A drainage gas recovery method based on flow field reconstruction theory

Abstract

The application provides a drainage gas production method based on a flow field reconstruction theory. The method breaks the gas-water flow field distribution under a water cone state by injecting gas, realizes gas production by using the time difference of the gas-water flow field recovery to the water cone state after well opening, and determines the reasonable gas injection amount, the well closing end time and the well opening reasonable gas production intensity by using a numerical simulation method, so as to realize the drainage gas production of deep and super deep water-bearing gas reservoirs. The method has the advantages that the drainage gas production of deep and super deep water-bearing gas reservoirs in the edge and bottom water breakthrough stage is realized, and the drainage gas production of deep and super deep water-bearing gas reservoirs in the edge and bottom water breakthrough stage by other drainage gas production methods cannot be applied. On the other hand, a new theory of "flow field reconstruction" of water-bearing gas reservoirs is provided, the gas-water two-phase seepage theory of water-bearing gas reservoirs is enriched, and the theory is applied to the actual gas reservoir development, so that the theory and practice are closely combined.

Description

Technical Field

[0001] This invention belongs to the field of natural gas extraction technology, specifically relating to a method for gas extraction from deep and ultra-deep gas reservoirs and for drainage based on water flow field reconstruction. Background Technology

[0002] If water production occurs during gas reservoir development, the pressure loss in the wellbore increases, causing the pressure of natural gas reaching the wellhead to be lower than the pressure of the surface pipeline. On the other hand, during the flow of gas and water in the wellbore, the gravity difference between gas and water causes water to fall back, creating back pressure at the bottom of the well and reducing the effective production pressure differential. The combined effect of these two factors leads to a decrease in gas well productivity, ultimately reducing the recovery rate of the gas reservoir. Traditional water-drive gas reservoir development technologies include early water-controlled gas production, followed by drainage gas production combined with water shut-off after water breakthrough. However, these methods are not suitable for the development of deep and ultra-deep gas reservoirs. Deep and ultra-deep gas reservoirs have well-developed fractures, early water breakthrough, severe water production, and are difficult to control for gas production. Water-driven gas reservoir development inevitably leads to edge and bottom water intrusion and conical advance: gas production creates a low-pressure area in the gas field, triggering the overall intrusion of edge and bottom water into the gas reservoir, which then advances along fractures towards the well. Water control and gas production using well point production measures (points) are insufficient to suppress the overall advance trend and intensity of edge and bottom water (surface). Fractures are the main water intrusion pathway for deep and ultra-deep gas reservoirs. After water is encountered, water production rises rapidly, resulting in large water production and ineffective water control and gas production. Deep and ultra-deep gas reservoirs are buried at depths (>5000m), and the energy consumption of liquid lifting for drainage and gas production is high, resulting in poor feasibility. Among all drainage and gas production measures, only the foaming process can reach a depth of about 6000m. However, the temperature of deep and ultra-deep gas reservoirs is generally high, such as the Kesheng 2 reservoir with a temperature greater than 160℃. Existing foaming agents become unstable above 120℃, making it more difficult to select reliable foaming agents. Other drainage and gas production processes (such as tubing optimization, electric submersible pumps, beam pumping units, and plunger gas lift) have a maximum depth of no more than 4000m. Although gas lift can reach 6000m, the pressure requirements for surface pumps are high beyond 6000m, greatly increasing the construction difficulty and drastically reducing economic efficiency. Gas lift is also unsuitable for drainage and gas production in deep and ultra-deep gas reservoirs. Fractures are both pathways for edge and bottom water intrusion and major channels for natural gas flow. Water plugging can easily block both water and natural gas. The large volume of produced water, along with the complex composition of minerals and drilling fluid chemicals, makes surface water treatment difficult and increases the environmental impact assessment burden for reinjection. Therefore, traditional water management and drainage measures for gas reservoirs are extremely challenging to apply in deep and ultra-deep gas reservoirs, and most are unsuitable for the economical and efficient development of these reservoirs. Summary of the Invention

