Bioclastic deposits constitute important carbonate Pre-salt reservoirs, sustaining for decades a significant oil production from offshore Brazilian fields. Nuclear magnetic resonance (NMR) logs show highly homogeneous reservoirs, contrasting with formation tests and production logging data, which indicate heterogeneous productivity from these reservoirs. In order to understand the effects of pore types on the mercury intrusion capillary pressure (MICP), NMR and conventional petrophysical data, we conduct a comparative study based on petrography, conventional petrophysics (porosity and permeability), MICP, laboratory NMR analyses to determine the T2 distribution (samples at SW=1 brine saturated, and at SWirr brine/isoparaffin), and determination of SWirr by centrifugation.
The selected samples were characterized according to conventional petrography, including the identification of predominant pore type, clasts and pore sizes. As revealed by the petrographic characterization, samples were divided in two distinct sets: one with moldic porosity and another with interparticle porosity, the former presenting higher irreducible water saturation values than the latter. Analyzes of MICP in rocks with predominance of moldic porosity indicate narrower pores throats than rocks with predominantly interparticle pores, which might reflect on low permeability. When NMR calculation was performed (e.g. Coates equation), rocks with moldic porosity showed slightly lower permeability values than those with interparticle porosity. However, according to petrophysical laboratory data, rocks with moldic porosity have significantly lower permeability values. As for centrifuge analyses, SWirr values were about 9% higher than those obtained with standard 100 ms cutoff on T2 relaxing time, showing weak correlated with each other. The distributions of T2 in water saturated samples show consistency with MICP in some samples; although, distribution of T2 and MICP pore size distribution show poor correlation. Likewise, the analysis of T2 distribution with samples at SWirr shows no relation with the pore size or distribution of pore throats. NMR technique is not able to identify the pore size when the reservoir is saturated with a wetting phase (water) and a non-wetting phase (oil/oil based drilling mud). MICP cannot be related to pore size in bioclastic rocks, considering that moldic pores are often connected by narrow pore throats. Moldic porosity rocks are less permeable and have slightly higher SWirr content, which is detected by NMR.
This work shows that the evaluation of bioclastic carbonate reservoirs by NMR without rock data support may fail on predicting permeability as the duality of porous system of these rocks generates opposite permeability patterns which cannot be easily identified in NMR logging.
