Chapter 6 - The role of natural fractures in reservoir quality assessment of tight gas sandstones

Abstract

Fracture characterization of tight gas sandstones is fundamental for making reservoir static and dynamic models. Production from tight gas sandstones is highly dependent on the development of the natural fracture. In this chapter, various sources of fracture data acquisition and interpretation in tight sandstones, including outcrop studies, core description, borehole image logs, and seismic data are discussed. Integration of the outcrop studies with the subsurface data provides fundamental inputs for fractures characterization and modeling. The main fracture parameters include fracture dip/strike, length, spacing, aperture, density, intensity, porosity, permeability, asperity, and conductivity. Several fracture classification schemes are proposed based on fracture geometry, origin, stress orientation, steepness, filling, aperture, spacing, and length. Fractures can occur parallel, perpendicular, or oblique to the bedding direction. In the anticlinal structures hosting tight gas sandstones, fractures occur normally, parallel and oblique to the hinge line. Hinge-perpendicular fractures are prefolding, while hinge-parallel and oblique fractures are syn-folding and postfolding, respectively. The source of fractures can either be tectonic or diagenetic. Tectonic fractures are caused by the stress’s orientation, while diagenetic fractures originate from the diagenetic processes altering the rock strength, pore pressure, and subsurface stress balance. Normally fractures zones are associated with a high productivity index, higher rates of mud loss, and higher anomalies in the curvature map. By constructing a continuous fracture network and discrete fracture network, the impact of fractures on production can be investigated.