CN102168545A - Coiled tubing supercritical CO2 injection fracturing method - Google Patents

Abstract

The invention relates to a coiled tubing supercritical _CO2 injection fracturing method. The method uses supercritical _CO2 as the fracturing fluid to jet-fracture oil well reservoirs. The present invention uses supercritical CO ₂ fluid as the fracturing fluid for jet fracturing, supercritical CO ₂ sand blasting and perforation can reduce system pressure, and at the same time, the low viscosity and high diffusion characteristics of supercritical CO ₂ fluid can also further reduce fracture propagation required pressure; the most important thing is that supercritical CO ₂ injection fracturing does not cause any pollution to the reservoir, on the contrary, entering the reservoir with supercritical CO ₂ can further improve oil and gas recovery; at the same time, no flowback is required after fracturing When the oil tubing is subjected to staged injection fracturing, the pipe string can be lifted or lowered without pressure relief from the wellbore, which reduces operating procedures and operating costs, so it is very suitable for heavy oil with little effect from conventional water-based fracturing fluids Oil reservoirs, low-permeability and ultra-low-permeability oil and gas reservoirs, shale gas reservoirs, coalbed methane reservoirs and other unconventional oil and gas reservoirs are fracturing.

Description

Coiled tubing supercritical CO2 injection fracturing method

technical field

The invention relates to an oil well jet fracturing method, in particular to a coiled tubing supercritical _CO2 jet fracturing method, which belongs to the field of oil drilling.

Background technique

Hydraulic fracturing technology was first proposed in the 1940s. At that time, it was mainly used in the reconstruction of old wells and enhanced oil recovery. At present, hydraulic fracturing technology has become an unconventional technology for low-permeability and ultra-low-permeability oil fields, shale gas reservoirs, and coalbed methane reservoirs. One of the main measures to increase production of oil and gas reservoirs. It uses surface high-pressure pumps to inject fracturing fluid into the wellbore with a displacement exceeding the absorption capacity of the formation (reservoir), and generates high pressure near the bottom of the well. When the pressure overcomes the in-situ stress near the well wall and the tensile strength of the rock When the fracturing fluid is continuously pumped, the fracture will continue to extend forward. After the fracture reaches the required size, the sand-carrying fluid mixed with proppant will be pumped and the sand-carrying fluid will continue to Extend the fracture and transport the proppant into the fracture; after the sand-carrying fluid is pumped, the gel breaker is pumped to reduce the high-viscosity fracturing fluid to a low-viscosity fluid and flow it back to the wellbore and back to the surface. After fracturing, only the proppant is left in the fracture to support the fracture wall, forming a fracture channel with high conductivity.

As we all know, the purpose of hydraulic fracturing is to create a fracture channel with high conductivity, penetrate the damaged zone near the wellbore, and restore the natural productivity of the oil well. However, for the above-mentioned traditional fracturing, it is necessary to inject a large amount of high-viscosity fracturing fluid, then pump the gel breaker, and then flow back the fracturing fluid. This is not only costly, but also complicated. The most important thing is that a large amount of water After entering the reservoir, it will cause secondary damage to the reservoir, reduce the permeability of the reservoir, and thus reduce the fracturing effect. Although a variety of clean fracturing fluids have appeared to reduce the reservoir pollution, it is still impossible to fundamentally eliminate the secondary pollution of the reservoir during fracturing.

CO ₂ is a common gas. When it is heated and pressurized above the critical point (T _c >31.1°C, P _c >7.38MPa), it becomes a supercritical CO ₂ fluid (SC-CO ₂ -super critical CO ₂ ), Under formation temperature and pressure conditions, _CO2 can reach a supercritical state generally above 750m. Supercritical CO ₂ fluid is different from both gas and liquid. It has low viscosity and high diffusivity close to gas, high density close to liquid, and zero surface tension. If CO ₂ is used for jet fracturing of oil wells, the above-mentioned characteristics will make supercritical CO ₂ jet rock breaking have higher efficiency and lower rock breaking threshold pressure. It will help the flow of _CO2 fluid, which is easy to generate many and complex fractures in the reservoir. Coiled tubing can work under pressure and is easy to get off and on. Therefore, if the combination of supercritical CO ₂ fluid and coiled tubing is used for reservoir injection fracturing, the fracturing cost can be greatly reduced and the fracturing effect can be improved.

