Towards a de-carbonized energy system in north-eastern Morocco: Prospective geothermal resource

Geothermal data has been indicating promising potentialities in the north-eastern Morocco. This paper presents new temperature data, recently recorded in water boreholes located in the Berkane and Oujda areas. Generally, the observed temperature gradients are rather high. One hole near Berkane, revealed an average geothermal gradient of more than 110 °C/km at depths greater than 300 m. This result confirms the geothermal gradient estimated in a mining borehole located about 30 km west of the Berkane borehole, in which water temperature of 96 °C is reached at a depth of about 700 m. Such a high geothermal gradient, exceeding by far the ones already determined for northeastern Morocco, could act as a stimulus to programs aimed at the geothermal exploitation of high temperature aquifers.     

Temperature front progression associated with a geothermal well doublet: a numerical study

The temperature front progression associated with a geothermal well doublet is studied using aquifer parameters which are appropriate for an area of west-central Alberta for which investigations of the temperature regime and formation properties have been under way at the University of Alberta. The distance from the injection well to which the injection fluid disturbs the ambient aquifer temperature as a function of time is investigated for different flow rates, aquifer thicknesses and separations between the wells. It is found that breakthrough time to the producing well depends on all these parameters, and for relatively small aquifer thicknesses or short distances between wells there is evidence of premature breakthrough by a “finger” of fluid. The results should prove to be an important guide for practical designs.     

Renewable hot dry rock geothermal energy source and its potential in Pakistan

Geothermal energy source, one of the viable renewable energy sources, has encouraging potential to generate full base-load electricity, which has not been explored so far in Pakistan. Though the country can be benefited by harnessing the hydro-geothermal options of energy generation in areas where sources exist, but most of these sources lie in extreme remote and inaccessible rugged mountainous ranges away from the urban-industrial centers. On the other hand, the present study shows that the HDR geothermal option is one of the most viable renewable sources considering the tectonic setup of Pakistan. Results of the study highlight the HDR geothermal energy prospects at relatively deeper depths than hydro-geothermal resources in water-free condition. The basement tectonic analyses reveal that the HDR prospects could be found even just below the urban-industrial centers of Pakistan where there are no hot springs and/or geysers like southern Indus basin in Sindh province or the Kharan trough in the western Balochistan province. Presence of high earth-skin temperature gradient trends derived from satellite temperature data and the high geothermal gradient anomalous zone derived from scanty data of bottom-hole temperatures of some of the oil and gas exploratory wells, indicates encouraging prospects for HDR energy sources in southern Indus and Thar Desert regions inclusive of Karachi synclinorium area. These high geothermal gradients have been inferred to be the result of the deep-seated southern Indus and the Thar fossil-rift structures. Moreover, the prospects of the HDR geothermal energy sources have also been inferred in the Chagai Arc region and the Kharan–Panjgur tectonic depression in the western part of Pakistan based on the analysis of integrated geophysical data. If HDR prospects are developed, they can offer the sustainable, CO₂-free and independent of time, of day, of weather or season, and the base-load energy-generation resource.     

Shallow geothermal studies in Tunisia: Comparison with deep subsurface information

Shallow geothermal prospecting has been undertaken in three zones in Tunisia for which few deep thermal data are available. Five areas have been included in this work, and temperature data from depths between 35 and 335 m from 46 wells in these areas have been analyzed. In addition, thermal measurements have been made on 112 limestones, dolomites, and sandstones to add to the data base.Temperature profiles from the different areas vary substantially from well to well, and these variations depend on both the local and regional geological and hydrological conditions. The shallow thermal gradients vary from 11 mkm^-1 to 132 mkm^-1, compared with the maximum deep gradient so far observed in Tunisia of 52 mkm^-1 (Ben Dhia, 1988). The discrepancies between shallow and deep gradients in the various regions appear to strongly depend on the geological continuity between the shallow and deeper layers, especially in zones where substantial tectonism has occurred.It is concluded that although near surface geothermal data are useful in areas where few deep data are available, great care must be taken in their interpretation, and both regional and local conditions must be considered. Furthermore, predicting the temperature conditions at depth from these observations can be uncertain, and so combining results from such studies with those from deeper data must be done with care.The Zazghouan area appears as an interesting geothermal prospect, while the others need more investigation to be conclusive.     

