The emerging technology of borehole imaging coupled with the advances ingraphics software will shape the future of formation evaluation because
imaging reveals new opportunities for exploration in unconventional and previously neglected reservoirs,
imaging exhibits the complex distribution of porosity and permeability throughout the reservoir, and
imaging enhances the knowledge of the physical dynamics of the reservoir for improved stimulation techniques.
Traditional methods of formation evaluation have relied upon the interpretation of data recorded from a wireline tool in the borehole. While no wireline tool directly measures the porosity or the resistivity of the formation, these properties have served as the basis for analysis of formations for some seventy years. The advantage of borehole imaging is the ability tovisualize the many variations of porosity types, permeability, and lithology changes within the rock. Borehole imaging utilizes either acoustic or resistivity recording devices to graphically display the surface features of the borehole wall in the open hole environment. This paper presents only theresistivity based tool, the electrical micro-imaging tool as the source for the following case studies.
The electrical micro-imaging tool is a six arm pad device with twenty-five button electrodes mounted on each pad (Fig. 1). Each button electrode emits an electrical current into the formation for a distance of fifteen inches. The changes in the permeability, porosity and lithology of the formations near the borehole wall are reflected as resistivity variations in the electrical current return to the individual electrodes. A system of magnetometers references the tool's orientation to magnetic north. Utilizing a computer workstation, these resistivity variations are processed into a two dimensional image that represents the surface of the borehole wall with respect to magnetic north(Fig. 2). The images are presented in either a gray-scale or color display with a standardized color code representing the• variation in formation features. Low porosity, low permeability, and high resistivity features represent the light end of the color spectrum. The high porosity, permeable, and low resistivity events are scaled as the darker colors.