A calculation method for evaluating deep granite geothermal potential

Abstract

The present application relates to the technical field of deep granite geothermal, and discloses a calculation method for evaluating deep granite geothermal potential, which comprises the following steps: according to the content of three types of radioactive heat generating elements, i.e., uranium, thorium and potassium, in the rock mass, and in combination with the element radioactive heat generation rate parameter, the radioactive heat generation rate of the rock mass is calculated; then, zircon single minerals are selected from the rock mass sample and are made into targets; the closure time of the zircon U-Pb isotope system and the fission track is measured; in combination with the radioactive heat generation rate, the two types of closure time and the specific heat capacity of the granite, the total sum of the multi-cycle cooling crystallization delay time generated by the continuous accumulation of the radioactive heat generation is calculated; meanwhile, the radioactive cause heat of the granite is less than the cooling conduction and dissipation heat, so that the delay time is ensured to be a convergent analytic function; and finally, according to the cooling crystallization delay time, the deep granite geothermal potential is evaluated. The present application quantitatively inverts the radioactive cause heat of the deep granite, scientifically and accurately evaluates the geothermal potential, and provides a reliable basis for the exploration and development of geothermal resources.

Description

Technical Field

[0001] This invention relates to the field of deep granite geothermal technology, and more specifically to a calculation method for assessing the geothermal potential of deep granite. Background Technology

[0002] Hot dry rocks are important geothermal resources, but their heat source formation mechanism remains highly controversial internationally, with two main viewpoints: one posits that heat conduction from deep basic magma chambers (capsules) is the primary heat source, with granite serving only as a low-porosity, fractured water-conducting layer; the other argues that radioactive heat generation from granite is dominant, and the fundamental cause of high geothermal gradients and high heat flow is this radioactive heat generation from granite. Deep basic magma chambers (capsules) and high mantle heat conduction are often found at the margins of subducting convergent plates and in areas of mantle hotspot activity. Existing research often associates high heat flow characteristics with these tectonic zones, neglecting the geothermal potential of tectonically stable regions.

[0003] In structurally stable regions, the crust is thick and the mantle conduction heat is low, yet massive amounts of granite are widely developed. These regions are less affected by the mantle and have underdeveloped deep active basic magma chambers (capsules). The surface heat flow mainly originates from the internal crustal heat, especially the internal crustal heat dominated by the radiogenic heat of granite. The differences in heat flow values ​​within the continental crust are also mainly caused by the radiogenic heat generation of granite, making these regions ideal for studying the radiogenic heat generation of granite.

[0004] However, current research on radioactive heat generation in granite still lacks an effective method to invert the geothermal potential of deep granite. Existing methods, such as deploying shallow drills, can only obtain shallow geothermal gradient thermal changes and cannot effectively assess the geothermal potential of deep dry hot rocks.

[0005] In summary, existing studies mostly focus on geothermal resources in tectonic active zones, neglecting tectonic stable zones, and lack effective methods to invert the geothermal potential of deep granites. Shallow drilling can only detect shallow geothermal temperatures and cannot assess the potential of deep dry hot rocks. Summary of the Invention

[0006] The purpose of this invention is to provide a calculation method for evaluating the geothermal potential of deep granite, in order to solve the technical problems in the prior art, which is that existing studies mostly focus on geothermal resources in tectonic active zones, neglecting tectonic stable zones, and lack effective methods to invert the geothermal potential of deep granite, while shallow drilling can only detect shallow geothermal temperatures and cannot evaluate the potential of deep dry hot rocks.

[0007] To solve the above-mentioned technical problems, the present invention specifically provides the following technical solution:

[0008] A calculation method for assessing the geothermal potential of deep granite includes the following steps:

[0009] Step 100: Obtain a sample of the target granite rock mass and test and analyze the content of radioactive heat-generating elements uranium, thorium, and potassium in the target granite rock mass;

[0010] Step 200: Based on the relevant parameters of the radioactive heat generation rate of uranium, thorium, and potassium, and combined with the measured contents of uranium, thorium, and potassium, calculate the radioactive heat generation rate of the target granite rock mass;

[0011] Step 300: Separate zircon single mineral from the target granite rock sample and prepare a target. Determine the zircon U-Pb isotope sealing time using SHRIMP or LA-ICP-MS and the zircon fission track sealing time using an external detector method.

