A method for predicting the temperature of the produced fluid in the karst geothermal well production and irrigation system

Abstract

The present invention relates to a method for predicting the temperature of production fluid in a karst geothermal well-injection system. The method involves pre-collecting relevant geological data on the geothermal system and data related to the operation of the well-injection system, determining tracer composition and calculating the injection amount. Tracer tests are then conducted in the karst well-injection system to obtain tracer test results and plot curves. Based on the tracer test results, Qtracer2 software is used to calculate the tracer recovery rate, cross-sectional area, and height of the karst channel. The boundary conditions of the karst geothermal heat transfer model are determined, a time period is set, and the temperature changes in the production well are calculated based on the heat transfer model. This method for predicting the temperature of production fluid in the well is accurate and reliable, highly operational, and quick and convenient. It solves the problem of predicting the temperature of production fluid in the well of the well-injection system under long-term well recharge conditions and plays an important guiding role in the selection of karst geothermal production and recharge plans and the well location layout for later projects.

Description

Technical Field

[0001] The present invention belongs to the technical field of hydrothermal geothermal resource reinjection, and in particular relates to a method for predicting the temperature of a produced fluid in a karst geothermal well production and injection system. Background Art

[0002] Geothermal tailwater reinjection is essential for the sustainable development and utilization of hydrothermal geothermal resources. It can address environmental issues such as thermal and chemical pollution caused by geothermal tailwater discharge. It also plays a crucial role in maintaining or restoring reservoir pressure, stabilizing reservoir production conditions, and preventing ground subsidence. Reinjected "cold water" flows from the reservoir to the production well. Unsound reinjection can cause a drop in well temperature, shortening the lifespan of the geothermal system or even rendering it useless. To ensure this, it is necessary to predict the thermal breakthrough of different reinjection water quantities. Based on this prediction, the production volume can be determined. This scientifically determines the recovery and reinjection volumes, using the reinjection volume to determine the recovery volume, ensuring 100% reinjection. Hydrothermal geothermal wells are often deep and complex, exposed to high temperatures and pressures. Conventional methods cannot directly measure certain reservoir geological parameters. Without understanding the hydraulic connections between production and injection wells, it is impossible to predict well temperature changes under long-term reinjection conditions. Karst geothermal resources, as a type of hydrothermal geothermal resources, face the same problem. In existing projects, the empirical coefficient method is usually used to predict the changes in the temperature of the mining flow diagram. However, due to the huge differences in the properties of geothermal reservoirs in different projects, this method cannot meet the engineering needs. Summary of the Invention

[0003] In order to overcome the problem that the existing empirical coefficient method cannot meet the needs of predicting the changes in the temperature of the mining flow diagram of karst geothermal resources.

[0004] The method for predicting the temperature of the produced fluid in a karst geothermal well production and irrigation system according to the present invention comprises the following steps:

[0005] Step 1: Collect geothermal geological data and data related to the operation of the extraction and irrigation system;

[0006] Step 2: Determine the tracer composition and calculate the dosage through preliminary experiments, and conduct a well recharge test. After the tracer is injected, sample the production well once a day. After on-site sampling, test and analyze the tracer concentration. When the tracer concentration in the production well is first detected to increase, the sampling frequency is increased until the recharge test is completed.

[0007] Step 3: Obtain tracer test data and a tracer concentration versus time curve;

[0008] Step 4: Based on the tracer test data, Qtracer2 software is used to perform inversion calculations to calculate the tracer recovery rate, cross-sectional area, and height of the karst channel;

[0009] Step 5: Determine the initial and boundary conditions of the karst-to-well system heat transfer model, consider the heat convection in the karst channel and the heat conduction in the surrounding rock, set the time period, and calculate the temperature change of the production well fluid according to the heat transfer model.

[0010] Furthermore, the geothermal geological data and data related to the operation of the production and irrigation system include the original heat reservoir temperature, the reinjection water temperature, the production volume of the production and irrigation system, the reinjection volume of the production and irrigation system, the specific heat capacity of the geothermal water, the thermal conductivity of the heat reservoir, the density of the geothermal water, the density of the heat reservoir, the specific heat capacity of the heat reservoir, the porosity of the heat reservoir, and the distance between the reinjection well and the production well.

