Applying Magnetic Susceptibility to Estimate Permeability From Drill Cuttings: A Case Study Constraining Uncertainty in the Culzean Triassic Reservoir
Authors:   TotalEngeries E&P UK Limited, Banks John Y. , Tugwell Andrew G. ,   Advanced Downhole Petrophysics, Potter David K. ,   University of Alberta
Published: August 2024

The accurate prediction of well productivity and zonal contribution depends on robust reservoir permeability characterization. Establishing reliable permeability models for heterogeneous and heterolithic Central North Sea Triassic sandstone reservoirs is extremely challenging due to complex and variable relationships between the depositional environments, lithologies, grain-size distributions, and diagenetic history (Kape et al., 2010; Aro et al., 2023). Permeability predictions outside core control of these reservoirs involve multicomponent relationships with wide ranges in the uncertainty of each input (Farmer, 2023). The consequence is a significant residual well productivity prediction uncertainty (Van der Post et al., 2015). To improve the productivity prediction, a quick, cheap, independent estimate of permeability was sought for the entire reservoir intervals of appraisal and development wells in Culzean. The solution was to estimate overburden-corrected permeability from a high-field magnetic susceptibility measurement technique applied to initially wet drill cuttings. Confidence was established by comparison with measured permeability from core plugs and slabbed core in the cored control intervals. In particular, permeability estimates derived from magnetic susceptibility measurements on drill cuttings showed an excellent correlation (R2 = 0.949) with the measured Gaussian-averaged overburden-corrected core plug control permeabilities. This indicated the potential usefulness of the magnetic susceptibility technique in intervals beyond core control. The successful results provided a significant reduction in permeability profile prediction uncertainty throughout the reservoir interval. The method is quick, cost effective, and utilizes drill cuttings that are readily available in all drilled wells. The study provided insights into how to judiciously apply the technique to estimate permeability profiles in other wells with little or no core. In particular, the use of high-field (rather than low-field) magnetic susceptibility removes most of the magnetic susceptibility signal from ferrimagnetic minerals (e.g., magnetite) or ferromagnetic metallic contaminants (e.g., from the drill bit and casing wear) since these components should saturate in high fields. This is important since permeability in this study is controlled by paramagnetic clays like illite. Ferrimagnetic and ferromagnetic components have high positive magnetic susceptibilities and could skew the results even in low concentrations. In practice, some ferromagnetic contaminant signals remained after the high-field treatment, so we introduced a further step to remove such contaminants with a magnet prior to each magnetic susceptibility measurement. Regarding limitations, the current methodology assumes paramagnetic clay is the key control on permeability, is dependent on the cuttings sampling frequency, and doesn’t give anisotropic permeability information.