[0012] Step 400: Combine the calculated radioactive heat generation rate of the granite rock mass, the measured zircon U-Pb isotope system closure time and zircon fission track closure time, and the specific heat capacity of the granite rock mass, calculate the cooling and crystallization delay time of the target granite rock mass caused by radioactive heat generation.

[0013] The cooling crystallization delay time is the sum of multiple delay times generated by the continuous accumulation of radioactive heat generation. The radiogenic heat of granite is usually less than the heat lost through conduction during rock cooling, ensuring that the cooling crystallization delay time is a convergent analytical function. The calculated cooling crystallization delay time is used to assess the geothermal potential of deep granite.

[0014] In a preferred embodiment of the present invention, in step 200, the measured uranium content is recorded as... Thorium content is recorded as All units are The measured potassium content is recorded as , The unit is %;

[0015] In step 300, the proposed uranium, thorium, 40 The radioactive heat production rates of K are respectively , and , 40 K is the only radioactive isotope of potassium, with an abundance of [missing information]. Calculate the rate of radioactive heat generation The formula is:

[0016] .

[0017] As a preferred embodiment of the present invention, in step 400, before using the LA-ICP-MS method to determine the sealing time of the zircon U-Pb isotope system, the zircon is first photographed by cathodoluminescence and the measurement point is determined before testing point by point.

[0018] ICPMSDATACAL software was used to correct the isotope ratios of the experimental data to remove the influence of common lead, and the obtained weighted age of the rock mass was used as the zircon U-Pb isotope sealing time t1.

[0019] In a preferred embodiment of the present invention, in step 400, when determining the closure time of zircon fission tracks using the external detector method, mica sheets are covered on the mineral surface and irradiated with thermal neutrons to generate induced tracks. The number of tracks on the mica external detector is then counted to determine the closure time. 235 U content, based on natural sources 238 U / 235 U=7.2527×10 -3 The fixed abundance ratio was converted to 238 The actual U content, combined with the spontaneous track count on mineral grains and the induced track count on the mica external detector, is used to calculate the fission track age using the Zeta method. This fission track age is taken as the zircon fission track sealing time t2, and satisfies the following conditions: .

[0020] As a preferred embodiment of the present invention, the zircon U-Pb isotope system sealing temperature and the zircon fission track FT annealing temperature of the granite mass are set to 950℃ and 200℃, respectively. Cooling is then gradually initiated from 950℃. Let A represent the radioactive elements U, Th, and... 40 The heat generation rate of K represents the radiogenic heat generated per gram of rock per year, and the radiogenic heat generated during cooling from 950°C to 200°C is: .

[0021] As a preferred embodiment of the present invention, when the heat transfer efficiency of the target granite rock sample in a non-surface environment is at a set threshold, the delay time is... The larger the value, the greater the geothermal potential of the deep granite, and vice versa.

[0022] As a preferred embodiment of the present invention, the total cooling crystallization delay time The calculation model is the superposition of multiple rounds of delay time, as detailed below:

[0023] The specific heat capacity of the granite mass is denoted as . First round of cooling crystallization delay time for:

[0024] ;

[0025] Second round of cooling crystallization delay time for:

[0026] ;

[0027] Third round of cooling crystallization delay time for:

[0028] ;

[0029] ...;

[0030] No. Wheel cooling crystallization delay time for:

[0031] ;

[0032] Total cooling crystallization delay time for:

[0033] .

[0034] As a preferred embodiment of the present invention, based on the premise that the radiogenic heat of the granite is less than the heat lost through conduction during the cooling of the rock mass, the following conditions are met: ,Sure It is a convergent analytic function that can yield a definite value.

