CN106089166A - A CO2 foam huff and puff method for enhanced recovery in tight oil reservoirs - Google Patents

Abstract

The invention relates to a method for _CO2 foam huff and puff in tight oil reservoirs to enhance recovery. The method injects high-pressure _CO2 slugs into the fractured tight oil reservoirs. At this time, the _CO2 mainly fills the fractures, and the well is soaked. , through extraction, volume expansion, dissolution and viscosity reduction, etc., the crude oil enters the fracture from the matrix; after the wellhead pressure is stabilized, the well is opened for production, and during the formation seepage process, CO ₂ contacts with the surfactant aqueous solution and foams, and the CO ₂ foam displaces the crude oil It flows from the fracture to the wellbore, and finally to the surface. This method does not foam during the CO ₂ injection process, preventing the generated CO ₂ foam from displacing crude oil to the deep part of the oil reservoir; while CO ₂ contacts with the surfactant aqueous solution during production can form foam in the formation, which can pass through the CO ₂ foam Mechanisms such as oil washing in fractures and effective control of CO ₂ mobility allow more crude oil in fractures to be displaced to the wellbore, improving the recovery of tight oil layers.

Description

A CO2 foam huff and puff method for enhanced recovery in tight oil reservoirs

technical field

The invention relates to a method for enhancing recovery by _CO2 foam huff and puff in tight oil reservoirs, and belongs to the technical field of oil and gas field development engineering.

Background technique

The increasingly serious environmental problems prompt people to pay more attention to the impact of CO ₂ on the environment, and at the same time place high hopes on the development of CO ₂ capture technology. The Chinese government has also made a promise: by 2020, the CO ₂ emission per unit of GDP will be reduced by 40-45% on the basis of 2005. The use of CO ₂ flooding can realize CO ₂ capture, storage, utilization, etc., and reduce the content of CO ₂ in the atmosphere.

Tight oil refers to oil stored in tight sandstone, tight carbonate rock and other reservoirs with matrix permeability less than or equal to 0.2mD (air permeability less than 2mD). Single wells in tight oil reservoirs generally have no natural productivity or natural productivity is lower than the lower limit of industrial oil flow, but industrial oil production can be obtained under certain economic conditions and technical measures. According to preliminary estimates, the tight oil resources in various exploration areas in China can reach (121.5-200)×10 ⁸ t. In recent years, with the widespread application of fracturing wells in various tight oil regions in China, significant development results have been achieved, and tight oil has also become an important replacement resource for increasing reserves and production in various oil regions. For fracturing wells in tight oil reservoirs, on the one hand, due to the relatively tight reservoir matrix, the fluid is difficult to flow, resulting in a sharp decline in the production of fracturing wells, and energy supplementation is urgently needed; on the other hand, due to large-scale artificial fracturing, underground seepage If water injection is used for development, the injected water will easily rush along the fractures, and the injected water will not be able to enter the matrix pores, thus affecting the production effect of the fractured well. Therefore, it is necessary to propose a new energy supplement method for fracturing wells. However, at present, there are few techniques and methods for replenishing formation energy in tight oil reservoirs at home and abroad, resulting in fewer ways to replenish energy for fracturing wells.

A large number of laboratory studies and field tests have been carried out on CO ₂ huff and puff to improve single well oil recovery in low permeability reservoirs. The results show that CO ₂ huff and puff can enhance production stimulation of low production wells. However, compared with low-permeability reservoirs, the permeability of tight oil reservoirs is lower, the heterogeneity of tight oil matrices and fractures is greater, the energy of depletion recovery decreases rapidly, and the effect of CO ₂ huff and puff is limited, so it is necessary to carry out effective mobility analysis. control. If only CO ₂ huff and puff is used to improve the recovery of tight oil reservoirs, CO ₂ gas channeling is prone to occur, the mobility of gas cannot be effectively controlled, and the effect of plugging and displacement cannot be exerted, and the effect is not obvious enough. If water flooding is used alone, it can only fill fractures but cannot spread to the matrix, and oil flooding and recovery cannot be performed, which is a waste of resources.

Contents of the invention

Aiming at the deficiencies of the existing tight oil reservoir development technology, the invention provides a method for enhancing recovery by CO ₂ foam huff and puff in tight oil reservoirs.

