Abstract

To determine formation fluid saturations, thermal decay tools rely on the existence of a sufficient contrast between the thermal neutron capture cross section of a hydrocarbon- bearing formation and that of a water-bearing formation. This contrast is dependent upon the formation's effective porosity (Φe) and upon the contrast between formation water cross section (∑W) and hydrocarbon cross section (∑HC).

Because of their low porosities which generally range from 3 to 14%, Western Canadian carbonate reefs present difficult logging environments for thermal decay tools. To obtain meaningful water saturations (∑W), accurate input of all parameters in the thermal decay saturation equation is necessary. Methods to obtain the most accurate input parameters include the use of porosity obtained from openhole logs and core data and, more recently, from through-casing acoustic logs. Accuracy is also enhanced by utilizing elemental analysis for matrix cross section values (∑MA) and by averaging data from multiple log runs to improve the statistical accuracy of the formation cross section (∑FM).

Before logging, an indication of the quality of the saturation that can be calculated can be determined by evaluating the expression Φe(∑W) ∑HC). In the subject reefs we have observed that, if the value is greater than 3, SW is quantitative; if between 1.5 and 3 inclusive, SW is qualitative; and if less than 1.5, W is ambiguous or unusable.

Current thermal decay logging is commonly performed with the well shut in. Even if the well has been shut in for an extended period of time, this can produce erroneous results opposite perforations, due to borehole fluid encroachment into the formation. To evaluate perforated zones, we have found that thermal decay logging should be performed while the well is flowing. This eliminates invasion effects near the borehole and can allow water production problems to be analyzed (e.g., locating fluid entry into the wellbore and indicating vertical water flow in fractures or in channels behind casing). This analysis is made possible by the oxygen activation measurement produced by the thermal decay tool.

Introduction

The Thermal Multigate Decay logging tool is designed to measure accurate ∑FM in the cased hole environment regardless of borehole salinity. The theory and operation ot the TMD* tool has been previously explained in the literature.1,2,3 To determine formation fluid saturations, the tool relies on the existence of a sufficient ∑FM contrast between a hydrocarbon-bearing formation and a waterbearing formation. This contrast is dependent in turn upon Φe and upon the contrast between ∑W and ∑HC.

FM has been successfully used to determine SW in high porosity (20 to 30%) formations such as exist along the U.S. Gulf Coast and in the Middle East. Western Canadian carbonates present a more difficult problem due to their generally lower porosity (3 to 14%). Figure 1 illustrates that, at these low porosities, the ∑FM contrast between hydrocarbon-filled and water-filled pore spaces is small. This contrast increases with increases in water salinity (CW). In the Western Canadian reefs, it is fortunate that CW is generally high (150,000 to 200,000 ppm NaCI), providing for optimal contrast between ∑W and ∑HC.

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