Geothermal Study of a Tailings Deposit: Frost Line Modelling and Comparison to Field Data

Seasonal freezing and thawing can have significant effects on tailings management. Tailings delivery, depositional schemes and water treatment are examples of activities that must be dealt with extra concern in sub-zero temperatures. Changes in mechanical properties, drainage possibilities or embedded frozen tailings layers are effects that can arise in poorly managed facilities. To avoid such consequences, a good understanding of the seasonal effects on the tailings deposit is needed. To get a better understanding of the geothermal regime in tailings, this paper presents a case study with geothermal modelling performed for the Laiva tailings facility in Finland, where major seasonal freezing and thawing periods are present. Ground temperatures and frost lines were predicted via one-dimensional modelling using air temperatures and snow cover depths from adjacent weather stations, and basic soil properties from the facility. Simulated results were compared to data obtained from thermal instruments in the field. The snow cover and its estimated thermal properties were shown to have large influence on the results. The model was able to accurately predict the thermal regime measured in the field. Strong agreement was shown, both in terms of ground temperatures and frost front positions. The methodology presented is useful for tailings management schemes in cold regions. Keywords: Geothermal Monitoring, Geothermal Modelling, Tailings, Soil Temperature, Frost Tubes, Snow Cover.

Estimation of subsurface formation temperature in the Upper Yangtze Area, Southwest China

<p>Subsurface formation temperature in the upper Yangtze area, southwest China, is significant for assessment of hydrocarbon generation and preservation, especially that of shale gas. The upper Yangtze area, with well-developed marine carbonate rocks, is one of the important preferred areas of shale gas exploration and development in China. Previous studies have analyzed the accumulation mechanism, development characteristics, hydrocarbon generation potential and occurrence modes of shale gas. However, the analysis of subsurface formation temperature is rare due to a lack of highly accurate temperature data. Here we combined new steady-state temperature logging data, drill-stem test temperature data and measured rock thermal properties, to investigate the geothermal regime and to estimate the formation temperature at specific depths in the range 1000~6000 m in this area.</p><p>Our results show that the present-day geothermal gradient for this area ranges from 10 to 74℃/km, with a mean of 24℃/km; While the heat flow varies from 27 to 118mW/m<sup>2</sup>, with a mean of 64mW/m<sup>2</sup>, indicating a moderate-high geothermal regime. Formation temperature at the depth of 1000 m is estimated to be between 26 °C and 71°C, with a mean of 40°C; the temperature at 2000 m ranges from 36~125°C with an average of 64°C; 45~180°C is for that at the depth of 3000 m, and the mean is 88°C; the temperature at 4000 m varies from 88 to 235°C, with a mean of 112°C; 65~290°C is for that at 5000 m depth, with a mean of 136°C; 75~344°C is for that at the depth of 6000 and the mean is 160°C. Generally, the pattern of the estimated subsurface temperatures in different depths is similar and has an obvious sub-area characterization, showing a trend of gradually increasing of temperature from northeast to southwest area. Most areas in the south and southeast of Sichuan Basin are with moderate temperature area, which maybe is the “sweet spot area” for shale gas exploration.</p>

Present-day geothermal regime of the Uliastai Depression, Erlian Basin, North China

In this study, we calculated the present-day terrestrial heat flow of the Uliastai Depression in Erlian Basin by using systematical steady-state temperature data obtained from four deep boreholes and 89 thermal conductivity measurements from 22 boreholes. Then, we calculated the lithospheric thermal structure, thermal lithospheric thickness, and lithospheric thermo-rheological structure by combining crustal structure, thermal conductivity, heat production, and rheological parameter data. Research from the Depression shows that the present-day terrestrial heat flow ( qs) is 86.3 ± 2.3 mW/m2, higher than the average of 60.4 ± 12.3 mW/m2 of the continental area of China. Mantle heat flow ( qm) in the Depression ranges from 33.7 to 39.3 mW/m2, qm/ qs ranges from 40 to 44%, show that the crust plays the dominant position in the terrestrial heat flow. The thermal thickness of the lithosphere is about 74–88 km and characterized by a “strong crust–weak mantle” rheological characteristic. The total lithospheric strength is 1.5 × 1012 N/m under wet mantle conditions. Present-day geothermal regime indicates that the Uliastai Depression has a high thermal background, the activity of the deep-seated lithosphere is relatively intense. This result differs significantly from the earlier understanding that the area belongs to a cold basin. However, a hot basin should be better consistent with the evidences from lithochemistry and geophysical observations. The results also show the melts/fluids in the study area may be related to the subduction of the Paleo-Asian Ocean. The study of the geothermal regime in the Uliastai Depression provides new geothermal evidence for the volcanic activity in the eastern part of the Central Asian Orogenic Belt and has significant implications for the geodynamic characteristics.

Geothermal Regime and Deep Temperatures of the Siberian Platform

The geothermal regime of the southern segment of the East Siberian platform, where more than 200 heat-flow measurements have been carried out, is well-understood. The present work deals with the study of deep temperatures of the southern Siberian platform, based on results of geothermal measurements in more than 70 boreholes. In addition, measurements of thermal properties have been made mostly on core samples representing the Vendian terrigenous deposits and Riphean magmatic and metamorphic basement rocks. The basement rocks may be subdivided into two groups, with thermal conductivity coefficients varying in the range of 2 and 3 W/m/K. Higher coefficients indicate the presence of carbonate-halogen admixtures. Studies have also been made of the borehole thermograms and temperatures at the bottom and top of the Moti suite, of lower Cambrian age. These boreholes vary in depth from 1300 to 6000 m, and the borehole temperatures attain values as high as 70оC. In this region average heat flow is 38±4 mW/m2. Higher heat flow values (45±6 mW/m2) are observed in the anticlinal domes and salt-dome crests, while low heat flow seems to be typical of marginal uplifts. This peculiar geothermal condition is also closely related to hydrodynamic features of the area, where underground seepage flow penetrates to depths of 3-5 km while conductive diffusion of heat prevails in the deeper crust. It is argued that such anomalous conditions exert influence on the dynamics of hydrocarbon accumulation, which in turn is also predetermined by geothermal conditions.