[0003] The purpose of this invention is to provide a method for drainage and gas production in deep and ultra-deep gas reservoirs based on gas-water flow field reconstruction, so as to achieve economical and efficient development of deep and ultra-deep water-bearing gas reservoirs, and at the same time, it can be extended to the development of conventional water-bearing gas reservoirs, thus creating a new concept and method for gas reservoir development.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: (1) Injecting gas drives away the formation water that accumulates near the wellbore and changes the reservoir gas-water flow field. After bottom water coning, formation water accumulates in the formation around the production well, forming a water pocket that isolates natural gas from flowing into the wellbore. A stable and unobstructed formation water flow channel is formed between the gas well and the water body, causing the gas well to be flooded and lose its production capacity. Injecting gas (which can be natural gas or nitrogen) displaces the formation water accumulated around the wellbore and simultaneously creates a reverse effect on the formation water flow field, changing or weakening the intensity and direction of the formation water's reverse intrusion into the wellbore. This causes the formation water flow field to intrude into other parts of the reservoir in the reverse direction, also displacing the remaining gas in the gas reservoir.

[0005] (2) Calculate the gas injection volume for the formation water accumulation around the breakthrough gas well. 1) Collect geological and dynamic data of gas reservoirs, and establish geological models and numerical models of gas reservoirs. 2) Using numerical simulation methods, fit the production history of gas wells, especially the production dynamics from water production to flooding, with a focus on fitting the water production rate. 3) Based on the fitted numerical model of the gas reservoir, design a sensitivity evaluation scheme for the gas injection rate and the time of water-flooded gas wells communicating with the gas-bearing area, and select a reasonable gas injection rate. (3) After completing the reasonable gas injection volume, the gas well is shut in and enters the well shut-in stage. After gas injection, the process begins to gradually displace the accumulated water, which takes a certain amount of time. During this period, the injected water gradually alters the bottom water's trajectory: in the reservoir, the injected gas exerts a drainage effect on the water cone, which gradually transfers to the water body over time. If a dominant channel exists at this point, the formation water at the edge of the cone will advance along both sides. The period from when gas injection stops until the effect of the injected gas reaches the water body is known as the "shutdown" stage.

[0006] This stage mainly monitors changes in wellhead pressure: the optimal wellhead pressure value can be selected through numerical simulation, and the well shut-in stage is stopped when the wellhead pressure reaches this value.

[0007] During the well-blocking stage, the injected gas drives away the formation water accumulated around the well, changes the bottom water trajectory, and disrupts the gas-water flow field: the injected gas enables communication between the gas well and the gas-bearing area; the water intrusion direction changes its direction under the action of the injected gas.

[0008] (4) When the well is opened for gas production, the gas-water flow field enters the reconstruction stage. The time difference of reconstruction is the gas production stage. After gas injection ends and gas production begins, the gas flow moves into the wellbore, and the water cone also begins to resume its conical advance. The period during which the water cone resumes its advance is the gas production period of the gas well: the longer the water cone resumes its advance, the longer the gas production period will be, and the more natural gas will be produced.

[0009] In this process, numerical simulation technology is needed to design a sensitivity study on the gas extraction intensity and the time required for gas-water flow field reconstruction, and to select a reasonable gas extraction intensity to achieve the maximum gas extraction benefit.

[0010] Compared with the prior art, the advantages and positive effects of the present invention are: This invention is the first to propose the concept of flow field reconstruction drainage gas production. Traditional views consider this to be akin to gas injection and huff-and-puff, without clearly defining what happens to the injected gas within the reservoir. This invention elaborates on the mechanism by which injected gas affects the gas-water flow field in the reservoir, and based on this understanding, proposes the concept of flow field reconstruction drainage gas production.

[0011] The innovation of this invention lies in step (2), which proposes a method for calculating the "reasonable gas injection volume of water accumulated in the formation around the gas well", and uses numerical simulation technology to simulate and calculate the critical gas injection volume (minimum gas injection volume) that changes the gas-water flow field in the reservoir.

[0012] The innovation of this invention lies in step (3), which proposes a method for determining the reasonable well-sealing time from a mechanistic perspective. The well-sealing stage is the time from the cessation of gas injection to the reaction of the injected gas with the water body, and the wellhead pressure during this time is used to determine the end time of well-sealing, making it more suitable for practical applications.

[0013] The innovation of this invention lies in step (4), which proposes the concept of "flow field reconstruction" and proposes that the effective drainage and gas collection time for flow field reconstruction is the gas-water flow field reconstruction time difference.