[0035] Compared with the prior art, the present invention has the following advantages:

[0036] This invention conducts a systematic geological survey of the target granite mass, tests the content of radioactive heat-generating elements and calculates the heat generation rate. After accurately determining the two types of sealing times of zircon, it calculates the cooling and crystallization delay time caused by radioactive heat generation based on the specific heat capacity of granite. Relying on this delay time, it quantitatively inverts the relative magnitude of radiogenic heat in deep granite, realizing a scientific, quantitative and accurate assessment of the geothermal potential of deep granite. This solves the technical problem that traditional methods are difficult to accurately assess the geothermal potential of deep granite. Furthermore, because the formula for calculating the delay time is a convergent analytical function, the assessment results have reliable numerical support. Attached Figure Description

[0037] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0038] Figure 1 This is a schematic diagram of the calculation method for evaluating the geothermal potential of deep granite in an embodiment of the present invention;

[0039] Figure 2 This is a schematic diagram illustrating the principle of evaluating the geothermal potential of deep granite in an embodiment of the present invention. Detailed Implementation

[0040] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] like Figure 1 and Figure 2 As shown, the present invention provides a calculation method for evaluating the geothermal potential of deep granite, comprising the following steps:

[0042] Step 100: Obtain a sample of the target granite rock mass and test and analyze the content of radioactive heat-generating elements uranium, thorium, and potassium in the target granite rock mass;

[0043] Step 200: Based on the relevant parameters of the radioactive heat generation rate of uranium, thorium, and potassium, and combined with the measured contents of uranium, thorium, and potassium, calculate the radioactive heat generation rate of the target granite rock mass;

[0044] Step 300: Separate zircon single mineral from the target granite rock sample and prepare a target. Determine the zircon U-Pb isotope sealing time using LA-ICP-MS or SHRIMP method, and determine the zircon fission track sealing time using external detector method.

[0045] Step 400: Combine the calculated radioactive heat generation rate of the granite rock mass, the measured zircon U-Pb isotope system closure time and zircon fission track closure time, and the specific heat capacity of the granite rock mass, calculate the cooling and crystallization delay time of the target granite rock mass caused by radioactive heat generation.

[0046] The cooling crystallization delay time is the sum of multiple delay times generated by the continuous accumulation of radioactive heat generation. The radiogenic heat of granite is usually less than the heat lost through conduction during rock cooling, ensuring that the cooling crystallization delay time is a convergent analytical function. The calculated cooling crystallization delay time is used to assess the geothermal potential of deep granite.

[0047] Based on systematic field reconnaissance and petrographic, zircon U-Pb geochronology, and geochemistry studies of the target rock mass, this implementation method mainly includes the following aspects:

[0048] 1. Field geological survey and rock and mineral identification:

[0049] Through field geological surveys, we carefully observe and record the lithological characteristics, distribution characteristics, and emplacement periods of granite bodies. We investigate and understand the structural types and distribution, mineral composition, and stratigraphic sequence within the rock bodies, and grasp the intrusive contact relationships between various intrusive bodies and their temporal and spatial distribution characteristics.

[0050] The rock type, mineral composition, structure, lineation and foliation features (flow and deformation structures) and sampling location are recorded in detail. The granite rock type is identified under a microscope, and the rock mineral assemblage, structure, accessory mineral characteristics, etc. are accurately described.

[0051] 2. Determination of U, Th, and K content in granite:

[0052] 238 U、 235 U、 232 Th、 40 K and 87 Rb is the most abundant naturally occurring radioactive nuclide on Earth with the longest half-life. During its radioactive decay, it releases a series of alpha, beta, and gamma particles, which possess heat. The content and heat generation rate of Rb are extremely low, accounting for a very small proportion (<1%) of the total radioactive heat generation. Therefore, the main focus of the analysis was on the content of the radioactive heat-generating elements U, Th, and K in the granite, denoted as […]. , and .

[0053] 3. Calculate the radioactive heat generation rate of granite:

[0054] According to the parameters proposed by Van Schmus (1984), , , 40 The radiothermal yields of K are 3.098 J / g·a, 0.833 J / g·a, and 0.921 J / g·a, respectively. K has three naturally occurring isotopes, namely... 39 K, 40 K and 41 K, where 39 K, 41 K is a stable isotope with high abundance, at 93.258% and 6.730%. 40 K is the only radioactive isotope, with an abundance (abundance percentage) of 1.17 × 10⁻⁶. -4 The formula for calculating the rate of radioactive heat generation is:

[0055] ,

[0056] In the formula, the unit of heat generation rate A is μJ / g·a.

[0057] , and These represent the contents of U, Th, and K in the rock, respectively, with U and Th measured in units of 10⁻⁶. -6 The unit of K is %.

[0058] Given that U, Th, and K have relatively long half-lives, 4.468 × 10⁻⁶ respectively... 9 y ( 238 U), 7.038×10 8 y ( 235 U), 1.41×10 10 y ( 232 Th) and 1.28×10 9 y ( 40 K), this calculation formula does not take into account the effect of decay.

[0059] 4. Determination of U-Pb isotopic sealing time and fission track FT sealing time in granite zircon:

[0060] The samples were separated into zircon single minerals using conventional gravity separation and magnetic separation methods, and the zircon particles were fixed with epoxy resin.

[0061] The U-Pb isotopic age of zircon was determined using the LA-ICP-MS or SHRIMP method. After zircon cathodoluminescence imaging, the locations of the zircon LA-ICP-MS measurement points were determined and the tests were performed point by point. The experimental data were corrected for isotopic ratios using ICPMSDATACAL software to remove the influence of ordinary Pb, and finally the weighted average age of the granite intrusion was obtained. The number of test points was no less than 20.

[0062] Fission track analysis was performed using the external detector method (EDM). This method mainly utilizes a mica external detector to analyze the fission track within apatite particles. 238 The U content is determined indirectly. The specific procedure involves covering the mineral surface with mica sheets, which are then irradiated with thermal neutrons to generate induced tracks.

[0063] Determining by counting the number of tracks on the mica-external detector 235 U content, and based on the natural environment 238 U and 235 The fixed abundance ratio of U ( 238 U / 235 U=7.2527×10 -3 ) Convert 238 The actual content of U. Finally, the age was calculated using the Zeta method, combining the spontaneous track number on the mineral grains and the induced track number on the corresponding mica external detector.

[0064] Due to its standardized experimental procedures, well-developed calculation formulas, and stable parameter system, this method has become the mainstream analytical tool in fission track laboratories worldwide.

[0065] Zircon U-Pb dating and fission track FT dating are essentially in-situ isotopic analyses of mineral micro-areas, and the age results obtained are relatively reliable.

[0066] 5. Calculate the cooling and crystallization delay time caused by radioactive heat generation. :

[0067] Dowty (1980) indicated through granite melting experiments that the liquidus temperature of granite containing 3.5% H2O is 950℃, and the zircon closure temperature can represent the liquidus temperature of granite. Yamada et al. (1995) indicated through zircon fission track experiments that the annealing temperature of zircon is approximately 200℃.

[0068] Based on zircon U-Pb isotope sealing time ( Starting from the thermal evolution to the FT closure time of the zircon fission track, the closure temperature (950℃) was calculated. ) and annealing temperature (200℃), the delay time caused by radiogenic heat And satisfy The calculation process is briefly described below:

[0069] The zircon U-Pb isotope sealing temperature and zircon fission track FT annealing temperature of the granite body were 950℃ and 200℃, respectively. Figure 1 Starting at 950℃ and gradually cooled, let A be a radioactive element (U, Th, and...). 40 The heat generation rate of K), in units of This represents the radiogenic heat generated per gram of rock per year. The radiogenic heat generated during cooling from 950°C to 200°C is: .

[0070] After radiogenic heat is generated in the granite, the heat is gradually transferred to the surrounding rock via thermal conduction, with a transfer efficiency of: .

[0071] in is the specific heat capacity of granite rock mass, expressed in J / (kg·℃), representing the amount of heat required / released to raise / lower the temperature of one kilogram of rock by 1 degree Celsius.

