Structural and Petrophysical Characterization of Dolomites

Abstract

In order to characterize main structural and petrophysical properties of characteristic pre-salt carbonate rock samples (Silurian dolomites) High Speed Centrifuge (HSC), Nuclear Magnetic Resonance (NMR) and Mercury Intrusion Porosimetry (MICP) were used. X-ray diffraction (XRD) mineralogy analysis and Energy-Dispersive X-ray Spectroscopy (EDS) showed that samples were composed manly by dolomite. From transverse relaxation time (T2) spectrum of the samples at water-saturated condition the pore size distribution was obtained and pore-throat size distribution was obtained by MICP measurements. The surface relaxivity and the specific pore surface area distribution were estimated by correlating NMR and MICP data. The T2cutoff values were obtained using the Coates methodology and it was observed that the desaturation occurred mainly from the macropores, and it allowed to infer about pore geometry complexity, in a qualitative way. Samples presented multimodal distribution and the continuous T2 spectrums reflected good connectivity between small pores and large pores. Capillary pressure curves obtained by HSC are function of sample average water saturation, which presents a non-uniform saturation profile along the core. In order to convert mean saturation for the inlet face, three different models were used (Hassler and Brunner, Skuse, and Forbes), and it was observed a shifting of the capillary pressure curves to lower irreducible water saturations. From these curves pore size distribution index and relative permeability curves where obtained using Brooks and Corey correlation. The wettability was quantified by 3 different methods: Amott-Harvey Index, USBM Method and RNM Wettability Index. The measurements obtained on the two first methods showed a preferential water-wet surfaces while NMR index did not provide a correlated data compared these methods. Structural and petrophysical characterization of rock samples can be performed using techniques that are relatively fast without compromising the accuracy of the results. In the present study multiple techniques were used in order to characterize main properties of typical carbonate reservoir rock (dolomites): mineralogy, porosity, pore structure, capillary pressure and wettability. X-ray diffraction (XRD) mineralogy analysis and Energy-Dispersive X-ray Spectroscopy (EDS) indicated that samples were composed manly of dolomite. From Nuclear Magnetic Resonance (NMR) and Mercury Intrusion Porosimetry (MICP) measurements was Around 90% of the porous structure was composed by macrostructures (macropores and macrothroats). The test of mercury injection apparatus showed that the pore size of samples ranged from XXX to XXX. The surface relaxivity and the specific pore surface area distribution were estimated by correlating NMR and MICP data.

AAPG Datapages/Search and Discovery Article #90325 © 2018 AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin, Lisbon, Portugal, May 2-3, 2018