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Hati, Samhita (Schlumberger) | Chawla, Hemlata (Schlumberger) | Ghosh, Arnab (Schlumberger) | Guru, Udit (Independent Consultant) | Ray, B. B. (Independent Consultant) | Guru, Rakesh (Oil and Natural Gas Corporation) | Pattanaik, Sambit (Schlumberger)
Abstract As oil and gas exploration and development forays into unconventional plays, more specifically, basement exploration, its characterization and understanding have become very important. The present study aims at understanding the reservoir quality in terms of complex mineralogy and lithology variations, porosity, fracture properties and distribution near and away from the borehole using an integrated approach with the help of elemental spectroscopy, borehole acoustic imager, borehole micro-resistivity imager, nuclear magnetic resonance and borehole acoustic reflection survey. A comprehensive petrophysical characterization of different mineralo-facies of basement was carried out using elemental spectroscopy, formation micro-resistivity imager, borehole acoustic imager and combinable magnetic resonance along with basic open-hole data. Two distinct rock groups were identified – silica rich, iron poor zones having open fractures with good fracture density, porosity and aperture and silica poor, iron rich zones with no open fractures, poor fracture density, porosity and apertures. The zones with open fractures were the prime zones identified for further testing and completion. However, the near well bore analysis could not explain the oil flow from one zone having open fractures, whereas another similar zone showed no flow. Borehole Acoustic Reflection Survey processing was attempted to understand how extent of fractures beyond the borehole wall contributed to productivity from a well. The presence of laterally continuous fracture network at an interval that coincides with the depths from which the well is flowing, in turn validated from production log data, explained fluid flow from basement. Furthermore, the absence of such network can cause no flow even though near well-bore possible open fractures are present. Present study established the fact that, identification of potential open fractured zones in basement is a lead for reservoir zone delineation, however, a lateral extent of such basement reservoir facies is the key for successful basement hydrocarbon exploration.
Ritzmann, Nicklas (Baker Hughes, a GE company) | Steinsiek, Roger (Baker Hughes, a GE company) | Dymmock, Stephen (Baker Hughes, a GE company) | Moody, Brian (Baker Hughes, a GE company) | Morris, Stephen (Baker Hughes, a GE company) | Oldervoll, Alf (Baker Hughes, a GE company) | Aarnes, Ingelinn (Baker Hughes, a GE company)
This paper introduces high-resolution acoustic borehole imaging as a new service available in logging-while-drilling (LWD). Borehole images (BHI) are an important part of the formation evaluation portfolio and contribute significantly to the understanding of subsurface structures and formation characteristics throughout the entire lifecycle of a field; from exploration, where BHIs yield detailed information about the depositional environments and the inferred depositional trends, to late field life where the accuracy of the estimated hydrocarbons in place are determined by the precision of the geological model. This information is crucial for the planning phase of new wells, determining recoverable reserves in place, analysis in production behaviour and a detailed understanding of the target formations. Acoustic images can also provide a detailed picture of the borehole shape that is used to reduce well construction and completions risk on a well and provides valuable geomechanical information.
The new method is based on ultrasonic transducers that scan the borehole wall with acoustic signals, while the tool is rotating in the borehole. The tool sends a high frequency acoustic signal towards the wellbore wall and measures the travel-time and the amplitude of the returning signal. Each measurement is recorded with its azimuthal position and can therefore be used to create a borehole image. Whereas the amplitude image contains information about the scanned formation, the traveltime represents the borehole shape and can be converted into a distance when the borehole environmental conditions are known. Both images can be acquired while drilling or tripping (reaming/washing). Since the selected transducers are highly focused they can reveal small scale features such as fractures, cross-bedding and mud-cracks. The high acquisition frequency enables the tool to acquire high resolution images for almost any ROP/RPM combination and the physical acquisition method results in an independence of the used mud type. High resolution images significantly improve the understanding of reservoir architecture, presence and type of fractures or faults and are now available in WBM and OBM.
Borehole acoustic reflection survey (BARS) is the name given to the data acquisition and processing for imaging near-borehole structures. The waveforms are acquired by a sonic tool using monopole and dipole sources whose frequency ranges are approximately 8 and 3 kHz, respectively. The main objectives of BARS are to image caprocks for confirmation of geosteering and well placement, to observe extensions of fractures for efficient production of gas and oil, and to find other wells for avoiding hazardous situations. The event signals used for imaging are the reflected P- and S-waves and the transmitted PS- and SP-waves. Before applying the imaging method, the direct waves, such as the P-, S-, Stoneley and flexural waves, are removed, and the reflected signals are extracted. Because the amplitudes of direct waves are significantly larger than those of reflected waves, the waveform separation is not straightforward and customized methods are used. In the imaging process, conventional Kirchhoff migration is often used. In this paper, a new coherency-based weighting method is applied and tested to obtain high-resolution images. This paper reviews parts of the processing workflow, such as waveform separation and migration, also tested on fracture imaging examples.
Near-borehole structures are imaged using the sonic waveforms acquired by a sonic logging tool (Hornby, 1989; Esmersoy et al., 1998; Tang et al., 2007; Tang and Patterson, 2009; Haldorsen et al., 2010). The main objectives of a borehole acoustic reflection survey (BARS) are to image caprocks for confirmation of geosteering and well placement (Borland et al., 2007; Haldorsen et al., 2010), to observe extensions of fractures for efficient production of gas and oil (Yamamoto et al., 1999; Hirabayashi et al., 2010), and to find other wells (Jervis et al., 2012). The source types and event signals to be processed are selected depending on the imaging targets. Because the central frequencies of monopole and dipole sources of the sonic tool are approximately 8 and 3 kHz, respectively, the monopole images have higher resolutions and the dipole images have deeper depths of investigation.