The Geothermal Gradient
The rate at which temperature (T) rises with increasing depth (Z) to the earth's interior is a geothermal gradient. Near the earth's surface, it grows by an average of 25°C per kilometer. The planet's internal heat comes from a combination of residual heat from the earth's accretion, radioactive decay heat, and other sources.
Radioactive Decay and Heat Production
Potassium-40, thorium-232, and uranium 238 and 235 are essential isotopes that produce heat. The temperature and pressure at the planet's core may be around 7000K and 360GPa, respectively. Radioactive decay has a lot of heat; thus, scientists believe that before the depletion of short-chain fatty acids, heat levels were much higher. Much heat is as a result of radioactive decay; therefore scientists consider that heat was much higher, before the depletion of short half-life isotopes. Early in the history of the earth, approximately three billion years ago, the production of heat was assumed to be twice that of today. As a result, the gradient of the temperature within the earth was enormous, more substantial mantle convention rates as well as tectonic of plates, encouraging igneous rocks production for instance komatiites. These types of rocks are not formed anymore presently (Liu et al., 2016 February).
Variations in Geothermal Gradient
Geothermal gradient varies from place to place with much higher heat flows recorded in areas with rifts, faults, active tectonic plates, and volcanic regions. This heat can make drilling and mining practices difficult and uncomfortable in deep wells. Some of the factors that affect the geothermal gradient are climate variations, tectonic subsurface water flows, and thermal properties of the rock and convection currents. It is essential to calculate the geothermal gradient primarily for the oil, geothermal energy, and gas industries because the tools required for downhole logging may need to be hardened to function in deep wells which are located in places with a high gradient.
Geothermal Gradient in the Earth's Interior
It is established well in the earth that increase in temperature is directly proportional to depth. This implies that heat at depth is generated also transferred to the surface by rocks as well as layers of sediments. Subsequently, the flow of heat, as well as data of thermal conductivity, are not often accessible to the applications for petroleum, temperatures measured in boreholes form the principle background for geothermal gradient calculations (Aigbadon et al,. 2017).
Geothermal Gradient in Different Regimes
In both countries, shallow as well as deep thermal regimes are demonstrated by geothermal gradient. The regimes are parted through a thermal conductivity zone that is high. The shallow thermal regime extends from the surface of the land to 8000ft. Also, it has locations with low as well as high up-dip also down-dip. The deep thermal regime extends from a range of 10,000 to 19,000ft, having no gradient differences between the areas of up-dip as well as down-dip. The zone with high thermal conductivity has a low gradient and lies in a range of 8,000 to 10,000ft (Al-Mahmoud, 2015).
Geothermal Energy Applications in Saudi Arabia and the United Arab Emirates
In Saudi Arabia, geothermal gradient has found various applications including the production of geothermal energy as compared to the United Arab Emirates. Heat from the earth's interior is being used as a source of energy. Geothermal gradient has found applications in space heating as well as bathing. In Saudi Arabia, geothermal energy exploration is taking place in provinces including Jizan provinces. The country has engaged itself in geothermal power because the population is continuously growing, therefore causing an interest in finding renewable energy sources as well as have an emission of greenhouse gas reduction. Currently, 240 terawatt electricity hours is consumed. An initial assessment was carried out on the hot springs geothermal potential encountered at the sites of Jizan as well as Al-Lith. Wide reservoir temperature ranges, as well as heat flows, are assessed from geothermometers. Based on the areas geothermal gradient, characteristics of the reservoir, rate of flow, flow of heat, also the volcanic flow surface area of the provinces with geothermal capacity, it is projected that geothermal systems that are wet can produce approximately 23 x 109 kWh. In the United Arab Emirates, the first geothermal plant construction has been proposed to be constructed in the city of Masdar. The plant would be the first facility for geothermal energy in the gulf (Lashin & Al Arifi, 2014).
Geothermal Gradient and Oil Exploration
In both countries, bottom-hole temperatures as well as geothermal gradient mapping have been generated to aid oil exploration and determine well anomalies. Explorers of oil have realized that hydrocarbon that penetrates wells reveals irregular geothermal gradient. High anomalies of geothermal gradient characterize sections of sedimentary basins that are thermally impending. The process of migration of subsurface fluid is accountable for entrapment of lighter components in the reservoirs that are shallow if there is a hydrocarbon migration along with the relatively higher water density. Dry holes, as well as suspended wells, were established to gather low Compensated Geothermal Gradients, characterizing settings of thermal impedance that is low with ascending fluids as well as seals that are poor or else breached (Swart et al., 2016).
