A method for determining hydrate dissociation control mechanisms

Abstract

This invention belongs to the technical field of natural gas hydrate reservoir exploitation, and relates to a method for determining the control mechanism of hydrate decomposition. This invention utilizes the temperature-pressure driving ratio to determine the control mechanism of hydrate decomposition. The steps are as follows: establishing a hydrate reservoir exploitation model; selecting the temperature and pressure at the point of complete decomposition during the hydrate reservoir exploitation process; calculating the temperature difference driving force and pressure difference driving force of hydrate decomposition based on the temperature and pressure; calculating the temperature difference driving coefficient and pressure difference driving coefficient during hydrate decomposition; and calculating the change in the temperature-pressure driving ratio during hydrate decomposition to determine the control mechanism of hydrate decomposition. This invention selects the point of complete hydrate decomposition for study and uses the temperature-pressure driving ratio to determine the control mechanism of hydrate decomposition. The method is simple and easy to implement, better reflects the control mechanism of hydrate decomposition, and can consider the dynamic changes of the control mechanism during hydrate decomposition at different times, which is the foundation for efficient development of hydrate reservoirs.

Description

Technical Field

[0001] This invention belongs to the technical field of natural gas hydrate reservoir exploitation, specifically relating to a method for determining the control mechanism of hydrate decomposition. Background Technology

[0002] Natural gas hydrate, often called "combustible ice," is a cage-like crystalline compound formed by gas molecules (mainly methane) and water molecules under high pressure and low temperature. Due to its vast reserves, wide distribution, shallow burial, and favorable physicochemical conditions, it has been regarded by countries worldwide as a highly promising strategic alternative energy source. However, current pilot production of natural gas hydrate faces numerous challenges, such as sand production, low single-well gas production, and short stable production periods. Pilot production output falls far short of the requirements for safe, stable, efficient, and long-term commercial exploitation. One key reason for these problems is the extreme complexity of the natural gas hydrate extraction process. Its decomposition mechanism involves chemical fields related to the hydrate decomposition process, seepage fields related to the gas-water flow after decomposition, temperature fields related to the endothermic process of hydrate decomposition, and mechanical fields related to the stability of the hydrate reservoir—a typical example of multi-field coupling and comprehensive effects. Therefore, clarifying the mechanisms of action of various complex factors in the hydrate extraction process is crucial. Currently, our understanding of the hydrate decomposition control mechanism is still unclear, which significantly limits our in-depth understanding and effective control of the hydrate extraction process.

[0003] During hydrate decomposition, the stability of hydrates is jointly controlled by temperature and pressure. The decomposition and formation of hydrates, as well as the compression and expansion of gases, all trigger changes in temperature and pressure. These changes, in turn, affect the decomposition and formation of hydrates, thus impacting the stability of hydrate-bearing reservoirs. Therefore, accurately understanding the temperature and pressure changes during hydrate decomposition and clarifying the control mechanisms of hydrate decomposition is crucial for achieving efficient extraction of natural gas hydrates.

[0004] Current research on the control mechanisms of hydrate decomposition remains unclear and has certain limitations. For example, while conventional experiments and numerical simulations can analyze pressure and temperature changes at different locations during hydrate decomposition, the main control mechanisms influencing temperature and pressure changes and hydrate decomposition are not well understood, leading to an inability to accurately grasp and control the hydrate decomposition process. Furthermore, the control mechanisms of hydrate decomposition are constantly changing during hydrate extraction. Currently, there is a lack of an effective method that can comprehensively consider temperature and pressure factors to accurately and dynamically determine the control mechanisms during hydrate extraction. Summary of the Invention

[0005] To address the aforementioned problems, this invention proposes a method for determining the hydrate decomposition control mechanism. By deeply studying the intrinsic relationship between the temperature-pressure driving ratio and the hydrate decomposition control mechanism, a method is proposed to determine the hydrate decomposition control mechanism using the temperature-pressure driving ratio, thereby achieving dynamic determination of the hydrate decomposition control mechanism during the mining process.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for determining the control mechanism of hydrate decomposition, utilizing the temperature-pressure driving ratio, includes the following steps:

[0008] (1) Establish a hydrate reservoir mining model and select the temperature and pressure at the location where the hydrates completely decompose during the mining process;

[0009] (2) Calculate the temperature difference driving force for hydrate decomposition based on temperature and pressure. TDF With pressure difference driving force PDF ;

[0010] (3) Calculate the temperature difference driving coefficient during the decomposition of hydrates. K 1 and pressure difference driving coefficient K 2;

[0011] (4) Calculate the change in temperature-pressure driving ratio during hydrate decomposition, and determine the control mechanism of hydrate decomposition by the magnitude of the temperature-pressure driving ratio.

[0012] In step (1), the temperature and pressure changes are greater at the location where the hydrate is completely decomposed, and this location is more likely to reflect the control mechanism of hydrate decomposition.

[0013] In step (1), the temperature and pressure at the location where the hydrate completely decomposes at different times during the mining process are selected.

[0014] In step (2), the temperature difference driving force for hydrate decomposition TDF This represents the difference between the hydrate reservoir temperature and the phase equilibrium temperature.

[0015] TDF = T - T e ( P (1)

[0016] In the formula, T Temperature of hydrate reservoir, in °C; T e ( P (for pressure) P The phase equilibrium temperature at which the phase equilibrium occurs is expressed in °C.

[0017] Pressure differential driving force for hydrate decomposition PDF This is the difference between the phase equilibrium pressure and the hydrate reservoir pressure.

[0018] PDF = P e ( T )- P (2)

[0019] In the formula, P The pressure of the hydrate reservoir is expressed in MPa. P e ( T (The temperature is) T The phase equilibrium pressure at that time is expressed in MPa.

[0020] Temperature difference driving force TDF and pressure difference driving force PDF Schematic diagram as follows Figure 1 As shown.

[0021] In step (3), the temperature difference driving coefficient K 1 represents the ratio of the temperature difference driving force of the hydrate to the phase equilibrium temperature, reflecting the degree of temperature driving force on the decomposition of the hydrate.

[0022] K 1= TDF / T e ( P (3)

[0023] In the formula, TDF The temperature difference driving force for the decomposition of hydrates, expressed in °C; T e ( P (for pressure) P The phase equilibrium temperature at which the phase equilibrium occurs is expressed in °C.

[0024] Differential pressure driving coefficient K 2 represents the ratio of the pressure difference driving force of the hydrate to the phase equilibrium pressure, reflecting the degree of pressure driving the decomposition of the hydrate.

[0025] K 2= PDF / P e ( T (4)

[0026] In the formula, PDF The pressure difference driving force for hydrate decomposition, expressed in MPa; P e ( T (The temperature is) T The phase equilibrium pressure at that time is expressed in MPa.

[0027] In step (4), the temperature and pressure driving ratio of hydrate decomposition K This is the ratio of the temperature-pressure driving coefficient to the pressure-pressure driving coefficient for hydrate decomposition. When the temperature-pressure driving ratio is less than 1, the movement of the hydrate decomposition front is controlled by the flow mechanism; when the temperature-pressure driving ratio is greater than 3, the movement of the hydrate decomposition front is controlled by the heat transfer mechanism; and when the temperature-pressure driving ratio is greater than 1 and less than 3, the movement of the hydrate decomposition front is controlled by both the flow mechanism and the heat transfer mechanism.

[0028] (5)

[0029] In the formula, PDF The pressure difference driving force of the hydrate, in MPa; P e ( T (The temperature is) T The phase equilibrium pressure at that time, in MPa; TDF The temperature difference driving force for hydrates, expressed in °C; T e ( P (for pressure) P The phase equilibrium temperature at which the phase equilibrium occurs is expressed in °C.

[0030] Previous analyses of hydrate decomposition control mechanisms relied on calculating and comparing characteristic times, which presented two main problems: first, the calculation of characteristic times required numerous parameters, some of which were difficult to determine, leading to poor reliability; second, the decomposition control mechanism was not static, and the characteristic time method failed to account for its dynamic changes. This invention selects the location of complete hydrate decomposition for investigation and utilizes the temperature-pressure driving ratio to determine the hydrate decomposition control mechanism. This method is simple and easy to implement, better reflects the control mechanism of hydrate decomposition, and considers the dynamic changes of the control mechanism at different times during decomposition, resulting in more reliable results and laying the foundation for efficient development of hydrate reservoirs. Attached Figure Description

[0031] The accompanying drawings, which form part of this invention, are used to provide a further understanding of this application. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0032] Figure 1 A schematic diagram showing the driving forces of temperature difference and pressure difference for hydrate decomposition;

[0033] Figure 2 This is a schematic diagram of the hydrate extraction model in Example 1;

[0034] Figure 3This is a graph showing the temperature and pressure changes over time at the location where the hydrate completely decomposes in Example 1.

[0035] Figure 4 This is a graph showing the changes in temperature difference driving force and pressure difference driving force over time in Example 1;

[0036] Figure 5 This is a graph showing the temperature-pressure drive ratio as a function of time in Example 1;

[0037] Figure 6 The graph shows the temperature-pressure drive ratio as a function of time in Example 2. Detailed Implementation

[0038] The present invention will be further described below.

[0039] Example 1

[0040] A method for determining the hydrate decomposition control mechanism using the temperature-pressure driving ratio includes the following steps:

[0041] (1) Establish a hydrate reservoir mining model and select the temperature and pressure at the location where the hydrate is completely decomposed at different times during the mining process.

