On the possibility of wettability assessment of oil and gas reservoirs by their main collector properties

Abstract

Problem statement. In some publications, the possibility of determining the wettability according to geophysical studies of wells (GIS), in particular, defined in the complex GIS residual water saturation or water retention capacity. As the main quantitative indicator of wettability, the thickness of the fictitious film of residual water is used. If this idea is true, then the calculation of wettability is possible and the results of determining the same parameters in the course of laboratory studies of core material.

The purpose of this work is to test the proposed method for calculating wettability on the basis of data on the main reservoir properties of rocks obtained in the laboratory and to assess the possibility of practical application of this technique.

Scientific and practical significance. The wettability of the rock surface is an important parameter on which the main indicators of the development of hydrocarbon deposits depend. At the moment, many oil and gas companies are experiencing difficulties in developing long-term fields. This is a breakthrough of water during water flooding, selective flooding of the wells and increased water-repellency of the reservoir in the development process. Taken together, this leads to a decrease in the rate of hydrocarbon extraction, a significant increase in flooding and, as a result, to a decrease in the final indicators of hydrocarbon recovery and significant economic losses. There are many methods of influencing the oil and gas reservoir in order to obtain a cost-effective inflow of hydrocarbons. But whatever method was used, there is a question of control and adjustment of wettability, the solution of which is impossible without determining the real relative wettability of the reservoir with water and hydrocarbons. Core material, which can determine the wettability of standard methods, is not selected enough and the possibility of calculating the wettability of GIS data could fundamentally improve the situation.

Analysis of available publications on the topic. In the course of the work, the theoretical background of the proposed technique, the conclusion of the formula for calculating the thickness of the dummy film and the data on practical application given in the available works were analyzed. According to the authors of the tested method, the degree of hydrophilicity of the productive formation is qualitatively characterized by the content of residual water: the higher its content, the more hydrophilic the rock. To quantify the degree of hydrophilicity, it is proposed to use the thickness of the fictitious film of residual water on the surface of pore channels, which should increase in direct proportion to the degree of hydrophilicity and which is determined by the values of the water-holding capacity of rocks.

Materials of own researches. On the basis of laboratory studies, the results of which are presented in the article, it can be argued that the values of residual water saturation with increasing hydrophilicity can both increase and decrease. The relationship between residual water saturation and wetting angle is ambiguous and should be used with great care to assess the degree of hydrophilicity of rocks.

Quantification of hydrophilicity on the thickness of the dummy film of residual water is complicated by the fact that, first, to determine the film thickness for water-holding capacity is highly problematic, they are too weakly bound, and secondly, the film thickness is poorly and ambiguously connected with the wetting, which used the wetting angle, and the index "M" defined by standard methods. Perhaps it is the proposed formulas for calculating the thickness of the fictitious film and the idea can be further developed, however, at this stage to replace the direct definition of wettability on the calculation of known values of reservoir properties is impossible.

Conclusion. Calculation of wettability by known values of basic reservoir properties determined in laboratory conditions is impossible. We stipulate that this conclusion cannot be unconditionally transferred to the assessment of wettability according to GIS (determination of both residual water saturation and gas saturation coefficient according to GIS has its own specifics), but the impossibility of constructing the desired connections in the laboratory forces caution to approach such calculations. The performed work will help to avoid gross errors in the assessment of wettability, performed for various practical purposes, in particular, in the development of methods to prevent selective flooding of wells.