Summary of the invention:

The method of the present invention injects a high-pressure _CO2 slug into the tight oil reservoir after fracturing. At this time, the _CO2 mainly fills the cracks, and then injects a small amount of aqueous solution slugs dissolved in surfactants. When injecting, the _CO2 and the aqueous solution do not contact, no foam will be generated; after the injection of the surfactant aqueous solution, the well will be soaked, and the aqueous solution slug can continue to push the CO ₂ slug to migrate to the deep, which is conducive to the mass transfer and diffusion of CO ₂ from the fracture to the matrix, through extraction and volume expansion , dissolution and viscosity reduction, etc., the crude oil enters the fracture from the matrix; after the wellhead pressure is stabilized, the well is opened for production. During the seepage process of the formation, CO ₂ contacts and foams with the surfactant aqueous solution, and the CO ₂ foam displaces the crude oil and flows from the fracture to the wellbore. Finally mined to the ground. This method does not foam during the CO ₂ injection process, preventing the generated CO ₂ foam from displacing crude oil to the deep part of the oil reservoir; while CO ₂ contacts with the surfactant aqueous solution during production can form foam in the formation, which can pass through the CO ₂ foam Mechanisms such as oil washing in fractures and effective control of CO ₂ mobility allow more crude oil in fractures to be displaced to the wellbore, improving the recovery of tight oil layers.

Detailed description of the invention:

Technical scheme of the present invention is as follows:

A kind of tight oil reservoir CO ₂ The method for foam huff and puff to enhance recovery, comprising steps as follows:

(1) High-pressure CO ₂ slug injection: inject a CO ₂ slug of 0.01PV to 0.1PV under reservoir conditions into tight oil reservoirs through the wellbore to fill the fractures after fracturing;

( ₂ ) Surfactant aqueous solution slug injection: The surfactant aqueous solution is fully mixed and then injected into the tight oil layer. The mass concentration of the active agent aqueous solution is 0.1-1.0%;

(3) Simmering diffusion: After injecting the aqueous surfactant solution into the tight oil layer, the well is soaked, and the pressure change during the soaking process is recorded. During the soaking process, CO ₂ undergoes mass transfer and diffusion from the fracture to the matrix, through extraction and volume expansion. , Dissolution and viscosity reduction, the crude oil enters the fracture from the matrix;

(4) Open well production: After the wellhead pressure is stabilized, the well is opened for production. During the seepage process of the formation, CO ₂ contacts and foams with the surfactant aqueous solution, effectively controlling the CO ₂ mobility, and the CO ₂ foam displaces crude oil from the fracture to flow into the wellbore , and finally to the ground.

Preferably in the present invention, after one round of CO ₂ foam huff and puff is completed, steps (1) to (4) are repeated to perform 2-5 rounds of huff and puff of CO 2 to increase the oil recovery rate of tight oil reservoirs.

In the present invention, preferably, the injection amount of the CO ₂ slug is 0.03PV-0.08PV.

Preferably in the present invention, in step (2), the surfactant is di-(2-ethylhexyl)-sodium sulfosuccinate (AOT), and the mass concentration of the surfactant aqueous solution is 0.3-0.8%.

Preferably in the present invention, in step (2), the ratio of the volume of the surfactant aqueous solution to the volume of CO under injected reservoir conditions is 1: ₂ .

Preferably in the present invention, in step (3), the soaking time is 5 hours to 10 days, preferably, the soaking time is 7 days.

Preferably in the present invention, in step (1), the CO ₂ slug is injected into the tight oil reservoir from the wellbore through the surface pressurization equipment.

In the present invention, no CO ₂ foam is generated after the injection of the high-pressure CO ₂ slug and the surfactant aqueous solution slug until the brine process, which can effectively prevent the CO ₂ foam generated in the early stage from displacing the crude oil to the deep part of the reservoir, causing the crude oil to be difficult to mining.

In the present invention, when the huff and puff wells are opened for production, high-pressure CO ₂ and surfactant aqueous solution contact at the fractures to generate a large amount of foam, which can effectively control the CO ₂ fluidity, prevent CO ₂ gas channeling, and increase the spread of CO ₂ foam Volume and oil washing efficiency, effectively displacing crude oil in fractures to the wellbore.

