We have conducted computational fluid dynamics simulations of the flow in standard oilfield Couette rheometer cells (often referred to as Fann rheometers), in order to calculate the correction factor linking torque measurements to wall shear stress in the annular gap between the rheometer rotor and the stator. Our results show that for visco-plastic fluids, correction factors required for a correct determination of rheological parameters are significantly larger than the values estimated for Newtonian liquids and vary with the imposed shear rate. We have developed an inversion method which dynamically includes shear-rate-dependent correction factors. Results are validated with synthetic data, as well as capillary tube experiments. (C) 2017 The Society of Rheology.