Australian Geothermal Conference 2010 1 Estimates of sustainable pumping in Hot Sedimentary Aquifers: Theoretical considerations, numerical simulations and their application to resource mapping J. F. Wellmann *, F. G. Horowitz, L. P. Ricard, K. Regenauer-Lieb Western Australian Geothermal Centre of Excellence, The University of Western Australia, 35 Stirling Hwy, WA-6009 Crawley, Australia *Corresponding author: wellmann@cyllene.uwa.edu.au A method for a spatial analysis of potential sustainability for the early stage of exploration in Hot Sedimentary Aquifers (HSA) is presented here. Our analyses are based on well established estimations for the thermal breakthrough in a doublet well setting. We consider two significantly different scenarios: the placement of a well doublet in an aquifer without significant natural flow, and the case where a natural groundwater flow exists. We integrate these two analytical estimations into one workflow with geological modelling and geothermal simulation. As a result, we obtain spatial analyses of theoretical sustainable pumping rates for a whole resource area. These maps are specifically suitable for the early stage of exploration where a potential target area has to be determined based on limited information. We present the application of our method to a geothermal resource area in the North Perth Basin, from geological modelling, to the simulation of fluid and heat flow, and finally to map the analysis of sustainable pumping rates for one aquifer. The results contain a high degree of uncertainty, but indicate the distribution of future prospective areas. These maps can be combined with other spatial datasets, e.g. infrastructure. Also, as they are integrated into one workflow, an update of the analyses is directly possible when new data become available. Keywords: Hot Sedimentary Aquifer (HSA); Resource Analysis; Sustainable Pumping Rates; Geothermal Simulation; Geological Modelling Introduction This paper presents a novel exploration method to identify geothermal prospects based on thermal and hydraulic properties of the subsurface. We combine estimates of sustainable pumping rates with simulations of fluid and heat flow, and derive maps of estimations for sustainable pumping rates. Our work regards estimates of sustainability for well doublet systems. After a certain time t B , the reinjected cold water front may reach the extraction well and cool down the extracted temperature (Fig. 1, red curve). This will affect the geothermal application and, at some stage, rule out further effective usage of the site. An estimation of this breakthrough time t B is required to evaluate the sustainability of a project. Temperature development at production well 90 95 100 0 10000 20000 30000 40000 50000 production time [days] temperature [C] flow parallel no flow flow perpendicular Figure 1: Comparison of temperature development at the extraction well for three different scenarios: i) If no advective flow is present, the cold reinjected water may reach the production/ extraction well (red curve) and the temperature of the pumped water will decrease; ii) For advective groundwater flow perpendicular to the wells, the temperature decrease is significantly slower (green curve); and iii) For the case that the reinjection well is directly downstream of the production well, no thermal breakthrough occurs (blue curve). Analytical estimates of a sustainable long-term use for geothermal installations have been applied for many years (e.g. Gringarten, 1978, Lippmann and Tsang, 1980). Most of the approaches are based on many simplifications and assumptions. They nonetheless deliver an important insight into the distribution of promising areas for sustainable flow in the subsurface, especially in the early exploration phase, as not only available temperature and heat in place are considered, but also hydraulic parameters like permeability and porosity. Another standard tool in geothermal exploration is numerical simulation of subsurface fluid and heat flow. (See e.g. O’Sullivan et. al. 2001 for a detailed revision of applications.) A thoroughly performed study can deliver detailed insight into fluid and heat movement in the subsurface, within the usual limitations of data availability and model accuracy. One problem with both estimations, analytical and numerical, is that they are usually only performed at one location, i.e. at a previously identified target, to evaluate its long term behaviour. We