Summary

We have studied the induced polarization (IP) response of multiphase porous systems by conducting complex resistivity (CR) frequency-domain IP measurements for sands and sandstones samples containing salt water in pores and those whose unsaturated pores were filled with synthetic oil. The results of our study show that the oil - saturated sands and sandstone samples are characterized by a significant IP response. We used a generalized effective-medium theory of induced polarization (GEMTIP) model to analyze the IP parameters of the measured responses. A conceptual GEMTIP model of polarizing clusters is proposed to explain the observed IP phenomena. Our studies confirm earlier geophysical experiments with the application of the IP method for hydrocarbon (HC) exploration.

Introduction

The measurement of the electrical induced polarization (IP) effect has proven to be one of a few geophysical methods providing in situ information about rock mineralogy, especially in the search for disseminated minerals with electronic conductivity. At the same time, the method has been applied to study the earth materials that do not contain conductive minerals, like sedimentary rocks. The previous IP studies of nonmetallic earth materials were focused on clay mineral soils, sandy and shaly sediments containing clay minerals (Klein and Sill, 1982). Laboratory studies of the electrical characteristics of such rocks show diagnostic signatures of what they consist of and thus, can lead to a proper classification of rocks in terms of the presence of clay and other materials. At the same time, the study of reservoir rocks was limited and did not include a quantitative analysis of the relationships between the petrophysical parameters of the rock samples and the IP responses.

The polarization phenomenon was previously studied in detail by Wait (1959) and its modern development stems largely from the work done by Bleil (1953). The theoretical foundations of the IP effect in complex multiphase heterogeneous rocks were developed by Zhdanov (2006; 2008a, b).

In this paper we study the IP response of multiphase porous rocks by conducting complex resistivity (CR) frequency-domain IP measurements. This study is based on laboratory analysis and modeling of induced polarization and resistivity measurements on sandstone samples from southern Utah. These results are compared with those obtained for synthetic rock samples, prepared using chemically pure sand, de-ionized (DI) water, and synthetic oil.

Preparation of the samples

We prepared two types of samples for a measurement. The first type is represented by the sandstone samples collected in southern Utah. They were cut to 30×30 mm cross section and 40 mm length size and polished to produce a rectangular shape. The second type of samples was formed by artificially prepared water and oil saturated sands and oiled sands. Figure 1 shows the photograph of these samples. The following steps describe the sample preparation.

1. Sieved commercial EMD Co. chemically purified quartz sand (0.1- to 0.25-mm grain size) was cleaned from clay and dust particles by repeated washing. The quartz mineral density was about 2.62 g cm-3. After saturation in saltwater at the first stage and in synthetic oil at the second stage, as described below, the samples of the second type were placed in a sample holder, a polyvinyl chloride (PVC) tube (a cartridge).

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