Stresses
A large number of the tasks in rock mechanics (ensuring the stability of underground openings, evaluating in situ deformations, planning hydraulic fracs etc.) depend for a reliable result on knowing the 3D in situ stresses.
There are very limited possibilities for directly measuring the in situ stresses. Most of these only enable one or two components of the real loading to be estimated. For that reason RACOS® was developed to enable the complete 3D in situ loading to be determined. This package also enables estimates to be made of the new stresses which will apply following changes in pore pressure.
For all tasks it is essential to consider the pore pressure, and its likely alteration, and so to distinguish clearly between total and effective stresses.
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Total stresses These describe the external loading on an element of rock. The most obvious example is the vertical total stress resulting from the weight of the overlying strata. This stress can be determined directly from the depth and the average density. Another (directly measurable) value is the closure pressure in a hydraulic frac. This corresponds to the magnitude of the minimum component of total in situ stress.
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Pore pressures These are the pressures exerted by the fluids which most rocks contain. Because of the real rock structure the pore pressure is mechanically only partially effective, and the degree to which it is effective generally varies with direction. This reduction in its mechanical effectiveness is described by a directional parameter, the pore pressure effectiveness.
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Effective stresses These stresses always describe the relevant loadings for the evaluation of deformations and of stability. They are defined as the (direction dependent) difference of the total stress and the effective pore pressure.
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Changes of the in situ stresses Changes in the in situ stresses can occur following changes made in the pore pressure, and independent of the local secondary stresses forming around underground openings. Knowledge of the changes of in situ stress is essential for prior assessments to be made of in situ deformations and of the related modifications of porosity and permeability. |
For further information on the in situ stresses see:
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
Braun,
R.: Critical Borehole Orientations - Rock Mechanics Aspects. OIL
GAS European Magazine, II/2010, pp. OG75 - OG79
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