Since the introduction of the first micro-electrical imaging tool in 1986, wireline resistivity images have proven to be an invaluable tool for geological and petrophysical formation evaluation in wells drilled with conductive water-base drilling mud (WBM). However, until recently, wellbore images acquired in non-conductive mud had been met with some less success due to poor borehole coverage, relatively low image resolution and electrical artefacts.
In 2014, an OBM-adapted imaging tool was introduced. The new tool was designed to provide improved resolution and borehole coverage as well as geological representativeness of the images. From an operations perspective, the tool sonde and hardware were designed to increase robustness and ease of logging for field engineers, and to improve operational efficiency and reduce rig time in consideration of high spread rates for deep-water drilling rigs and the overall high costs of deepwater wells.
The sonde design with two sets of pads supported by spring loaded arms allow both logging down and logging up of the tool to minimize logging time. Unlike previous imaging tools, pads are applied to the formation using spring load and not pad pressure, in order to minimize stick-slip of the tool. Pads are fully gimballed, are free to tilt, and rotate around the pad axis to enable maximum contact with the borehole wall.
As for the measurement physics, a high frequency current is sent into the formation which reduces the non-conductive mud electrical impedance. Amplitude and phase of this current are measured and used in the processing to create an electrical impedivity measurement. In order to cover the full range of formation resistivities, two frequency ranges are used. After acquisition, a "composite" processing technique is used in which amplitude and phase measurements from the two frequencies are processed to generate a final impedivity image that is a function of formation resistivity and dielectric permittivity.
The case study presented in this paper is an Oligocene-Miocene age deep-water turbidite deposits on the passive margin of West Africa, and comprises a complex of channels and sheet sands with localized intense faulting, and tilting due to salt tectonics and diapirism. The high-resolution image enabled highconfidence classification of geologic features. The variety of geologic features ranges from fine-scale laminations and syn-depositional micro-faults with displacement of a few centimeters to variable-scale injectite features and erosive surfaces. Also, a wide variety of formation textures that represent turbidite channel and levee facies are observed, and include coarse-grained basal conglomerates, rip-up clasts and large clay clasts, debrites, dewatering and flame structures, dish structures, internal injectite structures, pyrite nodules/streaks, and deformed facies. The high resolution image can be used for a wide range of quantitative image analyses such as net pay computation, textural attribute extraction, as well as other quantitative and semi-quantitative interpretations.
Today, with more than 13 case studies in West Africa and more than 250 worldwide, the image quality from this new formation imaging technology shows a great deal of improvement over previous generations of non-conductive mud imagers. The ultrahigh-resolution images from the new imaging service enables a wide spectrum of interpretations that can be directly incorporated to enhance the reservoir model and reduce geological and petrophysical uncertainties.
