SPE Reservoir Evaluation & Engineering
Volume 8, Number 1, February 2005, pp. 77-87

Summary

Resistivity logs, while used extensively in the oil industry for the determination of water-saturation profiles and, consequently, for the quantification of hydrocarbon originally in place (HOIP), are strongly affected by environmental effects such as borehole, shoulder-bed resistivity contrasts, mud-filtrate invasion, dipping beds, and electrical anisotropy. It is well known by log interpreters that the combination of the different effects may strongly affect the estimation of hydrocarbon in place and hydrocarbon reserves. This paper highlights the strong reduction of the uncertainties in water-saturation determination and, consequently, the petrophysical characterization of the reservoir achieved by applying the appropriate 2D resistivity-modeling and -inversion techniques to two wells of the Norwegian offshore area. Both wells were drilled in a sandstone reservoir, with some thin-bedded intervals, and affected by the presence of anomalous invasion profiles.

Introduction

Resistivity logs, as directly used for the determination of water-saturation profiles, have always been of focal interest for the oil industry; it is clear that the quality of these measurements, currently used in the net-pay and hydrocarbon-in-place determinations, must be very high. As a consequence, more accurate and flexible resistivity tools have been developed in recent years. We will address the family of array tools, particularly the HRLA,* which makes available a set of five galvanic resistivity measurements at different depths of investigation.

Unfortunately, the most common types of environmental noise (borehole effects, shoulder-bed resistivity contrasts, invasion, the presence of dips, and anisotropy) still alter the measured resistivity, thus affecting the estimation of the true resistivity in hydrocarbon-bearing levels. To remove these alterations, we have developed a 2D resistivity modeling and inversion technique that can correct a number of environmental effects simultaneously.

This paper presents the results obtained in two wells of the same reservoir in the offshore Norway area, where the sandstone bodies are interbedded with deltaic shales. The values of porosity and permeability are generally very high, and a complete set of data [conventional and special core analysis, conventional wireline logs, microresistivity imaging logs, nuclear magnetic resonance (NMR), and sedimentological analysis from core and images] is available. The 2D modeling provides a better definition of the water saturation in the thinner sandstone bodies of the sequence and in the presence of anomalous invasion profiles.

When comparing the resistivity-modeling results with those obtained by standard interpretation techniques, we can see the effectiveness of the developed methodologies (both hardware and software) in improving the reservoir characterization and in maximizing the return of the investments in logging and well-data measurements.

The aim of this paper is two-fold: the authors want to show how complex reservoir studies can benefit from the correct integration of heterogeneous geological data, while addressing at the same time the added value of applying a 2D modeling and inversion numerical technique to resistivity measurements to compute accurate water-saturation profiles.

One of the most important issues of the formation-evaluation process is the correct estimation of all the petrophysical parameters necessary to determine the hydrocarbon content of the reservoir. This implies the need to compute a saturation profile as correct as possible. Because Sw (and, consequently, Sh) strongly depends on resistivity, porosity, and shale volume, it is of the utmost importance that the uncertainty on these measurements be kept very low. In recent years, the accuracy of resistivity tools has been improved greatly by the introduction of array measurements1,2; unfortunately, the utter complexity of real formations can often lessen the intrinsic advantages of the available logs. The most common environmental noise sources, as listed in many well-known works,3–5 are:

• Thin beds and/or dips.

• Deep and/or exotic invasion profiles.

• High resistivity contrasts between mineralized (porous) and tight layers (shoulder effects).

• Electrical anisotropy (usually related to laminations and grain-size variations).

In most cases, their combined effects cannot be removed separately but must be treated as a unique, nonlinear problem. In previous work,6–9 it has been shown how resistivity modeling and inversion techniques can solve these kinds of problems, provided that an appropriate and fast forward model (2D or 3D) is available for all the acquired tools and that a robust and efficient inversion algorithm can be implemented.

In the following paragraphs, we will show how the integration of different types of data [geological studies, wireline logs, nuclear magnetic resonance (NMR) measurements, core data], together with the most advanced numerical interpretation techniques, can produce accurate and robust results for many formation-evaluation problems, thus reducing the uncertainty of the estimation of the petrophysical parameters that are relevant in reservoir studies. The importance of geological and petrophysical information in defining a correct formation model was also addressed in a recent paper,10 which shows how this is also useful in constraining the inversion process.

For this reason, we will first describe the geological setting of the reservoir and the available data, highlighting the interpretation process and the problems encountered; we will then focus on the methodology used for the evaluation of the correct water-saturation profile from resistivity measurements, demonstrating how this methodology, based on modeling and inversion techniques, can enhance the robustness of the results, as confirmed by different sources of information. Because the field study has not been yet completed, from the reservoir point of view, the conclusions will not be definitive, and the paper will end with a work-in-progress description of future activities. We will, however, be able to state the advantages of the proposed numerical modeling and inversion technique applied to laterolog array measurements, especially when in the presence of data of different qualities.

© 2005. Society of Petroleum Engineers

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