--> Abstract: Pore Pressure - Stress Coupling and the Requirements for Seal Breach, by Richard Swarbrick, Richard Lahann, and Stephen O'Connor; #90078 (2008)

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Pore Pressure - Stress Coupling and the Requirements for Seal Breach

Richard Swarbrick1, Richard Lahann2, and Stephen O'Connor1
1GeoPressure Technology, Durham, United Kingdom
2Lahann Geoservices, Nashville, IN

High pore fluid pressure increases the risk of seal breach, i.e. faulting and hydraulic fracturing leading to upwards re-migration of subsurface fluids. The requirements for hydraulic fracturing are that the pore fluid pressures reach the fracture strength of the seal. Pore fluid pressure can be measured directly by wireline tools and production tests. Fracture strength is estimated from rock stress relationships combined with borehole Leak Off Test data, and may relate to horizontal stress magnitude, which is also influenced by pore fluid pressure through poroelastic fluid-stress interaction.

Using data from the North Sea and mid-Norway the pore pressure:stress coupling ratio is determined to be close to 0.3. This coupling ratio value is lower than previously published estimates for a worldwide set of data which suggested values closer to 0.7. This paper will explain a new and robust method to determine the appropriate coupling value and its underlying assumptions and will examine the reasons for a lower stress coupling ratio in these basins relative to previous coupling studies.

Pore pressure - stress coupling can be used in the development of pre-drill fracture strength estimates for well planning. In addition it can be used to assess risk of seal breach. In the Central North Sea HPHT area a clear relationship exists between seal capacity (difference between pore fluid pressure and fracture strength) and dry holes/discoveries for Jurassic/Cretaceous seals and reservoirs. The relationship between seal capacity and dry holes/discoveries is less clear in mid-Norway despite similar geological conditions and pore pressure - stress coupling values.

 

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas