Drill-bit SWD and seismic interferometry for imaging around geothermal wells Flavio Poletto * , OGS, Piero Corubolo, OGS, Biancamaria Farina, OGS, Andrea Schleifer, OGS, Joseph Pollard, DHI, Marco Peronio, OGS, and Gualtiero B¨ ohm, OGS Summary In this work we present the results of a seismic while drilling (SWD) application using the drill-bit as a seismic source in geothermal wells. The survey was performed in a well drilled in the Nevada desert with the aim of providing geophysical information while drilling and images of the geological structures around the well. We present a summary description of the drill-bit seismic technology adapted for geothermal purposes, and the SWD results obtained along 2D profiles passing through the well. The good-quality seismic-while-drilling results have been subsequently used to obtain images of the well area in the direction normal to the main fault system, with SWD reverse vertical seismic profiling (RVSP) data migration, using seismic interferometry to extend laterally the coverage of the SWD RVSP images. The migrated seismic interferometry results confirm the main trends of the 2D geological model in the geothermal area. Introduction Drill-bit seismic while drilling is a known methodology (Rector and Marion, 1991; Poletto and Miranda, 2004) used in oil wells to predict the formation and structures ahead of the bit and support drilling geophysically. The drill-bit SWD technology is a tool useful also in geother- mal wells, where drilling vertical wells in hard rocks with roller-cone bits makes the application favorable for obtaining good-quality data and imaging around the well purposes. In this context the drill-bit SWD technology has the advantage with respect to conventional wireline borehole seismic methods of providing multioffset infor- mation around the well without the need of recording tools in the well, where high temperature may be a critical condition for the recording equipment. In this paper we describe a SWD application performed in Nevada (US), where the standard SWD technology and processing flow used for oil wells was adapted to provide while-drilling and after-drilling imaging results. The sur- vey area is located in a regional trans-tensional system characterized by the presence of major NW-SE trend- ing strike-slip faults leading to very complex fault pat- terns and structures on the small scale (Faulds et al., 2005). The well was drilled close to a NE-SW striking nor- mal fault, possibly intersecting two of these right-lateral strike-slip faults. The fault zone dips NW and is associ- ated with several synthetic, antithetic and Riedel faults on both its hanging wall and footwall. Hydrothermal flu- ids upflow occurs at locations associated with fractures and fault intersections and results, in this case, in the presence of silica caps due to hydrothermal alteration of quaternary sediments. A significant difference with respect to conventional SWD survey preparation was that surface reflection seismic lines are not available in this area. This makes the addi- tional seismic information from SWD even more impor- tant for the reconstruction of the structural and geological characteristics of the complex subsurface around the well. At the same time, this makes more crucial the task of estimating the preliminary seismic parameters using the existing information, i.e., geological and that obtained by gravity and magneto-telluric surface profiles, for survey design purposes and interpretation of the final results in zones where the SWD coverage is lower. The main results consist of conventional-SWD (Poletto and Miranda, 2004) and new products, which include check shot, velocity profile and while-drilling prediction ahead by single and multioffset VSP, while-drilling diffrac- tion analysis, while- and after-drilling tomography and data migration, drill-bit seismic interferometry (Poletto et al., 2009), and fault model tuning by waveform anal- ysis. In this work we present while drilling prediction, migration and interferometry results. SWD survey description The survey was performed using a cross of two seismic lines of surface geophones, with a layout designed on the basis of the geophysical and geological information. The main surface acquisition line was deployed in the NW-SE direction, passing through the well and perpen- dicular to the normal fault system. A second and shorter surface receiver line crossing the main line in the well po- sition, was deployed approximately in the perpendicular SW-NE direction to monitor possible lateral effects due to the strike-slip fault systems. The length of the main line was set to cover the expected zone of reflections from the dipping fault/silicified zone by one line branch (negative offsets), and to cover the diffraction bodies interpreted in the subsurface geological model derived from gravity profiles by the opposite-side branch of the line (positive offsets). The offset of the main line ranges approximately from -750 to 870 m, and that of the secondary line from -700 to 300 m from wellhead. The distance between re- ceiver groups is 30 m, with the nearest offset from the well in each line branch is 60 m. Drill-bit seismic data have been recorded using a Seisbit R system hosted in the mudlogging cabin, during near- vertical drilling between 180 m to 750 m depth with roller cone bits. Acquisition was performed in automated mode driven by drilling parameters, and average depth-level ac- © 2011 SEG SEG San Antonio 2011 Annual Meeting 4319 4319 Downloaded 03/30/19 to 151.36.135.42. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/