RACOS^®

 

RACOS^® (Rock Anisotropy Characterization On Samples) can be used to determine the 3D in situ stresses, and their changes resulting from modification of pore pressure, and also the related deformations and the changes in significant rock parameters (pore volume, hydraulic conductivity etc.). The procedure involves special tests on a small quantity of core. It requires no on-site activity, can be carried out at any time after the core was taken and makes no prior assumptions about the rock behaviour.

 

The analyses are based on the measurement of the directional propagation characteristics of shear and compression waves in loaded samples of rock to determine the magnitudes and directions of the principal propagation velocities.

 

If oriented coring was not used the core to be tested can be geographically reorientated using various procedures. One relies on viscous remnant magnetism in the rock and another on the comparison of scans of the core surface with images of the geographically reorientated wellbore wall. The information can also be checked in RACOS^® using geological and tectonic data for the cored area.

 

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Stresses In RACOS® the 3D effective stresses are determined directly using special analyses of the changes in the rock structure which occur at the time of coring. The total stresses are then calculated using the pore pressure and the pore pressure effectiveness (derived from the deformation analysis). Effective paleo loading conditions can also be determined. With RACOS® it is also possible to calculate in advance the in situ stress changes resulting from modification of the pore pressure.

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Deformations For these, in addition to the elastic wave velocities on loaded samples, the density and porosity on unloaded samples are also measured. The loading-dependent axial deformations of a loaded plug are measured in order to calibrate and check the analysis results.   ● Elastic deformation and pore pressure effectiveness Loading-dependent elastic deformation parameters are determined in RACOS® on the basis of elastic theory with the measured characteristics of the elastic waves and the density of the samples. From these parameters the elastic components of deformation and the pore pressure effectiveness can be calculated directly.   ● Complete deformation To evaluate complete deformations in RACOS® the measured loading-dependent compression wave velocities and the initial density and porosity of the relevant samples are used to determine the loading-dependent changes in porosity. The 3D deformations of the rock and of its pore space are then determined using the deformation parameters for the solid components derived from the measured characteristics of the elastic waves.

 

For more information on RACOS^® see:

Racos^®-Info
Braun, R.; Jahns, E. & Stromeyer, D.: Ermittlung von Gebirgsstrukturen und -belastungen. Erdöl Erdgas Kohle, Vol 115, No.4, S. 191-197, 1999
Braun, R.: Predicting Production Induced Changes in Reservoirs. OIL GAS European Magazine, 3/2006, pp OG124 - OG129
Braun, R.: A Commonly Neglected Factor in Rock Mass and Borehole Stability. OIL GAS European Magazine, 2/2007, pp. OG79 - OG82

Braun, R.: Consideration of 3D Rock Data for Improved Analysis of Stability and Sanding. OIL GAS European Magazine, 2/2008, pp. OG64 - OG68

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