The advantages of the present invention are:

1. The _CO2 foam huff and puff process method for improving oil recovery in tight oil reservoirs of the present invention can effectively solve the problems of tight oil reservoirs where water cannot be injected into the matrix and high-pressure _CO2 gas channeling, and can effectively control the flow of supercritical _CO2 fluid degree, give full play to the role of CO ₂ solution flooding and foam washing, and greatly increase the recovery of tight oil reservoirs.

2. In the method of the present invention, after the injection of the high-pressure _CO2 slug and the surfactant aqueous solution slug, no _CO2 foam is generated until the brine process, which can effectively prevent the _CO2 foam generated in the early stage from displacing the crude oil to the deep part of the reservoir, making it difficult to extract crude oil. When the huff and puff wells are in production, CO ₂ and surfactant aqueous solution contact the fractures to produce a large amount of foam, which can not only effectively control the CO ₂ mobility, prevent CO ₂ gas channeling, but also increase the swept volume of CO ₂ foam and wash oil Efficiency, the crude oil in the fracture is displaced to the wellbore and recovered to the surface.

CO ₂ foam huff and puff can enter the reservoir matrix from fractures through CO ₂ mass transfer and diffusion, which can not only effectively replenish formation energy, but also prevent CO ₂ gas channeling, improve mobility ratio, greatly increase swept volume and oil washing efficiency, and thus Enhanced oil recovery.

Description of drawings:

Fig. 1 is a schematic diagram of the technical method of _CO2 foam huff and puff in tight oil reservoirs of the present invention to enhance recovery, and the pressurization equipment is a high-pressure _CO2 storage tank, which is the prior art.

In the figure, 1. High-pressure _CO2 storage tank; 2. Surface injection device; 3. Injection well; 4. Foaming agent aqueous solution storage tank; 5. Horizontal well; 6. Fracturing fracture; 7. Tight oil reservoir.

Fig. ₂ is a histogram showing the effect of CO ₂ froth huff and huff on recovery factor for 4 rounds of CO ₂ huff and puff in the method of the present invention and CO 2 huff and puff in the prior art.

detailed description

The present invention will be described in detail below in conjunction with some embodiments and accompanying drawings, but is not limited thereto.

Example 1

A method for _CO2 foam huff and puff in tight oil reservoirs to enhance recovery, the process is shown in Figure 1, comprising a high-pressure _CO2 storage tank 1, the outlet of the high-pressure _CO2 storage tank 1 is connected to a ground injection device 2 through a pipeline, The ground injection device 2 and the injection well 3 are connected through pipelines, and the foaming agent aqueous solution storage tank 4 is connected with the ground injection equipment 2 through pipelines, and then connected with the injection well 3 .

Specific steps are as follows:

(1) High-pressure CO ₂ slug injection process: A 0.03PV liquid CO ₂ slug is injected into the tight oil reservoir from the wellbore through the surface pressurization equipment, mainly filling the fractures;

(2) Surfactant aqueous solution slug injection process: Dissolve di-(2-ethylhexyl)-sodium sulfosuccinate (AOT) in water, mix well and inject into tight oil layer. The CO ₂ slug volume ratio is 1:2, and the surfactant mass concentration is 0.3%;

( ₃ ) Simmering diffusion process: After injecting the surfactant aqueous solution into the tight oil layer, carry out a week of simmering, and record the pressure changes during the simmering process. , dissolution and viscosity reduction, etc., the crude oil enters the fracture from the matrix;

(4) Production process of well opening: After one week, the wellhead pressure will no longer decrease, and the well will start production. Since CO ₂ foams in contact with the surfactant aqueous solution in the formation, the CO _{2 mobility can be effectively controlled, and the CO 2 foam} will displace crude oil from fractures. Flow to the wellbore, and finally to the surface;

(5) After one round of CO ₂ foam huff and puff is over, repeat steps (1) to (4) several times to realize multiple rounds of CO ₂ foam huff and puff, and greatly increase the recovery of tight oil layers.

Example 2

The experimental device for simulating CO ₂ foam huff and puff displacement of crude oil in tight oil reservoirs mainly consists of core holders, back pressure valves, pressure gauges, high-pressure plunger pumps, gas flow meters, oil and gas separators, gas cylinders, intermediate containers, and thermostats equal composition, the whole experiment simulates reservoir conditions and is carried out at a constant reservoir temperature.

