X-ray Tomography of Preserved Samples from the Geysers Scientific Corehole

Approximately 800 ft. of continuous core was recovered from borehole SB-15 D (on unit 15, near the site of the abandoned Geysers Resort) during a recently completed drilling operation funded by the USDOE. Sections of this core were collected at 50 ft intervals for subsequent examination as drilling proceeded. Five foot sections were not removed at the drill site, but were sealed in the innermost sleeve of a triple tube coring system to minimize drying and disturbance of the core. All cores remained sealed and were radiographed within 72 hours of drilling: the five foot core from near 1400 ft. was scanned within 18 hours of drilling. A third generation x-ray scanner, which uses high energy radiation to penetrate the aluminum sleeve and 3.5 inch cores, was used to make preliminary radiographs and to collect multiple views of the sample as the core is rotated in front of the beam. True three dimensional tomographs are then reconstructed from the data. At present, the images have a spatial resolution of approximately 140 micrometers and can resolve contrast differences of 0.2%. The tomographs clearly show differences in lithology with depth in the reservoir. Partially filled fractures, vein selvage and vuggy porosity are all evident in parts of the core. A principle goal of the imaging effort is to help determine the fluid content of the reservoir. Important questions to investigate include water loss during core recovery, infiltration of drilling fluid, and the heterogeneous distribution of pore fluid. Images show that radial gradients in x-ray attenuation commonly occur in jacketed cores. Regions of excess attenuation extend about halfway into the 3.5 in. core, and are probably caused by mud invasion induced by capillarity of the small scale porosity of the graywacke matrix. X-ray measurements will be coordinated with other independent measurements of fluid content underway in separate studies, particularly NMR spectroscopy of frozen ‘pressure core,’ and compressional velocity and electrical resistivity measurements. INTRODUCTION The successful completion of The Geysers scientific corehole at the site of SB-15 (near the abandoned Geysers resort) provides an unprecedented opportunity to study rocks from the upper regions of The Geysers steam reservoir. High quality core was recovered starting near 800 ft extending to -1600 ft depth from a sidetrack starting from the former production well, SB-15. The rock was characterized at the drill site by Hulen and Nielson as “highly fractured, heavily veined, but weakly altered Franciscan graywacke with minor argillite.” The graywacke‘s origin appears to be turbiditic as indicated by the graded beds, load structures and the interbedded argillite, a dark organicrich shale which exhibits flame structures into the graywacke. Details of the drilling operation and a first analysis of the rocks encountered are given by Hulen et al., 1994. Sections of this core were collected at 50 ft intervals for subsequent examination at Lawrence Livermore National, Laboratory. At alternating intervals, either 1.5 ft of core was selected and sealed, or a 5 ft section was preserved. The five foot sections were not removed from the core tubes, but were sealed at the drill site with caps fitted with ‘0’ rings to minimize drying and disturbance of the core. The jacketed cores are examined with a third generation x-ray scanner, which uses high energy radiation to penetrate the aluminum sleeve and three inch cores. Two types of data are collected: digital radiographs and tomographs. Radiographs are made by collecting two dimensional projections of x-ray transmission through the sample on high resolution film. Tomographs are produced by collecting multiple views of the sample as the core is rotated in front of the beam, and then’reconstructing true three dimensional images using computational techniques. Both types of images are useful for locating highly fractured regions, for detecting structural features such as filled veins and folds and for detecting changes in lithology. Improved estimates of the amount of stored water at The Geysers are also of interest because these data are critical for predicting the performance and lifetime of the reservoir. Water