[0014] Other features and advantages of the present invention will become clearer after reading the detailed description of the embodiments of the present invention in conjunction with the accompanying drawings. Attached Figure Description

[0015] The present invention will be further described below with reference to the accompanying drawings and embodiments. In the accompanying drawings: Figure 1 This is a schematic diagram of the steps of the drainage gas extraction method based on flow field reconstruction theory described in this invention; Figure 2 This is a schematic diagram of the bottom water coning of the bottom water gas reservoir described in this invention; Figure 3 This is a schematic diagram of the bottom water cone inlet and subsequent air injection as described in this invention; Figure 4 This is a schematic diagram of the gas-water flow field changes during the well-sealing stage after gas injection, as described in this invention. Figure 5This is a schematic diagram of flow field reconstruction gas production during the gas production stage as described in this invention; Figure 6 It is a gas reservoir geological model; Figure 7 It is a fitting of the water production history of gas wells; Figure 8 It is a time sensitivity analysis of communication in gas-bearing regions; Figure 9 This is a wellhead pressure sensitivity analysis at the end of the well stagnation phase.

[0016] Figure 1 The specific implementation steps of the flow field reconstruction drainage gas production method include: ① injecting gas to change the reservoir gas-water flow field, ② optimizing the gas injection rate, ③ shutting in the well and suffocating the well, and ④ opening the well to produce gas. Figure 2 Illustration: The distribution of gas and water formed after bottom water coning in a bottom water gas reservoir; Figure 3 Schematic illustration: After gas injection, the injected gas displaces the formation water enriched near the wellbore, gradually changing the direction of formation water displacement. Figure 4 Illustration: After a period of time, the direction of the injected gas in the formation corresponds to a displacement state of the formation water in the gas zone. At this time, the gas-water flow field distribution of the reservoir forms a gas production window. Figure 5 Illustration: After well production, the process by which the formation water displacement direction returns to the wellbore is called the gas-water flow field reconstruction process, which is the main gas production process. Figure 6 Schematic illustration: The gas reservoir geological model established in Example 1; Figure 7 Illustration: The fitting results of numerical simulation on water production from gas wells in Example 1 confirm that the numerical model can be used to demonstrate the reasonable gas injection rate. Figure 8 Illustration: Sensitivity analysis data graph of reasonable gas injection volume in Example 1. The inflection point determines that the reasonable gas injection volume for this example is 20 × 10. 4 m 3 / d; Figure 9 Illustration: In Example 1, the well shut-in time was reasonably determined. Through numerical simulation, the sensitivity between the wellhead pressure and the cumulative gas production at the end of the well shut-in stage was analyzed, and it was found that the reasonable wellhead pressure at the end of the well shut-in stage in Example 1 was 8~10MPa. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the embodiments described herein are merely illustrative and not intended to limit the invention. Example

[0018] refer to Figure 1 As shown, the present invention provides a drainage gas extraction method based on flow field reconstruction theory, comprising the following steps: Step 1: As Figure 2 Injecting gas drives away the formation water that accumulates near the wellbore, changing the gas-water flow field of the reservoir. The injected gas drives away the formation water that accumulates around the wellbore, and at the same time, it has a reverse effect on the formation water flow field, changing or weakening the intensity and direction of the formation water intrusion into the wellbore, so that the formation water flow field intrudes into other parts of the reservoir in the reverse direction, and also plays the role of displacing the remaining gas in the gas reservoir.

[0019] Step 2: Calculate the gas injection volume for the formation water accumulation around the breakthrough well. 1) Establish geological models and gas reservoir numerical models, such as Figure 5 ; Numerical simulation methods were used to fit the production history of gas wells, especially the production dynamics from water production to flooding, with a focus on fitting the water production rate, such as... Figure 6 ; Based on the fitted numerical model of the gas reservoir, a sensitivity evaluation scheme for the gas injection rate and the time of water-flooded gas well communication with the gas-bearing area is designed to optimize the appropriate gas injection rate, such as... Figure 7 The appropriate gas injection volume is 20×10. 4 m 3 / d.