[0011] Furthermore, the tracer preliminary test in step 2 includes a thermal stability test, an adsorption test, and a compatibility test.

[0012] Furthermore, the calculation formula for the tracer dosage in step 2 is:

[0013] G=πR 2 Hαγc(1)where: G is the amount of tracer injected, kg; R is the distance between the production well and the recharge well, m; H is the thickness of the thermal reservoir aquifer, m; α is the dispersion constant factor, which is 0.0153; γ is the correction coefficient of the dominant flow concentration field, 0.1; Cp is the peak tracer concentration (5-10 times the concentration of the geothermal water tracer), mg / L.

[0014] Furthermore, the initial conditions and boundary conditions of the karst-to-well system heat transfer model are determined, including the original temperature of the reservoir, the recharge water temperature, the production volume of the production and injection system, the recharge volume of the production and injection system, the specific heat capacity of geothermal water, the distance between the recharge well and the production well, the thermal conductivity of the reservoir, the porosity of the thermal reservoir, the density of the geothermal water, the density of the thermal reservoir, the specific heat capacity of the thermal reservoir, the cross-sectional area of the karst channel, the height of the karst channel and the tracer recovery rate.

[0015] Furthermore, in the heat transfer model, the fluid temperature of the production well is expressed as

[0016]

[0017] in:

[0018]

[0019]

[0020]

[0021] (ρc) f =ρ w c w φ+ρ r c r (1-φ); (5)

[0022] Where T(t) is the outlet water temperature of the production well, °C; i is the number of seepage channels between the production and injection wells; T in is the recharge water temperature, ℃; T0 is the original reservoir temperature, ℃; Q is the production volume of the production and injection system, kg / s; q in is the return injection volume of the water collection and injection system, kg / s; c w is the specific heat capacity of geothermal water D, J / kg / ℃; x is the distance between the recharge well and the production well, m; k r Thermal conductivity of the reservoir, W / m / ℃; h is the height of the karst channel, m; q i is the recharge rate in the i-th channel, kg / s; ρ w is the density of geothermal water, kg / m 3 ρ r is the density of the thermal reservoir, kg / m 3 ;c r is the specific heat capacity of the thermal reservoir, J / kg / ℃; A is the cross-sectional area of the karst channel, m 2 ; φ is the porosity of the thermal reservoir; is the tracer mass recovery rate.

[0023] Furthermore, the inversion calculation was performed using Qtracer2 software to obtain the tracer recovery rate, cross-sectional area and height of the karst channel. The calculation formula is:

[0024]

[0025]

[0026] in

[0027]

[0028] Where C is the tracer mass concentration; t is time; Q is the production volume of the production and injection system; x is the distance between the reinjection well and the production well; M is the tracer injection volume; M i is the tracer recovery, is the tracer mass recovery rate; A is the cross-sectional area of the karst channel.

[0029] The beneficial effects of the present invention are as follows: the method for predicting the temperature of the production fluid of the karst geothermal well production and injection system provided by the present invention is accurate and reliable, highly operational, convenient and fast, and solves the difficult problem of predicting the temperature of the production fluid of the production and injection system under the condition of long-term well re-injection. It plays an important guiding role in the selection of karst geothermal production and re-injection plans and the well location layout of later projects.

[0030] The present invention will be described in further detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a flow chart of a method for predicting the fluid temperature in a production well of a karst geothermal production and irrigation system according to an embodiment of the present application.

[0032] Figure 2 Schematic diagram of the reinjection tracer test of the karst well production and injection system.

[0033] Figure 3 A graph showing the change in tracer concentration over time in a production well.

[0034] Figure 4 To calculate the temperature change of produced fluid in a well production and irrigation system within 100 years. DETAILED DESCRIPTION

[0035] In order to further illustrate the technical means and effects adopted by the present invention to achieve the predetermined purpose, the specific implementation methods, structural features and effects of the present invention are described in detail below with reference to the accompanying drawings and examples.

