(1) Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China
Abstract: Natural Hydraulic Fracturing: Episodic or Continuous? - Modeling and Examples
Natural hydraulic fracturing caused by overpressure plays an important role in geopressure evolution and hydrocarbon migration in petroliferous basins. Its mechanism is only understood through artificial hydraulic fracturing triggered by high-pressured fluid injection in a well. Natural hydraulic fracturing is therefore generally thought to be episodic. If this were true for all cases, the case of formation pressure close to the lithostatic pressure would be rare, because pore pressure decreases quickly and a large amount of pore fluid releases when fractures open. However, in some low-stress basins, such as the Yinggehai Basin, South China Sea, high formation pressures with a pressure coefficient equal to or larger than 2.0 are common.
The mechanism of natural hydraulic fracturing is studied using a fracturing model coupled to the physical processes in basin evolution. In the model, hydraulic fracturing limit lies between the classic rupture limit and the beginning of dilatancy. Fluid pressure evolution is calculated iteratively to allow dynamic permeability adjustment so that the fracturing limit is always preserved. The increase of permeability is interpreted on the basis of equivalent fractures. Observations in Yinggehai Basin and model results strongly suggest that natural hydraulic fracturing occur continuously to keep the pressure at the fracturing limit under relaxed stress conditions. In order to keep the stress level at rock's hydraulic fracturing limit, the fracture permeability needs to be an order of magnitude larger than the non-fracture permeability.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana