The state of effective stress within a reservoir is extremely important if the permeability of the formation is stress sensitive. Generally the lower the permeability of the formation the more sensitive to effective stress changes it is.
This is particularly the case for coals where an increase in effective stress of 3 MPa (435 psi) may lead to an order of magnitude drop in permeability.
Fluid withdrawal from the reservoir leads immediately to an increase in effective stress. In the case of coals and other carbonaceous reservoir rocks another effect takes place. When gas desorbs from the coal it tends to shrink, which leads to a stress reduction. The two effects oppose each other.
The stress path is the term used to describe the changes in effective stress with fluid withdrawal.
Sigra models the stress path using as inputs:
- Initial stress measurement from
- Tectonic strain determination from field measurements of stress + Young’s modulus + Poisson’s ratio
- Hydrofracture field measurements
- Mechanical Properties
- Young’s modulus versus effective stress characteristic
- Poisson’s ratio
- Shrinkage with gas desorption
- Absorption Behaviour
- Reservoir pressure
- Gas sorption pressure/gas content
- Sorption isotherm
Using this information Sigra creates plots of the effective stress change that may be expected with fluid pressure reduction. An example of such a plot is given in the Figure 1. below.
Figure 1. Example of changes in seam effective stress with a high shrinkage behaviour ; stiff coal with a sorption of 2.9 MPa. The upper two curves (red and blue) indicate the range of the major horizontal permeability while the lower two curves (black and mauve) indicate the range of the minor permeability.