[0036] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0037] In the description of the present invention, it should be understood that the terms "center", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "aligned", "overlap", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention.

[0038] The terms "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of the technical features indicated. Therefore, features specified as "first" or "second" may explicitly or implicitly include one or more of such features; and in the description of the present invention, unless otherwise specified, "plurality" means two or more.

[0039] Example 1

[0040] This embodiment provides a Figures 1 to 4 The method for predicting the temperature of the produced fluid in the karst geothermal well production and irrigation system shown includes the following steps:

[0041] Step 1: Collect geothermal geological data and data related to the operation of the extraction and irrigation system;

[0042] Step 2: Determine the tracer composition and dosage through preliminary experiments, and conduct a well recharge test. After the tracer is injected, sample the production well once a day. After on-site sampling, test and analyze the tracer concentration. When the tracer concentration in the production well is first detected to increase, increase the sampling frequency (sample the production well several times a day, and after on-site sampling, test and analyze the tracer concentration) until the recharge test is completed.

[0043] Step 3: Obtain tracer test data and a tracer concentration versus time curve;

[0044] Step 4: Based on the tracer test data, perform inversion calculations using Qtracer2 software to determine the tracer recovery rate, cross-sectional area, and karst channel height. Qtracer2, developed by the U.S. Environmental Protection Agency, is specifically designed for quantitative interpretation of tracer results. It calculates the geometry of the aquifer and related hydrogeological parameters based on the temporal variation of tracer mass concentration. It is applicable to hydrogeological systems including surface rivers, granite and clastic aquifers, and groundwater channels (such as karst pipes and mining tunnels).

[0045] Step 5: Determine the initial and boundary conditions of the karst-to-well system heat transfer model, consider the heat convection in the karst channel and the heat conduction in the surrounding rock, set the time period, and calculate the temperature change of the production well fluid according to the heat transfer model.

[0046] Furthermore, the geothermal geological data and data related to the operation of the production and injection system include the original heat reservoir temperature, the reinjection water temperature, the production volume of the production and injection system, the reinjection volume of the production and injection system, the specific heat capacity of the geothermal water, the thermal conductivity of the heat reservoir, the density of the geothermal water, the density of the heat reservoir, the specific heat capacity of the heat reservoir, the porosity of the heat reservoir, and the distance between the reinjection well and the production well. The above data can be obtained through preliminary data collection, drilling data, pumping tests, etc.

[0047] Furthermore, the tracer preliminary test in step 2 includes a thermal stability test, an adsorption test, and a compatibility test.

[0048] Furthermore, the calculation formula for the tracer dosage in step 2 is:

[0049] G=πR 2Hαγc(1)where: G is the amount of tracer injected, kg; R is the distance between the production well and the recharge well, m; H is the thickness of the thermal reservoir aquifer, m; α is the dispersion constant factor, which is 0.0153; γ is the correction coefficient of the dominant flow concentration field, 0.1; Cp is the peak tracer concentration (5-10 times the concentration of the geothermal water tracer), mg / L.

[0050] Furthermore, the initial conditions and boundary conditions of the karst-to-well system heat transfer model are determined, including the original temperature of the reservoir, the recharge water temperature, the production volume of the production and injection system, the recharge volume of the production and injection system, the specific heat capacity of geothermal water, the distance between the recharge well and the production well, the thermal conductivity of the reservoir, the porosity of the thermal reservoir, the density of the geothermal water, the density of the thermal reservoir, the specific heat capacity of the thermal reservoir, the cross-sectional area of the karst channel, the height of the karst channel and the tracer recovery rate.

