This study provides a new rock-typing approach for low-resistive and low-permeable clastic rocks. The approach includes integrated interpretation of routine core analysis data with microstructural characteristics, acquired from computed tomography (CT) and nuclear-magnetic resonance (NMR) data. The studied formation comprises siltstones in its bottom, which are replaced by sandstones in its top. Sandstones form the main part of the oil reservoir, whereas siltstones were originally considered as water-saturated. The reserves calculation was performed based on a single Archie equation for the whole formation. Despite on apparent water saturation and low permeability of the siltstones, incidental perforation showed considerable oil inflow from them as well. In order to delineate missed productive intervals within the low-resistive siltstones, we had to develop a new rock-typing approach, acknowledging rock multimineral composition, diversity of microstructures, a wide range of porosity, permeability, and residual water saturation values. Designed laboratory program included porosity, permeability, electrical resistivity measurements, capillary, NMR and CT tests. The experiments were performed on the same core samples that enabled reliable correlation between measured parameters. The joint interpretation of flow zone indicator, calculated as a function of porosity and residual water saturation, together with the results of petrophysical and microstructural measurements allowed reliable rock-typing of the clastic formation. It will serve as a petrophysical basis for identification of the missed productive intervals. The developed laboratory program and rock-typing algorithm can be implemented in other oilfields.

This article describes the effects of different physico-chemical factors on formation damage caused by migration of in situ clay particles as a result of water injection into a clastic reservoir.

The proposed method of predicting spatial distribution of leaching zones in carbonate reservoirs includes integrated interpretation of well logging data, including electrical microimages (Formation MicroImager – FMI), together with analysis of lithofacies within depositional cycles. Based on the comparison of FMI data with results of lithological and petrophysical studies of the core, an atlas of diagenetic porosity images for the studied formation was developed. Choquette and Pray international classification was used for pore typing. Applying the developed atlas to FMI logs, karstification zones were identified and classified in reference wells. In wells, where only standard well loggings was performed, karstification zones were identified using a developed decision tree. Sedimentological and sequence stratigraphic analysis made it possible to identify sedimentation cycles and to correlate the main lithofacies within the target reservoir. It was established that microbial and shallow facies form the most productive part of the reservoir. These facies with initially high primary porosity were subjected to the most intensive karstification along sequence stratigraphic boundaries due to subaerial exposure. The established relationships between leaching zones, lithofacies, and sequence-stratigraphic boundaries made it possible to carry out inter-well correlation of karstified intervals. The correlation will allow construction of leaching zones maps, that will be further applied for geological modeling.

The article describes the effect of different physico-chemical factors on formation damaged caused by in-situ clay particles migration resulted from water injection into clastic reservoir.

The paper suggests a new laboratory tests workflow, aimed at reservoir characterization and rock typing of a polimineral clastic formation, based on microstructural characteristics. The proposed approach is accompanied by a comparative study of various laboratory techniques, including CT, NMR, MICP, centrifuging, and thin section analysis. The methodology for combining quantitative porous medium characteristics, derived from these multi-scale tests is also provided. The developed laboratory test program includes porosity and permeability measurements, capillary pressure, electrical resistivity, NMR and CT-scanning tests. For initial differentiation of samples based on porosity, permeability and irreducible water saturation, we applied methods of flow zone indicator (FZI) and Winland R35. Afterwards, we performed joint interpretation of FZI with the results of centrifuging, resistivity, NMR and CT-scanning tests. Finally, for the most representative samples from the rock types, defined at previous steps, we performed X-ray fluorescence spectroscopy, XRD, MICP and thin section analyses. In our study, we applied FZI, calculated as a function of irreducible water and porosity. Compared to conventional FZI, derived from porosity and permeability, our approach enabled more reliable rock typing in both sandstones and low-permeable siltstones. CT tests showed that within the distinguished 4 rock types grain and pore sizes increase from the 1st to the 4th rock types. CT-scanning and X-ray fluorescence spectroscopy detected presence of pyrite in all samples. Siltstones contain more pyrite than sandstones that reflects deeper marine and consequently more anoxic depositional environment. The obtained NMR T2-spectra appeared to be effected by both porous medium and mineralogical composition. Pyrite inclusions caused shifting of the T2-spectra to lower values, while carbonate presence – to higher ones. Therefore, since NMR signal is effected by multiple factors, applying of a single T2cutoff value for reservoir characterization and rock typing can lead to ambiguous interpretation. The proposed algorithm for combining quantitative parameters of the porous medium allowed integration of pore size distributions, derived from CT and capillary tests. This enabled characterization of the whole range of pore sizes covered only partly by the different methods. The resulted pore size distribution enabled calibrating NMR Т2 spectra and defining individual surface relaxivity values for each rock type. The selected set of techniques allowed confident differentiation and characterization of the rock samples based on multiple parameters, which were interpreted in relationship with each other. Moreover, the developed methodology was verified by applying additional laboratory tests, which confirmed the results of the proposed rock typing and reservoir characterization.