A method and device for regulating supercritical state of bottom fluid of a well fractured by supercritical carbon dioxide

Abstract

The present application relates to the technical field of oil and gas well completion, and discloses a method and device for regulating supercritical state of bottom fluid of a well fractured by supercritical carbon dioxide. The method comprises the following steps: liquid CO2 fluid in a storage tank is injected into a wellbore in sequence after being heated and pressurized by a fracturing pump truck, and the CO2 fluid at the entrance of the wellbore reaches a preset temperature; the CO2 fluid at the preset temperature is in a supercritical state when reaching the bottom of the well through the wellbore; when the temperature of the CO2 fluid at the entrance of the wellbore is lower than the preset temperature, the input power of a heating device is increased; when the temperature of the CO2 fluid at the entrance of the wellbore is higher than the preset temperature, the input power of the heating device is reduced. The method keeps the temperature of the CO2 fluid near the bottom of the well at about 32 DEG C during the whole fracturing process, prevents the decrease of the carrying capacity of supercritical CO2 for proppants due to excessively high temperature, avoids the accelerated settlement of the proppants due to the existence of a large section of supercritical CO2 fluid in the wellbore, and saves the cost of heating low-temperature CO2 fluid on the ground.

Description

Technical Field

[0001] This invention relates to the field of oil and gas well completion technology, and in particular to a method and apparatus for controlling the supercritical state of bottom hole fluid in supercritical carbon dioxide fracturing. Background Technology

[0002] The principle of oil and gas well fracturing is to artificially apply external energy to underground oil and gas reservoirs for a short period of time, creating fractures in the reservoir, increasing the flow channels for oil and gas, improving well production, and enabling the economical development of oil and gas reservoirs. Supercritical carbon dioxide fracturing technology uses CO2 as the medium to transfer external energy to the reservoir. When CO2 flows from the wellhead into the reservoir, it enters a supercritical state. Compared to using water as the energy transfer medium, supercritical carbon dioxide has lower surface tension, lower viscosity, and stronger diffusivity, making it easier to penetrate into reservoir pores. Therefore, compared to hydraulic fracturing, supercritical carbon dioxide fracturing can reduce reservoir initiation pressure, generate more microfractures, and form a more complex fracture network. Furthermore, for reservoirs with high clay mineral content, hydraulic fracturing easily causes clay expansion, inducing micropore throat blockage and damaging the reservoir, while supercritical carbon dioxide does not cause clay mineral expansion. Supercritical carbon dioxide fracturing can also bury some CO2 in the formation, making it a green fracturing method.

[0003] During supercritical carbon dioxide fracturing operations, liquid CO2 enters the booster pump from the storage tank, is slightly pressurized, and then transported to the mixing tank where it is thoroughly mixed with proppant and additives (this step is omitted if proppant is not transported during the fracturing stage). Afterward, it is significantly pressurized by the fracturing pump truck and enters the wellbore through the surface manifold. At this point, the CO2 injected at the wellhead is in a low-temperature liquid state. As the CO2 flows towards the bottom of the well, it exchanges heat with the formation, and its temperature gradually rises. Existing research indicates that in the early stages of fracturing operations, the temperature of the CO2 reaching the bottom of the well can reach or exceed the critical temperature of 31.1°C required for it to enter the supercritical state. However, due to the large pumping volume of the CO2 fluid, after a certain period, the heat transfer from the surrounding formation is insufficient to raise the temperature of the CO2 at the bottom of the well to 31.1°C. This means the CO2 at the bottom of the well is not in a supercritical state, and the advantages of supercritical carbon dioxide fracturing cannot be fully realized. Therefore, certain measures need to be taken to ensure that the temperature of the CO2 fluid at the bottom of the well does not fall below 31.1°C throughout the entire fracturing process. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a method and apparatus for controlling the supercritical state of the bottom-hole fluid in supercritical carbon dioxide fracturing. This method heats the CO2 fluid introduced into the well to a certain temperature at the surface, ensuring that the temperature of the CO2 fluid at the bottom of the well does not fall below 31.1°C throughout the fracturing process; that is, the CO2 fluid at the bottom of the well remains in a supercritical state at all times. The invention provides the following technical solution:

[0005] This invention provides a method for controlling the supercritical state of fluid at the bottom of a supercritical carbon dioxide fracturing well, the method comprising:

[0006] The liquid CO2 fluid in the storage tank is heated by a heating device and pressurized by a fracturing pump truck before being injected into the wellbore. The temperature of the CO2 fluid at the wellbore inlet reaches the preset temperature.

[0007] CO2 fluid at the preset temperature is in a supercritical state when it reaches the bottom of the well through the wellbore; wherein,

[0008] When the CO2 fluid temperature at the wellbore inlet is lower than the preset temperature, increase the input power of the heating device;

[0009] When the CO2 fluid temperature at the wellbore inlet is higher than the preset temperature, reduce the input power of the heating device.

[0010] Furthermore, the method also includes:

[0011] The liquid CO2 fluid in the storage tank is heated by a heating device, mixed with proppant and additives in a sand mixing tank, and then injected into the wellbore after being pressurized by a fracturing pump truck. The temperature of the CO2 fluid at the wellbore inlet reaches the preset temperature.

[0012] Further, determining the preset temperature includes the following steps:

[0013] Based on the basic data of the target fractured well, a physical model of the wellbore and reservoir of the target fractured well is constructed.

[0014] Based on the physical model of the target fractured wellbore and reservoir, a simulation model of CO2 fracturing fluid flow and heat transfer in the wellbore is constructed.

[0015] Based on the simulation model of CO2 fracturing fluid flow and heat transfer in the wellbore, the preset temperature corresponding to the supercritical state of CO2 at the bottom of the well is determined.

[0016] Furthermore, the basic data of the target fracturing well includes: the geometric dimensions of the wellbore, the tubing string, and the cement sheath and formation, as well as their positional relationships.

[0017] Furthermore, based on the fundamental data of the target fractured well, a physical model of the wellbore and reservoir is constructed, including:

[0018] The physical model of the target fracturing wellbore and reservoir is meshed.

[0019] Based on the physical property data of CO2 fluid, the fluid boundary constraints and the control equations for the flow and heat transfer of CO2 fluid in the wellbore and reservoir of the target fractured well were determined.

