Validation of Porosity of NMR Cuttings

Drill cuttings are ubiquitous in oil and gas well drilling operations, but beyond mud logging, cuttings are often discarded without analysis. Recently, there has been an up-tick in employing nuclear magnetic resonance (NMR) rock core analysis techniques to cuttings to unlock more information about oil and gas wells. Deriving porosity of drill cuttings via NMR T2-pore size distribution can be significantly influenced by surface fluid on the cuttings, acquired either during saturation of the cuttings in the lab or from cutting fluid at the well site, which can easily be misinterpreted as pore volume. Previous attempts to eliminate the surface fluid signal employed a cutoff to the pore size distribution curve to distinguish fluid in the pores (below the cutoff) from fluid on the surface (above the cutoff). Unfortunately, the use of cutoffs is prone to error as the position of the cutoff is difficult to determine accurately as signal from fluids in the pores can easily overlap with signal from fluids on the surface of the cuttings. In recent years, we have developed a technique which eliminates signal from fluid on the surface of the cuttings removing the need to apply a cutoff. In this paper, we present a validation of this technique for determining porosity of cuttings based on NMR T2 distribution measurements. A survey of hundreds of samples has been completed validating this technique for the porosity determination of cuttings. The paper will also include a discussion on the utility of the method on various rock types and reservoirs as well as an investigation of the advantages and limitations of the method. Specifically, the NMR porosity of cuttings measurement (applied to both polycrystalline diamond compact (PDC) and non-PDC cuttings) has shown porosities within 5% of the NMR porosities obtained from rock core plugs and logging data. In addition to deriving the porosity, the T2 distributions (related to pore size distributions) can unlock a whole new suite of parameters which can be derived from cuttings including the relative size of the pores and the bound vs. free fluid content in a well. Results will be shown that validate the T2 distribution measured from cuttings in comparison with the T2 distribution measured from core plugs. Most recently, work has focused on extending this NMR cuttings measurement with other techniques commonly found in NMR rock core analysis and NMR well logging. T1-T2 NMR in traditional measurements has been used for fluid typing and organic content characterization.