Determination of T2 cut-off for shale reservoirs: a case study from the Carynginia formation, Perth Basin, Western Australia

Abstract

Over recent decades, the low-field Nuclear Magnetic Resonance (NMR) method has been consistently used in the petroleum industry for the petrophysical characterisation of conventional reservoirs. Through this non-invasive technique, the porosity, pore size distribution and fluid properties can be determined from the signal emitted by fluids present in the porous media. Transverse relaxation (T2) data, in particular, are one of the most valuable sources of information in an NMR measurement, as the resulting signal decay can be inverted to obtain the T2 distribution of the rock, which can in turn be correlated with porosity and pore size distribution. The complex pore network of shales, which can have a large portion of pore sizes in the nanopore and mesopore range, restricts the techniques that can be used to investigate their pore structure and porosity. The ability of the NMR technique to detect signals from a wide range of pores has therefore prompted the quest for more standardised interpretation methods suitable for shales. Using low-field NMR, T2 experiments were performed on shale samples from the Carynginia formation, Perth Basin, at different saturation levels. The shale samples were initially saturated with brine and the T2 spectrum for each sample was obtained. Then, they were placed in a vacuum oven and their weight monitored until a constant value was reached. T2 curves were subsequently obtained for each of the oven-dried samples and a cut-off value for clay-bound water was calculated.

Determination of Clay Bound Water in Shales from NMR Signals: The Fractal Theory

“…Testamanti and Rezaee 9 found that conventional centrifuge speed would not be enough to remove the water from the tiny pores, which could lead to inaccurate T 2 data for estimating the CBW in shale samples. Increasing the centrifuge speed could extract the water in the small pores.…”

“…Increasing the centrifuge speed could extract the water in the small pores. 9 However, Yuan et al 10 found that under 4500 rpm, a maximum capillary pressure of 315 psi will be created, which might eventually damage the samples. Furthermore, the thermal treatment provided by Testamanti and Rezaee 9 could be an alternate way for more accurate CBW estimation.…”

“…9 However, Yuan et al 10 found that under 4500 rpm, a maximum capillary pressure of 315 psi will be created, which might eventually damage the samples. Furthermore, the thermal treatment provided by Testamanti and Rezaee 9 could be an alternate way for more accurate CBW estimation. In their study, the samples were heated in a vacuum oven with a series of elevated temperatures to dehydrate the shale samples and the T at each stage.…”