Method for improving recovery of shale oil well group by using carbon dioxide huff and puff + drive

Abstract

The application discloses a method for improving the recovery ratio of a shale oil well group by using carbon dioxide huff and puff + drive, and recovery steps include: selecting suitable well groups according to well selection principles; designing a recovery scheme according to the number of wells contained in the well group and the interwell connectivity, and implementing the scheme in two steps, and then implementing multiple cycles; designing two injection modes of tubing positive injection and casing reverse injection according to the different structures of oil well pipe columns; designing the injection slug sequence and dosage according to the pipe column material conditions; carrying out field construction according to the implementation mode designed in step 2, and the design dosage and injection sequence of each slug in step 4; and implementing well group blowout according to the carbon dioxide huff and puff blowout operation specification, and opening wells for production according to the original lifting process of each well. The application uses the double action of carbon dioxide huff and carbon dioxide drive, expands the action range of carbon dioxide, not only uses the remaining oil around single wells, but also starts the interwell remaining oil, maximally improves the shale oil recovery ratio, and achieves good oil increasing effect.

Description

Technical Field

[0001] This invention relates to the field of oil and gas field development technology, specifically to a method for improving the recovery rate of shale oil well groups using carbon dioxide huff and puff + flooding. Background Technology

[0002] Shale oil reservoirs have a high proportion of nanoscale pores and throats, which are even finer. The seepage capacity mainly comes from submicron-level pores and throats. They have high displacement pressure, low maximum mercury saturation, poor availability, and are difficult to develop.

[0003] The current development method is horizontal wells + segmented multi-cluster large-scale volumetric fracturing, which mostly does not have artificial energy supplementation. In the early stage, it produces high output through self-flowing, but the self-flowing period is short and the output declines rapidly. Artificial lift can rapidly increase output, but the dilemma of short high-production period and rapid decline still exists. There is an urgent need for replacement technologies to increase production and efficiency and improve shale oil recovery rate.

[0004] Therefore, a method to improve the recovery rate of shale oil well groups by using carbon dioxide huff and puff combined with flooding is proposed to address the above problems. Summary of the Invention

[0005] The purpose of this invention is to provide a method for improving the recovery rate of shale oil well groups using carbon dioxide huff and puff + flooding, wherein the recovery steps include:

[0006] Step 1: Select a suitable well group according to the well selection principles;

[0007] Well selection principles include:

[0008] (11) The well group had a certain production in the early stage;

[0009] (12) Production is declining rapidly and is currently low;

[0010] (13) The target formation in the well group has good connectivity or a displacement relationship can be established;

[0011] A well group refers to a shale oil well group containing two or more oil wells, or a shale oil well group or development unit with an injection-production relationship.

[0012] Step 2: Design an oil production plan based on the number of wells in the well group and the connectivity between wells, and implement it in two steps, followed by multiple rounds of cyclical implementation;

[0013] The well group consists of three wells, and the specific operating methods include:

[0014] Step 21: Inject gas into the intermediate well. If the connectivity between the two wells is good, shut in the well and apply pressure. Once all three wells reach the conditions for venting, vent and produce oil simultaneously. If the connectivity between the two wells is average and only a micro-permeability displacement relationship can be established, slowly inject carbon dioxide into the intermediate well and continue producing oil from the two wells until the oil production is lower than the production before the measures. Then, the first step is considered to have failed.

[0015] Step 22: After the first step fails, gas injection is carried out on the two wells. If the well connectivity is good, the middle well is shut in and pressure treatment is carried out. After all three wells reach the conditions for venting, they are vented and oil is produced at the same time. If the well connectivity is average and only a non-smooth displacement relationship can be established, carbon dioxide is slowly injected into the two wells and the middle well continues to produce oil until the oil production is lower than the oil production before the measures are taken. Then the second step is considered to have failed.

