Electric Resistivity of Vuggy Fractured Media

Abstract

A theoretical and experimental study on electric resistivity of vuggy fractured media is presented. In order to have a rigorous control of the involved variables, a vuggy fractured medium is idealized by a cubic array of cubes having hemispherical cavities drilled in each cube face. In this model, the spaces between the cubes represent fractures and the hemispherical cavities represent vugs. The theoretical developments lead to a simple relationship expressing formation resistivity factor as the product of two factors, one depending on vug porosity and the other on fracture porosity. This formulation has been validated with experimental data obtained with a special resistivity cell. The proposed formulation can easily be generalized to be applicable to real rocks, and so it is a useful tool for interpretation of electric well logs of vuggy fractured formations. Introduction It is well known that an important part of the world oil production comes from vuggy fractured reservoirs. This type of reservoirs are commonly found in Saudi Arabia, Iran, Iraq, Mexico, Oman and Syria, hence the importance of developing reliable analytical formulations concerning the geometrical properties, storage capacity, and flow properties of the porous space. A practical way of knowing the internal geometry of a porous medium consists in using the so called formation resistivity factor.1 By knowing the formation resistivity factor, it is possible to determine, for instance, the magnitude and type of porosity, and the tortuosity. However, in the case of vuggy fractured media, no previous well established expressions relating formation resistivity factor and the various kinds of porosities have been proposed to date. In this paper, expressions for the formation resistivity factor of vuggy fractured media are developed in terms of fracture and vug porosities. To establish these formulations, the vuggy fractured medium is idealized by a cubic array of cubes with hemispherical cavities drilled in each face. In this model, the spaces between the cubes represent fractures, and the hemispherical cavities represent vugs.2 The theoretical work is based on the idea that, in a vuggy fractured medium saturated with a conducting fluid, the vugs are zones of very low resistivity (or very high conductivity) in comparison with that of the fractures. In this way, one arrives at an equation expressing formation resistivity factor as the product of two components, one depending exclusively on fracture porosity, and the other on vug porosity. This equation was validated with experimental data obtained with a special resistivity cell. The Fractured Medium Due to the geometrical complexity of a vuggy fractured medium, a rigorous analytical study of the formation resistivity factor is not an easy task; however, by making certain logical idealizations it is possible to manage this problem in a relatively simple way. But before entering the study of vuggy fractured media, the case containing no vugs will be considered in this section. The simple fractured medium (i.e., with no vugs), has previously been treated analytically by several authors. For its study, the following basic considerations are made:When a potential difference is applied across the medium saturated with a conducting fluid, the electric current flows much more easily through the fractures than through de matrix, so that the current through the matrix can be considered as negligible, andthe fractured medium can be idealized by a cubic array of cubes (Warren and Root model3), as shown in Fig. 1. This same model has been used by Towle4 and by Aguilera5 for studying the relationship that exists between porosity and formation resistivity factor of fractured media.

Experimental Study of Water and CO2 Flooding in the Tight Main Pay Zone and Vuggy Residual Oil Zone of a Carbonate Reservoir

“…23 The detailed experimental setup and procedure were described in the previous work. 17 The measured equilibrium IFTs between the Steelman light crude oil and CO 2 at 17 different equilibrium pressures of P eq = 1.08−18.66 MPa and T res = 51.1°C are 0.40 C 29 0.65 C 4 1.59 C 30 0.57 C 5 3.94 C 31 0.57 C 6 8.26 C 32 0.53 C 7 12.27 C 33 0.37 C 8 8.19 C 34 0.34 C 9 7.66 C 35 0.41 C 10 6.52 C 36 0.31 C 11 6.04 C 37 0.30 C 12 4.58 C 38 0.21 C 13 4.41 C 39 0.27 C 14 3.92 C 40 0.27 C 15 3.65 C 41 0.13 C 16 2.95 C 42 0.12 C 17 2.55 C 43 0.17 C 18 2.43 C 44 0.17 C 19 2.12 C 45 0.10 C 20 1.66 C 46 0.10 C 21 1.84 C 47 0.09 C 22 1.02 C 48 0.08 C 23 1.31 C 49 0.08 C 24 1.12 C 50+ 1.34 C 25 1.07 total 100.00 C 26 0.96 24 On the basis of the measured data in Figure 2, the equilibrium IFT γ eq (mJ/m 2 ) is correlated to the equilibrium pressure P eq (MPa) by applying the linear regression in the above-mentioned two equilibrium pressure ranges, respectively. …”

Characterization of a Vuggy Fractured Media: A Practical Approach

“…Experimental results 9 obtained with this physical model show that, when vug porosity is zero (no cavities), and fracture porosity is small (cubes near each other), the relationship between fracture porosity, φ f , and formation resistivity factor is approximately given by…”

Detecting vuggy porosity in carbonate reservoirs by well logs

“…Vugs plays an important role in reservoir quality, storage capacity and flow properties of porous space, and can be found in two classes: separate vugs and touching vugs (Lucia 1983;Perez-Rosales et al 2002). In general, the productivity of a reservoir will increase where the fractures are decreasing and the touching vugs are expected (Akram et al 2010).…”

“…For high effect of reliable analytical formulations on storage capacity, geometrical properties, and flow properties of porous space, these formulations have been developed (Perez-Rosales et al 2002). Vugs can be defined as visible pores that are significantly larger than adjacent grains or crystals.…”

“…Matrix porosity of Towle's models is considered as zero (Towle 1962). The previous studies on detecting vuggy porosities include: a study by Aguilera and Aguilera on carbonate reservoirs by triple porosity model which are composed of mainly matrix, fractures and non-connected vugs using petrophysical evaluation (Aguilera and Aguilera 2004) developing an expression for the formation resistivity factor of vuggy fractured media in terms of fracture and vug porosities (Perez-Rosales et al 2002).…”