Hydraulic fractures and their implications regarding the state of stress in a sedimentary sequence during burial

Abstract

Hydraulic fracture is the brittle failure of a material resulting from an increase in the internal fluid pressure. Because of the intimate relationship between the type of brittle failure and the differential stress and the orientation of the fractures and the stress orientation it is possible to use fractures to determine the orientation and state of stress that existed during fracture formation, or conversely to predict the type and orientation of the fractures that would form in various stress environments within the crust if the fluid pressure were sufficiently high to cause hydraulic fracturing.

Using these basic ideas the pattern of hydraulic fractures likely to develop in sediments as they are buried in a basin are considered, together with the stage in sediment burial when conditions for hydraulic fracturing are first met.

The experimental investigation of the initiation and development of hydraulic fractures is extremely difficult. However, if it can be argued that the process is directly analogous to the formation of cooling fractures or desiccation fractures then the processes can be modelled relatively easily and analogue experiments can be carried out to investigate the formation and spatial organization of hydraulic fractures under a variety of geologically realistic boundary conditions.