[0020] Based on the physical model of the wellbore and reservoir after gridding, the fluid boundary constraints, and the control equations for the flow and heat transfer of CO2 fluid in the wellbore, a simulation model of CO2 fracturing fluid flow and heat transfer in the wellbore is constructed.

[0021] Furthermore, the physical properties of CO2 fluid include: density, viscosity, thermal conductivity, and specific heat capacity under temperature and pressure conditions.

[0022] Furthermore, the control equations for the flow and heat transfer of the CO2 fluid in the wellbore include:

[0023] The heat transfer equations for CO2 in the central tube during heat extraction in fracturing, the energy equations for heat exchange between the annular fluid and the surrounding medium, and the heat transfer equations within the formation.

[0024] Furthermore, the heat transfer equation for CO2 in the central tube during the heat extraction process in fracturing is as follows:

[0025]

[0026] In the formula, ρ f1 The density of the fluid inside the central tube; A c1 C is the cross-sectional area of ​​the flow channel inside the central tube. f1 T is the specific heat capacity of the fluid inside the central tube. f1 The temperature of the fluid inside the central tube. is the derivative with respect to the fluid temperature inside the central tube; t is the fluid flow time. Q is the derivative with respect to time. 热对流1 For thermal convection of the fluid inside the central tube; Q 热传导1 For heat conduction of the fluid inside the central tube; Q 摩擦热1 Q1 represents the frictional heat between the fluid inside the central tube and the tube wall; Q2 represents the heat exchange between the fluid inside the central tube and the fluid in the annulus; where...

[0027]

[0028]

[0029]

[0030]

[0031] In the formula, u f1 The velocity of the fluid inside the central tube; For divergence, λ represents the divergence of the fluid temperature inside the central tube; f1 f is the thermal conductivity of the fluid inside the central tube. D1 Darcy friction factor for fluid flow within the central tube; d p1T is the hydraulic diameter of the central pipe. f2 R1 is the temperature of the fluid in the annulus; R2 is the thermal resistance between the fluid in the central tube and the fluid in the annulus.

[0032] Furthermore, the energy equation for the heat transfer process between the annular fluid and the surrounding medium is expressed as follows:

[0033]

[0034] In the formula, ρ f2 A represents the density of the fluid within the annulus. c2 C is the cross-sectional area of ​​the flow channel within the annulus. f2 T is the specific heat capacity of the fluid within the annulus. f2 The temperature of the fluid inside the annulus. is the derivative with respect to the fluid temperature in the annulus; t is the fluid flow time. Q is the derivative with respect to time. 热对流2 For thermal convection of fluid within the annulus; Q 热传导2 For heat conduction of the fluid within the annulus; Q 摩擦热2 Q1 is the frictional heat between the fluid in the annulus and the pipe wall; Q2 is the heat exchange between the fluid in the central pipe and the fluid in the annulus; Q3 is the heat exchange between the fluid in the annulus and the formation.

[0035]

[0036]

[0037]

[0038]

[0039] In the formula, u f2 The velocity of the fluid within the annulus; For divergence, λ represents the divergence of the fluid temperature within the annulus. f2 f is the thermal conductivity of the fluid within the annulus. D2 d represents the Darcy friction factor for fluid flow within the annulus. p2 R2 is the hydraulic diameter of the annulus; R2 is the thermal resistance between the fluid in the annulus and the surrounding formation; Ts is the temperature of the surrounding formation.

[0040] Furthermore, the heat transfer equation within the formation is expressed as:

[0041]

[0042] In the formula, (ρC) eff and λ eff , respectively, represent the equivalent specific heat capacity and equivalent thermal conductivity of the formation; t is the fluid flow time. is the derivative with respect to time; Ts is the formation temperature around the well. For the derivative with respect to the formation temperature around the well, T represents the divergence of formation temperature around the well. f2 R1 is the temperature of the fluid within the annulus; R2 is the thermal resistance between the fluid within the annulus and the surrounding formation; where

[0043]

[0044] In the formula, ρ represents the porosity of the formation. s C is the density of the rock strata. s ρ is the specific heat capacity of the strata rocks. f3 C is the density of the formation fluid. f3 λ is the specific heat capacity of the formation fluid; s λ is the thermal conductivity of the rock formation. f3 denoted as , where is the thermal conductivity of the formation fluid.

[0045] Furthermore, based on the simulation model of CO2 fracturing fluid flow and heat transfer in the wellbore, the preset temperature of the CO2 fluid at the wellbore inlet when the CO2 fluid at the bottom of the well is in a supercritical state is determined, including:

[0046] The CO2 physical properties, the thermal conductivity parameters of the wellbore and reservoir, and the boundary conditions are input into the CO2 fracturing fluid wellbore flow and heat transfer simulation model for iterative solution to obtain the CO2 fluid temperature at the wellbore inlet when the CO2 fluid at the bottom of the well is in a supercritical state.

[0047] When the CO2 fluid at the bottom of the well is in a supercritical state, the corresponding CO2 fluid temperature at the wellbore inlet is the preset temperature.

[0048] The present invention also provides a device for controlling the supercritical state of the bottom fluid in a supercritical carbon dioxide fracturing well. The device is used to heat the CO2 fluid before it is injected into the wellbore. The heated CO2 fluid is in a supercritical state when it enters the wellbore and reaches the bottom of the well.

[0049] The device includes a booster pump, a heating device, and a fracturing pump truck connected in sequence.

[0050] The booster pump is used to draw and pressurize the liquid CO2 fluid in the storage tank and then deliver it to the heating device. It is also used to deliver the CO2 fluid flowing out of the heating device to the fracturing pump truck.

[0051] The heating device is used to heat CO2 fluid;

[0052] The fracturing pump truck is used to inject heated CO2 fluid into the wellbore after pressurization. The temperature of the CO2 fluid at the wellbore inlet is a preset temperature. When the CO2 fluid reaches the bottom of the wellbore after reaching the preset temperature, it is in a supercritical state.

[0053] The device also includes a temperature sensor, which is disposed at the outlet of the heating device and the inlet of the well.

[0054] A temperature sensor is installed at the outlet of the heating device to monitor the temperature of the heated CO2 fluid.

