New Ideas in Petrophysics: Different Mechanisms for Changing the Initial Hydrophilic Wettability of Reservoirs as a Factor Influencing the Oil Saturation Coefficient According to Well Logging

One of the important properties of the rock is the degree of water wettability of the pore space’s surface, which affects most petrophysical, geophysical and hydrodynamic parameters: residual water saturation, electrical resistivity of oil and gas saturated rocks, oil and gas saturation coefficients (S_o), displacements, relative phase permeability. Omission of wettability leads to geological and recoverable reserves distortion. Therefore, a comprehensive analysis of the geological conditions and petrophysical properties of reservoirs should begin with establishing the type of sediment wettability. When establishing a heterogeneous (non-hydrophilic) type of wettability, the standard method for determining So using the electrical GIS model must be adjusted using a special technology for core research, taking into account surface properties or using the P_n-s_w dependence of the “core-GIS” type. There are a number of diagnostic features of rock wettability, such as the mineral composition of the rock and clay components, the properties of water and oil, the structure of the void space, electrical resistance along the core and GIS, knowing which, it is possible to make qualitative assessments of the rock wettability type. The combination of various diagnostic factors leads to different mechanisms of sediment hydrophobization, which have different effects on the P_n-s_w relationships for determining S_o and on reserves estimation. Therefore, it is important to study the combination of signs that influence on wettability on the rock in natural conditions, rather than the influence of individual parameters.

When studying sections of productive wells using geophysical GIS and core methods, it was found that the GIS WPP in the productive section is a reliable value of the natural wettability of productive rocks in the well section. The values of RP, as a standard for the wettability of productive rocks according to GIS, allow us to select reasonable petrophysical P_n-s_o dependences for determining reliable coefficients of oil and gas saturation according to GIS. This paper collects and analyzes the main and most important factors affecting wettability, obtained from extensive studies of deposits in the TimanPechora oil and gas basin from north to south, in productive formations covering almost the entire productive section from P₂ to S₁. The article considers an example of the formation of different mechanisms of sediment hydrophobization for two types of heterogeneous reservoirs of terrigenous composition P₂-T₁ and D₂st.

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