Geothermal Gradient Mapping Techniques
The exploration groups used the software of CGG-ESTI to input as well as generate a database of bottom-hole temperature, correct the raw readings of BHT, test the BHT measurements' statistical significance. Moreover, an analysis as well as plotting a graph of the relationship between the compensated geothermal temperatures against the extrapolated surface temperature was done. The chart had a relationship with the intercepts of the development wells being explored in the areas that had been studied. An application of a procedure of descriptive computer contouring was applied to the statistically important wells, as well as the less important to act as control wells with multi borehole temperatures, and others that have geothermal gradients with single surface temperatures to produce regional and anomalous contours of CGG-ESTI. A cross-plot analysis considered to be interactive was applied to substantial control wells to find the geothermal cluster regimes of discovery against dry holes that have been corrected. Descriptive computer contouring was used for the data set that was adjusted to explain regional geothermal contours as well as anomalies of the local geothermal gradient (Zare-Reisabadi et al., 2015).
Limitations in Geothermal Gradient Information
In both countries, information concerning the measurements of the flow of heat has been insufficient. For instance, In Saudi Arabia, the measurements have been made at the line of the Saudi Arabian seismic deep-refraction, at five onland shot points. The exercise sampled significant Arabian shied tectonic elements along Ar Riyad to the islands of Farasan profile. Because of each shot’s pattern drilling, several 60m deep holes could be temperature logged, therefore allow a better geothermal gradient estimate. Mapping, as well as sampling, was done in detail at each site and representative samples analyzed in the lab. Computation of thermal conductivities was made from the modal analyses as well as information of single mineral conductivity. The places that were having granitic rocks, a technic involving spectrometry of gamma-ray was applied to approximate the concentrations of potassium, thorium, as well as uranium. A graph showing the relationship of heat generation against the flow of heat suggests that in the shield of Arabia the flow of heat also heat production relationship is not linear. More data related to the flow of heat is required. Research done in Zakum, an oil town in the United Arab Emirates also indicates lack of insufficient heat flow information (Fattah et al., 2014).
Works Cited
Aigbadon, G., Okoro, A., Akpunonu, E., Nimnu, R., & Ocheli, A. (2017). Geothermal modelling and its application to hydrocarbon generation from agbada formation. a case study of USANi field, Niger delta basin, Nigeria. International Journal of Advanced Geosciences, 5(2), 75-80.
Al-Mahmoud, M. J. (2015). Pressure and thermal regimes and systems in the sedimentary sequence of central and eastern Saudi Arabia. Arabian Journal of Geosciences, 8(8), 6249-6266.
Fattah, R. A., Meekes, S., Bouman, J., Ebbing, J., & Haagmans, R. (2014, January). Modeling tectonic heat flow and source rock maturity in the Rub’Al-Khali Basin (Saudi Arabia), with the help of GOCE satellite gravity data. In IPTC 2014: International Petroleum Technology Conference.
Lashin, A., & Al Arifi, N. (2014). Geothermal energy potential of southwestern of Saudi Arabia” exploration and possible power generation”: A case study at Al Khouba area–Jizan. Renewable and Sustainable Energy Reviews, 30, 771-789.
Liu, C., Li, K., Chen, Y., & Chen, J. (2016, February). Geothermal Gradient in the Oilfields in China. In Proceedings of the 41st Workshop on Geothermal Reservoir Engineering, Stanford, CA, USA (Vol. 2224).
Swart, P. K., Cantrell, D. L., Arienzo, M. M., & Murray, S. T. (2016). Evidence for high temperature and 18O‐enriched fluids in the Arab‐D of the Ghawar Field, Saudi Arabia. Sedimentology, 63(6), 1739-1752.
Zare-Reisabadi, M., Kamali, M. R., Mohammadnia, M., & Shabani, F. (2015). Estimation of true formation temperature from well logs for basin modeling in Persian Gulf. Journal of Petroleum Science and Engineering, 125, 13-22.
