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An experimental study on the complex resistivity of fractured rock under different saturation conditions

Pan Baozhi, Ruhan A., Guo Yuhang, Zhang Lihua, Wei Boyang, Zhou Weiyi

Acta Geophysica

2022

Vol. 70, no 3

1061--1081

The fracture can be a good channel for oil and gas migration, which has a great influence on the permeability of the reservoir. Therefore, it is of major significance to identify fractures and determine the characterization parameters and physical proper ties of fractured reservoirs. In this study, homogeneous sandstone was used to simulate different artificially fractured rocks. The fractured rock samples had different fracture widths, fracture numbers, and fracture dip angles. In addition, the complex impedance and weights of the rock samples were measured during the process of natural evaporation, and the relationships between the water saturation and the complex resistivity values at different frequencies were examined. The frequency range is 100 Hz–10 kHz. It was found that the influence effects of frequency on the resistivity, dielectric constant, and loss factor had differed among the homogeneous samples and the fractured rock sample. The fracturing had led to the resistivity index and the water saturation curves separating under the different frequencies, and the degree of the dielectric constant index and water saturation curve separation became larger. Furthermore, the influencing effects of the fracture widths, fracture numbers, and fracture dip angles mainly occurred in terms of three aspects. The first was the slope of the resistivity index and water saturation curves (IR–Sw). The second was the slope of the dielectric constant index and water saturation curves (Iε–Sw), and the third was the loss tangent D and water saturation Sw curves.

Fractured-bedrock aquifer studies based on a descriptive statistics of well-logging data: A case study from the Dajia River basin, Taiwan

Chou P.-Y, Hsu S.-M, Chen P.-J, Lin J.-J., Lo H.-C

Acta Geophysica

2014

Vol. 62, no. 3

564--584

A simple descriptive-statistical approach is proposed for evaluating the transmissivity of fractured-bedrock under field conditions. Based on standard well logging and identification of structural discontinuities from borehole televiewer, the approach consists of four steps: (1) determination of discontinuity properties, namely the frequency of discontinuities and the ratio of open fractures along the borehole; (2) ranking the three well logs and the discontinuity properties into tertiles (high-to-low); (3) performing statistical analysis (F-test) for the three well logs to assess the significance of open fractures on the water-transmitting potential and then assigned a weighting factor; and (4) a semi-quantitative indicator, RGVFO, is obtained by multiplying all predictive indexes and weighting factors. It is a multi-criteria evaluation procedure that elucidates the quantitative and qualitative importance of all selected geophysical indexes. A high cross-correlation between transmissivity to RGVFO value is found, which indicates that by a combined consideration of well logging and structural discontinuities, an appropriate estimate of bedrock water-transmitting potential can be derived.

Reassessment of the effect of water saturation on intact rock and fractured Rrck

Reddish D. J., Li Z., Sheng Y.

Archives of Mining Sciences

2006

Vol. 51, no 1

13-34

Water is one of the many factors influencing the strength of geomaterials. In this paper, multistage triaxial compressions were performed to investigate the weakening effect of water on intact and fractured sandstone specimens. Laboratory testing results are presented and discussed with particular reference to the change of the mechanical properties of the rock. A trend of strength reduction for both intact and fractured rocks can be generalized from the testing results. It is also observed that the fractured rock is more reactive to the water even though no apparent soluble gouge is sandwiched in the related rock fractures. Additionally, theoretical formulations are derived to characterize the strength reduction on the basis of the understanding of the physical process. Properties of a single rock joint are correlated with the tangential plastic energy of the shearing plane with the non-associated plastic flow rule during shearing. The degree of saturation Sw is introduced into Patton's model as an additional parameter to predict the shear strength evolvement with the degree of water saturation.

Woda jest jednym z wielu czynników decydujących o wytrzymałości geo-materiałow. W niniejszej pracy przeprowadzono próby wielostopniowego trójosiowego ściskania skał, w celu zbadania w jaki sposób woda będzie zmniejszać wytrzymałość nienaruszonych i popękanych próbek piaskowca. Przedstawiono i przedyskutowano wyniki badań laboratoryjnych, zwłaszcza w kontekście zmian właściwości mechanicznych skał. Na podstawie wyników badań zauważono wyraźny trend do obniżenia wytrzymałości zarówno próbek skał nienaruszonych jak i popękanych. Zauważono także, że skały popękane bardziej reagują na wodę, pomimo, że w pęknięciach nie stwierdzono żadnych substancji rozpuszczalnych. Ponadto, opracowano teoretyczne wzory określające obniżenie wytrzymałości, w oparciu o zachodzące procesy fizyczne. Właściwości pojedynczego ciosu skały skorelowano z energią pochodzącą od odkształcenia stycznego płaszczyzny ścinania bez powiązania z zasadą płynięcia plastycznego w czasie ścinania. Do modelu Pattona wprowadzono stopień nasycenia Sw, jako dodatkowy parametr pozwalający na przewidywanie zmian odporności na ścinanie przy zmianie stopnia nasycenia wodą.