Cooling method of drilling fluid

Abstract

The invention relates to the technical field of drilling fluid cooling in petroleum drilling engineering, in particular to a cooling method for drilling fluid. The method comprises the steps that a phase change material and a dispersing agent are added into the drilling fluid, and the underground operation conditions of the drilling fluid include that the flow velocity of the drilling fluid in a drill column is 30-100 L / s, and the pressure difference between the drill column and the annulus is 5-30 MPa. According to the cooling method for the drilling fluid, by controlling the flow speed of the drilling fluid and the pressure difference between the drill column and the annulus, the temperature difference between the descending drilling fluid in the drill column and the ascending drilling fluid in the annulus can be reduced, and then convection heat exchange between the descending drilling fluid in the drill column and the ascending drilling fluid in the annulus is reduced; the phase-change material in the drill string generates phase change at the bottom of the well as much as possible, and the phase-change material with phase change in the annulus does not generate phase change again at the stage close to the bottom of the well as much as possible, so that heat at the bottom of the well is carried out as much as possible, and the temperature of drilling fluid at the bottom of the well is remarkably reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of drilling fluid cooling in oil drilling engineering, and particularly relates to a method for cooling drilling fluid. Background Art

[0002] Drilling fluid is the general term for various circulating fluids that meet the needs of drilling operations with its multiple functions during the drilling process. Drilling fluid is the blood of drilling, and stable drilling fluid performance is crucial for the drilling fluid to perform its main functions such as cleaning the wellbore, carrying cuttings, balancing the bottom hole pressure, and protecting the formation. Another important function of drilling fluid is to cool the drill string, especially for cooling measurement-while-drilling instruments that contain a large number of electronic components.

[0003] The flow process of drilling fluid in the wellbore is divided into three stages: (1) The drilling fluid is pumped into the drill string and flows downward in the drill string. During the downward flow, the temperature of the drilling fluid gradually increases; (2) The drilling fluid flows out through the drill bit and enters the annulus; (3) The drilling fluid flows upward in the annulus and returns to the wellhead. During the upward flow, the temperature of the drilling fluid gradually decreases. The temperature of the drilling fluid at different stages depends on different heat exchange processes. In the first stage, drilling fluid with a certain temperature enters the drill string. When the liquid flows downward through the drill string, the change in its temperature is determined by the rate of downward heat convection of the liquid and the rate of radial heat transfer (i.e., the heat transfer process, which consists of three sub-processes: convective heat transfer between the hot liquid and the drill string wall, heat conduction of the drill string wall, and convective heat transfer between the cold liquid and the drill string wall) between the liquid, the drill string wall, and the liquid in the annulus. In the second stage, obviously the temperature of the liquid at the drill string outlet is equal to the temperature of the liquid at the annulus inlet. In the third stage, when the liquid entering the annulus flows upward, its temperature depends on the rate of upward heat convection along the annulus; the rate of radial heat transfer between the annulus liquid, the drill string wall, and the liquid inside the drill string, and the rate of radial convective heat transfer through the wellbore wall.

[0004] During the drilling process, the temperature of the drilling fluid in the wellbore continuously increases with the increase of well depth. Especially during the drilling of deep wells and ultra-deep wells, the increase in formation temperature causes the temperature of the circulating drilling fluid to also continuously increase, which brings a series of problems: First, high temperature causes chemical changes in the components of the drilling fluid, seriously affecting the stability of the drilling fluid; second, high temperature leads to poor mechanical properties of the formation, increasing the risk of instability of the wellbore rock formation; third, high temperature affects the stability of downhole instruments, resulting in problems such as increased drill bit wear, reduced performance of rubber parts, and instrument failures caused by damage to electronic components. The above three adverse effects brought by high temperature greatly extend the drilling cycle and increase the drilling cost.

[0005] In order to reduce the temperature of the circulating drilling fluid, a ground cooling technology of "air cooling + water cooling" heat exchange has been developed currently. Before the circulated drilling fluid is pumped back into the well again, the temperature of the drilling fluid is reduced to a level close to the temperature of the exchange medium, which can reduce the temperature of the bottom-hole drilling fluid by about 10 °C after circulating to the ground. However, this cooling technology is for ground cooling. With the increase of well depth, the cooling effect is significantly reduced, and even the cooling effect cannot be achieved, with relatively large technical limitations.