Positive heat flow anomaly in the Carpathian basin

Intensive geothermal investigations in the central Hungarian Tertiary sedimentary basin show uniform high temperature gradients between 45 and 70°C/km. New temperature measurements between 3000–5800 m depth confirm previous values between 400–2000 meters. Sixteen heat flow measurements showed values between 2.0 and 3.3 μcal/cm² s (84 and 138 mW/m² respectively). Sporadic measurements outside the Carpathian basin have shown invariably average or low heat flow and low temperature gradients.The investigation of the crustal structure in Hungary along 5 profiles indicates the depth of the Moho as being between 24.5 and 30.4 km. Comparing the isobaths of the Moho with the temperature gradient map there is an evident relation between high temperature gradients, the consequent high heat flow and the elevated position of the Moho. Areas with high heat flow are found where the Moho is 24.5–26 km deep. The Carpathian basin can be compared with the Black Sea depression where the Moho is 20 km deep.An interesting geothermal similarity exists between the Carpathian basin and the marginal basins of the western Pacific. The Okhotsk, Japan, Shikoku, Parace-Vela basins have a high mean heat flow above 2 μcal/cm² s. In the southwestern Pacific, the Fiji plateau and the Lau basin are also characterized by high heat flow. The Japan and Okhotsk seas may represent a subsidence similar to the Carpathian basin, caused by the uplift of the surrounding mountain ranges e.g., Kurili island arc.     

Structure and hydrology of the Ogiri field, West Kirishima geothermal area, Kyushu, Japan

The geothermal system in the West Kirishima area is controlled by a system of faults and fractures oriented along two main directions, northwest to southeast and east–northeast to west–southwest. The Ginyu fault extends through the Ogiri field in the Ginyu area, which is one of the east–northeast to west–southwest striking faults in this area. This fault is the reservoir target for developing the geothermal resources in the Ogiri field. The Ginyu fault is a near planar fracture with a uniform temperature of 232°C and has near-neutral pH, chloride fluids. Based on the results of a detailed analysis of the Ginyu fault, all production wells drilled in the Ogiri field intersected the Ginyu fault reservoir successfully, securing steam production for a 30 MW_e power plant. A typical fracture-type geothermal model for the Ogiri field was developed on the basis of the geology, electric and geophysical logs, fluid chemistry, and well test data.     

Prediction of downhole circulating and shut-in temperatures

An accurate prediction of downhole circulating temperatures during geothermal well drilling is needed for cement slurry and mud design. At present, API correlations are used to predict bottom-hole circulating temperatures for geothermal wells. Presented in this paper are field data which show that the API predictions overestimate the bottom-hole circulating temperatures for wells with high geothermal gradients. New empirical relationships are suggested to predict downhole circulating temperatures for deep, high temperature wells. Also, an analytical formula for predicting downhole shut-in temperature is presented.     

西藏羊易EGS开发储层温度场与开采寿命影响因素数值模拟研究

西藏羊易地区具有丰富的地热能,单井开发潜力接近10 MW,对其深部热储进行EGS开采,可缓解西部能源紧缺问题。本文建立二维理想EGS开发模型,探讨深层地热开采过程中开采流量、注采方式、注入温度等参数对热储温度场分布及开采寿命的影响。基于羊易温度信息设计了12个数值模型,对比研究发现,开采流量对EGS开采的影响较大,为保证开采50年内的商业利用价值,最大开采流量应控制在0.028 kg/s以下;考虑到钻井成本,注采方式的选择以高注高采和中注高采为最佳;注入温度对热储开采影响较小,可选择40℃ ~ 80℃之间任意温度的地热尾水进行回灌,实现地热资源梯级利用。     

Modeling heat extraction from hot dry rock in a multi-well system

A mathematical model is developed for describing the heat energy extracted from a hot dry rock in a multi-well system. The solutions for the water temperature, accounting for a geothermal gradient in a geothermal reservoir, are given in the Laplace domain and computed by numerical inversion, the modified Crump method. The results show that the heat extraction effectiveness is affected significantly by the well spacing, well radius, reservoir thickness, and pumped flow rate in a multi-well system. The water temperature decreases with increasing pumping rate and increases with the well spacing, well radius, and reservoir thickness. The geothermal gradient affects only the early time heat extraction effectiveness significantly and has direct impact on the water temperature all the time if the vertical thickness of geothermal reservoir is large. The present solution is useful for designing and simulating the heat extraction project of geothermal energy exploitation in a multi-well system.     

