2D NMR - Quantifying Oil Volume in High Clay Content Low-Salinity Reservoirs
Authors: Boyle Keith , Zuraidah Zuraidah , Manescu Adrian , Bal Adriaan 
Published: December 2009

Oil was discovered in Jurassic shaly sand reservoirs in central Australia in the mid-1960's. Following the initial discovery numerous fields and a considerable number of wells have been drilled targeting these formations. The oil-bearing formations were deposited under fluviatile-lacustrine conditions with fluviatile, low-energy mixed load the predominant facies type. Historically shaly sand evaluations from conventional logs in the presence of fresh formation waters (salinity in the range of 3 – 4 kppm NaCl equivalents) were problematic due to the subtle and sometimes non-existent resistivity contrast between the hydrocarbon and water-bearing zones. The complex mineralogy of the oil-bearing sandstone is characterised by feldspathic litharenites with volcanic lithic components. The oil varies across the basin but is generally light (API 35 – 50°) with low GOR (under 200 scf/bbl). This paper shows how the acquisition and interpretation of Nuclear Magnetic Resonance (NMR) data provides, not only porosity and pore size distribution, but also identifies and quantifies oil shows in the invaded zone over the logged intervals.

Earlier generation NMR tools acquired only one (or a maximum of two) hydrogen spin-echo decay curves. These measurements were inverted into apparent T2 spectrums, which are separated into clay-bound, capillary-bound and movable fluid volumes,. However, this technology does not provide sufficient information to type and quantify reservoir fluids.

New-generation NMR tools acquire many more hydrogen spin-echo decay curves. Several spin-echo decay curves focus on the very early T2 decay times, with the goal of improving the response of clay-bound fluids. Other decay curves focus on medium T2 decay times with the target of improving capillary-bound fluid definition.

The biggest advantage of the new-generation NMR tools is the acquisition of decay curves at multiple inter-echo times (TE), wait times (TW) and frequency acquisition with additional focus on longer T2 decay times. The acquisition of these long T2 echo decay curves allows for reservoir fluid typing and the quantification of fluid saturation based on 2D-NMR techniques. These techniques invert the multitude of decay curves by treating two NMR properties as independent variables. The main NMR properties are: T2 – Transverse Relaxation Time; T1– Longitudinal Relaxation time, and D – Diffusivity. The inversion process produces 2D maps of two of these NMR properties which are particularly useful at identifying hydrocarbon type and for discerning water from hydrocarbon.

NMR data MR ExplorerSM (MReXSM) service was recently acquired in five wells across a number of fields. The acquisition mode was Porosity Permeability Light Oil (PPlo), in which more than twenty spin-echo decay curves are recorded with the target of characterizing light-oil reservoir fluids. The 2D-NMR maps of Diffusivity vs. T2,int were used to quantify the oil volume. This technique produced along-hole maps of diffusivity and intrinsic T2 distributions. Because D and T2 are related to viscosity an estimation of the oil viscosity was also possible.