It is very common that effective stresses increase as reservoir fluids being produced from both shallow and deep reservoirs. It may seem reasonable to assume that permeability and porosity decrease as pore pressure declines, since effective volumetrical stresses become intensified during reservoir depletion. However, laboratory results show that this is not always the case.
A series of very delicate experimental procedures was conducted to reveal some of the most interesting phenomena in pore collapse and their impact on permeability. Sandstone samples were tested using a triaxial set-up. Experimental results show that porosity is certainly decreases as a result of the compaction process, which allows the breakage of grain-to-grain cement bonds. Grain particles will become more compacted as both lateral and axial effective stresses increase. On the other hand, permeability shows no definite trend.
In weak reservoir formation, pore collapse does not occur suddenly. Rather, rocks gradually compact as grain-to-grain cement bonds break down. It was found that permeability indeed changes as effective stresses increase. Nevertheless, the pathway to permeability was found to be much more complex than previously stipulated. It was discovered that enhancement or damage to permeability is not a function of pore collapse alone. Other factors, such as stress path, initial porosity, particle size, and particle shape and distribution play a major role in determining the type of permeability alteration and the severity of this change.
As reservoir production continues during pressure depletion process, effective stresses within the reservoir increase. It may seem reasonable to assume that the effect of stresses on porosity and permeability of the reservoir is more severe when porosity and permeability are high, although some experimental studies like Hubbert and Willis1, Voight2, and Rosepiler3 showed this effect is still significant even at low porosity and permeability. It is also understood that stress paths have a large influence on horizontal and vertical permeability and also on porosity.
The elastic uniaxial strain model is mostly used in reservoir engineering to describe production-induced changes in horizontal stress due to pore pressure decline (pressure depletion). It predicts the total horizontal stress by using overburden stress, reservoir pressure decrease, and material mechanical parameters. The principal assumption in this model is that there is no lateral deformation (zero horizontal strain condition) during the depletion process.
For a sandstone rock, Ruistuen et al.4 showed that the ratio of change in minimum effective horizontal stress to the change in effective vertical stress in a reservoir depletion process was 0.53. This effective stress relationship was believed to be the same for production-induced or geologically-induced changes in pore pressure, i.e. before reservoir disturbed by production. Schutjens et al.5 concluded that in an elastic domain of deformations, permeability reduction is predominantly controlled by mean effective stress increase and not by stress path.
Depletion of the reservoir may contribute to the failure of the formation in two ways. Pore collapse is one of the mechanisms which in fact is a volumetric failure. This mechanism is mainly activated where lateral displacement is either zero or small. In this case, shear failure cannot take place and the only mechanism for material disaggregation would be pore collapse through volumetric failure. For this mechanism to be activated, material must have a high porosity and low strength. If the stress path meets the cap, i.e. volumetric failure surface, volumetric failure takes place. Another failure mechanism induced by depletion is shear. As the reservoir pressure depletes, effective stresses increase. The increase of effective stresses around the wellbore deforms perforation cavities and shears them. Depletion induces shear stress increment, which adds to the shear stress induced by pressure drawdown. As depletion increases, shear failure develops, which, in the worst condition, fails it.