New Methodology for NMR Interpretation in Lower Cretaceous Chalk Reservoirs

It is well known that interpretation of NMR logs on the basis of core-derived parameters often fails because the down-hole situation is much different from that in the laboratory. Core measurements at full in-situ conditions are in principle possible, but very expensive and therefore bound to span a limited range of properties. We demonstrate that the NMR response at reservoir conditions can be accurately obtained by forward modelling, using water-saturated core samples measured at ambient conditions only. In-situ modelling was used to calibrate a variable T2-cutoff for the prediction of bound water from the NMR log in good agreement with the resistivity in well A, drilled with water-based mud. Permeability correlations calibrated to in-situ conditions, produced values that compare favourably with core data where available. Well B, drilled with oil-based mud, was easier to interpret because the oil filtrate does not change the water saturation; the bound water signal is clearly separated from that of the oil, yielding robust saturation values without the need of any calibration parameters.