Hot springs and geothermal gradients in northern Thailand

Chemical geothermometry of hot springs in northern Thailand indicates that many have reservoir temperatures in excess of 150°C and some in excess of 180°C. Measurements of temperatures in abandoned oil wells in Fang Basin indicate geothermal gradients of 70 – 130 mK/m. The high geothermal gradient may be the result of extensional tectonics in northern Thailand, caused indirectly by sea-floor spreading in the Andaman Sea. Relatively high reservoir temperatures and shallow reservoir depths suggest that hot spring areas in northern Thailand may be potential sources of geothermal energy.     

Forecast and evaluation of hot dry rock geothermal resource in China

Utilizing information from plate tectonics characteristics, volcanic activities, and geothermal anomaly, this paper identifies areas where hot dry rock (HDR) may exist as potential geothermal resource in China. Further investigations are also carried out in the paper based on results from regional tectonics, volcanic geology and lithology, as well as data from geothermal displays, geochemistry, geophysics, and shallow borehole temperature measurements. The study reveals several promising areas of HDR geothermal resource in China, including Tengchong of Yunnan province, Qiongbei of Hainan province, Changbaishan of Jilin province, Wudalianchi of Heilongjiang province, and the Southern Tibet area. A 3D static heat conduction model was developed to study the underground temperature gradient characteristics of the Rehai geothermal field in Tengchong and the Yangbajing geothermal field in Tibet. The model adopted is a geological block 10 km deep from the ground surface and 50 km wide in each of the horizontal directions (2500 km² area). The numerical simulation results in evaluations on the quantities of the HDR geothermal resource in Rehai and Yangbajing geothermal fields. The paper shows that there is abundant HDR geothermal resource with large exploitation value in China. If developed with a power capacity of 1×10⁸ kW, the Rehai and Yangbajing fields along would be able to generate electricity for 1560 years.     

San Jacinto-Tizate geothermal field,Nicaragua: Exploration and conceptual model

The results of the exploration of the San Jacinto-Tizate geothermal field during 1992–1995 included geological, hydrogeological and geophysical investigations (magnetotelluric, frequency soundings, subsurface temperature, and soil-gas surveys), and the drilling and testing of seven deep wells (728–2339 m). The geothermal field, located within a composite volcano-tectonic depression, can be divided into two main areas: San Jacinto and Tizate. The San Jacinto area shows evidence of a high-temperature (250–300°C) fossil geothermal system that at present has reservoir temperatures in the 180–190°C range. In the Tizate area there is an active geothermal system with temperatures of 250–285°C. An upflow zone with an excess pressure gradient exists in the central part of this area. Two hydraulically connected reservoirs exist: a shallow one at 550–1200 m depth, and a deeper one below 1600m. Two-phase conditions exist in the upper part of the shallow reservoir. Production tests demonstrate the commercial potential of both Tizate reservoirs.     

Introduction to geothermal exploration on ascension island, South Atlantic Ocean

The exploration for a geothermal resource on Ascension Island utilized a strategy that initially employed geologic mapping. On the basis of this, subsurface faults were mapped using an aeromagnetic survey. The faults were then explored using electrical resistivity surveys to define areas of potential hydrothermal fluid up-welling. The results of all of these techniques were used to site temperature gradient holes. A deep geothermal exploration well was then drilled in the area with highest heat flow adjacent to a rhyolite-to-trachyte volcanic complex.     

