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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe analysis of deep-reading electromagnetic measurements is critical to the evaluation of hydrocarbon reserves. However, in thin bed formations, poor tool vertical resolution and corresponding low sensitivity to hydrocarbon presence make interpretation in the virgin zone difficult. A priori knowledge such as the formation geometry or auxiliary petrophysical information is necessary to overcome these difficulties. This paper presents a prototype code developed by Schlumberger S-RPC in collaboration with AGIP. Using this code, wireline or LWD, laterolog and induction measurements can be more correctly analyzed in thinly bedded environments (2-D geometry, fluid invaded layers perpendicular to the borehole).This code has been implemented in a software framework that provides a common environment specifically designed for electrical tool interpretation. Processing modules have a common interface and share common functionalities. Their organization reflects an implicit processing methodology, with progressive refinements that provides the interpreter with a robust and simple to use product, to better quantify reserves.A preliminary step is to determine the formation geometry, which is carried out by detecting bed boundaries and representing the formation as a vertical sequence of layers. Petrophysical analysis can be invoked to characterize certain formation properties such as shale volume and porosity. These steps are performed prior to resistivity log measurement analysis and serve as a form of a priori knowledge.Once the formation is described as a sequence of layers, wireline l ogging or logging while drilling (LWD) tool response can be computed using fast 2D simulators. The estimation of resistivity and the subsequent estimation of saturation will correspond to the minimization of a cost function, defined as the weighted squared difference between the measurement and the simulated response. Confidence outputs can be related to the local shape of the cost function at the end of the processing.Two important advantages of the new code must be emphasized: (1) the possibility to choose among several petrophysical models to better describe the environment and determine directly parameters such as hydrocarbon saturation, and (2) the possibility to group together beds which are too thin or too close to each other to be analyzed independently, into a so called single optimization interval described by a reduced set of parameters.This paper presents results obtained on selected benchmarks extracted from real data and compares them with those obtained through more traditional approaches.

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 2Dresistivity-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-knownworks,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.

The log interpretation proces is based on data acquired under environmental and thermodynamic conditions which may potentially affect the accuracy and the precision of the measurement system. All the possible sources of errors should be recognized and classified as systematic errors or random errors. Systematic errors should be corrected before interpretation whereas random errors propagation should be accounted for in the calculation process to evaluate the uncertainty associated to the final results. Uncertainty propagation can be calculated by two different approaches : the analytical method, and the numerical method. Both the analytical and the numerical methods veere applied to the deterministic log interpretation process in order to select the most suitable approach to evaluate the uncertainty affecting the reservoir petrophysical characterization. The results of the study showed that, depending on the rock and Huid properties, the analytical approach may overestimate or underestimate the uncertainties associated to porosity and water saturation values and may lead to results that do not honor the volume balance equations .

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractResistivity logs, as directly used for the determination of Water Saturation profiles, have always been of focal interest for the oil industry; it's clear that the quality of these measurements, currently used in the net pay and hydrocarbonin-place determination, must be very high. As a consequence, more accurate and flexible resistivity tools have been developed in recent years. We are addressing here the family of array tools, and especially the HRLA 1 , which makes available a set of 5 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, anisotropy), still alter the measured resistivity, thus affecting the estimation of the true resistivity in hydrocarbon bearing levels.In order to remove these alterations, Schlumberger, in cooperation with ENI-AGIP, has developed a 2D resistivity modeling & inversion technique that can simultaneously correct a number of environmental effects. This paper presents the results obtained in two wells of a 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 & special core analysis, conventional wireline logs, microresistivity imaging logs, NMR, sedimentological analysis from core and images) is available. Mark of SchlumbergerThe 2D modeling provided 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.