Regional estimation of Curie-point depths and succeeding geothermal parameters from recently acquired high-resolution aeromagnetic data of the entire Bida Basin, north-central Nigeria

<p><strong>Abstract.</strong> A regional estimation of Curie-point depths (CPDs) and succeeding geothermal gradients and subsurface crustal heat flow has been carried out from the spectral centroid analysis of the recently acquired high-resolution aeromagnetic (HRAM) data of the entire Bida Basin in north-central Nigeria. The HRAM data were divided into 28 overlapping blocks, and each block was analysed to obtain depths to the top, centroid, and bottom of the magnetic sources. The depth values were then used to assess the CPD, geothermal gradient, and subsurface crustal heat flow in the basin. The result shows that the CPD varies between 15.57 and 29.62<span class="thinspace"></span>km with an average of 21.65<span class="thinspace"></span>km, the geothermal gradient varies between 19.58 and 37.25<span class="thinspace"></span>°C<span class="thinspace"></span>km⁻¹ with an average of 27.25<span class="thinspace"></span>°C<span class="thinspace"></span>km⁻¹, and the crustal heat flow varies between 48.41 and 93.12<span class="thinspace"></span>mW<span class="thinspace"></span>m⁻² with an average of 68.80<span class="thinspace"></span>mW<span class="thinspace"></span>m⁻². Geodynamic processes are mainly controlled by the thermal structure of the Earth's crust; therefore this study is important for appraisal of the geo-processes, rheology, and understanding of the heat flow variations in the Bida Basin, north-central Nigeria.</p>

The role of the legislative and regulatory branches in promoting the use of geothermal energy in Latvia

<p><strong>Abstract.</strong> Latvia currently is self-sufficient in energy resources up to approximately 35<span class="thinspace"></span>%. Annual fossil energy prices rise and risks of security of energy supply promote the development legislation in the matter of renewable resources. One of the Latvian Ministry of Economics' recent products is a new draft law called the "Renewable Energy Law", which has been created due to one of the European Union and Latvian national energy policy objectives: to increase the share of renewable energy up to 40<span class="thinspace"></span>% by 2020 (Moore and Vanags, 2012). Currently, geothermal energy potential is assessed at 1 × 1013<span class="thinspace"></span>kWh; nevertheless, it is difficult for geothermal energy to compete with other renewable energy resources in the Latvian energy market. A great job has been done in recent years at the legislative branch to choose the right methods for supporting the use of renewable energy resources. This paper aims is analysis of current situation and assessment of Latvian legislation possibilities to promote the use of geothermal energy.</p>

Geothermometric evaluation of geothermal resources in southeastern Idaho

<p><strong>Abstract.</strong> Southeastern Idaho exhibits numerous warm springs, warm water from shallow wells, and hot water from oil and gas test wells that indicate a potential for geothermal development in the area. We have estimated reservoir temperatures from chemical composition of thermal waters in southeastern Idaho using an inverse geochemical modeling technique (Reservoir Temperature Estimator, RTEst) that calculates the temperature at which multiple minerals are simultaneously at equilibrium while explicitly accounting for the possible loss of volatile constituents (e.g., CO₂), boiling and/or water mixing. The temperature estimates in the region varied from moderately warm (59<span class="thinspace"></span>°C) to over 175<span class="thinspace"></span>°C. Specifically, hot springs near Preston, Idaho, resulted in the highest reservoir temperature estimates in the region.</p>