[0072] The extended time from the onset of consolidation to the closure of the fission track in a single granite unit is due to the accumulated radiogenic heat. The calculation formula is: .

[0073] Radioactive heat generation is continuous and uninterrupted; therefore, the extended time due to radioactive heat generation... This process also leads to the accumulation of radioactive heat, which in turn prolongs the cooling time (crystallization time of granite minerals). , This will lead to a longer time. This cycle continues until... , Therefore, the formula for calculating the radiogenic thermal extension time is: ,in,

[0074] ,

[0075] ;

[0076] ;

[0077] ...;

[0078] .

[0079] The total delay time can be calculated from this:

[0080] =(t1-t2)× [A×(t1-t2) / (750×C p )] n ;

[0081] Generally speaking, the heat generated by the radioactivity of granite is less than the heat lost through conduction during the cooling of the rock mass. Therefore... ,illustrate It is a convergent analytic function that can yield a definite value.

[0082] 6. Delay time The practical geological significance

[0083] Generally, geological surveys in the field are based on sampling from the surface. The surface is the result of crustal uplift and weathering. Exposed granite rock masses on the surface lose radioactive heat rapidly, leading to premature sealing of mineral isotopes. In contrast, deep rock masses are in a normal state of heat conduction and can better accumulate and preserve heat, including radiogenic heat.

[0084] Given a constant heat transfer efficiency, the more radiogenic heat a granite intrusion contains, the more it preserves. Furthermore, radiogenic heat can prolong the cooling and crystallization time of minerals (such as zircon) or the sealing time of their isotopic systems within the granite, thus allowing for the preservation of more minerals through delayed crystallization. The calculation quantitatively inverts the relative magnitude of the radiogenic heat of granite. Delay time The larger the size, the greater the geothermal potential of the deep granite, and vice versa.

[0085] like Figure 2 The image shows a comparison between the prolonged crystallization time of deep granites under normal heat transfer rates and the rapid dissipation of radiogenic heat in surface or near-surface granites. : The prolonged crystallization time due to radiogenic heat, in Ma or 10. 6 a. t1: Sealing time of zircon U-Pb isotopic system, unit: Ma or 10 6 a. t2: Zircon fission track FT closure time, unit: Ma or 10 6 a.

[0086] The following calculation demonstration uses a real-world example:

[0087] 1. Assume that the average contents of U, Th, and K measured in a certain granite body are U = 5.30 × 10⁻⁶. -6 , =23.0×10 -6 , =4.50%, according to the formula A=3.098 +0.833 +1.17×0.921×10 -4 The calculated radioactive heat generation rate of the rock mass is 37.31 × 10⁻⁶. -6 J / g·a.

[0088] 2. The zircon U-Pb isotopic age of this granite was determined to be 160 Ma by LA-ICP-MS, and the fission track FT isotopic age was determined to be 140 Ma. The zircon U-Pb isotopic sealing temperature was taken as 950℃, and the fission track FT sealing temperature was taken as 200℃.

[0089] 3. Through Calculate the cooling and crystallization delay time caused by radioactive heat generation. Cp was set to 1.3398 J / g·℃, and the calculation results were obtained. =57.71Ma.

[0090] 4. Similarly, in the target geothermal area or even a larger area, calculate the cooling and crystallization delay time caused by radioactive heat generation of different granites, and then compare them. The greater the delay time, the greater the geothermal potential of the deep rock mass.

[0091] The above embodiments are merely exemplary embodiments of this application and are not intended to limit this application. The scope of protection of this application is defined by the claims. Those skilled in the art can make various modifications or equivalent substitutions to this application within its substance and scope of protection, and such modifications or equivalent substitutions should also be considered to fall within the scope of protection of this application.