The specific steps are:

(1) Dissolving bis-(2-ethylhexyl)-sodium sulfosuccinate (AOT) in formation water and pouring it into an intermediate container with a mass concentration of 0.3%;

(2) Put the oil-bearing tight oil core obtained by drilling into the core holder for fracturing and creating fractures, and adjust the temperature of the constant temperature box to the formation temperature of 80°C for more than 6 hours to ensure that the core and crude oil evenly reach the formation temperature;

(3) Determine the back pressure according to the actual formation pressure, and set it to 20MPa;

(4) According to the design, inject 0.05PV high-pressure CO ₂ slug, then inject 0.025PV surfactant aqueous solution slug, close the valve and start the well, and record the pressure change during the well;

(5) Open the inlet valve after soaking the well for 5 hours, record the gas production and oil production, and observe and record the pressure changes;

(6) After one round of CO ₂ foam huffing and puffing, repeat steps (1) to (5) three times to achieve 4 rounds of CO ₂ froth huffing and puffing;

(7) Compared with CO ₂ huff and puff to displace crude oil, inject 0.05PV high-pressure CO ₂ , then do not inject water, close the valve to brine the well, and record the pressure changes during the brine process and the oil and gas production of the well, and also Perform 4 rounds of _CO puffs.

The experimental results are shown in Figure 2. Comparing CO ₂ foam huff and CO ₂ huff, the former has a higher recovery rate. After 4 huff and puff rounds, the total recovery rate reached 20.43%, and a better recovery effect was achieved.

Claims (7)

1. a kind of tight oil reservoir CO _The method that foam huff and puff improves the recovery rate, comprises steps as follows: (1) High-pressure CO ₂ slug injection: inject a CO ₂ slug of 0.01PV to 0.1PV under reservoir conditions into tight oil reservoirs through the wellbore to fill the fractures after fracturing; ( ₂ ) Surfactant aqueous solution slug injection: The surfactant aqueous solution is fully mixed and then injected into the tight oil layer. The mass concentration of the active agent aqueous solution is 0.1-1.0%; (3) Simmering diffusion: After injecting the aqueous surfactant solution into the tight oil layer, the well is soaked, and the pressure change during the soaking process is recorded. During the soaking process, CO ₂ undergoes mass transfer and diffusion from the fracture to the matrix, through extraction and volume expansion. , Dissolution and viscosity reduction, the crude oil enters the fracture from the matrix; (4) Open well production: After the wellhead pressure is stabilized, the well is opened for production. During the seepage process of the formation, CO ₂ contacts and foams with the surfactant aqueous solution, effectively controlling the CO ₂ mobility, and the CO ₂ foam displaces crude oil from the fracture to flow into the wellbore , and finally to the ground. 2. the tight oil reservoir CO according to claim ₁ The method for foam huff and puff to enhance oil recovery is characterized in that, after one round of _CO2 foam huff and puff ends, repeat steps (1) to (4) , to carry out 2-5 rounds of huffing and puffing to improve the oil recovery rate of tight oil reservoirs. 3. The method for CO ₂ foam huff and puff in tight oil reservoirs to enhance oil recovery according to claim 1, characterized in that the CO ₂ slug injection rate is 0.03PV-0.08PV. 4. tight oil reservoir CO according to claim ₁ The method for foam huff and puff to enhance recovery is characterized in that, in step (2), surfactant is two-(2-ethylhexyl)-sulfonic acid Sodium succinate (AOT), the mass concentration of the surfactant aqueous solution is 0.3-0.8%. 5. tight oil reservoir CO according to claim ₁ The method for foam huff and puff to enhance recovery is characterized in that, in step (2), the volume of the surfactant aqueous solution is the same as the injected oil reservoir condition _CO Volume The ratio is 1:2. 6. the tight oil reservoir CO according to claim ₁ The method for foam huff and puff to enhance oil recovery is characterized in that, in step (3), the soaking time is 5 hours-10 days. 7. the tight oil reservoir CO according to claim ₆ The method for foam huff and puff to enhance oil recovery is characterized in that, in step (3), the soaking time is 7 days.