Thermal modeling of the Clear Lake magmatic-hydrothermal system,California, USA

Recent volcanism, high heat flow (4 HFU or 167 mW/m²), and high conductive geothermal gradients (up to 120°C/km) indicate that heat from a shallow silicic intrusion in the Clear Lake region is largely being dissipated by conduction. The Geysers area has the highest heat flow in the region, consistent with the presence of shallow convective heat transport within the vapor-dominated geothermal system. Thermal modeling of the Clear Lake magmatic-hydrothermal system based on petrologic and geophysical constraints provides a test of petrologic models, and yields insight into the relationships between observed thermal gradient and magma chamber size, abundance, and emplacement history in the crust. A user-interactive two-dimensional (2-D) numerical model allowing for complex host rocks and multiple emplacements of magma was developed to simulate conductive and convective heat transport around magma bodies using a finite-difference approach. Conductive models that are broadly consistent with the petrologic history and observed thermal gradients of the Mt. Konocti and Borax Lake areas imply a combination of high background gradients, shallow magma bodies (roofs at 3–4 km), and recent shallow intrusion not represented by eruption. Models that include zones of convective heat transport directly above magma bodies and/or along overlying fault zones allow for deeper magma bodies (roofs at 4–6 km), but do not easily account for the large areal extent of the thermal anomaly in the Clear Lake region. Consideration of the entire Clear Lake magmatic system, including intrusive equivalents, leads us to conclude that: (1) emplacement of numerous small and shallow silicic magma bodies occurred over essentially the entire region of high heat flow (about 750 km²); (2) only a very small fraction (much less than 10%) of the silicic magma emplaced in the upper crust at Clear Lake was erupted; (3) high conductive thermal gradients are enhanced locally by fault-controlled zones of convective heat (geothermal fluid) transport; and (4) except for the Mt. Hannah and possibly the Borax Lake area, most of the silicic magma present in the upper crust has solidified or nearly solidified. These bodies are currently difficult to distinguish from surrounding hot basement rocks dominated by graywacke using geophysical methods. The Clear Lake region north of the Collayomi fault is one of the best prospects for hot dry rock (HDR) geothermal development in the US, but is unlikely to provide significant development opportunities for conventional geothermal power production. Modeling results suggest the possibility that granitic bodies similar to The Geysers felsite may underlie much of the Clear Lake region at shallow depths (3–6 km). This is significant because future HDR reservoirs could potentially be sited in granitoid plutons rather than in structurally complex Franciscan basement rocks.     

Heat flow in Albania

As part of a joint geothermal project between Albania and the Czech Republic, a field expedition was organized to Albania in summer 1993 to measure temperature profiles in selected boreholes and to collect rock samples to determine their thermal conductivity. Fourteen localities were visited and nine detailed temperature-versus-depth profiles were obtained. These results were completed with numerous industrial temperature records from other deep holes. The regional patterns of temperatures at 100 m depth and of characteristic near-surface temperature gradients were constructed, and the effect of the topography on the subsurface temperature field was assessed to calculate a total of 49 heat flow density data. A generally low geothermal gradient exists in all of the country, ranging from 7–11 mK/m in the synclinal belt, 11–13 mK/m in southernmost Albania, to a maximum of 18–20 mK/m in the central part of the Pre-Adriatic Depression. These values correspond to a low heat flow zone of 30 to 45–50 mW/m² extending from the north and bordering the Adriatic coast. Heat flow generafly increases from west to east, but its distribution in the Inner Albanides is not clear.     

Wellbore temperature and pressure calculation model for coiled tubing drilling with supercritical carbon dioxide

To better control the state of carbon dioxide during supercritical carbon dioxide drilling, a mathematical model is established to analyze the wellbore carbon dioxide temperature and pressure influencing factors. In this model, the influences of formation temperature change and fluid-friction-generated heat on wellbore temperature distribution are considered. Additionally, the impact of casing, tubing, and cement sheath thermal resistance on heat transfer are considered. The model is validated by comparing the wellbore temperature data calculated from this model with data from previous models. Based on the model, the factors that may affect the wellbore carbon dioxide temperature and pressure are analyzed. The results show that the downhole temperature decreases with the decrease in nozzle diameter and geothermal gradient, and with the increase in injection rate. The injection temperature significantly affects the wellbore temperature near the wellhead, but it does not affect the downhole temperature. Therefore, for low geothermal gradient formation, reducing the injection rate and increasing the nozzle diameter are two effective methods to maintain the CO₂ at the downhole in the supercritical state. The pressure inside the coiled tubing increases with the increase in injection rate and decrease in nozzle diameter, but the injection temperature and geothermal gradient has little effect on the pressure inside both the coiled tubing and annulus.     

