Interpretacja współczynnika nasycenia wodą w skałach klastycznych typu non-Archie
Authors: Lis-Śledziona Anita 
Published: September 2024

The monograph presents new techniques of water saturation calculation in non-Archie clastic rocks. The issue was discussed in three different areas where new proposed methods of water saturation calculation were tested. This was done because the term non-Archie rocks encompasses a wide variety of rocks in which is not possible to use one interpretation approach. Areas selected for the analysis cover documented oil and gas accumulations which have been the subject of interest of Polish oil and gas companies in the recent years. Due to the fact that water saturation coefficient is the final product of the borehole interpretation, it was also necessary to calculate other parameters that were used as inputs in Sw calculation. Those parameters include: clay volume, effective porosity and irreducible water saturation. The work consists of a theoretical and practical part. The first one describes models used in the interpretation of the Sw coefficient and the parameters closely related to the values of the water saturation: wettability, capillary pressure, irreducible water content. While the practical part consists of four chapters presenting the models and the possibilities of calculating the water saturation coefficient in various types of clastic rocks: • heterogeneous Middle Cambrian sandstones with significantly reduced values of the reservoir parameters: effective porosity and permeability • Lower Paleozoic shale plays • gas saturated multihorizons developed in a form of mudstones and shaly-sands in the Sarmatian and Upper Badenian sediments. Different types of clastic rocks require a separate interpretation approach. All the models presented in the practical part were applied and tested on the well log data from the selected areas. The proposed methods used mostly the basic well log data and the results of laboratory measurements carried out on the core samples. The analyses performed in oil and gas bearing heterogeneous Middle Cambrian sandstones with significantly reduced values of reservoir parameters (porosity and permeability) allowed to calculate the continuous curve of wettability through the reservoir interval. The wettability was expressed through the values of water connectivity index correlated with the values of the wettability measured on the core samples. The analyses showed that we can observe different wettability systems within the reservoir, depending on the lithology and the reservoir parameters - from the water-wet system in the claystones and mudstones, through the neutral-water wet system in tight sandstones, to the neutral to slightly oil wet system in intervals with relatively high porosity and permeability. Shale formations are rocks in which the source rock is simultaneously the reservoir rock. Kerogen present in the rock, similar to gas, influences the increase in recorded resistivity values. Additionally, besides free gas, adsorbed gas also occurs in the rock. Shale gas reservoirs require not only a different interpretive procedure but also costly stimulation techniques such as hydraulic fracturing and drilling a large number of directional wells. In gas saturated Miocene deposits, developed as shaly-sands and mudstones with higher content of clay minerals (40-45%), porosity logs: NPHI, RHOB, DT were additionally used in the detection of gas saturated intervals. The presence of hydrocarbons, especially gas, has an impact on the recorded values on these well logs and cause that they were considered as an additional valuable source of information that was used as input data in the modified Passey equation. An obvious advantage of the presented approach is the availability of input data, even in archival boreholes drilled in the 1980s and 1990s, which allows use of the presented methodology in almost every well, without the need for expensive laboratory measurements or the availability of advanced new technique of well logging. This fact is of a great importance in the context of reservoir modeling and reserves analysis, where it is important to perform a well log interpretation in a large number of boreholes. Keywords: water saturation coefficient, wettability, non-Archie rocks, Montaron’s model, Miocene, Middle Cambrian sandstones, gas-bearing mudstones