[0042] Based on geological data of hydrate deposits at station SH7 in the Shenhu area of ​​the South China Sea, a geological model for depressurization mining of hydrates was established. The model size is 400 m × 400 m × 332 m. A schematic diagram of the model is shown below. Figure 2 As shown, vertically, from top to bottom, the reservoir is divided into a 155m caprock, a 22m hydrate layer, and a 155m underlying layer. The hydrate saturation of the hydrate layer is 0.438. The seafloor mudline is at a depth of 1108 meters, the reservoir porosity is 0.41, and the initial permeability is 75 × 10⁻⁶. -3 μm 2 The initial pressure at the bottom layer of the hydrate layer is 13.83 MPa, and the initial temperature is 14.15℃. Both the upper and lower boundaries of the model are set as constant pressure boundaries to simulate the pressure environment of the actual reservoir. A vertical production well is set at the center of the model, and the entire hydrate layer is perforated for production. The bottom hole pressure is set to 3 MPa.

[0043] The grid division for the numerical simulation of depressurization mining of hydrate reservoirs is as follows: 79 grids are divided in both the X and Y directions, with grid sizes of 14×10m (i.e., 14 x 10m grids, hereinafter the same), 3×5m, 45×2m, 3×5m, and 14×10m respectively. This ensures a denser grid near the model's central well point and a sparser grid further away, thus reducing computational workload while maintaining accuracy. The Z direction is divided into 53 grids, with sizes from top to bottom of 10×10m, 11×5m, 11×2m, 11×5m, and 10×10m. The 22-meter-thick hydrate layer is the focus of this study, and its grid is relatively dense. The simulation period is 15 years. Taking the middle layer (the 6th of 11 hydrate layers) and the bottom layer (the 11th layer) as examples, temperature and pressure values ​​at the location of complete hydrate decomposition at different times are extracted. Figure 3 As shown, the temperature and pressure at the hydrate decomposition front gradually increase as mining progresses. The lowest layer has a higher temperature and pressure compared to the middle layer because it is closer to the underlying layer, resulting in a less significant pressure drop and easier temperature replenishment.

[0044] (2) Based on the temperature and pressure at the location where the hydrate is completely decomposed at a certain moment, calculate the temperature difference driving force TDF and pressure difference driving force PDF of the hydrate decomposition.

[0045] Taking the lowest layer of hydrate as an example, with an extraction time of one year, according to numerical simulation calculations, the temperature at the point of complete decomposition of the lowest layer of hydrate after one year of extraction is 11.33 ℃ (corresponding to a phase equilibrium pressure of 8.67 MPa), and the pressure is 8.30 MPa (corresponding to a phase equilibrium temperature of 10.91 ℃). Therefore, the calculation results of the temperature difference driving force (TDF) and the pressure difference driving force (PDF) are as follows:

[0046] , ;

[0047] These two values ​​reflect the magnitude of different driving forces during the decomposition of hydrates.

[0048] Taking the lowest layer of hydrate as an example, the driving forces of temperature difference and pressure difference at the location of complete hydrate decomposition at different times during the extraction process are statistically analyzed and calculated. The changes of the driving forces of temperature difference and pressure difference in the lowest layer of the hydrate reservoir over time are obtained as follows: Figure 4 It can be seen that after depressurization mining of hydrates, both the temperature difference driving force and the pressure difference driving force decrease rapidly, and then gradually stabilize.

[0049] (3) Calculate the temperature difference driving coefficient during the decomposition of hydrates. K 1 and pressure difference driving coefficient K 2。

[0050] Using data from when the lowest layer of hydrates was mined over a year as an example for calculation:

[0051] , ;

[0052] The temperature difference driving coefficient reflects the degree to which hydrate decomposition is driven by temperature, while the pressure difference driving coefficient reflects the degree to which hydrate decomposition is driven by pressure difference.

[0053] (4) Calculate the change in temperature-pressure driving ratio during hydrate decomposition, and determine the control mechanism of hydrate decomposition by the magnitude of the temperature-pressure driving ratio.

[0054] Using data from when the lowest layer of hydrates was mined over a year as an example for calculation:

[0055] ;

[0056] The temperature-pressure driving ratio reflects the ratio of the degree to which hydrate decomposition is controlled by heat transfer to the degree to which it is controlled by flow. When the temperature-pressure driving ratio is less than 1, the decomposition of hydrate is mainly controlled by the flow mechanism; when the temperature-pressure driving ratio is greater than 3, the decomposition of hydrate is mainly controlled by the heat transfer mechanism; and when the temperature-pressure driving ratio is greater than 1 and less than 3, the decomposition of hydrate is controlled by both the flow mechanism and the heat transfer mechanism.