[0016] Step 23: Implement steps 21 and 22 in sequence, and combine them with the oil production effect of the well group to carry out multiple rounds of implementation. The order of steps 21 and 22 can be adjusted according to the structural location of each well and production needs. For multi-well groups, the implementation method is not limited to the above two. The injection wells and production wells can be rotated according to production and research needs.

[0017] Step 3: Based on the different oil well tubing string structures, design two injection methods: tubing forward injection and casing reverse injection;

[0018] Positive injection: This means that all injection materials entering the wellbore and oil layer are injected through the tubing valve and the internal space of the tubing.

[0019] Reverse injection: This means that the injection is carried out from the annulus. All the injection material that enters the wellbore and the oil layer enters the wellbore and the oil layer through the casing valve and the annulus.

[0020] Step 4: Design the injection sequence and dosage of slugs based on the tubing material;

[0021] Step 41: Based on the tubing material, if the target well group tubing is made of non-low temperature resistant and non-carbonic acid resistant materials, it is necessary to consider that carbon dioxide is a low temperature liquid. The low temperature generated by the venting and throttling will cause freezing damage to the tubing and carbonic acid formed by carbon dioxide when it comes into contact with water, which will cause corrosion of the downhole tubing. Protective fluid and corrosion inhibitors need to be added before carbon dioxide injection to reduce low temperature damage and carbonic acid corrosion. At the same time, considering the water-sensitive characteristics of shale oil reservoirs, anti-swelling agent needs to be added to the aqueous solution entering the oil layer to prevent water-sensitive damage to shale oil reservoirs. The injection slug sequence is designed as a six-slug injection: protective fluid slug, anti-swelling agent displacement slug, pre-corrosion inhibitor slug, main carbon dioxide slug, post-corrosion inhibitor slug, and clear water displacement slug.

[0022] If the target well string is made of a low-temperature resistant and carbonic acid resistant material, then the protective fluid and corrosion inhibitor slug section can be omitted.

[0023] If the target well string is made of a low-temperature resistant material, the protective fluid slug section is omitted.

[0024] If the target well string is made of a carbonic acid resistant material, then the corrosion inhibitor slug section is omitted;

[0025] The functions and dosages of each plug are as follows:

[0026] ① Protective fluid slug

[0027] The protective fluid is a low-temperature resistant liquid. Its function is to fill the space of the protected oil pipe or oil casing annulus and protect the oil pipe or oil casing annulus from low-temperature freezing damage caused by liquid carbon dioxide. The designed injection volume is the volume inside the protected oil pipe or the volume of the oil casing annulus. Considering losses, the amount can be increased appropriately.

[0028] ② Anti-swelling agent displacement block

[0029] The anti-swelling agent displacement slug has two functions. First, as a displacement slug, it can replace the protective fluid into the protective space, reducing the amount of protective fluid used and saving money. Second, after the anti-swelling agent is added, the slug is replaced by the subsequently injected corrosion inhibitor slug and carbon dioxide into the oil layer, which can reduce water-sensitive damage to shale oil reservoirs. The amount of anti-swelling agent displacement slug used is the volume in the tubing or the annulus volume. Considering losses, the amount can be appropriately increased.

[0030] There are differences in the dosage and protection location for protective fluid slugs and anti-swelling agent displacement slugs under forward and reverse injection methods:

[0031] In the forward injection method, a protective fluid slug and an anti-swelling agent displacement slug are injected into the tubing until the protective fluid is backflowed from the casing. Then, a pre-corrosion inhibitor slug is injected, followed by the main carbon dioxide slug. Finally, a post-corrosion inhibitor slug and a water displacement slug are injected to complete the injection process. In this method, the protective fluid volume is the volume of the annulus up to the end of the tubing, and the anti-swelling agent displacement slug volume is the volume inside the tubing. The protective fluid slug fills the annulus, protecting it from low-temperature freezing damage caused by the liquid carbon dioxide injected into the tubing. The corrosion inhibitor slug forms a protective film on the inner wall of the tubing, protecting the metal from carbon dioxide corrosion.

