Thin Section Based Cutting Analysis as a New Approach in Rock Type Determination While Drilling Deep Geothermal Wells

Authors: Torsten STEIGER, Stephan UHLIG, Inga MOECK
Keywords: thin section, cutting analysis, microfacies, analysis while drilling, cutting form analysis
Conference: World Geothermal Congress Session: Exploration
Year: 2015 Language: English
Abstract: Microfacies analyses using thin sections made up from cutting material and side-wall cores are an essential tool in order to scientifically accompany geothermal deep drilling projects. Components and microfabrics which are included in washed cutting material give information about geological origin, mineral composition, diagenetic processes as well as porosity and permeability of the drilled rock formations. Components can be qualitatively observed and quantitatively estimated by particle analyses with the purpose to reconstruct depositional environments. Mineral shape and composition demonstrate diagenetic history and the development of microfabric characteristics. Finally, the occurrences of open porosity and precipitated cements can be identified in thin section based cutting material from deep wells helping to evaluate the porosity of the rock. 3D reconstruction of fracture systems is possible from oriented side-wall cores and indirectly by cutting form analysis from cutting material. In combination with stress field analysis and structural geological analysis from 3D seismic the cutting form analysis helps to identify the dominated fracture pattern in situ which might be below the seismic resolution. These important information about cap rock and reservoir rock quality needs to be gathered as quick as possible to steer the drilling process and refine targeting. Thin section analysis of cuttings after the drilling operation may help to evaluate reservoir rock properties but cannot steer the drilling process anymore. In a newly established mobile lab, fast thin section production while drilling enables well-site geologists to obtain early information about rock properties within a few hours. This method facilitates rock type determination immediately after sample recovery at the drill-site. This approach of thin-section based cutting analysis-while drilling is newly developed for geothermal wells and represents an essential part in reliable determination of casing shoes, reservoir rock identification and – if the suggested reservoir rock formation is not proven through thin section cutting analysis – drilling targeting. Thin section based cutting analysis requires a detailed understanding of facies and paleontology, opening obviously a new research field in geothermal exploration and well site geology.
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