[0055] The temperature sensor is installed at the wellbore inlet to monitor the CO2 fluid temperature at the wellbore inlet; wherein,

[0056] When the CO2 fluid temperature at the wellbore inlet is lower than the preset temperature, increase the input power of the heating device;

[0057] When the CO2 fluid temperature at the wellbore inlet is higher than the preset temperature, reduce the input power of the heating device.

[0058] Furthermore, the device also includes a sand mixing tank, which is located between the heating device and the fracturing pump truck, and is used to mix CO2 fluid heated to the temperature at the wellbore inlet with proppant and additives;

[0059] The fracturing pump truck is also used to receive CO2 fluid mixed with proppant and additives, and to pressurize the CO2 fluid before injecting it into the wellbore. The temperature of the CO2 fluid at the wellbore inlet is a preset temperature.

[0060] The technical effects and advantages of this invention are as follows:

[0061] The control method of this invention can increase the inlet temperature of CO2 fluid through a ground heating device, and control the CO2 fluid near the bottom of the well to be in a supercritical state throughout the fracturing stage, thus giving full play to the advantages of supercritical carbon dioxide technology.

[0062] The CO2 fluid temperature calculation process of this invention can solve the problem of maintaining the temperature of the CO2 fluid near the bottom of the well at about 32°C throughout the fracturing process. This prevents the supercritical CO2 from reducing its proppant carrying capacity due to excessively high temperatures, avoids the presence of large sections of supercritical CO2 fluid in the wellbore that would accelerate proppant settling, and saves the cost of heating the low-temperature CO2 fluid at the surface.

[0063] The low-temperature liquid CO2 heating process of this invention, due to the appropriate selection of heating timing, can heat low-temperature CO2 fluid in a relatively low-pressure environment, thereby improving heat exchange efficiency and reducing energy consumption and equipment material costs; it can avoid wear or blockage of the heat exchange pipeline of the heating device by CO2 fluid, thus improving the service life of the heating device; it can stably and rapidly supply liquid to the heating device, fully releasing the potential of the heating device, improving the efficiency of a single heating device, and reducing the number of heating devices used and the total floor space.

[0064] The wellhead CO2 fluid temperature monitoring and control process of the present invention can measure the CO2 fluid temperature at the outlet of the heating device and at the wellhead in real time. Through automatic software control or manual adjustment, the CO2 fluid inlet temperature is guaranteed to be consistent with the set value based on theoretical calculation.

[0065] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures pointed out in the description and the drawings. Attached Figure Description

[0066] Figure 1 This is a flowchart of the method for controlling the supercritical state of fluid at the bottom of a supercritical carbon dioxide fracturing well, provided in an embodiment of this application.

[0067] Figure 2 This is a schematic diagram of a cryogenic liquid CO2 fluid ground heating process provided in an embodiment of this application;

[0068] Figure 3 A flowchart for determining a preset temperature provided in an embodiment of this application;

[0069] Figure 4 This is a flowchart illustrating the implementation of the bottom hole fluid supercritical state control method provided in this application embodiment;

[0070] Figure 5 This is a schematic diagram of the CO2 fluid temperature calculation process provided in an embodiment of this application. Detailed Implementation

[0071] 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.

[0072] To address the shortcomings of existing technologies, this invention discloses a method for controlling the supercritical state of fluid at the bottom of a supercritical carbon dioxide fracturing well, such as... Figure 1 As shown, the method includes,

[0073] The liquid CO2 fluid in the storage tank is heated by a heating device and pressurized by a fracturing pump truck before being injected into the wellbore. The temperature of the CO2 fluid at the wellbore inlet reaches the preset temperature.

[0074] At the preset temperature, the CO2 fluid is in a supercritical state when it enters the wellbore and reaches the bottom; wherein,

[0075] When the CO2 fluid temperature at the wellbore inlet is lower than the preset temperature, increase the input power of the heating device;

[0076] When the CO2 fluid temperature at the wellbore inlet is higher than the preset temperature, reduce the input power of the heating device.

[0077] The selection of 32℃ is based on the fact that the critical temperature and critical pressure for CO2 to enter the supercritical state are 31.1℃ and 7.38 MPa, respectively. In supercritical carbon dioxide fracturing, reaching a bottomhole CO2 fluid pressure of 7.38 MPa is a prerequisite because the fracture initiation and propagation pressures in the fracturing reservoir are generally much higher than 7.38 MPa. Therefore, it is only necessary to maintain the bottomhole CO2 fluid temperature at no less than 31.1℃. To save energy for heating cryogenic liquid CO2 and reduce operating costs, setting the target at 32℃ is reasonable. Furthermore, the higher the temperature of the supercritical carbon dioxide fluid, the lower its viscosity, which is less conducive to carrying proppant. Therefore, it is not advisable to excessively increase the viscosity of supercritical carbon dioxide. It is worth noting that it is not advisable to heat a large section of the CO2 fluid in the wellbore to the supercritical state; it is only necessary to bring the CO2 fluid near the bottomhole into the supercritical state. Because the viscosity of liquid CO2 is higher than that of supercritical CO2, its ability to carry proppant is superior. After the CO2 fluid in the wellbore changes from a liquid state to a supercritical state, if a large section of the CO2 liquid in the wellbore remains in a supercritical state, the proppant is prone to settling and falling below or accumulating near the fracturing point, which is detrimental to enhancing the conductivity of the fracture. Furthermore, reducing the length of the supercritical CO2 liquid column in the wellbore can reduce the temperature of the CO2 fluid entering the well, saving on surface heating costs.

[0078] In this invention, cryogenic liquid CO2 heating is a key step in the supercritical state control of CO2 fluid at the bottom of the well. For ease of storage and transportation, the CO2 feedstock used in supercritical carbon dioxide fracturing is in a cryogenic liquid state, requiring a heating device to raise its temperature to the calculated inlet temperature. A booster pump draws and pressurizes the cryogenic liquid CO2 from the storage tank and delivers it to the heating device. After sufficient heat exchange in the heating device, it reaches the set temperature and then enters a mixing tank to be thoroughly mixed with proppant and additives. The mixture is then significantly pressurized by the fracturing pump truck and finally flows through the surface manifold into the wellbore. If proppant pumping is not required at a certain stage of fracturing, the CO2 fluid flowing from the heating device directly enters the fracturing pump truck and then flows through the surface manifold into the wellbore.

