Prediction of Petrophysical Parameters Based on Digital Video Core Images
- Lars Oyno (Reservoir Laboratories a.s.) | B.G. Tjetland (Reservoir Laboratories a.s.) | K.H. Esbensen (Telemark Inst. of Technology) | Rune Solberg (Norwegian Computing Center) | Aase Scheie (Norsk Hydro a.s) | Tore Larsen (Saga Petroleum a.s.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- February 1998
- Document Type
- Journal Paper
- 82 - 87
- 1998. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 5.1 Reservoir Characterisation, 4.1.5 Processing Equipment, 5.6.1 Open hole/cased hole log analysis, 6.1.5 Human Resources, Competence and Training, 5.6.2 Core Analysis, 1.6.9 Coring, Fishing, 4.3.4 Scale, 1.2.3 Rock properties, 5.5.2 Core Analysis
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Core-slab photography is a common way to document geological information from cores. Past practice has been to photograph core slabs with ordinary cameras that produce paper photographs. The presented method retrieves petrophysical properties from high-resolution digital video core images.
The procedures described in this work are based on video images (standard RIO/B camera) of cores taken with a digital recording system. The system is able to record in both visible and UV light at different illumination angles, store images, compress/decompress images, and display one or several images as a continuous long core. The seamless core image is marked with depth scale and can be scrolled, scaled, and zoomed. Facilities for correlation with other related data, such as wireline logs, discrete core data, and microscopy images, are also included in the system.
We used homogenous dry core plugs from three North Sea oil fields in this work. We recorded images of plug surface, together with conventional core-analysis data (i.e., porosity, gas permeability, average grain size, and mineralogy). The new method is based on processed digital images: light/shadow patterns are obtained by use of asymmetric, low-angle illumination in the green channel. Texture spectra of the rock material are obtained by dedicated image-analytical processing of these gray-scale images and by detecting textural features by use of a unique set of specially designed texture filters. We then calibrate these spectra with respect to measured petrophysical parameters by use of multivariate calibration [partial least squares (PLS)-regression]. Multivariate calibration is based on a set of representative training images, selected to span representative ranges of the intensive petrophysical parameters being modeled. On the basis of this calibration model, similar gray-level video images from new, unknown core sections (with geologically similar facies) are used to estimate properties of the core material by PLS-prediction. In this study it has been possible to model porosity, gas permeability, and average grain size (ORZ) of different formations with a relatively high accuracy and precision.
PLS-modeling/-prediction is a strict empirical calibration procedure. The present method is critically dependent upon a thorough, geologically well-documented training data set. Results show that the method is capable of predicting a continuous log of these three petrophysical parameters based on core images calibrated against a set of routine laboratory core-analysis data taken at discrete intervals for a particular formation. The advantages of the new method are rapid and cost-efficient methods for prediction of petrophysical parameters, particularly from slim cores, and improved integration of geological records with wireline data. The method is proposed to be included in future routine laboratory core analysis studies because of its low cost and ability to predict values continuously along the core.
In many cases where core material is available from a potential hydrocarbon reservoir, it is possible to perform conventional laboratory core analysis on selected zones or at regular intervals. These measurements are commonly used as input in numerical simulations predicting recovery from the field. The results are also commonly used for net pay calculations to provide a reserves estimate.1-4
Usually, conventional core plugs are taken at regular intervals (every 30 cm or every meter) in the reservoir zone. Core-analysis plugs are often neglected below the oil/water contact (OWC), sometimes also in other parts of the reservoir for various reasons. Core photography has been used for decades to document the geology in the reservoir for later study. The photographs are usually printed on paper with a few core lengths in each photograph. Obtaining a complete picture of the reservoir geology and petrophysics from the core photographs involves extensive leafing through numerous pages of core photographs. Also, paper photographs do not offer the possibility to perform image analysis.
Advances in digital storage and image analysis, together with decreasing costs of computers, have now allowed the use of digital storage of core information.5-8 The work described in this article makes exclusive use of digitally recorded imagery. Core images are taken continuously along the slabbed core. Software automatically combines the core images into a seamless, continuous core image of the complete length of the core's interval. This opens the door to easy access to image analysis.
In contrast with the routine core-analysis measurements, the present digital video images provide continuous information regarding the texture of the core material. If these images also could be used to extract petrophysical information, they could offer parameter values continuously along the entire cored material. Because reservoir material differs widely from field to field and also between wells, we expected some initial experimentation with optimal recording parameters as well as the geological calibration base to be necessary to tune a new type of image correlation model.
Consider an image of core material, say sandstone, where each grain can be seen at an appropriate resolution; it is not difficult to accept that image analysis should be able to extract grain-size (and grain-size distribution) information pertaining to the material in the field of view. Grains can be seen down almost throughout the fine range of the sandstone grain size. Moreover, when applying different data analytical techniques to postprocess, earlier-derived texture spectra, it became clear that even other petrophysical parameters like porosity and permeability could indeed also be predicted. Multivariate calibration,18,19 to be explained further later, is carried out from a number of calibration samples where the desired petrophysical parameters are known (from traditional methods). The camera field of view was maintained constant, and an analysis area large enough to be representative for all types of material in the present study was determined by initial sensitivity analysis.
The advantage of the presented method is that petrophysical parameters now can be predicted directly from identical video imagery on samples which then, of course, need not be measured in the laboratory. This approach can even be augmented so as also to produce results from layered zones, where routine core-analysis results are difficult to obtain. It can also provide results where routine core-analysis results are doubtful, for example, in unconsolidated cores. Last, it provides continuous petrophysical estimates from a core at a detail and at significantly lowered cost, which is both impractical and uneconomical to achieve with conventional core analysis.
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