No abstract
Quantitative methods taking into account the sedimentological characteristics will answer the needs of reservoir engineers. We propose here a geostatistical method for the conditional modelling of the facies of a sedimentary fluvio-deltaic series. This model was elaborated jointly by I.F.P. and the Paris School of Mines, with the aim of modelling reservoir heterogeneities. From the sedimentological study contained in the paper by C. Ravenne et al., we present several simulations, conditioned by "drill-core" taken from the outcrop. The block permeabilities are then calculated from the values given to the facies. Introduction Reservoir engineers have, for a long time, been asking what type of models could be entered into reservoir simulators. The geological models that are normally used are essentially qualitative and so it is difficult to numerize them, except by correlating the drillholes facies, which is not always self evident. This often leads to models with too many constraints for simulating reservoirs. Their dynamic behaviour worsens considerably going from very continuous layers of sandstone to disseminated lenses. (Fig. 1). This raises the question of how to characterize the geometry of the sandstone given the drillhole data, the geologist's interpretation and also other measurements (e.g. seismic recordings), and how to model the reservoir levels to suit this shape. As well as this, the models must match the lithology along the drillholes. In this article, we present a method for conditionally modelling the lithology which is designed for sedimentary processes. This approach was tested using the geological section of a cliff-face in Yorkshire (England) which shows a fluvio-deltaic environment similar to some of the levels in the Brent formation in the North Sea. A detailed description of the geology of the cliff-face studied and of the approach used in this project have been presented by Ravenne et al. Here we shall only consider the problem of modelling random sets by using a probabilistic method for representing the spatial distribution of the facies (sandstone, shaly sandstone, shale) in a heterogeneous reservoir. Before presenting the method, we review the main procedures for modelling random sets, that are used in the petroleum industry. REVIEW OF THE EXISTING METHODS Boolean Sets A simple way is to consider a heterogeneous medium as consisting of sandstone lenses in a shale matrix (or vice versa). Boolean sets (Matheron, Serra, Jeulin) are a mathematical way for modelling this type of deposit, that has been used for many years in other fields (Fig. 2). This method consists of putting lenses of a predetermined shape (e.g. ellipses or rectangles) at random points in the domain under study (i.e. the points are statistically uniformly distributed in space). Lenses are not correlated. The advantage of this approach is that it is easy to use in 2D or 3D spaces. It only depends on a few parameters: the number of seed points per unit space (called density), the shape of the lenses (fixed or variable), their size and orientation. The model is very flexible. The parameters can be modified locally in order to reproduce the real phenomenon more accurately. Clearly the more complicated the model is, the more parameters there are to fit but this can be overcome by fitting them by trial and error. P. 591^
No abstract
Sandstone-type uranium deposits contain approximately 28% of the world uranium resources. Many of these deposits are located below the water table in weakly lithified or non-consolidated sands, and therefore they can be exploited using in situ leach (ISL) technology. Such technology is based on dissolving uranium minerals directly in their host rocks (in situ) by reactive solutions that are injected through drill holes and then pumping the dissolved solution to the surface through some discharge drill holes. Uranium grade is determined by down-hole geophysics, in particular the prompt fission neutrons, or PFN, technique, coupled with sampling and assaying of the drill core. The drill grid which is used in Kazakhstan for definition of ISL uranium resources are as follows: (i) Measured: average 506100 m (range from 25650 to 506100 m); (ii) Indicated: average 506200 m (range from 506100 to 506200 m); and (iii) Inferred: average 506400 m (range from 506400 to 1006800 m). Estimation and reporting of uranium resources for ISL projects differ from hard rock mining projects in the need for quantitative estimation of the geotechnical and hydrogeological parameters which are specific for ISL technologies. The main parameters which need to be considered are as follows: (i) grade and geometry of mineralisation are estimated with accuracy sufficient for supporting the remote mining; (ii) if grade is estimated using the gamma logging technique secular disequilibrium should be studied and reported; (iii) hydrogeological confinement of the mineralised horizon; (iv) permeability of the mineralised horizon; (v) composition of the host rocks, in particular the carbonate content, in order to estimate if uranium mineralisation is amenable to dissolution by acid or alkaline solutions; (vi) groundwater flow; (vii) aquifer salinity; and (viii) rate of the in-situ dissolution of the uranium minerals. Hydrogeological and geotechnical information is obtained by testing the drill core samples and in the field, using the pump tests and the down-hole piezometers. Modifying factors for conversion resources to reserves are verified and corrected using field leach tests of uranium. This test is a strict requirement for feasibility studies of the ISL uranium projects in Kazakhstan.
Résumé -Intégration d'une contrainte dérivée de la sismique : revue des différentes étapes et quantification des incertitudes associées à la construction du modèle géologique -La construction d'un modèle géologique réaliste est l'un des premiers objectifs du processus de caractérisation de réservoir. Ce terme résume les différentes étapes menant à l'obtention d'un modèle 3D intégrant toute l'information a priori disponible, le modèle géologique conceptuel, les données géologiques, de sismique et de production, avant la simulation des écoulements. La définition du meilleur « modèle géologique réaliste » à obtenir doit être donnée en considérant l'objectif final de l'étude : il peut s'agir d'obtenir une réalisation qui respecte au mieux les paramètres géologiques et les contraintes dérivées de la sismique, ou de produire une réalisation qui reproduira au mieux les données de production. L'intégration d'une information sismique est cruciale pour cette modélisation. Beaucoup de travaux récents ont porté sur des méthodes visant à introduire dans les modèles probabilistes une contrainte secondaire, caractérisée par une relation souvent indirecte avec les propriétés réservoir. À cause des échelles de résolution différentes entre sismique et géologie, et de la relation complexe qui existe entre les propriétés des roches et la réponse sismique, il peut être efficace d'extraire dans un premier temps une propriété géologique des données sismiques, puis de contraindre la simulation de faciès en utilisant cette propriété. L'extraction d'une contrainte géologique moyenne des données sismiques se fait à partir d'une calibration statistique sur les données de puits. Deux approches sont principalement utilisées, basées sur des méthodes de segmentation ou sur des techniques d'estimation. Le résultat en terme de variables corrélées avec la distribution de faciès se présente sous forme de contraintes 2D c'est-à-dire de cartes reliées à l'épaisseur totale de l'unité stratigraphique étudiée, ou à de petits volumes à l'intérieur de l'unité (contrainte 3D). L'Institut français du pétrole et le Centre de Géostatistique de l'École des Mines ont développé une méthodologie innovante qui permet de générer des grilles de proportions de faciès à partir de données de puits et de modèles sédimentologiques conceptuels (cartes et zonations) tout en étant contraintes par une information dérivée de la sismique. L'objectif de ce papier est de présenter une analyse et une évaluation de l'impact réel de ces contraintes dérivées de la sismique sur le modèle géologique de réservoir en termes de quantification des incertitudes, de la distribution des hétérogénéités et des caractéristiques géologiques clés. Ces tests supposent que le modèle structural du réservoir est fixé et que le modèle géologique conceptuel est
International audienceThe interest of a digital model to represent the geological characteristics of the field is well established. However, the way to obtain it is not straightforward because this translation is necessarily a simplification of the actual field. This paper describes a stochastic model called truncated Gaussian simulations (TGS), which distributes a collection of facies or lithotypes over an area of interest. This method is based on facies proportions, spatial distribution and relationships, which can be easily tuned to produce numerous different textures. Initially developed for ordered facies, this model has been extended to complex organizations, where facies are not sequentially ordered. This method called pluri-Gaussian simulation (PGS) considers several Gaussian random functions, which can be correlated. PGS can produce a large variety of lithotype setups, as illustrated by several examples such as oriented deposits or high frequency layering
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