Laboratory investigation of effective stresses' influence on petrophysical properties of sandstone reservoirs during depletion

An increase in effective stresses takes place in reservoirs as a consequence of fluid production, a well-known phenomenon in both shallow and deep reservoirs. It may seem reasonable to assume that permeability and porosity decrease as pore pressure declines, since both effective radial and axial stresses become intensified during reservoir depletion. However laboratory results show that this is not always the case. Porosity 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 this paper, a series of delicate experimental procedures were conducted to reveal some of the most intriguing phenomena in pore collapse and their impact on permeability. Sandstone samples were tested using a triaxial set-up. Based on the experimental results of this study, in weak reservoir formations, 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. However, 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, particle shape and particle distribution play a major role in determining what type of alteration in permeability occurs and to what magnitude that alteration would be.