Improved oil recovery from chalk reservoirs by water-flooding may cause mechanical weakening and change in elasticity. Confined compressive strength testing of chalk from a North Sea reservoir was done in water-saturated and oil-saturated conditions. During testing, elastic wave velocities were sampled by ultrasonic transducers, so that subsequently Biot's coefficient could be modelled. The porosity declined via an 'elastic phase', a 'transitional phase', an 'elastoplastic phase' and a 'strain hardening phase', but Biot's coefficient indicates that these terms may be partly misleading. In the 'elastic phase', porosity and Biot's coefficient decrease, indicating elastoplastic deformation. In the 'transitional phase', Biot's coefficient increases as a reflection of breaking contact cement (pore collapse), whereas Biot's coefficient remains stable in the 'elastoplastic phase', indicating elastic deformation on the virgin curve. Plastic deformation takes place during phases of creep, where both porosity and Biot's coefficient decrease. Similarly, in the 'strain hardening phase', both porosity and Biot's coefficient decrease as a reflection of elastoplastic deformation. For chalk with 45%-47% porosity, the 'transitional phase' begins at 8 MPa axial stress when water-saturated and at 12 MPa when oil-saturated. For chalk with 41%-43% porosity, the corresponding stresses are 16 and 20 MPa. For chalk with 32%-36% porosity, the corresponding stresses are 23 and 31 MPa. Chalk samples with irreducible water saturation and movable oil were water-flooded. They yield at stresses close to corresponding oil-saturated samples, but after flooding show compaction trends not significantly different from the water-saturated samples. Water-flooding promotes pore collapse as reflected in an increasing Biot's coefficient. The consequent softening effect on acoustic impedance is small as compared with the effect of increasing fluid density. With respect to 4D seismic, water-flooding causes distinctly higher acoustic impedance and Poisson's ratio irrespective of compaction.