in mixed-salinity environments.

Traditionally, C/O technology was developed for single-cased hole completions but subsequently applied and characterized in even more complicated completion designs with gravel packs, blast joints or double/multiple strings 1–3. A majority of wells in Saudi Arabia are completed barefoot. Although this reduces the uncertainty of carbon-oxygen saturation predictions, since only borehole size, fluid type, and tool standoff need to be considered, limited application experience and tool characterization exists for this type of completion.

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