Thermo-analytical and spectroscopic characterisation of pore lining minerals in reservoir rocks.

Two methods have been developed for the characterisation of sandstone and carbonate based hydrocarbon reservoir rocks. Particular emphasis has been directed towards determining the surface mineralogy in order to understand the flow of fluids through rocks.The first method employs Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) to distinguish the constituent minerals of a reservoir rock using characteristic bands in their vibrational spectra. Spectra collected from reservoir rocks that have undergone various sample preparations have shown that the spectra are weighted to the surface mineralogy. Spectra that are specific to the surface mineralogy have been obtained by sedimentation and ultrasonication techniques. The feasibility of applying partial least squares (PLS) modelling to DRIFTS spectra of reservoir rocks for quantitative analysis has been discussed. Application of PLS modelling to mixtures of powdered mineral standards has shown that it is possible to estimate the mineral constituents. The second method uses organic molecules as chemical probes to elucidate the surface mineralogy of reservoir rocks. Clay mineral standards which have been exposed to certain chemical probes and subsequently heated show desorption profiles (monitored by thermogravimetric analysis (TGA)) that are characteristic for each mineral. The most suitable chemical probe studied so far is Dimethylformamide (DMF). The desorption profiles of DMF from Mg, Ca, Na and K exchanged montmorillonites are characteristic for each of the clays. Characteristic desorption maxima are observed at 420, 330, 220, 190°C, respectively. The desorption of DMF from Mg-SWy-2 within a powdered mixture of mineral standards can only be detected at the 5% level using TGA, whereas using infrared or mass spectrometry 2% can easily be detected with considerable scope for detecting smaller levels.Extensive studies using Variable Temperature-DRIFTS and Variable Temperature x-ray diffraction on the complexes formed between both DMF and N-Methylformamide (NMF), and, Mg, Ca, Na and K-montmorillonites have shown that molecules retained at higher temperatures are located in the interlayer space and are associated with exchangeable cations via their carbonyl groups. Two layers of DMF and NMF are present in the interlayers at low temperatures which reduces to one upon heating to 50-170°C. NMF is found to be present in hydrogen-bonded clusters.