[0032] In the reverse injection method, a protective fluid slug and an anti-swelling agent displacement slug are injected into the annulus until the protective fluid is backed out of the tubing. Then, a pre-corrosion inhibitor slug is injected, followed by a carbon dioxide main slug, and finally a post-corrosion inhibitor and a water displacement slug to complete the entire injection process. In this injection method, the volume of protective fluid used is the volume up to the tubing, and the volume of the anti-swelling agent displacement slug is the volume up to the end of the tubing in the annulus. The protective fluid fills the internal space of the tubing, protecting the tubing from freezing damage caused by liquid carbon dioxide at low temperatures. The pre-corrosion inhibitor forms a protective film on the inner wall of the casing and the outer wall of the tubing in the annulus, protecting the casing and tubing from carbon dioxide corrosion.

[0033] ③ Pre-corrosion inhibitor slug

[0034] The pre-installed corrosion inhibitor slug can form an isolation between the anti-swelling agent displacement slug and the main carbon dioxide slug, preventing the anti-swelling agent slug from freezing upon contact with low-temperature carbon dioxide. At the same time, the corrosion inhibitor can form a protective film on the metal surface, isolating the carbonic acid from contact with the metal surface and protecting the tubing or casing from carbonic acid corrosion. The designed injection volume is 1.0 cubic meters per meter of perforated section, and the dosage can be adjusted according to the actual effect of the protective film.

[0035] ④ Carbon dioxide main slug

[0036] First, carbon dioxide enters the shale oil reservoir and, after mixing with the shale oil, reduces the viscosity of the crude oil and increases the fluidity of the shale oil, making it easier for the shale oil to flow to the wellhead and be lifted, thereby increasing shale oil production. Second, after entering the reservoir, carbon dioxide expands in volume to replenish the energy of the oil layer, and has the functions of diffusion, dissolution, and extraction of shale oil in the matrix, further improving the shale oil recovery rate.

[0037] The amount of carbon dioxide used is determined based on the fracturing volume of a single well, the length of the horizontal section, the thickness of the shale oil layer, the content of free hydrocarbons, the content of organic carbon, and the radius of action of carbon dioxide. For well groups that are implemented in multiple rounds, the injection volume is increased in each round to expand the radius of action of carbon dioxide in each round and further improve the recovery rate of shale oil well groups.

[0038] ⑤ Post-corrosion inhibitor slug

[0039] The post-corrosion inhibitor slug forms an isolation between the main carbon dioxide slug and the water displacement slug, preventing the water displacement slug from freezing due to contact with carbon dioxide at low temperatures. At the same time, the corrosion inhibitor can form a protective film on the metal surface, isolating carbonic acid from contact with the metal surface and protecting the tubing or casing from carbonic acid corrosion.

[0040] The designed injection volume can be less than the amount of pre-corrosion inhibitor slug used, and should be adjusted according to the actual effect of the protective film.

[0041] ⑥. Use clear water to replace the plug.

[0042] The function of the water displacement slug is to displace residual carbon dioxide in the wellbore into the oil layer, thereby preventing carbon dioxide from remaining in the wellbore and causing corrosion. At the same time, it also prevents the carbon dioxide remaining in the wellbore from being heated by the oil layer and vaporizing, which would increase the pressure in the wellbore and cause leakage risk.

[0043] The amount of water used to replace the slug is one times the volume of the tubing or the annulus. Considering losses, the amount can be increased appropriately.

[0044] Step 5: Based on the implementation method designed in Step 2, and the designed dosage and injection sequence of each plug segment in Step 4, carry out on-site construction;

[0045] Step 51: For well groups with good inter-well connectivity, the production wells are shut in and pressure-bearing treatment is carried out. The pressure-bearing capacity is based on the lowest pressure-bearing capacity among all wellheads in the well group. The injection wells inject carbon dioxide at an appropriate discharge rate until the designed injection volume is reached. After the injection construction is completed, the well group is simultaneously sealed and diffused.