[0006] As for the downhole direct cooling technology, there are currently relevant research explorations on the phase change material (PCM) cooling technology, which can utilize its property of absorbing (or releasing) heat during the conversion process between solid state and liquid state to store (or release) energy to reduce the circulating temperature of the downhole drilling fluid. However, simply adding the phase change material as a treating agent to the drilling fluid can only achieve a slightly better cooling effect. This is because, during the process of the heated drilling fluid flowing upward into the annulus, the temperature of the drilling fluid in the annulus and the heat of the formation will be transferred along the direction pointing to the wellbore axis into the drill string, causing the temperature of the drilling fluid flowing downward in the drill string to increase and the temperature of the drilling fluid in the annulus to decrease. The temperature difference between the drilling fluid in the drill string and the annulus will, on the one hand, cause the liquid phase change material that has undergone phase change in the annulus to undergo phase change again near the bottom hole, changing from the liquid phase to the solid phase and releasing heat; on the other hand, it will cause the phase change material in the drill string to undergo phase change in advance before reaching the bottom hole, thus reducing the cooling effect of the phase change material at the bottom hole and unable to meet the drilling requirements of high-temperature deep wells.

[0007] A method for controlling the temperature of drilling fluid based on phase change materials is disclosed in CN109652028A, which includes the following steps: Selecting a phase change material for drilling fluid: Selecting a phase change material with a suitable phase change temperature according to the bottom hole temperature. The phase change temperature point should be selected at a certain point of the wellbore circulation temperature. The closer the phase change temperature of the phase change material is to the bottom hole temperature, the better the temperature control effect, and the phase change latent heat should be greater than 160 kJ / kg; (2) Using the selected phase change material as a drilling fluid treatment agent, it can be added before weighting together with other non-polymer solid particles when preparing the drilling fluid, and the addition amount is 5%-15% of the total weight of the drilling fluid; or it can be added at a concentration of 5-15 kg per cubic meter of drilling fluid during drilling. After uniform circulation, drilling can be carried out. The concentration of the phase change material is calculated based on the weight of the dry phase change material added per cubic meter of drilling fluid. Different bottom hole temperatures result in different concentrations of the added phase change material until the required drilling fluid circulation temperature is reached, but it does not exceed 20% of the total weight of the drilling fluid at most; When the drilling fluid containing the phase change material circulates from the wellhead to the bottom hole, the formation temperature gradually rises. When it reaches the phase change temperature, the phase change material undergoes a phase change, absorbs the heat in the drilling fluid, and reduces the circulation temperature of the drilling fluid; The absorbed heat is stored in the phase change material, and the temperature of the material itself remains almost unchanged before the phase change is completed, generating a wide temperature platform; When the drilling fluid circulates from the bottom hole to the wellhead and passes through external environments such as the surface circulation tank, sedimentation tank, purification system, and mud pit, the temperature gradually decreases. When it drops to the phase change temperature, the phase change material undergoes a phase change again, releases heat, transfers a large amount of phase change heat to the external environment along with the drilling fluid, and then the phase change material follows the drilling fluid into the drilling pump for recycling; (3) After drilling is completed, the phase change material can be recovered and reused. In this method, the phase change material can be recycled simultaneously with other drilling fluid treatment agents, without maintenance, power consumption, and is environmentally friendly, and is competitive in economic benefits. However, this method does not provide a solution to the problem that the phase change material undergoes a phase change again in the annulus bottom hole stage and the phase change material undergoes a phase change in advance before reaching the bottom hole in the drill string. Summary of the Invention

[0008] The object of the present invention is to solve the problem of poor cooling effect of the existing drilling fluid and provide a method for cooling the drilling fluid.

[0009] To achieve the above object, the present invention provides a method for cooling drilling fluid, wherein the method includes: adding a phase change material and a dispersant to the drilling fluid, and the operating conditions of the drilling fluid downhole include: the flow rate of the drilling fluid in the drill string is 30-100 L / s, and the pressure difference between the drill string and the annulus is 5-30 MPa.

[0010] Through the above technical solutions, the beneficial technical effects obtained by the present invention are as follows:

[0011] 1) In the method for cooling drilling fluid provided in the present invention, after adding a phase change material and a dispersant to the drilling fluid, by controlling the flow rate of the drilling fluid and the pressure difference between the drill string and the annulus, the phase change temperature of the phase change material in the drill string and the annulus can be controlled, effectively slowing down the convective heat exchange rate between the annulus return drilling fluid and the downward drilling fluid in the drill string. On the one hand, it can ensure that the phase change material in the drill string does not prematurely undergo a phase change due to the temperature increase caused by convective heat exchange, and as much as possible, the phase change material is pumped to the bottom of the well in a completely solid state to fully absorb the heat at the bottom of the well. On the other hand, it can minimize the heat released when the phase change material that has already undergone a phase change in the annulus undergoes a phase change again near the bottom of the well, thereby maximizing the cooling effect. The deeper the well depth, the more obvious the effect;

[0012] 2) In the method for cooling drilling fluid provided in the present invention, by efficiently cooling the drilling fluid, the damage to the formation caused by high temperature can be minimized, the performance of the drilling fluid can be maintained stable, the stability of downhole instruments can be ensured, the number of trips can be reduced, and the ineffective time during the drilling process can be reduced, thereby achieving the purpose of improving drilling efficiency and reducing drilling costs. Detailed implementation mode

[0013] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0014] The present invention provides a method for cooling drilling fluid, wherein the method includes: adding a phase change material and a dispersant to the drilling fluid, and the operating conditions of the drilling fluid downhole include: the flow rate of the drilling fluid in the drill string is 30 - 100 L / s, and the pressure difference between the drill string and the annulus is 5 - 30 MPa.

[0015] Among them, in the present invention, the pressure difference between the drill string and the annulus refers to the pressure loss of the drilling fluid flowing through the bottom hole assembly. The inventor has found through research that after adding a phase change material to the drilling fluid, by controlling the flow rate of the drilling fluid and the pressure difference between the drill string and the annulus, the temperature difference between the downward drilling fluid in the drill string and the upward drilling fluid in the annulus can be reduced, and further, the convective heat exchange between the downward drilling fluid in the drill string and the upward drilling fluid in the annulus can be reduced, avoiding the phase change of the phase change material in the drill string before reaching the bottom of the well, enabling the phase change material to undergo a phase change at the bottom of the well as much as possible, and as much as possible, preventing the phase change material that has already undergone a phase change in the annulus from undergoing a phase change again in the stage close to the bottom of the well, thereby carrying out as much heat as possible from the bottom of the well and significantly reducing the temperature of the drilling fluid at the bottom of the well.

[0016] In one embodiment of the present invention, the density of the phase change material is lower than that of the drilling fluid. Wherein, in the present invention, by controlling the density of the phase change material, the loss of the drilling fluid during the centrifugal separation of harmful solids can be minimized as much as possible, so as to ensure that the phase change material is not sieved out when passing through the centrifuge and can continue to participate in the drilling fluid circulation.

[0017] In one embodiment of the present invention, the density of the phase change material is 90-102% of the density of the drilling fluid. Wherein, in the present invention, the density of the phase change material can be 90%, 91%, 92%, 93%, 94%, 95%, 95%, 97%, 98%, 99%, 100%, 101%, 102% of the density of the drilling fluid, and any number between these values, preferably 93-96%.

[0018] In one embodiment of the present invention, the density of the phase change material is 1.45-1.55 g / cm 3 . Wherein, in the present invention, the density of the phase change material can be 1.45 g / cm 3 , 1.46 g / cm 3 , 1.47 g / cm 3 , 1.48 g / cm 3 , 1.49 g / cm 3 , 1.50 g / cm 3 , 1.51 g / cm 3 , 1.52 g / cm 3 , 1.53 g / cm 3 , 1.54 g / cm 3 , 1.55 g / cm 3 , and any number between these values, preferably 1.48-1.52%.

[0019] In one embodiment of the present invention, the melting temperature of the phase change material is lower than the maximum tolerance temperature of the electronic components and rubber parts in the bottom hole assembly. Wherein, in the present invention, even if the formation temperature is higher than the maximum tolerance temperature of the downhole instruments and tools, through the reasonable design of the pressure difference and flow rate, the temperature of the drilling fluid can still be maintained near the melting temperature of the phase change material.

[0020] In one embodiment of the present invention, the phase change material can adopt inorganic phase change materials such as crystalline hydrated salts, molten salts, metals or alloys, organic phase change materials such as paraffin wax, fatty acids, higher aliphatic hydrocarbons, alcohols, carboxylic acids and salts, or composite phase change materials such as shaped phase change materials and microcapsule phase change materials.

[0021] Among them, in the present invention, according to the latent heat value and phase change temperature of the phase change material, a phase change material with a phase change temperature close to the bottom hole drilling fluid temperature is preferably selected. Among them, the bottom hole drilling fluid temperature can be predicted according to the wellbore temperature or measured during actual drilling according to known methods. The phase change material can be circulated to the bottom hole with the drilling fluid, undergo a phase change, absorb heat, reduce the bottom hole drilling fluid temperature, and then be circulated to the ground with the drilling fluid, thereby carrying the heat out of the bottom hole, realizing controllable cooling of the wellbore, and maintaining the normal use of the drilling fluid in a high-temperature environment.

[0022] In an embodiment of the present invention, the phase change material is a microcapsule phase change material. Among them, the present invention does not make special limitations on the microcapsule phase change material, which can be a commercially available product or synthesized according to known methods, such as the W / O / W multiple emulsion solvent evaporation method. The W / O / W multiple emulsion solvent evaporation method includes the following steps: 1. Take 5-15 parts by weight of erythritol and dissolve it in 20-40 parts by weight of deionized water, and then sequentially add 1-3 parts by weight of diethylenetriamine, 0.02-0.08 parts by weight of polysorbate-80, and 0.05-0.2 parts by weight of span-80, and stir evenly to obtain an erythritol solution; 2. Dissolve 0.5-2.5 parts by weight of toluene diisocyanate in 10-30 parts by weight of liquid paraffin oil to obtain a toluene diisocyanate solution; 3. Under a stirring speed of 400-800 rpm, drop the erythritol solution prepared in step 1 into 30-80 parts by weight of liquid paraffin oil at a dropping speed of 5-15 g / min, and continue to stir for 0.5-1.5 h after dropping to obtain a water-in-oil emulsion; 4. Under a stirring speed of 800-1200 rpm at 40-60 °C, drop the toluene diisocyanate solution prepared in step 2 into the water-in-oil emulsion prepared in step 3 at a dropping speed of 0.2-0.8 g / min; 5. Heat the mixture in step 4 to 55-70 °C, continuously react for 0.5-2.5 h, then filter, wash the filtered solute with distilled water, and then put it into a vacuum oven for drying to obtain microcapsules with a particle size of 1-10 μm.

[0023] Among them, in the present invention, the inventors have found through research that compared with other phase change materials, the microcapsule phase change material can, on the one hand, improve the compatibility between the phase change material and the drilling fluid, which helps to improve the stability of the drilling fluid at high temperatures, and on the other hand, can improve the form stability of the phase change material, prevent the material from aggregating and causing blockages after multiple cyclic phase changes, thereby further improving the cooling effect of the phase change material on the drilling fluid.

[0024] In an embodiment of the present invention, the phase change temperature of the phase change material is 110-150 °C (100 kPa), and the phase change latent heat is 300-380 J / g.

[0025] In one embodiment of the present invention, the addition amount of the phase change material is 5-15% of the total weight of the drilling fluid. Among them, in the present invention, the addition amount of the phase change material can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15% of the total weight of the drilling fluid, and any number between these values, preferably 8-12%.

[0026] In one embodiment of the present invention, the dispersant is selected from cetyltrimethylammonium bromide and / or sodium dodecylbenzenesulfonate. Among them, in the present invention, the addition of the dispersant can effectively prevent the drilling fluid from precipitating and caking, and contribute to further improving the cooling effect.

[0027] In one embodiment of the present invention, the addition amount of the dispersant is 0.1-5% of the total weight of the drilling fluid. Among them, in the present invention, the addition amount of the dispersant can be 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5% of the total weight of the drilling fluid, and any number between these values, preferably 2-3%.

[0028] In one embodiment of the present invention, the present invention does not make special limitations on the drilling fluid, and conventional drilling fluids in the art can be used in the present invention.

[0029] In one embodiment of the present invention, the flow rate of the drilling fluid in the drill string can be 30L / s, 35L / s, 40L / s, 45L / s, 50L / s, 55L / s, 60L / s, 65L / s, 70L / s, 75L / s, 80L / s, 85L / s, 90L / s, 95L / s, 100L / s, and any number between these values, preferably 30-45L / s.