[0051] Furthermore, in the heat transfer model, the fluid temperature of the production well is expressed as

[0052]

[0053] in:

[0054]

[0055]

[0056]

[0057] (ρc)f=ρwcwφ+ρrcr(1-φ); (5)

[0058] Where T(t) is the outlet water temperature of the production well, °C; i is the number of seepage channels between the production and injection wells; T in is the recharge water temperature, ℃; T0 is the original reservoir temperature, ℃; Q is the production volume of the production and injection system, kg / s; q in is the return injection volume of the water collection and injection system, kg / s; c w is the specific heat capacity of geothermal water D, J / kg / ℃; x is the distance between the recharge well and the production well, m; k r Thermal conductivity of the reservoir, W / m / ℃; h is the height of the karst channel, m; q i is the recharge rate in the i-th channel, kg / s; ρ w is the density of geothermal water, kg / m 3 ρ r is the density of the thermal reservoir, kg / m 3 ;c r is the specific heat capacity of the thermal reservoir, J / kg / ℃; A is the cross-sectional area of the karst channel, m 2 ; φ is the porosity of the thermal reservoir; is the tracer mass recovery rate.

[0059] Furthermore, the inversion calculation was performed using Qtracer2 software to obtain the tracer recovery rate, cross-sectional area and height of the karst channel. The calculation formula is:

[0060]

[0061]

[0062] in

[0063]

[0064] Where C is the tracer mass concentration; t is time; Q is the production volume of the production and injection system; x is the distance between the reinjection well and the production well; M is the tracer injection volume; M i is the tracer recovery, is the tracer mass recovery rate; A is the cross-sectional area of the karst channel.

[0065] Example 2

[0066] Step 1: Collect relevant geological data and operation data of the production and irrigation system, including reinjection water temperature, original heat reservoir temperature, production volume of the production and irrigation system, reinjection volume of the production and irrigation system, specific heat capacity of geothermal water, thermal conductivity of the heat reservoir, density of geothermal water, density of the heat reservoir, specific heat capacity of the heat reservoir, porosity of the heat reservoir, and the distance between the reinjection well and the production well. The above data can be obtained through preliminary data collection, drilling data, pumping tests, etc. Through preliminary data collection, drilling data, pumping and recharge tests, etc., it was obtained that the original heat reservoir temperature T0 in this example is 82.6℃, the system production volume Q is 9.94kg / s, the recharge water temperature Tin is 51.8℃, the recharge volume qin is 9.94kg / s, the specific heat capacity cw of geothermal water is 4.2×103J / kg / ℃, the thermal conductivity coefficient of the heat reservoir kr is 2.5W / m / ℃, the density of geothermal water ρw is 1000kg / m3, the density of the heat reservoir ρr is 2400kg / m3, the specific heat capacity c of the heat reservoir is 900J / kg / ℃, and the porosity φ of the heat reservoir is 6%.

[0067] Step 2: Based on the preliminary investigation and relevant tests, the tracer composition and quantity are determined, and the well recharge tracer test is carried out. First, through extensive research and combined with previous work, ammonium thiocyanate (NH4SCN) is preliminarily selected as the tracer; then the background value test of the tracer in the geothermal water is carried out, and the test shows that the concentration of thiocyanate ion (SCN-) in the geothermal water is 0.093 mg / l; then the preliminary test of the tracer is carried out, including thermal stability, adsorption, and compatibility tests. The specific test methods are detailed in the literature. Through the preliminary test, it is determined that the selection of ammonium thiocyanate (NH4SCN) meets the requirements and can be used as the tracer in this example. Finally, the amount of tracer added is calculated. The calculation formula for the mass of ammonium thiocyanate (NH4SCN) added is:

[0068] G=πR2Hαγc (1)

[0069] Where: G is the amount of tracer added, kg; R is the distance between the production well and the reinjection well, m; H is the thickness of the thermal reservoir aquifer, m; α is the dispersion constant factor, which is 0.0153; γ is the correction coefficient for the dominant flow concentration field, 0.1; Cp is the peak tracer concentration (5-10 times the geothermal water tracer concentration), mg / L. In this example, the amount of ammonium thiocyanate added is 1.3 t.

