Deformations

 

Deformations are caused by changes in the effective stresses, e.g. those resulting from modification of the pore pressure. The complete deformation parameters in 3D can be determined with the RACOS^® analysis package. The effects of deformations on economically important in situ parameters can also be determined with this package, distinguishing thereby between the reversible and irreversible components of the deformations.

Measurements of stresses and strains in classical rock mechanics tests can also be used to determine deformations, but generally only in single directions.

 

►	Pore volume Rock consists of solid components with the voids between them making up the pore volume. Deformations of the rock always also affect the voids, and thus the pore volume available for storage.
►	Permeability Flow channels within a rock result from the presence of interconnected voids (pores, cracks, fissures etc.). Deformations change the cross-sections of the flow channels and so modify the permeability.
►	Vertical deformations In some circumstances the vertical deformations resulting from pore pressure changes can be observed as heave or subsidence of the ground surface above the reservoir.

 

The amount of deformation which occurs depends on the rock structure and the change of effective stress (see Stresses).

 

 

The complete deformation always has irreversible (permanent) and reversible (elastic) components. In the case of a change of loading the irreversible components usually dominate. These result in permanent changes in storage volume, permeability etc. With cyclic changes of the pore pressure (and thus of the effective stresses) within a given range the elastic components become increasingly dominant.

 

For further information on the deformation behaviour of porous rock see:

Braun, R.: A method for analysing the deformation of anisotropic rocks. Proceedings of the 7. International Congress on Rock Mechanics, pp 431 - 432, Aachen 1991

Braun, R.; Stammnitz, T.;Schelle, H.;Stromeyer, D.: A laboratory method for analyzing the pore volume change. Proceedings of the 7. International Congress on Rock Mechanics, pp 433 - 435, Aachen 1991

Braun, R.; Stromeyer, D.: Abstraktes Modell der Gebirgsdeformation. Neue Bergbautechnik, Vol 22, No 2, S. 46 - 50, 1992

Braun, R.: Predicting Production Induced Changes in Reservoirs. OIL GAS European Magazine, 3/2006, pp. OG124 - OG129

Braun, R.; Jahns, E.: Cracking and compaction behaviour of sandstone. SPE/ISRM 47377

Braun, R.: Reservoir Pressure and Mechanical Integrity of the Overburden. Erdöl Erdgas Kohle, Vol. 125, No.11, S. 427-432, 2009