[0079] Heating CO2 fluid at a time (via a heating device) between low-pressure pressurization (via a booster pump) and mixing with proppant and additives (via a sand mixing tank) or between low-pressure pressurization (via a booster pump) and high-pressure pressurization (via a fracturing pump truck) offers several advantages. First, relatively low-pressure heat exchange significantly reduces the performance requirements of the heat exchange pipelines in the heating device, such as strength and wall thickness, thereby improving heat exchange efficiency and reducing energy consumption and material costs. Second, mixing CO2 with proppant and additives after heating avoids the impact and wear of high-velocity solid particles on the heat exchange pipelines and prevents solid particles from clogging them, extending the service life of the heating device. Third, the low-temperature liquid CO2 is slightly pressurized by a booster pump before entering the heating device, increasing the liquid supply rate, fully utilizing the potential of the heating device, improving the efficiency of a single heating unit, and reducing the number of heating units and the total floor space required.

[0080] The heating device can use a single heating method or a combination of heating methods, including but not limited to electric heating and gas combustion heating.

[0081] Monitoring and controlling the CO2 fluid temperature (second predetermined temperature) at the wellhead is crucial for ensuring the supercritical state control of CO2 fluid at the bottom of the well. After flowing out of the heating device, the CO2 fluid passes through a mixing tank, a pressure pump truck, and surface manifolds. During this process, the CO2 fluid exchanges heat with the external environment, resulting in a temperature difference between its inlet temperature and the outlet temperature of the heating device. Therefore, this difference must be monitored constantly. Temperature sensors are installed at both the outlet of the heating device and the wellhead, and the readings are displayed in real time on the heating device's monitor.

[0082] In one specific embodiment of the present invention, the cryogenic liquid CO2 is heated by a heating device at the ground until it reaches the temperature of the CO2 fluid entering the well, i.e., a preset temperature. The process is as follows: Figure 2 As shown, the booster pump 22 draws and pressurizes the cryogenic liquid CO2 from the storage tank 21 and delivers it to the heating device 23. After sufficient heat exchange in the heating device 23, it reaches the set temperature and then enters the mixing tank 24 to be fully mixed with proppant and additives. It is then significantly pressurized by the fracturing pump truck 25 and finally flows through the surface manifold into the wellbore 26. If proppant pumping is not required at a certain stage of fracturing, the heated CO2 fluid flowing from the heating device 23 directly enters the fracturing pump truck 25, enters the wellbore 26 through the surface manifold, and reaches the bottom of the well in a supercritical state.

[0083] In one specific embodiment of the present invention, such as Figure 3 As shown, determining the preset temperature includes the following steps:

[0084] Step S101: Based on the basic data of the target fractured well, construct a physical model of the wellbore and reservoir of the target fractured well;

[0085] Step S1O2: Based on the physical model of the target fracturing wellbore and reservoir, construct a simulation model of CO2 fracturing fluid flow and heat transfer in the wellbore;

[0086] Step S103: Based on the simulation model of CO2 fracturing fluid wellbore flow and heat transfer, determine the preset temperature of CO2 fluid when CO2 at the bottom of the well is in a supercritical state;

[0087] Step S104: When the temperature at the well inlet is lower than the preset temperature, increase the input power of the heating device; when the temperature at the well inlet is higher than the preset temperature, decrease the input power of the heating device.

[0088] In a specific embodiment of the present invention, for step S101: the basic data of the target fracturing well includes: the geometric dimensions and positional relationships of the wellbore, the tubing string, the cement sheath, and the formation.

[0089] In a specific embodiment of the present invention, the specific steps of step S1O2 are as follows:

[0090] Step S1O21: Grid the physical model of the target fracturing wellbore and reservoir;

[0091] Step S1O22: Based on the physical property data of CO2 fluid, determine the fluid boundary constraints in the physical model of the target fractured wellbore and reservoir, as well as the flow and heat transfer control equations of CO2 fluid in the wellbore; wherein, the physical property data of CO2 fluid includes: density, viscosity, thermal conductivity, and specific heat capacity under temperature and pressure conditions.

[0092] Step S1O23: Based on the physical model of the wellbore and reservoir after gridding, the fluid boundary constraints, and the control equations for the flow and heat transfer of CO2 fluid in the wellbore, construct a simulation model of CO2 fracturing fluid flow and heat transfer in the wellbore.

[0093] Fluid boundary constraints include boundary conditions within the wellbore, the annulus, and the formation. Boundary conditions within the wellbore include the fluid pressure and flow rate at the wellhead, the flow rate and temperature at the bottom of the well, and the flow rate at the wellbore wall. In this embodiment of the invention, given the CO2 fluid flow rate and pressure at the wellhead and the CO2 fluid flow rate and temperature at the bottom of the well, the required fluid temperature at the wellhead (i.e., the temperature required for heating and control) and the fluid pressure at the bottom of the well are numerically calculated. This allows for the determination of whether the fluid at the bottom of the well has reached a supercritical state (a supercritical state requires the CO2 fluid temperature to be no less than 31.1℃ and no less than 7.38MPa).

[0094] The control equations for the flow and heat transfer of CO2 fluid in the wellbore include: the CO2 heat transfer equation in the central tube during the heat extraction process in fracturing, the energy equation during the heat exchange process between the annular fluid and the surrounding medium, and the heat transfer equation within the formation.

[0095] Furthermore, a pipe flow model is used to characterize the flow process inside the central pipe of the wellbore. The mass conservation equation and momentum equation are as follows:

[0096]

[0097]

[0098] In the formula, A c1 ρ is the cross-sectional area of ​​the flow channel inside the central tube; f1 ρ is the density of the fluid inside the central tube; t is the flow time of the fluid. This is the derivative with respect to time; For divergence; u f1 p is the flow velocity of the fluid inside the central tube. w f is the pressure of the fluid inside the wellbore. D1 Darcy friction factor for fluid flow within the central tube; d p1 Let f be the hydraulic diameter of the central pipe; g be the acceleration due to gravity. The second term on the right-hand side of the second equation above represents the pressure drop caused by friction between the fluid and the pipe wall, and the third term on the right-hand side represents the gravity of the fluid. D1 The table is calculated using the Churchill model:

[0099]

[0100] In the formula, e1 is the surface roughness of the inner wall of the central tube; Re1 is the Reynolds number of the fluid in the central tube, and the calculation expression is as follows:

[0101]

[0102] In the formula, μf1 is the viscosity of the fluid inside the central tube.

