Browsing by Author "Assteerawatt, Anongnart"

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Open Access
Flow and transport modelling of fractured aquifers based on a geostatistical approach
(2008) Assteerawatt, Anongnart; Helmig, Rainer (Prof. Dr.-Ing.)
Aquifer-analogue studies established in the petroleum reservoir have been widely used for characterizing fractured aquifer systems because detailed analysis can be performed practically and characteristics of fractured systems obtained on this scale can be upscaled to fractured systems on field scales. A discrete model approach is an attractive alternative compared with a continuum model approach for aquifer analogue studies because there is no a priori assumption that a fractured system behaves as a continuum, and the effect of individual fractures can be explicitly investigated. However, a generation of a representative'' fracture network remains a challenging task when the discrete model is applied. In a case where a fracture network is embedded in a porous matrix, known as a fracture-matrix system, the numerical study of flow and transport processes requires a full two- or three-dimensional description of the fractures and the surrounding matrix. This causes a rise in computational demand for the numerical study of the flow and transport behavior of such a fractured system. The overall purpose of this work is to improve the study of flow and transport processes in a fracture-matrix system on an analogue scale by using a discrete fracture model. An important prerequisite for this is the generation of a representative'' fracture network. Subsequently, an alternative approach to advective-dispersive transport which requires high computational demand for simulating transport in a fracture-matrix system has to be considered. In the first part of this work, a geostatistical fracture generation (GFG) which integrates statistical geometries and spatial characteristics has been developed and the technique for evaluating spatial characteristics in terms of a standardized variogram, neighborhoods, a fracture-cell density and a variance has been presented. In the following part, streamline tracing (STR) in a fracture-matrix system has been introduced as an alternative to advective-dispersive transport (ADT). The comparative study of geostatistical fracture generation (GFG) and statistical fracture generation (SFG) shows that the spatial characteristics of a fracture network observed from the field as well as the flow and transport behavior of a fracture-matrix system (such as discharge, peak arrival time and mean effective time) are better represented by the results of GFG than those of SFG. Hence, integrating the spatial characteristics and the statistical geometries in GFG have improved the discrete fracture generation and the fractured system behavior can be better predicted. Furthermore, the transport behavior in terms of an accumulated breakthrough curve (AccBTC) and a breakthrough curve (BTC) of fracture-matrix systems are investigated by using streamline tracing (STR) and compared with the results obtained by using advective-dispersive transport (ADT). STR shows significant reduction in computation time and clearly less numerical diffusion in comparison with ADT. In the cases considering a single fracture and systematically distributed fractures in a porous matrix, the effect of fast flow in fractures and slow flow in matrix, which is obviously noticed in STR, is smeared out due to the numerical diffusion in ADT. In complex fracture-matrix systems, numerical diffusion in ADT delays plume migration, whereas purely advective transport in STR leads to fast solute transport. As a result, the difference between the AccBTCs and the BTCs from the two approaches are clearly distinguished. Further investigations involving comparisons with experimental or field studies have to be carried out in order to validate the results of the two approaches.