[0070] The tracer is then stirred evenly with geothermal water to fully dissolve and then quickly injected from the reinjection well so that the tracer follows the reinjection water flow. Immediately after sampling in the production well, the tracer concentration is tested and recorded on site. The sampling frequency is once a day at the beginning of the test, and the sampling frequency is increased after the tracer concentration is first detected. In this embodiment, the tracer concentration obtained by sampling and on-site testing on the second day after the tracer was injected into the reinjection well was 0.275 mg / l, which exceeded the background value, indicating that the tracer front halo has reached the production well. Subsequently, the sampling frequency was increased to once every 4 hours until the reinjection test was completed. Schematic diagram of the reinjection tracer test for the well production and injection system, as shown below Figure 2 shown.

[0071] Step 3: Obtain tracer test result data and curve. In a specific application, after the recharge test is completed, a tracer concentration curve is drawn according to the measured data over time ( Figure 3 ).

[0072] Step 4: Calculate the equivalent seepage channel length, cross-sectional area, tracer recovery rate and other data using Qtracer2 inversion. Use Qtracer2 software for inversion calculation, input the data obtained from the tracer test into the software, and obtain the karst channel height h as 11.1m, the channel cross-sectional area A as 96.842m2; the tracer mass recovery rate It is 27.477%.

[0073] Step 5: Determine the boundary conditions and initial conditions and use the convection heat transfer theoretical model for calculation. Using the data obtained in step 4 as the boundary conditions, use the heat transfer model to calculate the temperature T(t) of the observation well after the reinjected fluid is heated through each seepage channel. The calculation formula is:

[0074]

[0075] in:

[0076]

[0077]

[0078]

[0079] (ρc)f=ρwcwφ+ρrcr(1-φ)(5);

[0080] T(t) is the outlet water temperature of the production well, ℃; Tin is the reinjection water temperature, ℃; T0 is the original temperature of the reservoir, ℃; Q is the production volume, kg / s; qin is the reinjection volume, kg / s; cw is the specific heat capacity of water, J / kg / ℃; erf() is the error function; x is the distance between the reinjection well and the production well, m; kr is the thermal conductivity of the reservoir, W / m / ℃; h is the height of the karst channel, m; qi is the reinjection volume in the i-th channel, kg / s; ρw is the density of water, kg / m3; ρr is the density of the thermal reservoir, kg / m3; cr is the specific heat capacity of the thermal reservoir, J / kg / ℃; A is the cross-sectional area of the channel, m2; φ is the porosity of the thermal reservoir; is the tracer mass recovery rate.

[0081] i is the number of seepage channels between production and injection wells, according to Figure 3 Determine i=1; the rest of the data can be obtained according to step 1: Ti=51.8℃; T0=82.6℃; Q=qin=9.94kg / s; cw=4.2×10 3 J / kg / ℃; x=450m; kr=2.5W / m / ℃; h=11.1m; ρw=1000kg / m 3 ;ρr=2400kg / m 3 ; cr = 900 J / kg / ℃; According to step 4, A = 96.842 m2; φ = 6%;

[0082] On the right side of the equation, only t is an unknown quantity. By setting t1, we can calculate the data of T(t1) at time t1. By setting t2, we can calculate the data of T(t2) at time t2. Similarly, by setting the value of t, we can calculate the data of T(tn) at time tn. If we set the time t to 100 years, we can find the change of the fluid temperature in the production well over 100 years. Figure 4 ,It can be seen that in this example, the temperature of the fluid in the karst well production and irrigation system dropped by 8.31℃ within 100 years.

[0083] The above is a further detailed description of the present invention in conjunction with specific preferred embodiments, and the specific implementation of the present invention should not be considered to be limited to these descriptions. For those skilled in the art of the present invention, without departing from the concept of the present invention, several simple deductions or substitutions can be made, which should be considered to fall within the scope of protection of the present invention.

