Utilisation of Extended Slow Heating (ESH) pyrolysis and core analysis for determination of oil saturations and porosity correction.

Extended Slow Heating (ESH) pyrolysis is applied to assess oil fractions in reservoir rocks. It
offers a novel approach to estimate oil saturation, as presented by a case study from the Nini West oilfield
(Danish North Sea). This method enables cost-effective saturation estimation from minimal sample sizes,
using cleaned core plugs to evaluate cleaning efficiency and adjust for porosity underestimation due to
incomplete cleaning. ESH involves gradual pyrolysis of 50 mg dry, ground samples (100–650 °C at
10 °C/min) using a HAWK Pyrolysis and TOC analyser calibrated against oils with known gravity and API
standards. Hydrocarbon release is continuously measured during pyrolysis, and oil fractions were defined
based on their densities. ESH-derived oil saturation and API values aligned with direct measurements,
confirming the method’s reliability. Saturation values in scCO₂-flooded plugs were significantly lower,
suggesting that lighter hydrocarbons were removed, leaving heavier, immobile fractions (solid
bitumen/asphaltenes), although with minor quantities. The study, combining ESH, chromatography,
porosity measurements, and cleaning techniques, reveals that solid bitumen and asphaltenes remain post-
scCO2 flooding and cleaning. ESH is a rapid and effective method for determining oil composition,
saturation, and API gravity. Its low sample requirement allows for spatial oil composition analysis from
cuttings, offering advantages over traditional core analysis methods.