Application of Multi-Well Normalization of Open Hole Logs in Integrated Reservoir Studies
Authors: Aly Ahmed M. , Hunt Ercill R. , Pursell David A. , McCain William D. 
Published: December 2007

Well logs are the foundation on which characterization of layered reservoirs is based. However, multi-well normalization of the well logs is necessary to reduce the probability of major errors and inconsistencies in the results of the log analysis. If not removed, these inconsistencies will cause failure in any attempt to integrate log analysis results with core, well test, and production data.

This paper presents a comprehensive procedure for multiwell normalization of well logs. Use of this method will ensure that the well log data can be used effectively in reservoir characterization.

The advantages of multi-well normalization are illustrated in three examples of integrated reservoir studies. Example 1 illustrates the multi-well normalization process using both histograms and M-N crossplots to verify the normalization. Example 2 from a 300 well reservoir study shows the use of multi-well dual-porosity crossplot with the histogram normalization. Example 3 introduces the benefits of multi-well normalization for permeability calculations from well-logs and the effect of removing the log errors in developing a relationship between the log response and permeability. This example also illustrates the integration between well-logs and core data.

Introduction

Practice in using well log analysis as a part of integrated reservoir studies has shown that for the results to be accurate, consistent and comparative well-to-well, the log data require corrections' with a process called multi-well normalization. This process ensures that each logging tool reads the correct values, i.e., that the density tool accurately records formation density, the neutron log accurately reflects hydrogen index, and so forth. Experience indicates that approximately 65 to 70 percent of gamma ray logs, 50 percent of density logs, 40 to 50 percent of neutron logs, and five to 10 percent of sonic logs require some normalization to correct for variances in field calibrations of the logging tools.

The normalized well log data can be effectively integrated, correlated, and calibrated with core data. The resulting correlations can be extended vertically to include layers which were not cored and laterally from well-to-well across the study area. The difference in the scale of measurement of the two sets of data must be taken into account. The core data have a scale of few cubic inches, while the log data have a scale of a few cubic meters and well test data have a scale of a few acres. Table 1 shows that even among the log measurements there are different volumes of investigation for the different tools.

Multi-Well Normalization

Multi-well normalization is a key activity to ensure accurate and consistent results from a multi-well log analysis study. Normalization is an iterative process that uses three tools; histograms, crossplots, and depth-based logs. These tools may be applied in the rock layers being analyzed or in nearby layers.

In a marine environment of deposition, the nearby or interbeded shales may be consistent enough over a large area for normalization purposes. For instance, the Bossier shale in East Texas can be used in the normalization process.

In many basins, there are often enough very low porosity carbonates with sufficient areal extent to be useful in the normalization process. This is especially true in evaporate deposition cycles.

When sandstone is abundant such as in the Travis Peak & Cotton Valley of the East Texas, Prairie du Chein of Michigan, Lorelle formation of Australia, the Miocene reservoirs of the Gulf of Suez in Egypt, and many others, the sandstone itself can be used in the normalization process.

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