Porosity With Nuclear Magnetic Resonance in Naturally Fractured Clastics Reservoirs in the Devonian of the Bolivian Sub-Andean

Aguirre, Omar Oscar (Repsol YPF) | Glorioso, Juan Carlos (Repsol YPF) | Morales Lujan, Jeannette Julieta (Schlumberger) | Mengual, Jean-Francois (Schlumberger)

OnePetro 

Abstract
We have validated with superior results that the direct measurement of porosity using Nuclear Magnetic Resonance (NMR), in Naturally Fractured Clastics Reservoirs of very low porosity (˜ 3.5%) in the Devonian of the Bolivian Sub-Andean, reveals information till now incoherent compared with core data. As it is well known, when the rock does not have paramagnetic elements, the porosity measured with the NMR is not affected by the minerals within the matrix and the tool answers mainly to the contained fluids in the pores of the rock. This peculiar characteristic of the NMR response in these low porosity reservoirs, with complex and variable lithology, become fundamentally beneficial at the time of determining an immediate porosity value with less uncertainty in comparison to the one from conventional logging tools, such as the Neutron, the Density and the Sonic, where there is a need to assume variable values of density and transit time for the matrix.

To corroborate that the obtained effective NMR porosity, is the best to be easily and truthfully correlated to true formation porosity, core data information are available. The key to obtain a reliable and precise measurement of porosity through NMR in these complex environments is based on the optimum selection of the acquisition parameters for the tool, like the polarization time, the echo-spacing and the use of a fit-for-purpose T2 cutoff time, tailored for this type of reservoirs. Furthermore, it will be demonstrated that the effects of nuclear diffusion on the transversal relaxation time distribution (T2 mode), primarily caused by gas, are not significant in these reservoirs, since an underestimate of the porosity was not noticed with regard to the one from cores.

Additionally with the aim of obtaining a better correlation among cores and NMR porosities, it has been used a specific high resolution acquisition and processing method, achieving continuous porosity measurements with a dynamic vertical resolution of 22 inches, more suitable to the sampling core interval and to the real petrophysical characteristics of our fields.

Introduction
The gas-condensate reservoirs from Devonian age of the Bolivian Sub-Andean that lie between the 3000 and 5500 m of depth, have been produced by the existence of a thick column of clastics sediments of scarce to null primary intergranular porosity, but with an important development of secondary porosity for fissures and fractures taking place during the characteristic tectonics of this region. The lithology of these Devonian formations is characterized mainly by shoreface sediments; quartzitic sandstones, litharenites, micaceous, and laminated sandstones together with shaly intervals that can be found in the productive formations of the region (i.e. Huamampampa, Icla and Santa Rosa). For e-log interpretation purposes, we have classified lithology of the Devonian formations under three main petrofacies: a) quartzitic sandstones, b) micaceous and laminated sandstones and c) shaly intervals. Sandstones composition is rather complex except for the massive quartzitic sandstone bodies because quartz, volcanic lithics, mica, and minor accessory heavy minerals are present in the rock composition concurrently with thin shale-silt laminations over certain intervals.

In these formations the petrophysical analysis is affected by strong limitations in core analysis and log interpretation, due to the very low porosity and naturally fractured reservoir environment. Despite of this, the measurement of a reliable porosity with the NMR technique in these naturally fractured clastics reservoirs, has demonstrated to be a viable and reliable alternative, this means, without the necessity of assuming a fixed lithology parameters that have turned out to be variable in these kind of formation and not easy to estimate with conventional logs.

The selection of the acquisition parameters for the tool takes here an important role. For the wells showed as example in this paper, the acquisition was done with a logging speed of 700 ft/hr corresponding to a wait time of 15.8 seconds, enough for the complete polarization of the fluids near the wall of the borehole, with a number of echoes of 3000 and an echo spacing of 0,2 ms (200ms). The high resolution EPM mode (Enhanced Precision Mode) allowed the acquisition of short CPMG pulses of 30 echoes each one with a repetition of 10 times for better signal/noise ratio at fast relaxation decay. After an exhaustive LQC, the data are reprocessed to improve the overall NMR response and signal by using the CMRTM application, part of the customary GeoframeTM software platform.