[0020] Step 3: Shutting down the gas well and entering the well shut-in phase. This stage primarily monitors changes in wellhead pressure: numerical simulations can be used to optimize the wellhead pressure value; once the wellhead pressure reaches this value, the well shut-in phase is terminated. Figure 3 , Figure 8 .

[0021] Step 4: Well opening and gas production. Gas flows into the wellbore, and the water cone begins to resume its conical advance. The period during which the water cone resumes its advance is the gas production period of the gas well. The longer the water cone resumes its advance, the longer the gas production period will be and the more natural gas will be produced.

[0022] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A drainage gas extraction method based on flow field reconstruction theory, characterized in that the steps include... include: (1) Injecting gas drives away the formation water that accumulates near the wellbore and changes the reservoir gas-water flow field. After bottom water coning, formation water accumulates in the formation around the production well, forming a water pocket that isolates natural gas from flowing into the wellbore. A stable and unobstructed formation water flow channel is formed between the gas well and the water body. The gas well is flooded and loses its production capacity. Injected gas cuts off this continuous formation water flow channel and establishes a channel between the formation gas and the wellbore. (2) Calculate the gas injection volume for the formation water accumulation around the breakthrough gas well. Collect geological and dynamic data of gas reservoirs, establish geological models and numerical models of gas reservoirs, and use numerical simulation methods to conduct sensitivity assessments and determine reasonable gas injection rates. (3) After completing the reasonable gas injection volume, the gas well is shut in and enters the well shut-in stage. After the injection gas is injected, the process begins to gradually displace the accumulated water, which takes a certain amount of time. During this period, the injected water gradually changes the trajectory of the bottom water: in the reservoir, the injected gas will have a drainage effect on the water cone, and this drainage effect will gradually be transmitted to the water body over time. If there is a dominant channel at this time, the formation water at the edge of the cone will advance along both sides. The period from when the gas injection stops to when the effect of the injected gas is reflected in the water body is the well-closing stage. (4) When the well is opened for gas production, the gas-water flow field enters the reconstruction stage. The time difference of reconstruction is the gas production stage. Based on a reasonable gas injection volume and well shut-in time, the well is opened and gas is extracted at a reasonable gas extraction volume. This avoids excessive gas extraction intensity, which can cause excessive stimulation of formation water, resulting in faster water cone recovery, shorter gas-water flow field reconstruction time, reduced gas extraction volume, and decreased flow field reconstruction drainage gas extraction effect. In step 1), the injected gas drives away the formation water accumulated around the wellbore, and at the same time produces a reverse effect on the formation water flow field, changing or weakening the intrusion intensity and flow direction of the formation water in the reverse direction towards the wellbore, so that the formation water flow field intrudes in the reverse direction towards other parts of the reservoir, and also plays the role of displacing the remaining gas in the gas reservoir. In step 2), the gas injection volume for water accumulation in the formation around the breakthrough gas well is calculated. The gas reservoir numerical simulation is used to fit the dynamics of water production from gas well to water flooding. Based on the fitted gas reservoir numerical model, a sensitivity evaluation scheme for the gas injection volume and the time of communication between the water-flooded gas well and the gas-bearing area is designed, and a reasonable gas injection volume is selected. In step 4), after the gas injection ends and the gas production begins, the gas flow moves towards the wellbore, and the water cone also begins to resume its conical advance. During the water cone recovery period, the gas well is in its production period: the longer the water cone recovery time, the longer the production period, and the more natural gas is produced.

2. The drainage gas extraction method based on flow field reconstruction theory according to claim 1, characterized in that, In step 3), the wellhead pressure value is optimized through numerical simulation. Once the wellhead pressure reaches this value, the well shut-in stage is stopped. During the well shut-in stage, the injected gas drives away the formation water accumulated around the well, changes the bottom water trajectory, and destroys the gas-water flow field. The injected gas enables communication between the gas well and the gas-bearing area. The water intrusion direction changes its intrusion direction under the action of the injected gas.

3. The drainage gas extraction method based on flow field reconstruction theory according to claim 2, characterized in that, In step 4), numerical simulation technology is used to design a sensitivity study on the gas extraction intensity and the time required for gas-water flow field reconstruction, and to select a reasonable gas extraction intensity to achieve the maximum gas extraction benefit.

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