[0046] Step 52: For well groups that can only establish micro-permeability, inject small-volume water into the injection well to slowly displace the remaining oil between wells and move it towards the production well. If the pressure of the production well does not change much or rises slightly, the injection can continue slowly. Adjust the production capacity of the production well in a timely manner according to the production situation. If the pressure-bearing capacity of the production well is reached, stop injecting carbon dioxide into the injection well, and implement the injection of corrosion inhibitor slug and water displacement slug to complete the injection construction well and implement well suffocation diffusion.

[0047] Step 6: Implement well group venting according to the carbon dioxide injection and release operation specifications, and start production according to the original lifting process of each well.

[0048] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0049] (1) This invention utilizes the dual effects of carbon dioxide injection and carbon dioxide flooding to expand the scope of carbon dioxide's action. It not only utilizes the remaining oil around a single well but also activates the remaining oil between wells, maximizing the recovery rate of shale oil and achieving better oil enhancement.

[0050] (2) This invention maximizes the role of injected carbon dioxide, improves carbon dioxide utilization, and makes full use of the energy of injected carbon dioxide gas to supplement the energy of single wells and inter-well formations.

[0051] (3) The present invention is highly operable and flexible. Under this method, there is no need for large-scale planning of injection and production well networks. Oil displacement between well groups can be completed by using a small-scale construction team. By using injection wells for churn and puffing, the remaining oil can be utilized to the maximum extent, thereby improving the recovery rate of shale oil. Attached Figure Description

[0052] Figure 1This is a dynamic liquid level data diagram of Embodiment 1 of the present invention. Detailed Implementation

[0053] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention. Example

[0054] Taking a horizontal well group with three interconnected wells in a shale oil block of Dagang Oilfield as an example, the specific implementation steps of this invention are illustrated below:

[0055] (1) Select well group;

[0056] The well group initially had high self-flowing production, with a maximum daily fluid production of 172 m³ and a maximum daily oil production of 50 tons. However, the high-production period was short-lived and declined rapidly, with a significant drop in fluid and oil production. Artificial lift was chosen in the later stages of production, which restored fluid and oil production to some extent, but the problem of rapid decline in production still exists.

[0057] Since this well group is a shale oil reservoir, there are currently no effective means to increase production. Given the multiple advantages of carbon dioxide in oilfield development, such as easy injection, viscosity reduction, and energy enhancement, the method of enhancing oil recovery by using carbon dioxide to reduce crude oil viscosity and increase shale oil fluidity after mixing with shale oil is adopted. After carbon dioxide enters the reservoir, it expands in volume to replenish the energy of the oil layer and has the functions of diffusion, dissolution, and extraction of shale oil in the matrix, thereby further improving the shale oil recovery rate. Based on the connectivity between wells in the well group, it was decided to conduct a field test of the carbon dioxide huff and puff + flooding method for enhancing the recovery rate of shale oil well groups in this well group.

[0058] (2) Design and implementation methods.

[0059] This well group adopts horizontal well segmented fracturing and production, with good inter-well connectivity. Therefore, the well group adopts a coordinated huff and puff and flooding method to utilize the remaining oil between wells and improve oil well productivity. According to the implementation guidance method of this invention, the well group carbon dioxide huff and puff + flooding implementation mode is implemented on three wells.

[0060] The method involves carbon dioxide injection and flushing from the intermediate well, and coordinated well-to-well oil production from both sides.

[0061] (3) Implementation of injection.

[0062] Oil production wells are shut in and pressure-bearing treatment is carried out. The pressure-bearing capacity is based on the lowest pressure-bearing capacity among all wellheads in the well group. Injection wells inject carbon dioxide at an appropriate discharge rate until the designed injection volume is reached.

[0063] (4) Muffled well.

[0064] (5) Flow-out production.

[0065] This method has already been applied in the field, and it has demonstrated good carbon dioxide injection and displacement synergy in terms of oil production status, dynamic fluid level recovery, pressure monitoring, and construction drawing analysis.

[0066] As attached Figure 1 As shown, the dynamic liquid level is related to the low-level liquid supply capacity. A decrease in the dynamic liquid level value indicates an increase in the low-level liquid supply capacity, while an increase in the dynamic liquid level value indicates a decrease in the low-level liquid supply capacity. The dynamic liquid level was relatively deep on June 2, 2021, i.e., before implementation. The dynamic liquid level data from June 18, 2021 onwards was shallower than before. As production progressed, the dynamic liquid level continued to decline.

[0067] The experiment achieved good oil production increase, with a total increase of 1,979 tons in the well group; of which the intermediate huff and puff wells increased oil production by 851 tons and the two side production wells increased oil production by 1,128 tons.

[0068] The well group has been producing for more than 15 days, and the dynamic fluid level monitoring data shows that the dynamic fluid level of each well has risen after the implementation of CO2 huff and puff + drive. All wells have been fully replenished with energy and have a significant oil production effect. The middle CO2 huff and puff well has been producing for 25 days, and the oil wells on both sides have been producing for 18 days. The dynamic fluid level of the wells on both sides has increased from no level before implementation to more than 460 meters before the release, and the casing pressure has increased significantly and the fluid supply is sufficient.

Claims

1. A method for enhancing the recovery rate of shale oil well groups using carbon dioxide huff and puff combined with flooding, characterized in that: The harvesting steps include: Step 1: Select a suitable well group according to the well selection principles; Step 2: Design an oil production plan based on the number of wells in the well group and the connectivity between wells, and implement it in two steps, followed by multiple rounds of cyclical implementation; Step 3: Based on the different oil well tubing string structures, design two injection methods: tubing forward injection and casing reverse injection; Step 4: Design the injection sequence and dosage of slugs based on the tubing material; Step 4 specifically involves: Step 41: Based on the tubing material, if the target well group tubing is made of non-low temperature resistant and non-carbonic acid resistant materials, it is necessary to consider that carbon dioxide is a low temperature liquid. The low temperature generated by the venting and throttling will cause freezing damage to the tubing and carbonic acid formed by carbon dioxide when it comes into contact with water, which will cause corrosion of the downhole tubing. Protective fluid and corrosion inhibitors need to be added before carbon dioxide injection to reduce low temperature damage and carbonic acid corrosion. At the same time, considering the water-sensitive characteristics of shale oil reservoirs, anti-swelling agent needs to be added to the aqueous solution entering the oil layer to prevent water-sensitive damage to shale oil reservoirs. The injection slug sequence is designed as a six-slug injection: protective fluid slug, anti-swelling agent displacement slug, pre-corrosion inhibitor slug, main carbon dioxide slug, post-corrosion inhibitor slug, and clear water displacement slug. The amounts of plugs used in each segment of step 41 are as follows: ① Protective fluid slug The designed injection volume is the volume inside the oil pipe or the annulus of the oil sleeve to be protected. Considering losses, the amount can be increased appropriately. ② Anti-swelling agent displacement block The designed injection volume is the volume inside the tubing or the volume of the annulus in the tubing. Considering losses, the amount can be increased appropriately. ③ Pre-corrosion inhibitor slug The designed injection volume is 1.0 cubic meters per meter of perforated well section, and the amount can be adjusted according to the actual effect of the protective film. ④ Carbon dioxide main slug The amount of carbon dioxide used is determined based on the fracturing volume of a single well, the length of the horizontal section, the thickness of the shale oil layer, the content of free hydrocarbons, the content of organic carbon, and the radius of action of carbon dioxide. For well groups that are implemented in multiple rounds, the injection volume is increased in each round to expand the radius of action of carbon dioxide in each round and further improve the recovery rate of shale oil well groups. ⑤ Post-corrosion inhibitor slug The designed injection volume can be less than the amount of pre-corrosion inhibitor slug used, and should be adjusted according to the actual effect of the protective film. ⑥. Use clear water to replace the plug. The designed injection volume is 1 times the volume inside the tubing or the volume of the annulus. Considering losses, the amount can be increased appropriately. Step 42: If the target well string is made of a low-temperature resistant and carbonic acid resistant material, then omit the protective fluid and corrosion inhibitor slug part; Step 43: If the target well string is made of a low-temperature resistant material, then omit the protective fluid slug part; Step 44: If the target well string is made of a carbonic acid resistant material, then omit the corrosion inhibitor slug part; Step 5: Based on the implementation method designed in Step 2, and the designed dosage and injection sequence of each plug segment in Step 4, carry out on-site construction; The specific steps in step 5 are as follows: Step 51: For well groups with good inter-well connectivity, the production wells are shut in and pressure-bearing treatment is carried out. The pressure-bearing capacity is based on the lowest pressure-bearing capacity among all wellheads in the well group. The injection wells inject carbon dioxide at an appropriate discharge rate until the designed injection volume is reached. After the injection construction is completed, the well group is simultaneously sealed and diffused. Step 52: For well groups that can only establish micro-permeability, inject small-volume water into the injection well to slowly displace the remaining oil between wells and move it towards the production well. If the pressure of the production well does not change much or rises slightly, the injection can continue slowly. Adjust the production capacity of the production well in a timely manner according to the production situation. If the pressure-bearing capacity of the production well is reached, stop injecting carbon dioxide into the injection well and implement the injection of corrosion inhibitor slug and water displacement slug. After the injection construction is completed, implement well suffocation diffusion. Step 6: Implement well group venting according to the carbon dioxide injection and release operation specifications, and start production according to the original lifting process of each well.

2. The method for improving the recovery rate of shale oil well groups using carbon dioxide huff and puff + flooding according to claim 1, characterized in that: The well selection principles in step 1 include: (11) The well group had a certain production in the early stage; (12) Production is declining rapidly and is currently low; (13) The target layer of the well group has good connectivity or can establish a displacement relationship.

3. The method for improving the recovery rate of shale oil well groups using carbon dioxide huff and puff + flooding as described in claim 1, characterized in that: In step 1, a well group refers to a shale oil well group containing two or more oil wells, or a shale oil well group or development unit with an injection-production relationship.

4. The method for improving the recovery rate of shale oil well groups using carbon dioxide huff and puff + flooding according to claim 1, characterized in that: Step 2 involves a three-well group, and the specific operation method includes: Step 21: Inject gas into the intermediate well. If the connectivity between the two wells is good, shut in the well and apply pressure. Once all three wells reach the conditions for venting, vent and produce oil simultaneously. If the connectivity between the two wells is average and only a micro-permeability displacement relationship can be established, slowly inject carbon dioxide into the intermediate well and continue producing oil from the two wells until the oil production is lower than the production before the measures. Then, the first step is considered to have failed. Step 22: After the first step fails, gas injection is carried out on the two wells. If the well connectivity is good, the middle well is shut in and pressure treatment is carried out. After all three wells reach the conditions for venting, they are vented and oil is produced at the same time. If the well connectivity is average and only a non-smooth displacement relationship can be established, carbon dioxide is slowly injected into the two wells and the middle well continues to produce oil until the oil production is lower than the oil production before the measures are taken. Then the second step is considered to have failed. Step 23: Implement steps 21 and 22 in sequence, and combine them with the oil production effect of the well group to carry out multiple rounds of implementation. The order of steps 21 and 22 can be adjusted according to the structural location of each well and production needs. For multi-well groups, the implementation method is not limited to the above two. The injection wells and production wells can be rotated according to production and research needs.

5. The method for improving the recovery rate of shale oil well groups using carbon dioxide huff and puff + flooding according to claim 1, characterized in that: The specific steps for forward and reverse injections in step 3 are as follows: Positive injection: This means that all injection materials entering the wellbore and oil layer are injected through the tubing valve and the internal space of the tubing. Reverse injection: This means that the injection is carried out from the annulus. All the injection material that enters the wellbore and the oil layer enters the wellbore and the oil layer through the casing valve and the annulus.

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