Contents of the invention

In order to solve the above-mentioned technical problems, the object of the present invention is to provide a method for jet fracturing of oil well reservoirs, which uses coiled tubing to transport liquid CO ₂ to make it supercritical CO ₂ at the bottom of the oil well and jet fracturing the reservoir The method has the characteristics of high jet rock breaking efficiency, low rock breaking threshold pressure, less operating procedures, and low fracturing cost.

In order to achieve the above object, the present invention provides a coiled tubing supercritical CO ₂ jet fracturing method, which uses supercritical CO ₂ as a fracturing fluid to jet fracturing a reservoir.

The above-mentioned coiled tubing supercritical _CO2 injection fracturing method provided by the present invention may comprise the following steps:

Well cleaning treatment: use clean water or well cleaning fluid to circulate and wash the well, and use the well drilling gauge to open the well;

Perforation treatment: Mix liquid _CO2 and jet abrasives to obtain a mixed fluid, transport the mixed fluid to the oil well through coiled tubing, and perform sandblasting and perforation through the jet fracturing device in the oil well to create pores in the reservoir;

Fracturing treatment: After removing the sundries at the bottom of the oil well, continue to input pure liquid CO ₂ , which will be transformed into supercritical _{CO 2 fluid} at a certain depth in the wellbore and fracturing the reservoir, causing fractures in the reservoir ;

Propping treatment: mix the proppant into the liquid CO ₂ and make it enter the generated fracture with the liquid CO ₂ . When the proppant entering the fracture reaches the predetermined amount, stop mixing the proppant and stop pumping the liquid CO ₂ to complete a stage. fracking operations.

According to the specific technical solution of the present invention, the above-mentioned spray fracturing method provided by the present invention is suitable for performing only one stage of fracturing, and can also be suitable for performing more than two stages of fracturing. When performing multi-stage fracturing, it is necessary to The fractured fractures and channels should be isolated to avoid liquid CO ₂ pouring into the first stage of fractures in subsequent fracturing operations. Infiltrate into the reservoir, and require the fracturing fluid to hydrate itself after a period of time at a certain temperature, so that it can flow back after the fracturing is completed. The spacer used in the present invention can be a spacer commonly used in the field, such as guar gum, Water-based vegetable glue spacer. When the last stage of fracturing operation is completed and fracturing is no longer performed, isolation treatment is no longer necessary. According to the specific technical solution of the present invention, preferably, the above-mentioned method also includes the following steps:

Isolation treatment: input isolating fluid, plugging cracks and channels;

Lift the coiled tubing to the next stage of fracturing, then repeat the steps of perforation, fracturing and support to complete the next stage of fracturing.

After the last fracturing operation is completed, isolation treatment is no longer needed, and the next step can be directly performed. Specifically, when the fracturing reservoir in the oil well is a heavy oil reservoir or a coalbed methane or shale gas reservoir, the After the fracturing treatment, borehole treatment can be carried out (make supercritical CO ₂ fully interact with heavy oil to reduce the viscosity of crude oil, or fully replace CH ₄ in coalbed methane and shale gas reservoirs before opening the well for production), that is, the above method can also It includes the step of performing flowback treatment or borehole treatment; when the fractured reservoir in the oil well is a conventional oil and gas reservoir, it can be directly produced without flowback treatment or borehole treatment.

In the above-mentioned fracturing method provided by the present invention, the _CO2 source that adopts is more extensive, can be directly obtained by _CO2 gas field, perhaps extracts from the tail gas that power plant, steel mill etc. produce (helps environmental protection like this), because supercritical The amount of CO ₂ used in CO ₂ fracturing is not very large, and it can be transported by a CO ₂ tank truck. The CO ₂ storage tank is preferably able to withstand a pressure of about 10 MPa to ensure that the CO ₂ is in a liquid state at a certain temperature and pressure. The low temperature required in the storage tank. Refrigeration units can be installed on the _CO2 tank truck as needed to ensure the needs during transportation and before pumping into the fracturing truck. The transportation pressure of CO ₂ can generally be controlled at 3-5MPa, and the temperature can be controlled at -20°C to 5°C, which can ensure safe transportation. In order to better ensure the state of liquid _CO2 , preferably, the pressure of liquid _CO2 stored in the _CO2 tanker can be controlled to be 4-5MPa, and the temperature can be controlled to be -10°C to 5°C.

