Pore-water Distribution from NMR to Study Water-induced Compaction in High-porosity Chalk

High porosity chalk at full and partial saturation was isotropically loaded to 5 MPa, and beyond yield by<br>use of a standard Triaxial cell. We investigate the NMR response when chalk cores were loaded beyond<br>yield and compare with the result before yield. Both the CPMG and pulse field gradient sequence were<br>run, in order to measure the T1 and T2 relaxation, and the restricted diffusion. Measurements were<br>performed at fully and partial saturated (Sw < 9%) cores. The measurement of the restricted diffusion<br>coefficient gave limited information, because of limitations in the experimental equipment. The 1/T1,2<br>relaxation rate correlated with the deformation. The NMR signal from the partial saturated cores showed<br>no response to the applied stress level. The signal was dominated by two relaxation rates, which we argue<br>corresponds to the surface film (~10 nm) and non invaded pores (~0.1 μm). The fact that the partial<br>saturated cores showed no correlation with the applied stress level could be an indication that the forces<br>controlling the strength of the rock are of much shorter range than what we probe with the NMR machine.