Natural Fractures Network Development Due to Fluid Production in Anisotropic and Impermeable Granular Materials: Application to Organic-Rich Shales

Abstract

Natural Fractures Network Development Due to Fluid Production in Anisotropic and Impermeable Granular Materials: Application to Organic-Rich Shales

Alain Zanella, Salomé Larmier, Thomas Junique, Régis Mourgues

Laboratoire de Géosciences, Le Mans Université, 72 085 Le Mans, France

Shales are often considered to be very impermeable. This is especially well illustrated in organic-rich shales that are often source rocks for petroleum. Nevertheless, natural examples of fluids migrations through these sediments are common, in particular by the development of natural fractures networks. The understanding of such fracture networks is crucial to estimate the evolution and then the potential redistribution of fluids (water, hydrocarbons) through times.

Several natural fracture types are present within shales as unfilled fractures, mineralized fractures (often by calcite minerals) as well as hydrocarbons filled fractures (such as bitumen veins). The development of these fractures is the consequence of several geological processes that affected the source rock and that have mobilized the fluids. In shales, some fluids are produced within the sediment (water and hydrocarbons). Indeed, the fluid use two different flow systems to migrate: i) a circulation through the pore space of the rock, similar to the percolation mechanism and ii) a massive channelized flow using open fractures. Previous studies have shown that mechanisms of flowing can be initiated during the generation of hydrocarbons and thus affected the redistribution of fluids within or near the source rock.

To study the processes of production and migration of fluids as well as the fractures networks induced by the fluids, we carried out a new 3D experimental model. In our models, we created a new analogue material to represent an impermeable and anisotropic material as shales. This material is composed by a mixture of micas, silica powder and rapeseed wax flakes. The micas are used to introduce an anisotropic shape, the silica powder for the impermeability and the wax represents the organic matter. The model is built in a box with an aluminum bottom which rests on a hot plate. By increasing the temperature of the hot plate, the wax melts (at 62°C) and we are able to generate a fluid from solid particles by a mechanism analogue to the maturation of organic matter in shales (phase transition). We follow the temperature evolution as well as the fluid pressure at the bottom of the model with 9 sensors. During experiments, the pressure rises up and generates open fractures. Due to the pressure gradient, the melted wax migrates into fractures and solidify. After the experiments, we made several cross-sections to be able to observe the geometry of the fracture network developing.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019