A new laboratory evaporation method (EM) of the water content measurement of the low-permeability rocks with a water content of less than 1 wt.% is developed. The EM offers a rapid, efficient, and accurate analysis (error range 0.2÷6.8 rel.% if applied to real rock samples) of the free and loosely clay-bound water.
The EM was practically implemented and tested on a representative collection of 24 whole core samples (Ø 10 cm) of source-rock Bazhenov formation (BF) with the maximum preserved natural pore water content. The BF is developed within the West Siberian Plate in the Russian Federation at depths of 2–3 km, age J3v, with a fairly stable thickness of 15–50 m. To date, a reliable determination of water saturation and content is one of the important, but unsolved problems in the development of oil assets within the BF.
Using the EM for the first time a presence of free and loosely physically bound water was quantified for the BF rocks. The residual water content, that is, free and part of physically bound water in the pore space of the rocks reducing its permeability to oil varies from 0.05 to 4.27 wt.%, while the content of free water is 0–3.81 wt.%. At the same time, there was no relationship between the pore water content and depth established for the target wells. The spatial variation of residual water content takes place most likely due to the heterogeneous mineral composition of the BF rocks. The presented research has shed more light on the presence and distribution of the free and loosely clay-bound water components in the reservoir rocks of the BF. The loosely clay-bound water content measured using the developed method directly correlates to the bulk clay content. The free water content, in contrast, does not depend on the rock mineral composition. Estimated values of residual water salinity reach tens of grams per liter; the corresponding isotopic composition indicates the deep formation genesis and generally correlates with that of the deep stratal waters of the West Siberia.
The dataset of the residual water content of the BF rocks will be used for reliable estimations of hydrocarbon reserves (oil, gas, bitumen), for thermodynamic modeling of the equilibrium composition of pore water and for construction of adequate petrophysical models for well log interpretation, including the spontaneous potential, electric, dielectric, neutron, and nuclear magnetic resonance logging methods. Moreover, the obtained results on the water content of various water types form a basis to review ideas about the predominant hydrophobicity of the BF rocks.
This work was supported by the Russian Science Foundation (grant No. 17-77-20120).