Fracturing of tight rocks during internal fluid production: implications for primary migration

Abstract

This thesis presents a study of fracture formation in low permeability rocks induced by generation of fluid or gas during chemical reaction. In nature, primary migration of hydrocarbons, i.e. generation of oil and gas from tight source rock and its expulsion to more permeable reservoirs, is driven by formation of microfractures in the heated organic-rich shale. The tomographic 3D imaging of the rocks samples during heating showed that micro-cracks develop in the shale due to internal pore pressure build-up. Generation of hydrocarbons drives fracture formation until a drainage network sufficient for expulsion is developed. Experiments were also conducted on analogue material, where gelatine was fractured during fermentation of yeast and sugar. Studies of the gel confined in Hele-Shaw cell showed that CO2 generation leads to formation of a drainage network which topology represents a new class of networks, intermediate between hierarchical fracture pattern and river\tree network. When CO2 escapes via fracture network, individual cracks open and close in intermittent manner. The variation of fracture opening in time exhibits a universal frequency distribution with 1/f and 1/f2 noise. These results have implication for understanding how the generated fluids escape from hydraulic fractures. The methods can be applied in studying dehydration of sediments, formation of mud volcanoes, methane hydrate exploration and assessment, geological sequestration of carbon dioxide CO2, hydraulic fracturing of unconventional reservoirs, hydrothermal energy.