Conventional fracturing equipment can be used for supercritical CO ₂ fracturing after simple improvement. Firstly, the gaskets of chemical products such as rubber in pipe valves, high-pressure pumps, and downhole tools are replaced with metal gaskets. The main reason is that CO ₂ penetrating It is very strong and can penetrate into rubber-like macromolecular materials. If the time is too long, the rubber pad will be leaked; secondly, a circulating cooling device is installed on the head of the high-pressure pump on the fracturing vehicle, and the temperature is generally controlled at 0-3°C Left and right, to prevent the friction between the piston and the sleeve to generate heat, vaporize the liquid CO ₂ and affect the pump efficiency.

During fracturing operations, at the wellhead, the liquid _CO2 in the coiled tubing is in a low-temperature, high-pressure liquid state. As the liquid _CO2 gradually descends, its temperature and pressure gradually increase. When the temperature and pressure exceed the critical point at the same time, the liquid CO2 CO ₂ is transformed into supercritical CO ₂ , and the mixed fluid of supercritical CO ₂ and abrasive passes through the nozzle of the jet fracturing device to generate high-pressure jets to spray the reservoir, so that the reservoir produces pores. Since the temperature of the formation rises gradually with the increase of depth, the temperature gradient is about 20-50°C/km, and the temperature will exceed the critical temperature of _CO2 (31.1°C) at a depth of several hundred meters underground. The pressure is generally high, and it is easy to reach the critical pressure of CO ₂ (7.38MPa). Therefore, supercritical CO ₂ injection fracturing can usually be realized at 750m downhole. And when the formation temperature is abnormal, or when the shallow formation fracturing cannot reach the critical temperature, the liquid _CO2 fluid can be heated on the ground, that is, the jet fracturing method provided by the present invention can also include in the perforation treatment. A step of heating the mixed fluid composed of liquid CO ₂ and jet abrasive fed into the coiled tubing; this heating treatment can be performed on the ground.

According to the specific technical solution of the present invention, preferably, the perforation treatment may include the following specific steps:

The liquid _CO2 stored in the _CO2 tanker is transported to the sand mixing vehicle through the pipeline, and the jet abrasive is mixed with the liquid _CO2 , and then transported to the fracturing vehicle through the high-pressure pipeline; through the fracturing vehicle (the number of fracturing vehicles can be Increase or decrease according to displacement needs) The mixed fluid composed of liquid CO ₂ and jet abrasive is sent to the coiled tubing through the high-pressure pipeline, and reaches the jet fracturing device at the bottom of the oil well for sand blasting and perforation to create pores in the reservoir. In the mixed fluid in the perforation treatment, the amount of the jet abrasive used can be controlled to account for 5-8 wt% of the weight of the mixed fluid, and the jet abrasive used can be commonly used in the art, such as fine The density is 60 mesh-120 mesh garnet sand or quartz sand.

In the above-mentioned fracturing method provided by the present invention, preferably, the fracturing treatment may include the following steps:

An annular back pressure valve is installed at the wellhead of the oil well to reasonably control the wellhead pressure (generally can be controlled at 5-8MPa), so that the _CO2 at the bottom of the oil well can remove impurities (smoothly carried out of the wellbore), even if the bottom of the oil well becomes supercritical CO ₂ carries the abrasive and cuttings produced by perforation out of the wellbore through the annulus (the space between the coiled tubing and the wellbore is called the annulus); then the annulus back pressure valve is closed, and pure liquid CO ₂ is continuously input through the coiled tubing Or input pure liquid CO ₂ through the coiled tubing and the annular space at the same time, the liquid CO ₂ becomes supercritical CO ₂ fluid in the oil well, and the supercritical CO ₂ fluid enters the pores, fracturing the reservoir and causing fractures in the reservoir .

