Applying Seismic Web-Avo Inversion For Uncertainty Reduction In Geothermal Exploration: An Example At Luttelgeest-01

Abstract

Geothermal energy is a growing energy (heat) source being developed over the last decade. Many new initiatives are currently created including the exploration for high enthalpy systems in the deeply buried Dinantian carbonates at over 5,000m depth. Only two wells have targeted this reservoir at that depth in the Netherlands. Reservoir properties were not good. Can we deploy innovative imaging/inversion techniques to extract more information from the existing seismic and well data to find and target zones with better reservoir properties before we drill a new expensive exploration well? To review this Panterra Geoconsultants and Delft Inversion ran a seismic inversion test on well Luttelgeest 01 (LTG-01). First the well logs were analysed and corrected, and the petrophysical results were calibrated with core data. Then we used wave-equation based AVO inversion (WEB-AVO) to extract quantitative information from synthetic pre-stack seismic data. The technique has several unique features [1] when compared to conventional AVO inversions, among which the ability to directly extract reservoir models in terms of compressibility and shear compliance. Compressibility is highly sensitive to softness of rocks (e.g. porosity) while shear compliance is an excellent indicator for shale volume. If these parameters could be reliably obtained as part of a geothermal exploration project, not only the well placement can be improved but also certainty estimates (P90, P50, etc.) can be calculated more confidently. The technique is well established for hydrocarbon exploration in clastic reservoirs but here we demonstrate the technique for the first time in the context of geothermal exploration in a carbonate target penetrated by the Luttelgeest-01 well. Modelling parameters were chosen to be representative for an UDG project (fc=20 Hz, fmax=40Hz, max. angle approx. 30ͦ). The inversion results including modeled and predicted synthetic data as well as the obtained residual are shown in Fig.1. WEB-AVO has successfully reconstructed the top carbonate interface, which is by far the strongest contrast over the target interval. There are two potential targets at z=4480m and z=4580m respectively. While both units are represented on the compressibility, inverted shear compliance correctly indicates the higher shale content of the deeper unit, effectively down rating its geothermal potential. This has also been confirmed by petrophysical interpretations. While this initial test on synthetic data led to promising results, in a next step we plan to invert the available pre-stack seismic data around the Luttelgeest-01 area. The authors are aware of the very specific challenges in terms of S/N and frequency content of seismic acquired for characterisation of such a deep and geologically difficult environment. It is unclear yet how much results will be effected when inverting field data and proper expectation management will be key. Still, we are convinced that any information that can be obtained beyond conventional structural interpretations should be valuable to increase the commerciality of future geothermal projects.

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