[0103] The heat transfer equation for CO2 fluid inside the central tube during the heat extraction process in fracturing is as follows:

[0104]

[0105] In the formula, ρ f1 Let A be the density of the fluid inside the central tube. c1 C is the cross-sectional area of ​​the flow channel inside the central tube. f1 T is the specific heat capacity of the fluid inside the central tube. f1 The temperature of the fluid inside the central tube. is the derivative with respect to the fluid temperature inside the central tube; t is the fluid flow time. Q is the derivative with respect to time. 热对流1 For thermal convection of the fluid inside the central tube; Q 热传导1 For heat conduction of the fluid inside the central tube; Q 摩擦热1 Q1 represents the frictional heat between the fluid inside the central tube and the tube wall; Q2 represents the heat exchange between the fluid inside the central tube and the fluid in the annulus; where...

[0106]

[0107]

[0108]

[0109]

[0110] In the formula, u f1 The velocity of the fluid inside the central tube; For divergence, λ represents the divergence of the fluid temperature inside the central tube; f1 f is the thermal conductivity of the fluid inside the central tube. D1 Darcy friction factor for fluid flow within the central tube; d p1 T is the hydraulic diameter of the central pipe. f2 R is the temperature of the fluid inside the annulus; R1 is the thermal resistance between the fluid in the central tube and the fluid inside the annulus; T f2 R1 is the temperature of the fluid in the annulus; R2 is the thermal resistance between the fluid in the central tube and the fluid in the annulus, and its calculation formula is:

[0111]

[0112] In the formula, d1 is the inner diameter of the central tube; d2 is the outer diameter of the central tube; λ m h1 is the thermal conductivity of the central tube; h2 is the convective heat transfer coefficient between the fluid inside the central tube and the wall of the central tube; h3 is the convective heat transfer coefficient between the annular fluid and the wall of the central tube.

[0113] The expression for the convective heat transfer coefficient is as follows:

[0114]

[0115]

[0116] In the formula, N uwell1 N is the Nusselt number of the fluid inside the central tube. uwell2 λ is the Nusselt number of the fluid within the annulus. f2 d is the thermal conductivity of the fluid inside the annulus; p2 Let be the hydraulic diameter of the annulus. Under turbulent conditions, the Nusselt number can be calculated using the Gnielinski equation:

[0117]

[0118] or,

[0119]

[0120] In the formula, f D1 The Darcy friction factor is the fluid flow rate within the central tube; Re1 is the Reynolds number of the fluid within the central tube; Pr is the Prandtl number; f D2 Let be the Darcy friction factor for the fluid flow within the annulus; Re2 is the Reynolds number of the fluid within the annulus, calculated as follows:

[0121]

[0122] In the formula, ρ f2 The density of the fluid within the annulus; u f2 μ represents the flow velocity of the fluid within the annulus. f2 The viscosity of the fluid within the annulus.

[0123] f D2 The Churchill model was also used for calculations:

[0124]

[0125] In the formula, e2 is the average surface roughness of the outer wall of the central tube and the inner wall of the casing.

[0126] Based on the heat transfer process between the annular fluid and the surrounding medium, the energy equation for this process is as follows:

[0127]

[0128] In the formula, ρ f2 A represents the density of the fluid within the annulus. c2 C is the cross-sectional area of ​​the flow channel within the annulus. f2 T is the specific heat capacity of the fluid within the annulus. f2 The temperature of the fluid inside the annulus. is the derivative with respect to the fluid temperature in the annulus; t is the fluid flow time. Q is the derivative with respect to time. 热对流2 For thermal convection of fluid within the annulus; Q 热传导2 For heat conduction of the fluid within the annulus; Q 摩擦热2 Q1 is the frictional heat between the fluid in the annulus and the pipe wall; Q2 is the heat exchange between the fluid in the central pipe and the fluid in the annulus; Q3 is the heat exchange between the fluid in the annulus and the formation.

[0129]

[0130]

[0131]

[0132]

[0133] In the formula, u f2 The velocity of the fluid within the annulus; For divergence, λ represents the divergence of the fluid temperature within the annulus. f2 f is the thermal conductivity of the fluid within the annulus. D2 d represents the Darcy friction factor for fluid flow within the annulus. p2 R2 is the hydraulic diameter of the annulus; R2 is the thermal resistance between the fluid in the annulus and the surrounding formation; Ts is the temperature of the surrounding formation.

[0134] According to the law of conservation of energy, the heat transfer equation within the formation is expressed as:

[0135]

[0136] In the formula, (ρC) eff and λ eff , respectively, represent the equivalent specific heat capacity and equivalent thermal conductivity of the formation; t is the fluid flow time. is the derivative with respect to time; Ts is the formation temperature around the well. For the derivative with respect to the formation temperature around the well, T represents the divergence of formation temperature around the well. f2 R1 is the temperature of the fluid within the annulus; R2 is the thermal resistance between the fluid within the annulus and the surrounding formation; where

[0137]

[0138]

[0139] In the formula, ρ represents the porosity of the formation. s C is the density of the rock strata. s ρ is the specific heat capacity of the strata rocks. f3 C is the density of the formation fluid. f3 λ is the specific heat capacity of the formation fluid; s λ is the thermal conductivity of the rock formation. f3 denoted as , where is the thermal conductivity of the formation fluid.

[0140]

[0141] In the formula, d3 is the inner diameter of the annulus; d4 is the outer diameter of the casing; d5 is the outer diameter of the cement ring; h3 is the convective heat transfer coefficient between the fluid in the annulus and the casing wall, and its value is consistent with h2; λw λ is the thermal conductivity of the sleeve; c is the thermal conductivity of the cement ring.

[0142] In a specific embodiment of the present invention, the specific steps of step S103 are as follows:

[0143] Step S1031: Input the CO2 physical properties, wellbore and reservoir thermal conductivity parameters into the CO2 fracturing fluid wellbore flow and heat transfer simulation model for iterative solution to obtain the CO2 fluid inlet temperature at a preset bottom-hole temperature. This temperature can be automatically controlled by a surface heating device (automatically changing the heat exchange rate according to the required discharge rate and the initial temperature of the CO2 cryogenic fluid). The boundary conditions or initial conditions required for calculating the CO2 temperature at the inlet (CO2 inlet temperature) include the inlet CO2 fluid discharge rate and pressure, and the bottom-hole CO2 fluid discharge rate and temperature.

