Change in Biot's Effective Stress Coefficient of Chalk During Pore Collapse

Abstract

Biot's effective stress coefficient ( α ) is a measure of how well grains in the rocks are connected with each other. The amount of contact cements between the grains determines the stiffness of rocks. Change in grain contact occurs during natural diagenesis of sedimentary rock. Contact between the grains could also change during elastic deformation of the grains in a rock mechanics test. Diagenetic change in grain contact cement of chalk can be compared with stress-induced change in the laboratory. The change in porosity is studied with reference to the change in effective stress on grain contacts ( σ ' m ). The porosity reduction trend with change in σ ' m indicates that in newly deposited calcareous sediment (in Kerguelen Plateau), porosity reduces at a faster rate, as σ ' m increases until α decreased below 0.95. As contact area between the grains increases (decrease in α ), σ ' m decreases and porosity reduces at a slower rate. We noticed that presence of non carbonates and hydrocarbon could increase σ ' m . During rock mechanics test in the lab, with increased applied stress, σ ' m increases, Biot's effective stress coefficient shows a decreasing trend, while a minor porosity reduction was observed.