Pre-drilling assessments of average porosity and permeability in the geothermal reservoirs of the Danish area

Abstract

Denmark constitutes a low-enthalpy geothermal area, and currently geothermal production takes places from two sandstone-rich formations: the Bunter Sandstone and the Gassum formations. These formations form major geothermal reservoirs in the Danish area, but exploration is associated with high geological uncertainty and information about reservoir permeability is difficult to obtain. Prediction of porosity and permeability prior to drilling is therefore essential in order to reduce risks. Geologically these two formations represent excellent examples of sandstone diversity, since they were deposited in a variety of environments during arid and humid climatic conditions. The study is based on geological and petrophysical data acquired in deep wells onshore Denmark, including conventional core analysis data and well-logs. A method for assessing and predicting the average porosity and permeability of geothermal prospects within the Danish area is presented. Firstly, a porosity-depth trend is established in order to predict porosity. Subsequently, in order to predict permeability, a porosity-permeability relation is established and then refined in steps. Both one basin-wide and one local permeability model are generated. Two porosity-depth models are established. It is shown that the average permeability of a geothermal prospect can be modelled (predicted) using a local permeability model, i.e. a model valid for a geological province including the prospect. The local permeability model is related to a general permeability model through a constant, and the general model thus acts as a template. The applied averaging technique reduces the scatter that is normally seen in a porosity-permeability plot including all raw core analysis measurements and thus narrows the uncertainty band attached to the average permeability estimate for a reservoir layer. A "best practice" technique for predicting average porosity and permeability of geothermal prospects on the basis of core analysis data and well-logs is suggested. The porosity is primarily related to depth, whereas the permeability also depends on porosity, mineralogy and grain size, which are controlled by the depositional environment. Our results indicate that porosity and permeability assessments should be based on averaged data and not raw conventional core analysis data. The uncertainty range of permeability values is significantly lower, when average values are used.

Review of hydrocarbon potential in East Denmark following 30 years of exploration activities

“…Good to excellent quality Lower Triassic reservoirs are widespread in Denmark (Mathiesen et al 2010;Kristensen et al 2016). The presence of N 2 gas in Bunter in the Tønder structure confirmed the possibility of a Triassic play in the area, but it is dependent on the presence of structural closures and migration from mature source rocks.…”

Overview of potential geothermal reservoirs in Denmark

“…In addition to the risk of limited reservoir continuity, the quality of the various reservoir sandstones expressed by porosity and permeability is a major challenge as the effective permeability determines the flow rates that can be obtained in the production and injection wells. With the objective of gaining capability to predict reservoir productivity prior to the drilling of expensive exploration wells, GEUS has investigated the petrography of the reservoir sandstones and compared it with porosity and permeability in order to understand the variations related to sediment source areas, entry points and the diagenetic alterations associated with burial depths, temperature, geochemical composition of brines, as well as late uplift (Kristensen et al, 2016;Olivarius et al, 2017;Weibel et al, 2017a,b). Based on the detailed petrographic database obtained, GEUS has initiated advanced diagenesis modelling with the ultimate goal of predicting reservoir quality in undrilled exploration areas (e.g.…”

“…• The more locally occurring Flyvbjerg and Frederikshavn formations as well as unnamed sandstones within the Fjerritslev Formation and the Lower Cretaceous units constitute local alternatives or supplements to the Gassum and Bunter Sandstone/Skagerrak formations. • In general, a thickness of at least 15 m of clean sandstones (containing less than 30% mud/shale) and a minimum transmissibility of c.10 Darcy metres is suggested as a conservative approach (Kristensen et al, 2016;.…”

Review of induced seismicity in geothermal systems worldwide and implications for geothermal systems in the Netherlands

“…The thickness of the sedimentary cover ranges from 1-2 km in the south to 9 km in the north of the basin (Frederiksen et al, 2001). The main geothermal targets are the Upper Triassic Gassum (Rhaetian in Germany) and the Lower Triassic Bundsandstein (Kristensen et al, 2016). Two geothermal plants are operative: Thisted (started in 1984) and Margretheholm (started in 2005) near Copenhagen.…”