Integrated analysis of two-dimensional and three-dimensional images from electrical micro-imaging devices provides a powerful link between core measurements, traditional logging information, and methods of reservoir characterization. Side-by-side comparisons of electrical images and core photographs illustrate the value of this technique for understanding stratigraphic and structural relationships in a given formation. The derivation of a high-resolution quantitative resistivity log from an electrical micro-imaging device offers enhanced petrophysical analysis in highly laminated reservoirs. This quantitative resistivity log, characterized through a three-dimensional electrical modeling code, exhibits a depth of investigation comparable to the shallow laterolog, digitally focused log, or short guard log.
Introduction
Electrical micro-images often create a natural and useful link between the geological characteristics of a given formation and its petrophysical attributes. In many instances where various components of the formation exhibit a large resistivity contrast, electrical images may reveal details that might not be evident from a visual inspection of a core. From the geological point of view, electrical images help describe depositional environments, structural and sedimentary features, diagenetic events, mechanical deformations, and more. Planar events, such as structural and sedimentary dips or faults and fractures, can be readily observed and quantified. Integration of two-dimensional (2D) and three-dimensional (3D) images with available core information helps visualize and understand formation characteristics.
From the petrophysical and the reservoir engineering point of view, electrical micro-images may be first perceived as simply pretty pictures, until they are integrated with a traditional suite of logs. The display of micro-electrical images side by side with the porosity-resistivity set of logs sheds an entirely new light on the observed responses. In particular, laminated reservoirs may now be readily differentiated from low-permeability shaly sands. The same observation applies to low-permeability fractured reservoirs.
One additional feature of modern electrical micro-imaging devices has recently emerged. Some of those devices, such as the Electrical Micro-Imaging tool (EMITM) are indeed resistivity-measuring devices with electrical focusing provided by metallic pads and a metallic tool body raised to an equipotential The mechanical design of the EMI device (Fig. 1), which is based on six fully independent arms and articulating pads, helps obtain reliable quantitative formation resistivity measurements by allowing the sensors to conform to the borehole wall, even where caves or washouts are present.
Computer modeling has been performed to characterize the EMI tool as a resistivity-measuring device. Modeling results show that except for its high bed definition, this electrical micro-imaging device behaves electrically much like a macro-resistivity electrode tool, such as the short guard (SG) or the digitally focused log (DFL). The quantitative EMI resistivity measurement exhibits a significant depth of investigation of several inches from the borehole wall and experiences a minimal mudcake effect. A normalization method is described that recalibrates the micro-imaging resistivity measurement against the more robust DFL measurement (or an equivalent) if high-resolution resistivity is desired.
Electrical micro-images can be used with the special interpretation algorithm, LARATM, in the evaluation of thinly laminated shaly sand reservoirs. LARA is an iterative deterministic model that uses the high-resolution resistivity curves for bed definition, then applies a deconvolution and a modified Waxman-Smits equation to compute hydrocarbon saturation. First, the EMI image is used to precisely identify the laminated intervals and bed boundaries. Then, through the LARA program, the high-definition quantitative EMI resistivity curve is integrated with the standard resistivity and porosity logs. The resulting data can be used to enhance hydrocarbon saturation computations and improve net pay estimates within the laminated reservoir.
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