Claims

1. A calculation method for assessing the geothermal potential of deep granite deposits, characterized in that, Includes the following steps: Step 100: Obtain a sample of the target granite rock mass and test and analyze the content of radioactive heat-generating elements uranium, thorium, and potassium in the target granite rock mass; Step 200: Based on the relevant parameters of elemental radioactive heat generation rate, and combined with the measured contents of uranium, thorium, and potassium, calculate the radioactive heat generation rate of the target granite rock mass; Step 300: Separate zircon single mineral from the target granite rock sample and prepare a target. Determine the zircon U-Pb isotope sealing time using SHRIMP or LA-ICP-MS and the zircon fission track sealing time using an external detector method. Step 400: Combine the calculated heat generation rate of the rock mass, the measured zircon U-Pb isotope sealing time and zircon fission track sealing time, and the specific heat capacity of the granite rock mass to calculate the cooling and crystallization delay time of the target granite rock mass caused by heat generation. The cooling crystallization delay time is the sum of multiple delay times generated by the continuous accumulation of radioactive heat generation, and the radiogenic heat of granite is usually less than the heat lost by the cooling conduction of the rock mass, so as to ensure that the cooling crystallization delay time is a convergent analytical function; the calculated cooling crystallization delay time is used to assess the geothermal potential of deep granite.

2. The calculation method for evaluating the geothermal potential of deep granite as described in claim 1, characterized in that, In step 200, the measured uranium content is recorded as follows: Thorium content is recorded as All units are The measured potassium content is recorded as , The unit is %; In step 300, the proposed uranium, thorium, 40 The radioactive heat production rates of K are respectively , and , 40 K is the only radioactive isotope of potassium, with an abundance of [missing information]. Calculate the rate of radioactive heat generation The formula is: 。 3. The calculation method for evaluating the geothermal potential of deep granite as described in claim 1, characterized in that, In step 400, before determining the sealing time of zircon U-Pb isotopes using LA-ICP-MS, the zircon is first photographed using cathodoluminescence and the measurement point locations are determined before testing is performed point by point. ICPMSDATACAL software was used to correct the isotope ratios of the experimental data to remove the influence of common lead, and the obtained weighted age of the rock mass was used as the zircon U-Pb isotope sealing time t1.

4. The calculation method for evaluating the geothermal potential of deep granite as described in claim 1, characterized in that, In step 400, when determining the closure time of zircon fission tracks using the external detector method, mica sheets are applied to the mineral surface and irradiated with thermal neutrons to generate induced tracks. The number of tracks on the mica external detector is then counted to determine the closure time. 235 U content, based on natural sources 238 U / 235 U=7.2527×10 -3 The fixed abundance ratio was converted to 238 The actual U content, combined with the spontaneous track count on mineral grains and the induced track count on the mica external detector, is used to calculate the fission track age using the Zeta method. This fission track age is taken as the zircon fission track sealing time t2, and satisfies the following conditions: .

5. The calculation method for evaluating the geothermal potential of deep granite as described in claim 1, characterized in that, The zircon U-Pb isotope sealing temperature and zircon fission track FT annealing temperature of the granite mass were 950℃ and 200℃, respectively. Gradual cooling was initiated from 950℃. Let A be the radioactive elements U, Th, and... 40 The heat generation rate of K represents the radiogenic heat generated per gram of rock per year, and the radiogenic heat generated during cooling from 950°C to 200°C is: .

6. The calculation method for evaluating the geothermal potential of deep granite as described in claim 5, characterized in that, When the heat transfer efficiency of the target granite rock sample in a non-surface environment is at a set threshold, the time delay is... The larger the value, the greater the geothermal potential of the deep granite, and vice versa.

7. The calculation method for evaluating the geothermal potential of deep granite as described in claim 5, characterized in that, Total cooling crystallization delay time The calculation model is the superposition of multiple rounds of delay time, as detailed below: The specific heat capacity of the granite mass is denoted as . First round of cooling crystallization delay time for: ； Second round of cooling crystallization delay time for: ； Third round of cooling crystallization delay time for: ； ……； No. Wheel cooling crystallization delay time for: ； Total cooling crystallization delay time for: 。 8. The calculation method for evaluating the geothermal potential of deep granite as described in claim 7, characterized in that, Given that the radiogenic heat of granite is less than the heat lost through conduction during the cooling of the rock mass, the following conditions must be met: ,Sure It is a convergent analytic function that can yield a definite value.

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