ABSTRACT:
The discrete fracture network (DFN) approach is attractive for applications in which the geometry and properties of discrete fractures play a significant role in hydraulics, geomechanics, and resource assessment. This paper describes the development of approaches to address the particularities of carbonate geologies, including karstic features, layer boundary and through-going fractures, correlations between fracture intensity and bedding thickness.
1 INTRODUCTION
The discrete fracture network (DFN) method has been used extensively for carbonate rocks since at least 1988 (Dershowitz & Doe 1988). This paper describes recent developments to improve the realism of DFN models, and their application to practical engineering problems. The following areas are addressed • Karstic Features • Bedding Controlled Fractures • Fracture Termination Modes Following the discussion of the implementation of DFN models for carbonate rocks, site characterization issues are addressed.
2 KARSTIC FEATURES
Carbonate rocks typically contain enhanced porosity caused by acid solution of carbonate minerals. The formation of these ¡°vugs¡± generally requires flowing water, and as a consequence vugs have a tendency to form on fracture surfaces, or at fracture intersections. Within the DFN approach, these fracture related vugs can be implemented as a local change to the fracture aperture pattern, with higher apertures (mm to meter scales) at locations where vugs occur. Vugs also occur which are related to flow through the matrix porosity of carbonate rocks, without a spatial relationship to fracture patterns. Depending on scale, these vugs can be implemented in DFN approaches as an enhancement to the rock matrix porosity, or as a separate class of discrete features, with their own spatial process.
2.1 Preferential vug porosity at fractureintersections Solution enhanced porosity at fracture intersections can be very significant for flow through carbonate rocks, as well as fluid storage within the rock.
