Time-dependent compaction of deep-water reservoirs: A study on the viscous behavior of unconsolidated sandstone

There have been many studies on reservoir compaction where different mechanisms being suggested as the reasons behind the changes of porosity, permeability, and integrity of reservoirs during production. The theory of poroelasticity is often used to evaluate the likelihood of reservoir compaction, however, it has failed to explain the time dependent viscous behavior of unconsolidated formations. In this study, attempts are made to have a closer look into the compaction mechanism of deep-water sandstone reservoirs by bringing a case study from a gas field in Australia. The results obtained indicated that depending on the type of clays presented in the rock matrix, confining pressure and the loading rate, unconsolidated/poorly consolidated sandstone may exhibit an elasto-plastic or viscoplastic behavior during compaction. It was also found that the deformations initiated during compaction is a time-dependent process and can be a function of clay type in the sandstone matrix. It appears that kaolinite pushes the sandstone to exhibit an elastoplastic behavior while sandstones with a huge amount (more than 45%) of illite/smectite show a viscous (time dependent) response to hydrostatic loading. It was also revealed that the viscous behavior is induced once the confining pressure reaches a certain threshold (above 10 MPa). It seems that as the loading rate increases from 0.5 MPa/min to 1 MPa/min, a viscoplastic deformation dominates but an elasto-plastic response is revealed once the loading rate reduces to 0.25 MPa/min. It was concluded that the type of clays and their contribution in the rock matrix should be analysed very carefully to determine the geomechanical responses of deep-water reservoirs under different in-situ stress conditions.