In the above-mentioned fracturing method provided by the present invention, when performing fracturing operations, support treatment is required for the fractured tunnels and fractures. Preferably, the support treatment includes the following steps:

When the fracture extends to a predetermined length (in this field, according to different geological conditions and actual production needs, different fracturing lengths can be predetermined, and then determine the fracturing time and displacement of the pumped fracturing fluid. Generally, The fracturing time of each fracturing operation can be controlled to 10-30 minutes, the pumping volume can be controlled to 5-9 cubic meters per minute), but not limited to this range), and the proppant is mixed into the liquid CO ₂ , let it enter the generated fracture with liquid CO ₂ to support the fracture; when the proppant entering the fracture reaches a predetermined amount, stop mixing proppant, stop inputting liquid CO ₂ , and complete a fracturing operation. In the propping treatment, preferably, the amount of proppant added can be controlled to account for 5-20 wt% of the sum of liquid CO ₂ and proppant mass, where liquid CO 2 refers to the liquid CO ₂ that is pumped in simultaneously with the proppant For CO ₂ , the proppant used may be a commonly used proppant in the field, preferably a hollow ceramic proppant.

The present invention uses supercritical _CO2 fluid as the fracturing fluid to carry out jet fracturing, which has the following advantages:

1. Supercritical CO ₂ jetting has high rock-breaking efficiency and low rock-breaking threshold pressure. Therefore, jet abrasives can be added to supercritical CO ₂ fluid to carry out jet fracturing with casing window opening, which not only reduces the injection pressure requirements of the system, but also The safety of fracturing construction is improved.

2. The supercritical CO ₂ fluid has a low viscosity and high diffusion characteristics. Even if it flows in a small-sized coiled tubing, its frictional resistance is very small and it flows very easily. At the same time, in the original micro-fractures of the reservoir, High-viscosity fracturing fluid cannot enter, but supercritical CO ₂ fluid can flow freely, which is helpful for the transmission of pressure in the wellbore, reduces the pressure of the fracturing system, and can make the reservoir produce many and complex micro-fractures. Form a fracture network, connect the wellbore, and increase the production and recovery of a single well.

3. Supercritical CO ₂ has no pollution to the reservoir, and does not contain solid particles or water. When it enters the reservoir, it can avoid pore clogging, reservoir clay expansion, rock wetting reversal, water sensitivity, etc. At the same time, under the action of its super solvating ability, supercritical CO ₂ can dissolve heavy oil components and other pollutants in the near wellbore area, reducing the oil and gas flow resistance in the near wellbore area. Therefore, supercritical CO ₂ injection After the fracturing is completed, there is no need to inject gel breaker or flowback, and it can be put into production directly, which not only reduces operating procedures and costs, but also prevents the proppant in the fracture from flowing back into the wellbore; in addition, if the crude oil in the reservoir has a high viscosity, Poor wells can be selected to fully interact with supercritical _CO2 and crude oil, increase reservoir energy, reduce crude oil viscosity, and enhance its fluidity, thereby further improving oil recovery.

4. The surface tension of supercritical CO ₂ fluid is zero, and it can enter any space larger than supercritical CO ₂ molecules. At the same time, for shale and coal seams, CO ₂ molecules and their adsorption capacity are much greater than CH ₄ molecules, so in the process of When shale gas reservoirs and coalbed methane reservoirs are fractured, supercritical _CO2 entering the reservoir can also replace coalbed methane and shale gas with an adsorbed gas content of up to 85%, further improving the recovery rate of gas reservoirs and gas well production, the most important This type of gas reservoir has extremely low permeability, relatively small fluid loss, and is more conducive to the formation of fractures, so it is very suitable for fracturing stimulation of shale gas reservoirs, coalbed methane reservoirs, and tight sandstone gas reservoirs.

5. When combining supercritical CO ₂ fluid and coiled tubing for multi-stage jet fracturing, there is no need to release pressure during the lifting of the string, and the coiled tubing can be directly operated under pressure, saving procedures and time.

