Flow And Performance Of Geothermal Doublets In Deep Dinantian Carbonates Of The Netherlands

Abstract

The occurrence and properties of natural faults in fractured reservoirs are key in determining reservoir flow properties, and thereby the success of geothermal energy or oil & gas production from these reservoirs. The quest for exploiting geothermal reservoirs in sedimentary basins with relatively low geothermal gradients has drawn attention to temperature anomalies associated with deep fractured reservoirs. However, low permeability remains a major concern as the likelihood that fractures are closed or sealed increases with increasing depth. While the E&P industry has been successful in developing tight fractured reservoirs by hydraulically stimulating many horizontal wells, different economics of geothermal projects requires a different approach that involves optimization of doublet placement to achieve optimum flow conditions. Existing subsurface data acquired for oil & gas production can be of great value in such doublet optimization approach. Exploration for new geothermal sites will particularly benefit from site-specific data on fault-related factors like damage zone fracture density, connectivity and permeability. In many cases, such data is lacking during geothermal exploration, but generic relations can be used to constrain typical fault zone architectures, spatial distribution of permeability and characteristics of damage zone fracture populations. Site-specific characteristics of fault and fracture populations can be determined using seismic surveys, outcrop analogues, core material, and laboratory experiments. In this study, flow and performance of geothermal doublets is modelled for a potential geothermal play in a fractured Dinantian carbonate platform near Luttelgeest in the North of the Netherlands. Fault populations in the carbonate formation were analysed using 2D and 3D seismic data. Distributions of fault azimuths, dips and lengths are derived from the interpretation of 2D and 3D seismics. These distributions can be used to determine the preferred orientation of faults and apply scaling relations to describe bulk permeability that incorporates specific fault populations from the seismic surveys, typical fault zone architectures from outcrops, and fracture permeability from laboratory experiments. A bulk permeability model is used that describes non-isotropic permeability of individual fault zone using 3D permeability tensors for fault core, damage zone and surrounding intact reservoir matrix, and fractured reservoir permeability by volume averaging the contribution of fault zones or fractures based on the frequency distribution of fault dimensions. The approach ensures that data available prior to drilling as well as uncertainties are taken into account in doublet performance assessment. Bulk permeability is represented by multiple uniform fracture sets with different orientations in a semi-analytical model for performance assessment of geothermal doublets to analyse the evolution of temperature and pressure for different doublet configurations. The geothermal power for doublet systems consisting of a surface heat exchanger, and multiple injection (“injector”) and production (“producer”) wells that is placed in the fractured Dinantian Carbonates was analysed. Factors such as preferred orientations and permeability of faults and fractures, local stress field, and injection/production rates interact to determine the geothermal power of such doublet systems. optimum placement of geothermal doublet systems can be determined. Optimization of geothermal doublet design can be performed based on multiple model realization based on data of fault and fracture populations from local seismic surveys, outcrop analogues and experiments. Such optimization helps de- risking geothermal exploration and exploitation as it outlines preferred placement of doublets in terms of optimum flow and cold water breakthrough.

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