Geothermal heat pump system assisted by geothermal hot spring

<p><strong>Abstract.</strong> The authors propose a hybrid geothermal heat pump system that could cool buildings in summer and melt snow on the pedestrian sidewalks in winter, utilizing cold mine water and hot spring water. In the proposed system, mine water would be used as cold thermal energy storage, and the heat from the hot spring after its commercial use would be used to melt snow for a certain section of sidewalks. Neither of these sources is viable for direct use application of geothermal resources, however, they become contributing energy factors without producing any greenhouse gases. To assess the feasibility of the proposed system, a series of temperature measurements in the Edgar Mine (Colorado School of Mines' experimental mine) in Idaho Springs, Colorado, were first conducted, and heat/mass transfer analyses of geothermal hot spring water was carried out. The result of the temperature measurements proved that the temperature of Edgar Mine would be low enough to store cold groundwater for use in summer. The heat loss of the hot spring water during its transportation was also calculated, and the heat requirement for snow melt was compared with the heat available from the hot spring water. It was concluded that the heat supply in the proposed usage of hot spring water was insufficient to melt the snow for the entire area that was initially proposed. This feasibility study should serve as an example of "local consumption of locally available energy". If communities start harnessing economically viable local energy in a responsible manner, there will be a foundation upon which to build a sustainable community.</p>

Thermodynamic and thermoeconomic analysis of combined geothermal space heating and thermal storage using phase change materials

<p><strong>Abstract.</strong> The present work discusses the utilization of phase change materials for energy storage in geothermal space heating systems. Thermodynamics and thermoeconomics of the combined heating and thermal storing system were studied to show the scope of energy storage and cost savings. A computational model of the combined space heating and thermal storage system was developed and used to perform thermodynamic studies of the heat storage process and heating system efficiency at different times and ambient temperatures. The basis for these studies is daily variations in heating demand that is higher during the night than during the day. The results show the scope of the utilization of phase change material for low ambient temperature conditions. Under proper conditions a sufficient amount of exergy is stored during the charging period at a low ambient temperature to fulfill the daytime heat load requirement. Under these conditions the cost flow rate of exergy storage is found to be lower than the radiator heating cost flow rate. Thus, the use of exergy storage at low ambient temperatures for heating at higher ambient temperatures makes a significant contribution to cost savings.</p>

Geochemical study on hot-spring water in West New Britain Province, Papua New Guinea

<p><strong>Abstract.</strong> West New Britain Province, which occupies the western part of New Britain Island in Papua New Guinea, is ideally located within an active tectonic region that influences volcanism creating an environment favourable for geothermal activity. Geothermal mapping of surface manifestations reveals high temperature geothermal prospects along the northern coastline of West New Britain Province that are further confirmed by geochemical analysis. The occurrence of geothermal features is confined to the Quaternary Kimbe Volcanics and alluvium in the lowland areas. The features in Talasea appear to be controlled by deep-seated northerly trending faults while structures in Hoskins also appear to be deep seated but have not been identified. The geothermal systems in West New Britain Province have not been drilled, but preliminary reconnaissance geothermal mapping and geochemical analysis reveals four high temperature geothermal prospects suitable for further investigation and development of geothermal energy. These are the Pangalu (Rabili) and Talasea Station geothermal prospects in Talasea and Kasiloli (Magouru) and Silanga (Bakama and Sakalu) geothermal prospects in Hoskins. The calculated reservoir temperatures for these fields are in the range of 245–310 °C. Recommendations are made for further follow-up exploratory investigations.</p>

Convective, intrusive geothermal plays: what about tectonics?

<p><strong>Abstract.</strong> We revised the concept of convective, intrusive geothermal plays, considering that the tectonic setting is not, in our opinion, a discriminant parameter suitable for a classification. We analysed and compared four case studies: (i) Larderello (Italy), (ii) Mt Amiata (Italy), (iii) The Geysers (USA) and (iv) Kizildere (Turkey). The tectonic settings of these geothermal systems are different and a matter of debate, so it is hard to use this parameter, and the results of classification are ambiguous. We suggest a classification based on the age and nature of the heat source and the related hydrothermal circulation. Finally we propose to distinguish the convective geothermal plays as volcanic, young intrusive and amagmatic.</p>

Reservoir characterization of the Upper Jurassic geothermal target formations (Molasse Basin, Germany): role of thermofacies as exploration tool