Description of drawings

The following drawings are only intended to illustrate and explain the present invention schematically, and do not limit the scope of the present invention. in:

Figure 1 is a schematic diagram of the coiled tubing supercritical _CO2 injection staged fracturing process;

Figure 2 is a schematic diagram of sandblasting and perforating before the first stage of fracturing;

Fig. 3 is the schematic diagram of the fracturing method of the first stage of fracturing;

Fig. 4 is another kind of mode schematic diagram of pressing open crack;

Fig. 5 is a schematic diagram of the input method of spacer fluid after the first section of fracture is pressed;

Fig. 6 is a schematic diagram of another input mode of spacer fluid;

Fig. 7 is a schematic diagram of the sandblasting and perforating method before the second stage of fracturing;

Fig. 8 is a schematic diagram of the fracturing method of the second stage of fracturing;

Fig. 9 is a schematic diagram of the flowback mode of the spacer;

Fig. 10 is a schematic diagram of another flowback mode of the spacer;

Fig. 11 is a schematic diagram of a well waiting for production without flowback spacer fluid;

Fig. 12 is a schematic diagram of the coiled tubing supercritical CO ₂ injection staged fracturing process with a surface heater.

Explanation of reference numbers:

1CO ₂ tank truck 2 vehicle-mounted refrigeration unit 3CO ₂ storage tank 4 adiabatic high-pressure pipeline 5 sand mixing truck 6 fracturing truck 7 coiled tubing 8 coiled tubing drum 9 coiled tubing injection head 10 wellhead device 11 ring control back pressure valve 12 well wall 13 ring Empty 14 Surface 15 Jet fracturing tool 16 Supercritical CO ₂ jet 17 Hole 18 Jet hole 19 Bottom of well 20 Reservoir 21 Surface heating device 22 Fracture

Detailed ways

In order to have a clearer understanding of the technical features, purposes and beneficial effects of the present invention, the technical solutions of the present invention will be described in detail below with reference to the accompanying drawings, but this should not be construed as limiting the scope of the present invention.

Example

The coiled tubing supercritical _CO2 jet fracturing method provided by the present invention introduces supercritical _CO2 fluid as the fracturing liquid, and can fully exert the respective advantages of supercritical _CO2 and coiled tubing, and the method may include the following specific steps:

1. Well cleaning treatment:

Use a drift gauge to drill the well, and wash the well with clean water or well flushing fluid (well flushing fluid commonly used in this field is enough), on the one hand, to prevent tools from being blocked or stuck, and on the other hand, to prevent debris in the wellbore from fracturing During the process, it enters the deep formation and pollutes the reservoir;

2. Perforation treatment:

Fig. 1 is a schematic diagram of the coiled tubing supercritical CO ₂ injection staged fracturing process. When performing the first stage of sandblasting and perforating, the fracturing tool 15 is lowered into the predetermined layer at the bottom of the well 19, and the error is not more than 0.5 meters;

Liquid _CO2 is transported by a _CO2 tanker 1 equipped with a refrigeration unit 2, the transportation pressure is controlled at 4-5MPa, and the temperature is controlled at -10°C to 5°C;

The liquid _CO2 in the _CO2 storage tank 3 is transported to the sand mixing vehicle 5 through the insulated high-pressure pipeline 4, and the sand mixing vehicle 5 fully mixes the jet abrasive and the liquid _CO2 to obtain a mixed fluid and then transports it to the fracturing vehicle 6;

The mixed fluid of liquid CO ₂ and jet abrasive is pressurized by the fracturing vehicle 6 and becomes a high-pressure mixed fluid, which is directly transported to the coiled tubing 7, and the coiled tubing 7 passes through the coiled tubing drum 8, the coiled tubing injection head 9, and the wellhead device 10 in sequence and the wellbore 13 to deliver the high-pressure mixed fluid to the downhole jet fracturing tool 15; when the high-pressure mixed fluid needs to be heated, a surface heating device 21 can be installed behind the fracturing vehicle 6, as shown in Figure 12;

The mixed fluid of high-pressure supercritical CO ₂ and jet abrasive passes through the injection hole 18 of the jet fracturing tool 15 to generate a high-speed abrasive jet 16, which shoots through the casing (casing completion) or directly acts on the reservoir 20 (open hole completion) ) to generate holes (holes) 17 of a certain size, as shown in Fig. During sand perforating, the annular back pressure valve 11 should be adjusted to a proper position so that the supercritical _CO2 density at the bottom of the well is in the optimum range (generally controlled above 700kg/ ^m3 ), and the efficiency of sandblasting and perforating can be improved. At the same time, it is also necessary to ensure that debris such as cuttings and abrasive waste generated by perforation can be carried out of the wellbore 13 by the supercritical _CO2 fluid through the annular space and reach the surface 14;

