Application of photoacoustic spectroscopy to characterize thermal diffusivity and porosity of caprocks

Abstract

Photoacoustic spectrometry (PAS) was used to determine thermal diffusivity and porosity of three sealing caprocks originating from oil and gas fields in Brazil. In the petroleum industry developments in the low permeability rocks characterization are a key issue for safe exploitation. Also to assess the potential for using depleted reservoirs as a geologic repository for radioactive waste disposal or CO2 sequestration involves the knowledge of thermal properties parameters in caprocks. The experimental PAS technique using the open photoacoustic cell (OPC) method was employed to determine the porosity through correlation based on the thermal diffusivity values for consolidated and unconsolidated caprocks using small amounts of rocks and a few adjustments of empirical equations. The values obtained for thermal diffusivity were between 0.01667 (±0.0013) and 0.09298±(0.00018) cm2/s while the porosity ranged from 1.42% to 9%. It was observed that thermal diffusivity declines as the apparent porosity of the samples decreases. The variations in thermal diffusivity between the analyzed gas and oil fields were supported by field emission scanning electron microscopy and X-ray diffraction analyses, while porosity values were in a good agreement from those measured using the combination of nitrogen gas adsorption and mercury intrusion capillary pressure techniques. The use of PAS data on determining thermal diffusivity and subsequently low-porosity of porous media, such as caprocks, has not, to the best of our knowledge, been previously reported in the literature.