Drill cuttings can provide quick and useful information for reservoir characterization, especially when cores are not available. So far, several petrophysical properties can be determined from cuttings: porosity, permeability and T2 NMR distributions (directly related to the pore size distribution) Several laboratories are working to improve the techniques of measurements and also the drilling tools to minimize the damage to the rock. In this paper, we present new results for porosity and permeability measurements down to cuttings of diameters around 0.5 mm and the application to a field test.
The methods for measuring the porosity on cuttings are the same as for cores when the sizes are larger than 3mm. For smaller cuttings (most of cases), the best results are obtained using a centrifuge or a porous plate to remove the interstitial liquid with a controlled capillary pressure. With this method, porosities were measured within 1-2 PU (Porosity Units), even for the flat cuttings with diameter around 1mm and thickness around 0.5 mm collected during the field case.
For permeability, we used the technique developed by IFP based on the diffusion of a pressure wave in a viscous liquid saturating the cuttings. The results were improved by a better choice of the viscosity of the liquid used to saturate the cuttings. The suitable viscosity must satisfy two opposite constrains: high viscosity for a relaxation time large enough to be measured (larger than 0.2 second), and low viscosity to allow spontaneous imbibition in a short time. The results of the study are presented in a diagram, showing the viscosity to be used as a function of cuttings sizes and permeability. Porosity and permeability methods have been tested on synthetic cuttings obtained by crushing cores of known parameters.
Finally, the methods were tested on real cuttings sampled especially for this study every 20 cm in a carbonate formation (125 samples). The results are discussed and compared to porosity logs obtained in the same well. The porosities measured on cuttings agree with the log value for values larger than 5 PU. The T2 NMR relaxation times (peaks) are also in good correlation with the amount of clay given by the gamma ray log. However, the porosities and the sizes of cuttings are two small for permeability determination. Both porosities and T2 relaxation times give a good depth matching prediction with log less than 1 meter.