3. Fracturing treatment:

After the sandblasting and perforating is completed, the sand mixing vehicle 5 stops working, and the annular back pressure valve 11 is closed, and the large displacement (can be controlled to 5-9 cubic meters per minute, and the time of each fracturing treatment can be controlled to 10-30 Minutes, preferably 20 minutes) to pump liquid CO ₂ into the holes 17 to perform fracturing on the reservoir 20, so that the fractures 22 are generated and extended at the stress concentration positions of the holes 17 in the reservoir;

4. Support processing:

When the length of the fracture 22 reaches the predetermined requirement, the sand mixer 5 is turned on, and the proppant for fracturing is pumped in, as shown in Fig. 3; due to the low viscosity of the supercritical _CO2 fluid, compared with the conventional water-based fracturing fluid , its sand-carrying ability is relatively poor, so when pumping proppant, it also needs a larger displacement (the displacement can be determined according to the actual situation according to the conventional practice in this field, and can generally be controlled at 5-9 cubic meters per minute), so Only in this way can the proppant smoothly enter the root of the fracture and realize the effective support of the fracture. For _CO2 gas, after the high-pressure supercritical _CO2 is ejected from the injection hole 18, the pressure drops sharply, and the volume expands rapidly, which produces the Joule-Thomson cooling effect, which makes the surrounding jet stream cool down sharply. When the temperature drops below the freezing point, such as It will freeze when it meets water, which will have a negative impact on the effect and safety of jet fracturing. Therefore, during the jet fracturing process, the pressure drop and displacement of the nozzle must be strictly controlled to avoid freezing at the bottom of the well. Common pump injection method to reduce flow pressure of coiled tubing, as shown in Figure 4;

5. Isolation treatment:

After the first stage of fracture proppant pumping is completed, stop pumping CO ₂ and pump a small amount of spacer fluid to seal the fractures 22 and holes 17 in the first stage of fracturing to prevent a large amount of CO ₂ influx during the second stage of fracturing In the fracture 22 of the first stage of fracturing, the spacer fluid at this time is required to have high viscosity and low fluid loss performance, so as to minimize the infiltration into the reservoir during the second stage formation fracturing, and it is required to be kept at a certain temperature for a certain period of time. After the stage, it will hydrate itself to facilitate flowback after fracturing; the pump injection method is shown in Fig. 5 and Fig. 6. Fig. 5 is to inject directly from the coiled tubing. The annulus injection method shown in Figure 6;

6. The second stage of fracturing operation:

After the spacer fluid is injected into the fractures of the first stage of fracturing, the coiled tubing 7 is lifted up to the next stage of fracturing (the coiled tubing 7 can be operated under pressure, which saves the cumbersome steps of pressure relief and connection of ordinary tubing strings, etc.), Repeat the perforation, fracturing and support treatment, and perform the second stage of jet fracturing (as shown in Figure 7 and Figure 8);

After the last stage of fracturing, there is no need to inject spacer fluid. After the previously injected spacer fluid is hydrated, the spacer fluid is flowed back. Since a large amount of CO ₂ enters the reservoir during fracturing, sufficient energy is added to the reservoir, and the return The flowback is relatively easy, and even the depleted oil and gas reservoirs in the later stage of production can flow back smoothly, and the flowback method can be any one of the methods shown in Figure 9 and Figure 10;

If the fractured reservoir is a heavy oil reservoir or a shale gas reservoir or a coalbed methane reservoir, it is not necessary to flow back spacer fluid after fracturing (the volume of spacer fluid is very small and has little impact on the reservoir permeability), and boring wells are used. The method makes the CO ₂ entering the reservoir fully interact with the heavy oil, reduce the viscosity, reduce the flow resistance of crude oil, or fully replace the CH ₄ in the shale gas reservoir and coalbed methane reservoir, and increase the free CH ₄ in the gas reservoir. Content, when the bored well is opened for production after a period of time, both production and recovery will be greatly improved.

The mechanism of supercritical CO ₂ injection fracturing to enhance oil recovery and single well production is as follows:

1. The supercritical CO ₂ fluid has a high density and strong solvation ability. It can dissolve heavy oil components and other organic matter near the wellbore and reduce the oil and gas flow resistance near the wellbore;

2. The supercritical CO ₂ fluid has a small viscosity (close to the gas viscosity), strong diffusion ability, and is easy to permeate and diffuse into the crude oil in the reservoir, so that the volume of the crude oil expands, reduces the viscosity of the crude oil, and increases the fluidity of the crude oil. At the same time, the surface of the supercritical CO ₂ fluid The tension is zero, and it is easy to enter any space larger than its molecules, which is conducive to oil displacement;

3. The supercritical CO ₂ fluid does not contain liquid water, so it will not expand the clay in the reservoir, on the contrary, it can also dehydrate the dense clay sand layer, open the sand layer pores, and reduce the skin coefficient of the well wall;

4. During the fracturing process, a large amount of _CO2 infiltrates into the reservoir, providing energy for the reservoir, increasing the flow capacity of crude oil, greatly reducing the interfacial tension of oil and water, reducing the saturation of residual oil, thereby improving oil recovery;

5. When supercritical CO ₂ injection fracturing is used to transform shale gas and coalbed methane reservoirs, since the adsorption capacity of CO ₂ and rock is stronger than that of CH ₄ and rock, CO ₂ can be replaced and adsorbed on the shale gas after entering the reservoir. CH ₄ in rock formations and coal seams can increase the content of free CH ₄ , thereby increasing production and recovery, and can also achieve CO ₂ storage and protect the environment;

6. Supercritical CO ₂ fracturing fluid is a clean fracturing fluid, without any damage to the reservoir, no flowback and no residue after fracturing, which protects the reservoir and simplifies the operation process;

7. The characteristics of supercritical CO _{2 ,} such as low viscosity and easy diffusion, can produce many and complex micro-fractures in the reservoir and increase the oil drainage area.

Reasonable determination of nozzle pressure drop:

For CO ₂ gas, after passing through the throttling nozzle, the pressure drops sharply and the volume expands rapidly, resulting in the Joule-Thomson cooling effect, which makes the temperature around the nozzle drop sharply. Security risks. The temperature drop is related to parameters such as gas displacement, nozzle diameter, upstream and downstream pressure of the nozzle, and _CO2 temperature upstream of the nozzle. Therefore, the above parameters should be reasonably selected during the fracturing process. The following examples illustrate.

Assuming that the upstream temperature of the nozzle at a depth of 1900m is constant at 340K, the pressure downstream of the nozzle (bottom hole) is 10MPa, and the diameter of the nozzle is 4mm, try to calculate the downstream temperature of the nozzle when the upstream pressure of the nozzle is 14, 18, 22, and 26MPa respectively.

According to the formula The downstream temperature of the nozzle can be obtained. The k value (isentropic index) of _CO2 in the table is 1.28, and the downstream temperature can be calculated at the upstream temperature of different nozzles by substituting the corresponding data, as shown in Table 1:

T _dn - temperature downstream of the nozzle, K; T _up - temperature upstream of the nozzle, K; P _dn - pressure downstream of the nozzle, MPa; P _up - pressure upstream of the nozzle, MPa.

Table 1

Upstream pressure (MPa) 14 18 twenty two 26 27.2 Downstream temperature (K) 315.87 298.98 286.14 275.87 273.15

It can be seen from the data in Table 1 that when the upstream pressure of the nozzle reaches 27.2MPa, the bottomhole temperature can drop to freezing point, that is, under the above conditions, the maximum pressure difference of the nozzle is designed to be 17.2MPa. In the actual fracturing process, the injection pressure can be reduced by reducing the pump displacement, thereby reducing the nozzle pressure difference. Those skilled in the art can determine the size of the pump displacement according to the required pressure difference. Generally, it can be controlled as 5-9 squares/minute.

