Exploration Of Dinantian As A Potential Geothermal Reservoir - A Methodological Review

Abstract

Introduction Exploration and production of geothermal prospects poses a number of challenges which have been dealt with in the oil&gas industry already for long. With the cross-over to geothermal energy well underway, an inventory of technology, workflows and tools which have proven their success and cost effectiveness in oil&gas exploration and production is timely. We present three such technologies and their integration for application on geothermal exploration: 1. technology for upgrading critical well log data (velocity and density) 2. high-resolution seismic diffraction imaging 3. gravity&magnetic modelling. We demonstrate the integration of these three techniques in an area around the Heeswijk-01 well and the challenging characterization of the ultradeep Dinantian as a potential geothermal reservoir. Well log upgrading The use of seismic well logs (sonic, density) for seismic-to-well matching, seismic interpretation, inversion and 3D model building by means of depth-to-time conversion using the checkshot correction is common, but sensitive to errors in VSP/ checkshot first-break picking as well as logging errors and unmeasured sections. To maximize interpretation value, in particular for geothermal exploration, we use a novel and robust log repair and upgrading method to transform well data (seismic logs and VSP/checkshots) into a complete and realistic velocity and density profile from surface to TD. Steps to accomplish this include upgraded first-break picking and QC, iterative raytracing to convert first-breaks to a unique continuous interval velocity profile, QC and upgrade compressional/shear and density logs using lithology, caliper, gamma ray and possibly pressure data. This results in a complete set of compressional/shear velocity and density data from surface to TD, which is physics compliant with VSP/checkshot data and matches geological (cuttings) descriptions. High-resolution seismic diffraction imaging The well log upgrade is used to provide a velocity model for seismic imaging through depth-migration of pre- or post- stack input seismic data, which results in optimal focusing and positioning of subsurface reflectors. We use reflection data for global interpretation of stratigraphy and identification of geologic units. In addition, we use diffraction imaging to better define small-scale structural features. Unlike reflection, diffraction imaging is not subject to illumination constraints and has the potential of high-/super resolution imaging, taking full benefit from accurate velocities obtained from well upgrading. This makes diffraction imaging the optimal candidate to image faults, thereby reducing drilling risks for geothermal exploration and subsequent production risks. Gravity&magnetic modelling Exploration for ultra-deep (>4 km) geothermal reservoirs depends on the exploitation of existing data, before spending on new seismic data sets is commissioned. Especially the Carboniferous and Permian formations host proven oil&gas reservoirs and are therefore also targets of ultra-deep geothermal exploration. Using an integrated approach exploiting several data sets, we demonstrate ways to shed light on the degree and type of faulting, and presence and lateral variation in thickness of deep Dinantian reservoirs in the Roer Valley Graben, which are currently out-of-reach of the existing seismic and well data. Our approach entails: gravity and magnetic modelling partly constrained by seismic interpretation, in turn tied to reprocessed velocity and density log data and additional information on Moho and intra-crustal discontinuity depth from other recent studies. Acknowledgements We thank CGG, Geosoft, TNO, EBN, Hydreco and Transmark Renewables for their kind cooperation.

AAPG Datapages/Search and Discovery Article #90345 © 2018 AAPG European Region, Geothermal Cross Over Technology Workshop, Part II, Utrecht, The Netherlands, April 17-18, 2018