<p><strong>Abstract.</strong> The Upper Jurassic carbonates of the southern German Molasse Basin are the target of numerous geothermal combined heat and power production projects since the year 2000. A production-orientated reservoir characterization is therefore of high economic interest. Outcrop analogue studies enable reservoir property prediction by determination and correlation of lithofacies-related thermo- and petrophysical parameters. A thermofacies classification of the carbonate formations serves to identify heterogeneities and production zones. The hydraulic conductivity is mainly controlled by tectonic structures and karstification, whilst the type and grade of karstification is facies related. The rock permeability has only a minor effect on the reservoir's sustainability. Physical parameters determined on oven-dried samples have to be corrected, applying reservoir transfer models to water-saturated reservoir conditions. To validate these calculated parameters, a Thermo-Triaxial-Cell simulating the temperature and pressure conditions of the reservoir is used and calorimetric and thermal conductivity measurements under elevated temperature conditions are performed. Additionally, core and cutting material from a 1600 m deep research drilling and a 4850 m (total vertical depth, measured depth: 6020 m) deep well is used to validate the reservoir property predictions. Under reservoir conditions a decrease in permeability of 2–3 magnitudes is observed due to the thermal expansion of the rock matrix. For tight carbonates the matrix permeability is temperature-controlled; the thermophysical matrix parameters are density-controlled. Density increases typically with depth and especially with higher dolomite content. Therefore, thermal conductivity increases; however the dominant factor temperature also decreases the thermal conductivity. Specific heat capacity typically increases with increasing depth and temperature. The lithofacies-related characterization and prediction of reservoir properties based on outcrop and drilling data demonstrates that this approach is a powerful tool for exploration and operation of geothermal reservoirs.</p>

Overcoming challenges in the classification of deep geothermal potential

<p><strong>Abstract.</strong> The geothermal community lacks a universal definition of deep geothermal systems. A minimum depth of 400 m is often assumed, with a further sub-classification into middle-deep geothermal systems for reservoirs found between 400 and 1000 m. Yet, the simplistic use of a depth cut-off is insufficient to uniquely determine the type of resource and its associated potential. Different definitions and criteria have been proposed in the past to frame deep geothermal systems. However, although they have valid assumptions, these frameworks lack systematic integration of correlated factors. To further complicate matters, new definitions such as hot dry rock (HDR), enhanced or engineered geothermal systems (EGSs) or deep heat mining have been introduced over the years. A clear and transparent approach is needed to estimate the potential of deep geothermal systems and be capable of distinguishing between resources of a different nature. In order to overcome the ambiguity associated with some past definitions such as EGS, this paper proposes the return to a more rigorous petrothermal versus hydrothermal classification. This would be superimposed with numerical criteria for the following: depth and temperature; predominance of conduction, convection or advection; formation type; rock properties; heat source type; requirement for formation stimulation and corresponding efficiency; requirement to provide the carrier fluid; well productivity (or injectivity); production (or circulation) flow rate; and heat recharge mode. Using the results from data mining of past and present deep geothermal projects worldwide, a classification of the same, according to the aforementioned criteria is proposed.</p>

Classification of geothermal resources by potential

<p><strong>Abstract.</strong> When considering and reporting resources, the term "geothermal potential" is often used without clearly stating what kind of potential is meant. For renewable energy resources it is nowadays common to use different potentials: theoretical, technical, economic, sustainable, developable – decreasing successively in size. In such a sequence, the potentials are progressively realizable and more and more rewarding financially. The theoretical potential describes the physically present energy, the technical potential the fraction of this energy that can be used by currently available technology and the economic potential the time- and location-dependent fraction of the previous category; the sustainable potential constrains the fraction of the economic potential that can be utilized in the long term; the developable potential is the fraction of the economic resource which can be developed under realistic conditions. In converting theoretical to technical potential, the recovery factor (the ratio extractable heat/heat present at depth) is of key importance. An example (global geothermal resources) is given, with numerical values of the various potentials. The proposed classification could and should be used as a kind of general template for future geothermal energy resources reporting.</p>