Claims

1. A method for predicting the temperature of the fluid produced by a karst geothermal well production and irrigation system, characterized in that: The steps include: Step 1: Collect geothermal geological data and data related to the operation of the extraction and irrigation system; Step 2: Determine the tracer composition and calculate the dosage through preliminary experiments, and conduct a well recharge test. After the tracer is injected, sample the production well once a day. After on-site sampling, test and analyze the tracer concentration. When the tracer concentration in the production well is first detected to increase, the sampling frequency is increased until the recharge test is completed. Step 3: Obtain tracer test data and a tracer concentration versus time curve; Step 4: Based on the tracer test data, Qtracer2 software is used to perform inversion calculations to calculate the tracer recovery rate, cross-sectional area, and height of the karst channel; Step 5: Determine the initial conditions and boundary conditions of the karst-to-well system heat transfer model, consider the heat convection in the karst channel and the heat conduction of the surrounding rock, set the time period, and calculate the temperature change of the production well fluid according to the heat transfer model; The calculation formula for the tracer dosage in step 2 is: G=πR 2 Hαγc (1) Where: G is the amount of tracer injected, kg; R is the distance between the production well and the recharge well, m; H is the thickness of the thermal reservoir aquifer, m; α is the dispersion constant factor, which is 0.0153; γ is the correction coefficient for the dominant flow concentration field, which is 0.1; Cp is the peak tracer concentration, which is 5-10 times the geothermal water tracer concentration, mg / L; In the heat transfer model, the formula for expressing the fluid temperature in the production well is: in: (p.c.) f =ρwcwφ+ρrcr(1-φ); (5) Where T(t) is the outlet water temperature of the production well, °C; i is the number of seepage channels between the production and injection wells; T in is the recharge water temperature, ℃; T0 is the original reservoir temperature, ℃; Q is the production volume of the production and injection system, kg / s; q in is the return injection volume of the water collection and injection system, kg / s; c w is the specific heat capacity of geothermal water, J / kg / ℃; x is the distance between the recharge well and the production well, m; k r Thermal conductivity of the reservoir, W / m / ℃; h is the height of the karst channel, m; q i is the recharge rate in the i-th channel, kg / s; ρ w is the density of geothermal water, kg / m 3 ρ r is the density of the thermal reservoir, kg / m 3 ;c r is the specific heat capacity of the thermal reservoir, J / kg / ℃; A is the cross-sectional area of the karst channel, m 2 ; φ is the porosity of the thermal reservoir; is the tracer mass recovery; The inversion calculation was performed using Qtracer2 software to obtain the tracer recovery rate, cross-sectional area and height of the karst channel. The calculation formula is: in, Where C is the tracer mass concentration; t is time; Q is the production volume of the production and injection system; x is the distance between the reinjection well and the production well; M is the tracer injection volume; M i is the tracer recovery, is the tracer mass recovery rate; A is the cross-sectional area of the karst channel.

2. The method for predicting the temperature of the produced fluid in a karst geothermal well production and irrigation system according to claim 1, characterized in that: The geothermal geological data and data related to the operation of the production and irrigation system include the original heat reservoir temperature, the reinjection water temperature, the production volume of the production and irrigation system, the reinjection volume of the production and irrigation system, the specific heat capacity of the geothermal water, the thermal conductivity of the heat reservoir, the density of the geothermal water, the density of the heat reservoir, the specific heat capacity of the heat reservoir, the porosity of the heat reservoir, and the distance between the reinjection well and the production well.

3. The method for predicting the temperature of the produced fluid in a karst geothermal well production and irrigation system according to claim 1, characterized in that: The tracer preliminary test in step 2 includes a thermal stability test, an adsorption test, and a compatibility test.

4. The method for predicting the temperature of the produced fluid in a karst geothermal well production and irrigation system according to claim 1, characterized in that: The initial conditions and boundary conditions for determining the karst-to-well system heat transfer model include the original temperature of the reservoir, the recharge water temperature, the production volume of the production and injection system, the recharge volume of the production and injection system, the specific heat capacity of geothermal water, the distance between the recharge well and the production well, the thermal conductivity of the reservoir, the porosity of the thermal reservoir, the density of the geothermal water, the density of the thermal reservoir, the specific heat capacity of the thermal reservoir, the cross-sectional area of the karst channel, the height of the karst channel and the tracer recovery rate.

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