Determining irreducible water saturation based on well log data and laboratory measurements

Abstract

Unconventional resources, explored recently in Poland, require alternative methods of interpretation as the methods used in the conventional reservoir characterization often lead to misleading solutions that differ from the results of field tests and laboratory analyses. This paper presents novel methods of estimating irreducible water saturation in the tight gas sandstones based on the well log data and the results of laboratory analyses, including the NMR results. The first stage of the interpretation included calculating the quartz and shale volume, and calibrating the results with the laboratory measurements (XRD). Respectively, porosity and permeability were calculated and the pore-size distribution was evaluated. Based on porosimetry measurements, there were two porosities determined with pore-sizes below 1 µm and above 1 µm. Based on the relationships between laboratory NMR results and parameters calculated using well log data, capillary water content and mobile water saturation was estimated. The final stage of the analysis involved the calculation of irreducible water content by the following methods: evaluating the NMR data, using the Hong model lv (Hong et al., 2017), and calculating irreducible water saturation according to Zawisza’s formula, and assuming that irreducible water saturation is related to pore space with pore-diameter below µm. Irreducible water saturation identified with critical water saturation is an important parameter which should be taken into account when determining the water saturation. When the Montaron model is used, the critical water saturation values, Sc, are crucial for the correctly calculating the Water Connectivity Index (WCI). The aim of this paper was to assess the suitability of using the results of NMR measurements performed in the tight gas formations in determining irreducible water saturation. The recognition of pore diameters enables to determine the pore space occupied by free hydrocarbons and hydrocarbons stored in “tight”, micro-pore space, the exploitation of which will require the fracturing process.