Improving borehole fracture characterization using oriented sidewall cores for geothermal assessment – an example in central New York State USA

Abstract

In New York State and Pennsylvania, USA, Precambrian metamorphic and intrusive rocks and Cambrian to Lower Ordovician sedimentary rocks are reservoir targets for deep direct-use geothermal development. Evaluation of natural fractures and structures in the potential reservoir units at the Cornell University Borehole Observatory site was conducted through cross-scale evaluation of oriented sidewall cores, borehole image (BHI), and far-field acoustic survey data. Oriented sidewall cores in the basement complex (Cayuta Formation) reveal metasediments containing foliations, lineations, mineral-filled fractures, and breccia intervals. Basement sidewall core fracture data aid identification of fractures in BHI surveys riddled with borehole breakouts. In contrast, sidewall and image log data for the Cambrian-Ordovician sedimentary section show that open fractures are present and allow orientation and abundance to be estimated. At various depths sandstone and dolostone sidewall cores contain quartz-filled or carbonate-filled bed-normal and -parallel microfractures. Four subvertical microfracture sets, formed sequentially, strike NW-SE (F1), NE-SW (F2), N-S (F3), and WSW-ENE (F4). Microfracture set orientations F1, F2, and F4 match interpretations of acoustic fracture anomalies (open fractures) located tens of meters from the wellbore. In the uppermost Galway Formation sandstone, common microfracture apertures are 0.001 to 0.01 mm. The widest microfractures transition to quartz-lined and bridged open macrofractures. An open vertical macrofracture in Galway sandstone is observed in BHI surveys and a sidewall core, effectively ground-truthing the F4 fracture set. Based on comparison of core fractures with borehole image survey features, differentiation of natural from drilling-induced fractures reveals three sedimentary rock zones of elevated natural fracture frequency.