Lab assessment of NMR and resistivity derived porosity in shallow marine sediments
Abstract:
A thorough characterization of geological and geotechnical parameters must be conducted for the design and execution of offshore wind turbine foundations.
In doing so, deriving a good estimate of the porosity of shallow deposits is challenging. The possibility of a stuck tool in an unconsolidated borehole makes the traditional gamma-gamma bulk density log prohibitive.
Also, laboratory measurements incur in a long waiting time, are costly, do not describe the whole borehole (point measurements, difficulty in sampling sands), and have a wide measurement spread (procedural inconsistencies in reconstituting samples).
In this context, a low-field nuclear magnetic resonance (NMR) tool could be used to generate continuous porosity logs along a borehole. In this study we investigated the feasibility of using NMR to characterize unconsolidated sediments by comparing its outputs with conventional lab methods for porosity.
As a possible alternative to NMR-derived porosity, which is not a particularly widespread technology in wireline logging and yet not available in cone penetration tests, we also derived porosity from electrical resistivity measurements, comparing accuracy and uncertainty of these two methods.