POROSITY LOSS IN SAND BY GRAIN CRUSHING-EXPERIMENTAL EVIDENCE AND RELEVANCE TO RESERVOIR QUALITY

Abstract

The compaction of loose sands has been tested up to high effective stresses (50MPa). More than 50 test runs were made on five different types of sands, which were of well-sorted fine- to coarse-grained, mono-quartz and lithic compositions. The compaction curves are related directly to observations made from thin sections prepared at different stress levels. The image analyses of the thin sections show that the degree of grain fracturing increases continuously as a function of stress level. Fracturing is more intense in coarse- than in fine-grained sands and a lithic sand, fractures more readily than a mono-quartz sand in a given grain size category. Petrographic observations and grain size analyses show that, although the average grain size reduces with increasing stress level, grain fracturing is most effective in producing grains between 50 and 200μm, which also reduces the sorting of the sands. Grain size reduction and porosity losses are higher in coarse-grained and lithic sands than in fine-grained and mono-crystalline quartz dominated sands. Fractures similar to those experimentally produced are also seen in the deeply buried Jurassic reservoir sandstones from Haltenbanken area. The experimental compaction data may provide a basis to predict reservoir quality prior to extensive quartz cementation.