Application of Fourier Transform Infra-Red Spectroscopy in determination of reservoir properties
Authors: Woods John , Gaafar Gamal Ragab 
Published: May 2016

Fourier Transform Infrared Spectroscopy (FTIR) is a technique to determine qualitative and quantitative features of IR-active molecules in organic or inorganic solid, liquid or gas samples. It is a rapid relatively inexpensive method for the analysis of solids that are crystalline, microcrystalline, amorphous, or films. New advances make sample preparation straightforward.

FTIR spectroscopy is used by geochemists to determine mineral structure (1), to quantify volatile element concentrations, structural changes in natural minerals and to calibrate data from remote sensing (2).

To obtain the best possible IR spectra of samples it is necessary to choose the appropriate IR source, detection method and accessories. For this study I have used attenuated total reflection (ATR) which involves transmitting the IR beam through a crystal that has a moderately high refractive index which results in near-total internal reflection. The instrument used was the Bruker Alpha with a diamond press ATR. This allowed the analysis of small quantities of ground powder with a typical mass of less than 0.5g. The procedure results in absorbance spectra that are reproducible to within +/-5% standard deviation. A set of mineral standards was produced from pure mineral powders of calcite, dolomite, quartz, siderite, apatite, plagioclase feldspar, k-feldspar, kaolinite, chlorite, illite and smectite. Further to this standards were produced from a hydrocarbon powder which allowed quantitative analysis to be undertaken of both organic and inorganic components simultaneously. The sample spectra were evaluated using specific peak picking which produced an average absolute difference between the known and derived mineral concentrations of +/- 3% wt%. The technique does not require a size separation step to assess the weight percent of clay minerals. This therefore means that it has been possible to apply this technology for accurate and comprehensive determination of the major rock forming minerals and organic components at wellsite. Further this the mineralogical and organic components can be used to provide a calculated lithology, acid insoluble component and brittleness index all available at low cost and with a speed that allows key wellsite decisions to be made.

Introduction

A fundamental component of sedimentary formation description is mineralogy. Traditionally the technique of X-ray diffraction (XRD) has been the prevalent methodology to provide this information. However, the technique has some inherent problems in quantitative analysis mostly due to particle size and orientation coupled with natural variability in mineral diffraction spectra, this is particularly notable in respect to clay minerals. The common practice of fines separation to enhance clay mineral identification introduces additional error since not all clay minerals are finer than the usual 2µm cutoff (3).

FTIR spectroscopy is an alternative method for acquiring quantitative mineralogy. The mineralogy of a mixture can be extracted from its FTIR spectrum because minerals exhibit most of their fundamental vibration modes in the mid-infrared (4000–400 cm-1) (4–7) and the absorbance bands of each component in the mixture are proportional to the pure mineral spectrum (8–11) The latter is known as Beer's Law: