Mechanical compaction of silt-clay mixtures: Simulating porosity and permeability development in mudstones during burial

Category Sedimentology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Mondol, Nazmul Haque; Fawad, Manzar; Jahren, Jens; Bjّrlykke, Knut
Holding Date 23 September 2008

The porosity and permeability development in mudstones as a function of burial is not well constrained making pore pressure prediction and fluid flow modelling difficult. Prediction of porosity and permeability in natural mudstones is challenging because of the uncertainties associated with textural and mineralogical composition. This experimental compaction study has recorded the porosity-permeability evolution of thirteen brine-saturated synthetic mudstones consisting of silt (quartz) and clay (kaolinite) mixtures. The oedometric mechanical compaction tests were performed under vertical effective stress up to 50 MPa.
Results show that mineralogy is a major factor controlling the evolutions of porosity and permeability in these synthetic mudstones. Clay-dominated samples have much lower porosity and permeability compared to silt-dominated samples along the whole stress path up to 50 MPa. At 20 MPa effective stress pure clay was compacted to about 20% porosity and 1 mD permeability whereas the pure silt sample had 35% porosity and 40 mD permeability at the same effective stress.
The porosity reduction was not systematic with increasing clay content in silt aggregates; however a systematic permeability reduction with increasing clay content in silt aggregates was recorded. The permeability difference between pure silt and pure clay aggregates was relatively less at low effective stresses but increased significantly with increasing effective stress. At high effective stresses (>25 MPa) the permeability were 2-3 orders of magnitude lower in pure clay than in pure silt aggregates. Calculating permeability from porosity without considering mineralogy and textural relations will therefore introduce significant error in permeability prediction in mudstones. Our experimental results provide valuable constraints on porosity-depth/stress and permeability-depth/stress relations in mudstones that will have practical use for pore pressure prediction and fluid flow modelling in shallower part of the basins (<80-100°C) where mechanical compaction is the dominant process.