[0030] Among them, the flow rate of the drilling fluid affects the time for the phase change material to pass near the bottom of the well, and will have a significant impact on the heat absorption effect of the phase change material. In the present invention, the inventors have found through research that controlling the flow rate of the drilling fluid within the above range can ensure that the phase change material in the drill string is pumped to the bottom of the well in a completely solid or at least partially solid state, and undergoes a phase change during the process of flowing through the bottom of the well, basically all changing from a solid state to a liquid state, and can reduce the probability of the phase change material that has already undergone a phase change in the annulus from undergoing a phase change again in the bottom hole section during the upward process.

[0031] In an embodiment of the present invention, the pressure difference between the drill string and the annulus can be 5 MPa, 6 MPa, 7 MPa, 8 MPa, 9 MPa, 10 MPa, 11 MPa, 12 MPa, 13 MPa, 14 MPa, 15 MPa, 16 MPa, 17 MPa, 18 MPa, 19 MPa, 20 MPa, 21 MPa, 22 MPa, 23 MPa, 24 MPa, 25 MPa, 26 MPa, 27 MPa, 28 MPa, 29 MPa, 30 MPa, and any number between these values, preferably 6 - 12 MPa.

[0032] Among them, in the present invention, the inventors have found through research that the phase change reaction usually occurs within a temperature range. Controlling the pressure difference between the drill string and the annulus within the above range can make the highest melting temperature of the phase change material in the annulus lower than or equal to the lowest melting temperature of the phase change material in the drill string, so as to prevent the phase change material that has already undergone a phase change in the annulus from undergoing a phase change again near the bottom of the well as much as possible.

[0033] In an embodiment of the present invention, the downhole operating conditions of the drilling fluid further include: the bit pressure drop is 2 - 3.5 MPa, the positive displacement motor pressure drop is 3 - 5 MPa, the pressure inside the drill string above the positive displacement motor is 55 - 60 MPa, and the pressure inside the annulus is 42 - 52 MPa.

[0034] Among them, in the present invention, by adjusting the bit pressure drop, the positive displacement motor pressure drop, the pressure inside the drill string above the positive displacement motor, and the pressure inside the annulus, the flow rate of the drilling fluid inside the drill string downhole can be adjusted, and the pressure difference between the drill string and the annulus can be adjusted. Among them, the inventors of the present invention have found through research that controlling the bit pressure drop, the positive displacement motor pressure drop, the pressure inside the drill string above the positive displacement motor, and the pressure inside the annulus within the above range helps to further improve the temperature reduction effect of the drilling fluid at the bottom of the well. The temperature reduction method of the drilling fluid provided in the present invention can significantly reduce the temperature of the drilling fluid at the bottom of the well, making the temperature of the drilling fluid at the bottom of the well decrease by at least 25 - 32 °C.

[0035] The present invention will be described in detail below through examples.

[0036] Simulation well: Well depth is 3600 meters, and the bottom hole temperature is 154.4 °C.

[0037] Preparation method of microcapsule phase change material:

[0038] 1. Take 10 g of erythritol and dissolve it in 30 g of deionized water, then sequentially add 2 g of diethylenetriamine, 0.05 g of polysorbate - 80, and 0.1 g of span - 80, and stir evenly to obtain an erythritol solution;

[0039] 2. Dissolve 1 g of toluene diisocyanate in 17 g of liquid paraffin oil to obtain a toluene diisocyanate solution;

[0040] 3. Under a stirring speed of 500 rpm, slowly add the erythritol solution prepared in step 1 to 50 g of liquid paraffin oil at a dropping rate of 10 g / min in a uniform manner. After the dropping is completed, continue stirring for 1 h to obtain a water-in-oil emulsion;

[0041] 4. Under a stirring speed of 1000 rpm at 50 °C, slowly add the toluene diisocyanate solution prepared in step 2 to the water-in-oil emulsion prepared in step 3 at a dropping rate of 0.5 g / min in a uniform manner;

[0042] 5. Heat the mixture in step 4 to 60 °C and continue the reaction for 1 h. Then filter, wash the filtered solute with distilled water, and place it in a vacuum oven to dry at a temperature of 80 °C to obtain microcapsules with a particle size of 1 - 10 μm.

[0043] Among them, the density of the prepared microcapsule phase change material is 1.50 g / cm 3 , the phase change temperature is 117 - 123 °C (100 kPa), and the phase change latent heat is 339.9 J / g.