[0144] Step S1032: The inlet temperature of the CO2 fluid when the CO2 fluid at the bottom of the well is in a supercritical state is used as the preset temperature. After determining the required temperature of the CO2 fluid at the bottom of the well, the required inlet CO2 fluid temperature at the wellhead is obtained through numerical simulation. Therefore, by dynamically adjusting the heating device, the temperature of the CO2 fluid at the wellhead is ensured to be slightly higher than the safe value. This ensures both the inlet temperature required for the CO2 fluid at the bottom of the well to be in a supercritical state and prevents the wellhead temperature from becoming too high, thus increasing costs.

[0145] In a specific embodiment of the present invention, the specific steps for step 104—when the calculated temperature of the CO2 fluid in the injection wellbore is lower than the temperature of the CO2 fluid itself—to heat the CO2 fluid to the target temperature are as follows:

[0146] The supercritical carbon dioxide fracturing well bottom fluid control method of the present invention can increase the inlet temperature of CO2 fluid by using a ground heating device, and control the CO2 fluid near the bottom of the well to be in a supercritical state throughout the fracturing stage, thus giving full play to the advantages of supercritical CO2 technology.

[0147] The CO2 fluid temperature calculation process of this invention can solve the problem of maintaining the temperature of the CO2 fluid near the bottom of the well at about 32°C throughout the fracturing process. This prevents the supercritical CO2 from reducing its proppant carrying capacity due to excessively high temperatures, avoids the presence of large sections of supercritical CO2 fluid in the wellbore that would accelerate proppant settling, and saves the cost of heating the low-temperature CO2 fluid at the surface.

[0148] The low-temperature liquid CO2 heating process of this invention, due to the appropriate selection of heating timing, can heat low-temperature CO2 fluid in a relatively low-pressure environment, thereby improving heat exchange efficiency and reducing energy consumption and equipment material costs; it can avoid wear or blockage of the heat exchange pipeline of the heating device by CO2 fluid, thus improving the service life of the heating device; it can stably and rapidly supply liquid to the heating device, fully releasing the potential of the heating device, improving the efficiency of a single heating device, and reducing the number of heating devices used and the total floor space.

[0149] The wellhead CO2 fluid temperature monitoring and control process of the present invention can measure the CO2 fluid temperature at the outlet of the heating device and at the wellhead in real time. Through automatic software control or manual adjustment, the CO2 fluid inlet temperature is guaranteed to be consistent with the set value based on theoretical calculation.

[0150] A device for controlling the supercritical state of fluid at the bottom of a supercritical carbon dioxide fracturing well is also provided, such as... Figure 4 As shown, the device is used to heat the CO2 fluid before it is injected into the wellbore. The heated CO2 fluid is in a supercritical state after it enters the wellbore and reaches the bottom of the well.

[0151] The device includes a booster pump, a heating device, and a fracturing pump truck connected in sequence.

[0152] The booster pump is used to draw and pressurize the liquid CO2 fluid in the storage tank and then deliver it to the heating device. It is also used to deliver the CO2 fluid flowing out of the heating device to the fracturing pump truck.

[0153] The heating device is used to heat the CO2 fluid to the temperature at the wellbore inlet.

[0154] The fracturing pump truck is used to pressurize CO2 fluid heated to the temperature at the wellbore inlet and inject it into the wellbore. The temperature of the CO2 fluid at the wellbore inlet is a preset temperature. When the CO2 fluid reaches the bottom of the well after entering the wellbore at the preset temperature, it is in a supercritical state.

[0155] The device also includes a temperature sensor, which is disposed at the outlet of the heating device and the inlet of the well.

[0156] A temperature sensor is installed at the outlet of the heating device to monitor whether the temperature of the CO2 fluid reaches the temperature at the well inlet.

[0157] The temperature sensor is installed at the wellbore inlet to monitor whether the temperature of the CO2 fluid has reached a preset temperature; wherein,

[0158] When the temperature at the wellhead is lower than the preset temperature, increase the input power of the heating device;

[0159] When the temperature at the wellhead is higher than the preset temperature, reduce the input power of the heating device.

[0160] The device also includes a sand mixing tank, which is located between the heating device and the fracturing pump truck, and is used to mix CO2 fluid heated to the temperature at the wellbore inlet with proppant and additives.

[0161] The fracturing pump truck is also used to receive CO2 fluid mixed with proppant and additives, and to pressurize the CO2 fluid before injecting it into the wellbore. The temperature of the CO2 fluid at the wellbore inlet is a preset temperature.

[0162] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0163] This invention proposes a method for controlling the supercritical state of bottomhole fluid in supercritical carbon dioxide fracturing wells, including steps such as calculating the CO2 fluid temperature at the wellhead (S1), heating the CO2 fluid at low temperature (S2), and monitoring and controlling the CO2 fluid temperature at the wellhead (S3). Figure 4 As shown.

[0164] The calculation step S1 for the CO2 fluid temperature entering the well consists of the following steps: dividing the calculation model (S11), establishing the governing equations (S12), substituting data to solve (S13), and optimizing the entry temperature (S14). Figure 5 As shown. The calculation model is divided as follows: S11: Based on the basic data of the supercritical carbon dioxide fracturing well, a model of the wellbore and reservoir is established, and an appropriate grid density is defined; S12: The governing equations for the flow and heat transfer of CO2 fluid in the wellbore are established; S13: Substituting the required calculation parameters such as CO2 physical properties, thermal conductivity parameters of the wellbore and reservoir, the changes in temperature and pressure of CO2 fluid in the wellbore with well depth and time at a certain injection temperature are iteratively solved; S14: The minimum injection temperature of CO2 fluid is optimized, with the goal of maintaining the CO2 fluid near the bottom of the well at 32℃ throughout the fracturing process.

