Determination of Formation Specific NMR Calibrations for Water Well Evaluation in a Semi-Consolidated Aquifer

Abstract

Downhole NMR data was collected through a fibreglass cased section of water well NG3A that intersected the semi-consolidated sediments of the Yaragadee Formation which is one of the main aquifers supplying domestic water for the city of Perth. The main purpose of this work was to empirically derive formation specific NMR processing parameters to match the hydraulic property estimates to a direct core measurements, to calibrate the system to deliver a more detailed log of porosity and permeability in the hole as well as other holes logged in a similar geologic environment.

Equations for NMR permeability estimation generally include lithology specific calibration coefficients. We show that application of “generic” calibration coefficients derived for unconsolidated aquifer materials overestimate permeability in the Yaragadee Formation when compared to core permeability measurements. This result is expected given that the Yaragadee formation is not unconsolidated but is partially consolidated/cemented. More appropriate site specific coefficients were derived by scaling the calibration coefficients to produce NMR permeability estimates that match the measurements derived from the core samples. The site specific coefficients determined for the Yaragadee are consistent with those derived in previous studies for partially consolidated aquifers in the United States.

The NMR T2 distribution is very sensitive to pore size changes which often reflect subtle changes in the sedimentary geology of the formation. This can provide additional geological information which may not always be apparent in mud or core logging. Detailed knowledge of both the sedimentary geology as well as the hydraulic properties of the formation that can be obtained by NMR are likely to be useful in identifying the best place to place screens during the design of a production water well to generate the best possible yield.