A computational method for determining segmental and overall geothermal gradients and geothermal heat flow values

A new computational method is presented which calculates geothermal heat flow values and geothermal gradients with more precision than permitted by previously published techniques. The data required are: geothermal temperature at a known depth, mean surface temperature, the rock types in the stratigraphic column and the thermal resistivity values for the different types of rocks. This method is valuable in areas that have no measured gradient values. Basic equation used was the Fourier heat transfer equation where is heat flux in μcal/(cm² s), ρ_i is thermal resistivity (°C s cm/μcal) and ∂T/∂x is the x component of the temperature gradient (°C/cm). The thermal resistivity was allowed to vary linearly with temperature ρ_i = ρ_i^o [1 + K_i (T − 30)] where ρ_i is thermal resistivity of the lithographic segment «iå at a temperature T, ρ_i^o is thermal resistivity at 30°C and K_i is the temperature coefficient of thermal resistivity. The procedure consisted of integrating the combined equation for heat flux in terms of temperature dependent resistivity.Two iterative solutions were used to simplify the calculations: exact and approximate. The heat flux for each well was assumed to be 1.0 HFU and segmental temperatures were calculated from the bottom (arbitrarily) up, until a surface temperature was obtained. The calculated surface temperature could then be compared with the mean surface temperature (MST). Correction in the heat flux value was made until the calculated surface temperature and MST agreed. An analysis of three deep Appalachian test wells was made and the results showed the critical importance of lithographic ordering and the temperature dependence of thermal resistivity upon calculated geothermal quantities.     

Cold groundwater temperatures and conductive heat flow in the Mt. Amiata geothermal area, Tuscany, Italy

The hydrogeochemical study of the cold springs present in the Mt. Amiata geothermal area, where the Bagnore and Piancastagnaio geothermal fields are situated, has defined the different shallow groundwater systems.The cold groundwater temperature of the volcanic phreatic aquifer is largely correlated with the geothermal heat flow. Through the analysis of the temperature of cold groundwaters, a possible method for geothermal prospection has been developed, supported by the results obtained in the studied area. Through the enthalpic balance of the aquifer and in agreement with available data, a geothermal flow of about 200 mW m^-2 has been calculated.     

Exploration of Lahendong geothermal field in North Sulawesi, Indonesia

This paper describes the progress made in developing the geothermal resources at Lahendong, North Sulawesi, Indonesia for utilization in power generation. Exploration of the whole region included a geophysical survey undertaken exclusively by the Volcanological Survey of Indonesia (VSI). A temperature survey at various depths was conducted through gradient boreholes. The results show that the area of anomalous temperature corresponds to the area of low resistivity revealed by the seismic survey. Two shallow exploratory boreholes (300–400 m) drilled by VSI confirmed the existence of the resources. The deep reservoir in Lahendong field extends over an area of 10 km²; the upper parts of the reservoir are presumed to be water dominated (temperatures in excess of 200°C) and to overlie a zone of hot chloride water at an undetermined depth. The potential of Lahendong field is estimated to about 90 MW.In Pelita IV (1984–1989), the fourth 5-year plan, the State Electricity Public Corporation plans to construct a 30 MW geothermal power-plant in the Lahendong field.     

Geochemistry of thermal springs around Lake Abhe,Western Djibouti

The Republic of Djibouti, occupying an area of 23,180 km², falls within the arid zone of East Africa and is located above the ‘Horn of Africa’, adjacent to the Red Sea. This country has several thermal springs and fumaroles distributed over three regions – Lake Assal, Lake Hanle and Lake Abhe. The most characteristic feature of Lake Abhe is the presence of several linear chains of travertine chimneys. The thermal waters are typical of the Na-Cl type near neutral waters rich in CO₂. These waters show an oxygen shift, indicating reservoir temperatures>200°C. The chemical signature of the thermal springs and the geology of the Lake Abhe region are very similar to the Tendaho geothermal area of Ethiopia. The geology, temperature gradient and its proximity to Damah Ale volcano make the Lake Abhe region a potential site for geothermal power development.