[0165] The cryogenic liquid CO2 heating step S2 involves heating the cryogenic liquid CO2 at the surface to the temperature calculated in step S1, which is the temperature of the CO2 fluid entering the well. The process is as follows: Figure 5 As shown, the booster pump 22 draws and pressurizes the cryogenic liquid CO2 from the storage tank 21 and delivers it to the heating device 23. After sufficient heat exchange in the heating device 23, it reaches the set temperature and then enters the mixing tank 24 to be fully mixed with proppant and additives. The mixture is then significantly pressurized by the fracturing pump truck 25 and finally flows through the surface manifold into the wellbore 26. If proppant pumping is not required at a certain stage of fracturing, the CO2 fluid flowing from the heating device 23 directly enters the fracturing pump truck 25 and then flows through the surface manifold into the wellbore 26.

[0166] The heating device can use a single heating method or a combination of heating methods, including but not limited to electric heating and gas combustion heating.

[0167] Step S3, the CO2 fluid temperature monitoring and control at the wellhead, is used to precisely control the temperature of the CO2 fluid at the wellhead. After flowing out of the heating device 23, the CO2 fluid needs to pass through devices such as the mixing tank 24 and the pressure pump truck 25. During this process, the CO2 fluid will exchange heat with the external environment, and its temperature entering the wellbore 26 will differ from the temperature at the outlet of the heating device 23. Therefore, this difference needs to be monitored at all times. Temperature sensors are installed at both the outlet of the heating device 23 and the inlet of the wellbore 26, and the temperature is displayed in real time on the display of the heating device 23. If the temperature of the CO2 fluid at the outlet of the heating device 23 is lower than the temperature at the inlet of the wellbore 26, the input power of the heating device 23 is increased; if the temperature of the CO2 fluid at the outlet of the heating device 23 is higher than the temperature at the inlet of the wellbore 26, the input power of the heating device 23 is decreased. This function can be achieved through manual operation or automatic software control.

[0168] The supercritical carbon dioxide fracturing well bottom fluid control method of the present invention can increase the inlet temperature of CO2 fluid by using a ground heating device, and control the CO2 fluid near the bottom of the well to be in a supercritical state throughout the fracturing stage, thus giving full play to the advantages of supercritical carbon dioxide technology.

[0169] The CO2 fluid temperature calculation process of this invention can solve the problem of maintaining the temperature of the CO2 fluid near the bottom of the well at about 32°C throughout the fracturing process. This prevents the supercritical CO2 from reducing its proppant carrying capacity due to excessively high temperatures, avoids the presence of large sections of supercritical CO2 fluid in the wellbore that would accelerate proppant settling, and saves the cost of heating the low-temperature CO2 fluid at the surface.

[0170] The low-temperature liquid CO2 heating process of this invention, due to the appropriate selection of heating timing, can heat low-temperature CO2 fluid in a relatively low-pressure environment, thereby improving heat exchange efficiency and reducing energy consumption and equipment material costs; it can avoid wear or blockage of the heat exchange pipeline of the heating device by CO2 fluid, thus improving the service life of the heating device; it can stably and rapidly supply liquid to the heating device, fully releasing the potential of the heating device, improving the efficiency of a single heating device, and reducing the number of heating devices used and the total floor space.

[0171] The wellhead CO2 fluid temperature monitoring and control process of the present invention can measure the CO2 fluid temperature at the outlet of the heating device and at the wellhead in real time. Through automatic software control or manual adjustment, the CO2 fluid inlet temperature is guaranteed to be consistent with the set value based on theoretical calculation.

[0172] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for controlling the supercritical state of bottomhole fluid in supercritical carbon dioxide fracturing wells, characterized in that, The method includes: The liquid CO2 fluid in the storage tank is heated by a heating device and pressurized by a fracturing pump truck before being injected into the wellbore. The temperature of the CO2 fluid at the wellbore inlet reaches the preset temperature. Determining the preset temperature includes the following steps: Based on the basic data of the target fractured well, a physical model of the wellbore and reservoir of the target fractured well is constructed. Based on the physical model of the target fractured wellbore and reservoir, a simulation model of CO2 fracturing fluid flow and heat transfer in the wellbore is constructed. Based on the simulation model of CO2 fracturing fluid wellbore flow and heat transfer, the preset temperature corresponding to the supercritical state of CO2 at the bottom of the well is determined; Based on the fundamental data of the target fractured well, a physical model of the wellbore and reservoir is constructed, including: The physical model of the target fracturing wellbore and reservoir is meshed. Based on the physical property data of CO2 fluid, the fluid boundary constraints and the control equations for the flow and heat transfer of CO2 fluid in the wellbore and reservoir of the target fractured well were determined. Based on the physical model of the wellbore and reservoir after gridding, fluid boundary constraints, and the control equations for CO2 fluid flow and heat transfer in the wellbore, a simulation model of CO2 fracturing fluid flow and heat transfer in the wellbore is constructed. The fluid boundary constraints include the boundary conditions inside the wellbore, the annulus, and the formation. Among them, the boundary conditions inside the wellbore include the pressure and displacement of the fluid at the wellhead, the displacement and temperature at the bottom of the well, and the displacement of the wellbore wall. The control equations for the flow and heat transfer of CO2 fluid in the wellbore include: The heat transfer equations for CO2 in the central tube during heat extraction in fracturing, the energy equations for heat exchange between the annular fluid and the surrounding medium, and the heat transfer equations within the formation. The heat transfer equation for CO2 fluid inside the central tube during fracturing heat extraction is as follows: In the formula, The density of the fluid inside the central tube; This refers to the cross-sectional area of ​​the flow channel within the central tube; The specific heat capacity of the fluid inside the central tube; The temperature of the fluid inside the central tube. This is the derivative with respect to the temperature of the fluid inside the central tube; For fluid flow time, t This is the derivative with respect to time; For thermal convection of the fluid inside the central tube; For heat conduction of the fluid inside the central tube; The frictional heat between the fluid inside the central tube and the tube wall; For the heat exchange between the fluid inside the central tube and the fluid inside the annulus; The energy equation for the heat transfer process between the annular fluid and the surrounding medium is expressed as follows: In the formula, The density of the fluid within the annulus; This represents the cross-sectional area of ​​the flow channel within the annulus. Specific heat capacity of the fluid within the annulus; The temperature of the fluid inside the annulus. This is the derivative with respect to the temperature of the fluid within the annulus; For fluid flow time, t This is the derivative with respect to time; This refers to the thermal convection of fluid within the annulus. For heat conduction of the fluid within the annulus; This is the frictional heat between the fluid inside the annulus and the pipe wall; Q 1 represents the heat exchange between the fluid inside the central tube and the fluid inside the annulus; Q 2 represents heat exchange between the fluid in the annulus and the formation; The heat transfer equation within the formation is expressed as: In the formula, and These are the equivalent specific heat capacity and equivalent thermal conductivity of the formation, respectively. For fluid flow time, t This is the derivative with respect to time; Ts The temperature of the formation around the well. For the derivative with respect to the formation temperature around the well, This represents the divergence of formation temperature around the well. T f2 The temperature of the fluid within the annulus; R 2 represents the thermal resistance between the fluid within the annulus and the surrounding formation; the CO2 fluid at the preset temperature is in a supercritical state when it reaches the bottom of the well through the wellbore; wherein, When the CO2 fluid temperature at the wellbore inlet is lower than the preset temperature, the input power of the heating device is increased; when the CO2 fluid temperature at the wellbore inlet is higher than the preset temperature, the input power of the heating device is reduced.

