Resistivity Modeling of Array Laterolog Tools: An Application in an Offshore Norway Clastic Reservoir
- M.T. Galli (Eni-E&P Div.) | M. Gonfalini (Eni-E&P Div.) | M. Mele (Eni-E&P Div.) | P. Belik (Schlumberger S-RPC) | O. Faivre (Schlumberger S-RPC) | L. Jammes (Schlumberger S-RPC) | A. Litman (Schlumberger S-RPC)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- February 2005
- Document Type
- Journal Paper
- 77 - 87
- 2005. Society of Petroleum Engineers
- 1.6.9 Coring, Fishing, 1.12.2 Logging While Drilling, 4.1.2 Separation and Treating, 5.6.1 Open hole/cased hole log analysis, 5.2 Reservoir Fluid Dynamics, 5.6.2 Core Analysis, 2.4.3 Sand/Solids Control, 5.7 Reserves Evaluation, 1.2.3 Rock properties, 1.6.7 Geosteering / Reservoir Navigation, 4.3.4 Scale, 5.1.5 Geologic Modeling, 5.1.3 Sedimentology, 5.5.11 Formation Testing (e.g., Wireline, LWD), 1.6 Drilling Operations, 5.5.2 Core Analysis, 4.1.5 Processing Equipment
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Resistivity logs, while used extensively in the oil industry for thedetermination of water-saturation profiles and, consequently, for thequantification of hydrocarbon originally in place (HOIP), are strongly affectedby environmental effects such as borehole, shoulder-bed resistivity contrasts,mud-filtrate invasion, dipping beds, and electrical anisotropy. It is wellknown by log interpreters that the combination of the different effects maystrongly affect the estimation of hydrocarbon in place and hydrocarbonreserves. This paper highlights the strong reduction of the uncertainties inwater-saturation determination and, consequently, the petrophysicalcharacterization of the reservoir achieved by applying the appropriate 2Dresistivity-modeling and -inversion techniques to two wells of the Norwegianoffshore area. Both wells were drilled in a sandstone reservoir, with somethin-bedded intervals, and affected by the presence of anomalous invasionprofiles.
Resistivity logs, as directly used for the determination of water-saturationprofiles, have always been of focal interest for the oil industry; it is clearthat the quality of these measurements, currently used in the net-pay andhydrocarbon-in-place determinations, must be very high. As a consequence, moreaccurate and flexible resistivity tools have been developed in recent years. Wewill address the family of array tools, particularly the HRLA,* which makesavailable a set of five galvanic resistivity measurements at different depthsof investigation.
Unfortunately, the most common types of environmental noise (boreholeeffects, shoulder-bed resistivity contrasts, invasion, the presence of dips,and anisotropy) still alter the measured resistivity, thus affecting theestimation of the true resistivity in hydrocarbon-bearing levels. To removethese alterations, we have developed a 2D resistivity modeling and inversiontechnique that can correct a number of environmental effectssimultaneously.
This paper presents the results obtained in two wells of the same reservoirin the offshore Norway area, where the sandstone bodies are interbedded withdeltaic shales. The values of porosity and permeability are generally veryhigh, and a complete set of data [conventional and special core analysis,conventional wireline logs, microresistivity imaging logs, nuclear magneticresonance (NMR), and sedimentological analysis from core and images] isavailable. The 2D modeling provides a better definition of the water saturationin the thinner sandstone bodies of the sequence and in the presence ofanomalous invasion profiles.
When comparing the resistivity-modeling results with those obtained bystandard interpretation techniques, we can see the effectiveness of thedeveloped methodologies (both hardware and software) in improving the reservoircharacterization and in maximizing the return of the investments in logging andwell-data measurements.
The aim of this paper is two-fold: the authors want to show how complexreservoir studies can benefit from the correct integration of heterogeneousgeological data, while addressing at the same time the added value of applyinga 2D modeling and inversion numerical technique to resistivity measurements tocompute accurate water-saturation profiles.
One of the most important issues of the formation-evaluation process is thecorrect estimation of all the petrophysical parameters necessary to determinethe hydrocarbon content of the reservoir. This implies the need to compute asaturation profile as correct as possible. Because Sw (and, consequently, Sh)strongly depends on resistivity, porosity, and shale volume, it is of theutmost importance that the uncertainty on these measurements be kept very low.In recent years, the accuracy of resistivity tools has been improved greatly bythe introduction of array measurements1,2; unfortunately, the utter complexityof real formations can often lessen the intrinsic advantages of the availablelogs. 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-sizevariations).
In most cases, their combined effects cannot be removed separately but mustbe treated as a unique, nonlinear problem. In previous work,6-9 it has beenshown how resistivity modeling and inversion techniques can solve these kindsof problems, provided that an appropriate and fast forward model (2D or 3D) isavailable for all the acquired tools and that a robust and efficient inversionalgorithm can be implemented.
In the following paragraphs, we will show how the integration of differenttypes of data [geological studies, wireline logs, nuclear magnetic resonance(NMR) measurements, core data], together with the most advanced numericalinterpretation techniques, can produce accurate and robust results for manyformation-evaluation problems, thus reducing the uncertainty of the estimationof the petrophysical parameters that are relevant in reservoir studies. Theimportance of geological and petrophysical information in defining a correctformation model was also addressed in a recent paper,10 which shows how this isalso useful in constraining the inversion process.
For this reason, we will first describe the geological setting of thereservoir and the available data, highlighting the interpretation process andthe problems encountered; we will then focus on the methodology used for theevaluation of the correct water-saturation profile from resistivitymeasurements, demonstrating how this methodology, based on modeling andinversion techniques, can enhance the robustness of the results, as confirmedby different sources of information. Because the field study has not been yetcompleted, from the reservoir point of view, the conclusions will not bedefinitive, and the paper will end with a work-in-progress description offuture activities. We will, however, be able to state the advantages of theproposed numerical modeling and inversion technique applied to laterolog arraymeasurements, especially when in the presence of data of differentqualities.
|File Size||2 MB||Number of Pages||11|
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