[0044] Example 1

[0045] Add microcapsules and cetyltrimethylammonium bromide to the drilling fluid with a density of 1.60 g / cm 3 , where the addition amount of microcapsules is 8% of the total weight of the drilling fluid, and the addition amount of cetyltrimethylammonium bromide is 2% of the total weight of the drilling fluid;

[0046] Pump the above drilling fluid into the drill string. The drilling fluid flows downward along the drill string. After reaching the bottom of the well, the microcapsules undergo a phase change, and then flow out of the drill string with the drilling fluid, enter the annulus, and flow upward in the annulus and return to the wellhead;

[0047] Among them, the bit pressure drop is 3.3 MPa, the positive displacement motor pressure drop is 4.2 MPa, the pressure in the drill string above the positive displacement motor is 57.4 MPa, the pressure in the annulus is 45.8 MPa, the pressure difference between the drill string and the annulus is 11.6 MPa, and the flow rate of the drilling fluid is 36 L / s;

[0048] Measure the bottom hole temperature as 124.1 °C through a downhole tool temperature sensor, and the temperature reduction range of the phase change material is 30.3 °C.

[0049] Example 2

[0050] Add microcapsules and cetyltrimethylammonium bromide to the drilling fluid with a density of 1.60 g / cm 3In the drilling fluid, the addition amount of the microcapsules is 10% of the total weight of the drilling fluid, and the addition amount of cetyltrimethylammonium bromide is 3% of the total weight of the drilling fluid;

[0051] Pump the above-mentioned drilling fluid into the drill string. The drilling fluid flows downward along the drill string. After reaching the bottom of the well, the microcapsules undergo a phase change, and then flow out of the drill string with the drilling fluid, enter the annulus, and flow upward in the annulus and return to the wellhead;

[0052] Among them, the bit pressure drop is 2.5 MPa, the downhole motor pressure drop is 3.3 MPa, the pressure inside the drill string above the downhole motor is 57.2 MPa, the pressure in the annulus is 50.6 MPa, the pressure difference between the drill string and the annulus is 6.6 MPa, and the flow rate of the drilling fluid is 40 L / s;

[0053] The bottom hole temperature measured by the downhole tool temperature sensor is 127.2 °C, and the temperature drop of the phase change material is 27.2 °C.

[0054] Comparative Example 1

[0055] It is the same as Example 1, except that the bit pressure drop, the downhole motor pressure drop, the pressure inside the drill string above the downhole motor, and the pressure in the annulus are adjusted so that the pressure difference between the drill string and the annulus is 4.4 MPa, and the flow rate of the drilling fluid is 20 L / s.

[0056] The bottom hole temperature measured by the downhole tool temperature sensor is 133.4 °C, and the temperature drop of the phase change material is 21 °C.

[0057] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A method for cooling a drilling fluid, characterized in that, The method includes: adding a phase change material and a dispersant to the drilling fluid, and the operating conditions of the drilling fluid downhole include: the flow rate of the drilling fluid in the drill string is 30 - 100 L / s, and the pressure difference between the drill string and the annulus is 5 - 30 MPa.

2. The cooling method according to claim 1, wherein The density of the phase change material is lower than the density of the drilling fluid.

3. The cooling method according to claim 2, wherein, The density of the phase change material is 90 - 102% of the density of the drilling fluid.

4. The cooling method according to claim 1, wherein, The phase change material is selected from microcapsule phase change materials.

5. The cooling method according to claim 1, wherein, The addition amount of the phase change material is 5 - 15% of the total weight of the drilling fluid.

6. The cooling method according to claim 1, wherein, The dispersant is selected from cetyltrimethylammonium bromide and / or sodium dodecylbenzenesulfonate.

7. The cooling method according to claim 1, wherein, The addition amount of the dispersant is 0.1 - 5% of the total weight of the drilling fluid.

8. The cooling method according to claim 1, wherein, The flow rate of the drilling fluid in the drill string is 30 - 45 L / s.

9. The cooling method according to claim 1, wherein, The pressure difference between the drill string and the annulus is 6 - 12 MPa.

10. The cooling method according to claim 1, wherein, The operating conditions of the drilling fluid downhole further include: the bit pressure drop is 2 - 3.5 MPa, the pressure drop of the positive displacement motor is 3 - 5 MPa, the pressure in the drill string above the positive displacement motor is 55 - 60 MPa, and the pressure in the annulus is 42 - 52 MPa.