2. The method for controlling the supercritical state of bottomhole fluid in supercritical carbon dioxide fracturing wells according to claim 1, characterized in that, The method further includes: The liquid CO2 fluid in the storage tank is heated by a heating device, mixed with proppant and additives in a sand mixing tank, and then injected into the wellbore after being pressurized by a fracturing pump truck. The temperature of the CO2 fluid at the wellbore inlet reaches the preset temperature.

3. The method for controlling the supercritical state of bottomhole fluid in supercritical carbon dioxide fracturing wells according to claim 1, characterized in that, The basic data of the target fracturing well includes: the geometric dimensions of the wellbore, the tubing string, and the cement sheath and formation, as well as their positional relationships.

4. The method for controlling the supercritical state of bottomhole fluid in supercritical carbon dioxide fracturing wells according to claim 1, characterized in that, The physical properties of CO2 fluid include: density, viscosity, thermal conductivity, and specific heat capacity under temperature and pressure conditions.

5. The method for controlling the supercritical state of bottomhole fluid in supercritical carbon dioxide fracturing wells according to claim 1, characterized in that, In the formula, The velocity of the fluid inside the central tube; For divergence, The divergence of the fluid temperature inside the central tube; The thermal conductivity of the fluid inside the central tube; The Darcy friction factor is the fluid flow coefficient within the central tube. The hydraulic diameter of the central pipe; T f2 The temperature of the fluid within the annulus; R 1 represents the thermal resistance between the fluid inside the central tube and the fluid inside the annulus.

6. The method for controlling the supercritical state of bottomhole fluid in supercritical carbon dioxide fracturing wells according to claim 5, characterized in that, In the formula, The velocity of the fluid within the annulus; For divergence, The divergence of the fluid temperature within the annulus; The thermal conductivity of the fluid within the annulus; The Darcy friction factor represents the fluid flow within the annulus. The hydraulic diameter of the annulus; The thermal resistance between the fluid within the annulus and the surrounding formation; Ts This refers to the formation temperature around the well.

7. The method for controlling the supercritical state of bottomhole fluid in supercritical carbon dioxide fracturing wells according to claim 6, characterized in that, In the formula, The porosity of the formation; The density of the rocks in the strata; Specific heat capacity of the strata rocks; The density of the formation fluid; Specific heat capacity of formation fluids; λ s The thermal conductivity of the rock formation; denoted as , where is the thermal conductivity of the formation fluid.

8. The method for controlling the supercritical state of bottomhole fluid in supercritical carbon dioxide fracturing wells according to claim 1, characterized in that, Based on a simulation model of CO2 fracturing fluid flow and heat transfer in the wellbore, the preset temperature of the CO2 fluid at the wellbore inlet when the CO2 fluid at the bottom of the well is in a supercritical state is determined, including: The CO2 physical properties, the thermal conductivity parameters of the wellbore and reservoir, and the boundary conditions are input into the CO2 fracturing fluid wellbore flow and heat transfer simulation model for iterative solution to obtain the CO2 fluid temperature at the wellbore inlet when the CO2 fluid at the bottom of the well is in a supercritical state. When the CO2 fluid at the bottom of the well is in a supercritical state, the corresponding CO2 fluid temperature at the wellbore inlet is the preset temperature.

9. A device for controlling the supercritical state of fluid at the bottom of a supercritical carbon dioxide fracturing well, characterized in that, The device is used to implement the method according to any one of claims 1-8, and the device is used to heat the CO2 fluid before injection into the wellbore, wherein the heated CO2 fluid is in a supercritical state when it enters the wellbore and reaches the bottom of the well. The device includes a booster pump, a heating device, and a fracturing pump truck connected in sequence. The booster pump is used to draw and pressurize the liquid CO2 fluid in the storage tank and then deliver it to the heating device. It is also used to deliver the CO2 fluid flowing out of the heating device to the fracturing pump truck. The heating device is used to heat CO2 fluid; The fracturing pump truck is used to inject heated CO2 fluid into the wellbore after pressurization. The temperature of the CO2 fluid at the wellbore inlet is a preset temperature. When the CO2 fluid reaches the bottom of the wellbore after reaching the preset temperature, it is in a supercritical state. The device also includes a temperature sensor, which is disposed at the outlet of the heating device and the inlet of the well. A temperature sensor is installed at the outlet of the heating device to monitor the temperature of the heated CO2 fluid. The temperature sensor is installed at the wellbore inlet to monitor the CO2 fluid temperature at the wellbore inlet; wherein, When the CO2 fluid temperature at the wellbore inlet is lower than the preset temperature, increase the input power of the heating device; When the CO2 fluid temperature at the wellbore inlet is higher than the preset temperature, reduce the input power of the heating device.

10. The device for controlling the supercritical state of fluid at the bottom of a supercritical carbon dioxide fracturing well according to claim 9, characterized in that, The device also includes a sand mixing tank, which is located between the heating device and the fracturing pump truck, and is used to mix CO2 fluid heated to the temperature at the wellbore inlet with proppant and additives. The fracturing pump truck is also used to receive CO2 fluid mixed with proppant and additives, and to pressurize the CO2 fluid before injecting it into the wellbore. The temperature of the CO2 fluid at the wellbore inlet is a preset temperature.

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