The specific embodiments described above have further described the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above descriptions are only specific embodiments of the present invention and are not intended to limit the scope of the present invention. Protection scope, within the spirit and principles of the present invention, any modification, equivalent replacement, improvement, etc., shall be included in the protection scope of the present invention.

Claims (10)

1. coiled tubing supercritical CO ₂Spray the method for pressure break, it is with supercritical CO ₂Oil reservoir is sprayed the method for pressure break for fracturing fluid.

2. method according to claim 1, wherein, this method may further comprise the steps:

Clear well is handled: utilize clear water or flushing fluid circulation well cleanup, and use the drift size gauge tool drifting;

Perforation is handled: with liquid CO ₂Mix to obtain fluid-mixing with jet with abrasive material mutually, fluid-mixing is transported in the oil well, and carry out abrasive perforating by the injection fracturing device in the oil well and make reservoir produce the duct by coiled tubing;

Frac treatment: after the foreign material removing with the oil well bottom, continue the pure liquid CO of input ₂, this liquid state CO ₂In oil well, become supercritical CO ₂Fluid also carries out pressure break along described duct to reservoir, makes reservoir produce the crack;

Support and handle: proppant is sneaked into liquid CO ₂In, make it with liquid CO ₂Enter in the crack that is produced, when the proppant that enters the crack reaches predetermined quantity, stop to sneak into proppant, stop to import liquid CO ₂, finish one section fracturing work.

3. method according to claim 2, wherein, this method is further comprising the steps of:

Isolation processing: input insulating liquid, shutoff crack and duct;

To carry on the coiled tubing, repeat perforation processing, frac treatment and support treatment step then, finish next section fracturing work to next section pressure break position.

4. according to claim 2 or 3 described methods, wherein, when the pressure break reservoir in the oil well was heavy crude reservoir or coal bed gas, shale gas reservoir, this method was included in also that the row of returning after the frac treatment handles or the step of vexed well processing.

5. method according to claim 2, wherein, described perforation is handled and is comprised following concrete steps:

To be stored in CO by pipeline ₂Liquid CO in the tank car ₂Be transported to fracturing blender truck, with jet abrasive material and liquid CO ₂Mix obtaining fluid-mixing, be transported to fracturing unit truck through high pressure line then;

By fracturing unit truck with liquid CO ₂Send into coiled tubing with the fluid-mixing that jet is formed with abrasive material via high pressure line, and the injection fracturing device that arrives the oil well bottom carries out abrasive perforating and makes reservoir produce the duct.

6. according to claim 2 or 5 described methods, wherein, in described fluid-mixing, described jet accounts for the 5-8wt% of described fluid-mixing weight with the addition of abrasive material, and it is 60 orders-120 purpose pomegranate stone sand that described jet is preferably fineness with abrasive material.

7. method according to claim 5 wherein, is stored in CO ₂Liquid CO in the tank car ₂Pressure be 3-5MPa, temperature is-20 ℃ to 5 ℃.

8. method according to claim 2, wherein, described frac treatment may further comprise the steps:

The well mouth of oil well place is provided with the annulus back pressure valve, and well head pressure is controlled at 5-8MPa, makes the CO of oil well bottom ₂Foreign material are removed;

Continue the pure liquid CO of input by coiled tubing ₂Perhaps import pure liquid CO simultaneously by coiled tubing and annular space ₂, this liquid state CO ₂To change supercritical CO in a certain degree of depth of pit shaft ₂Fluid, this supercritical CO ₂Fluid enters the duct and reservoir is carried out pressure break, makes reservoir produce the crack.

9. method according to claim 3, wherein, described support is handled and be may further comprise the steps:

When fracture extension behind predetermined length, by fracturing blender truck proppant is sneaked into liquid CO ₂In, make it with liquid CO ₂Enter in the crack that is produced, the crack is supported;

When the proppant that enters the crack reaches predetermined quantity, stop to sneak into proppant, stop to import liquid CO ₂, finish one section fracturing work;

The addition of described proppant accounts for described liquid CO ₂With the 5-20wt% of described proppant quality sum, described proppant is preferably the hollow ceramic proppant.

10. method according to claim 2, wherein, this method also is included in liquid towards CO in the perforation processing ₂The step that the fluid-mixing of forming with abrasive material with jet heats.

Images