[0057] The graph shows the time-varying temperature-pressure driving ratio of the lowest and middle layers of the hydrate reservoir. Figure 5 It can be seen that initially, hydrate decomposition is jointly controlled by heat transfer and flow mechanisms. As mining progresses, the temperature-pressure driving ratio gradually decreases, while the degree to which hydrate decomposition is controlled by the flow mechanism gradually increases. The temperature-pressure driving ratio in the middle layer of the hydrate reservoir is greater than that in the bottom layer. This is because the middle layer does not easily receive heat replenishment, resulting in a lower temperature in the middle layer compared to the bottom layer. Therefore, the heat transfer mechanism has a relatively greater influence on hydrate decomposition.

[0058] Example 2

[0059] To verify the accuracy of using the temperature-pressure drive ratio to determine the control mechanism, a heat transfer control model was used as an example, and the previous analytical method using dimensionless characteristic time was adopted for verification.

[0060] First, a brief explanation of the dimensionless characteristic time analysis methods of previous researchers: hydrate decomposition is controlled by three mechanisms: flowability, decomposition capacity, and heat transfer capacity. The characteristic time for each of these mechanisms is defined as: ① Characteristic time controlled by decomposition. τ 1: The time required for complete decomposition of hydrates per unit volume of hydrate reservoir; ② The characteristic time for flow control is... τ2: The time it takes for the gas phase to flow through the entire hydrate reservoir; ③ The characteristic time for heat transfer control is... τ 3: The time required for gas-phase heat diffusion and transfer within a unit length of hydrate reservoir. During depressurization mining of hydrate reservoirs, the longer the characteristic time of any controlling mechanism, the greater the degree to which the movement of the hydrate decomposition front is controlled by that mechanism.

[0061] Taking heat transfer control as an example, the calculation results of the main parameters and dimensionless characteristic time in the model are shown in Table 1.

[0062] Table 1. Main parameters and feature schedule of the model in Example 2

[0063] ;

[0064] It can be seen that the characteristic time satisfies and Based on the dimensionless characteristic time analysis method, it can be seen that the decomposition of hydrates is controlled by the heat transfer mechanism.

[0065] The temperature-pressure driving ratio proposed in this paper is used to analyze the control mechanism of hydrate decomposition. The reliability of the proposed method is verified by comparing it with the dimensionless characteristic time method.

[0066] Similar to steps 1 to 4 in Example 1, and still taking the lowest layer of the hydrate layer as an example, the temperature-pressure driving ratio changes at the location of complete hydrate decomposition at different times in the heat transfer control mechanism model are calculated as follows: Figure 6 In the first three years of mining, the temperature-pressure driving ratio could not be calculated because the lowest layer of hydrates did not completely decompose. After complete decomposition of the hydrates, the temperature-pressure driving ratio at the location of complete decomposition was greater than 3, indicating that hydrate decomposition was mainly controlled by a heat transfer mechanism. This analytical result is consistent with the results obtained using the dimensionless characteristic time method, thus verifying the reliability of this method.

[0067] While the specific embodiments of the present invention have been described above, they are not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for determining the control mechanism of hydrate decomposition, characterized in that, Determining the control mechanism of hydrate decomposition using the temperature-pressure driving ratio includes the following steps: (1) Establish a hydrate reservoir mining model and select the temperature and pressure at the location where the hydrates completely decompose during the mining process; (2) Calculate the temperature difference driving force for hydrate decomposition based on temperature and pressure. TDF With pressure difference driving force PDF ; (3) Calculate the temperature difference driving coefficient during the decomposition of hydrates. K 1 and pressure difference driving coefficient K 2; (4) Calculate the change in temperature-pressure driving ratio during hydrate decomposition, and determine the control mechanism of hydrate decomposition by the magnitude of the temperature-pressure driving ratio; Temperature difference driving force for hydrate decomposition TDF The difference between the hydrate reservoir temperature and the phase equilibrium temperature; the pressure difference driving force for hydrate decomposition. PDF The difference between phase equilibrium pressure and hydrate reservoir pressure; temperature difference driving coefficient. K 1 represents the ratio of the temperature difference driving force to the phase equilibrium temperature for hydrate decomposition; pressure difference driving coefficient. K 2 represents the ratio of the pressure differential driving force to the phase equilibrium pressure of the hydrate; the temperature and pressure driving ratio for hydrate decomposition. K The ratio of the temperature-driven coefficient to the pressure-driven coefficient for hydrate decomposition is calculated using the following formula: ； In the formula, PDF The pressure difference driving force for hydrate decomposition, expressed in MPa; P e ( T (The temperature is) T The phase equilibrium pressure at that time, in MPa; TDF The temperature difference driving force for the decomposition of hydrates, expressed in °C; T e ( P (for pressure) P The phase equilibrium temperature at that time, expressed in °C.

2. The method for determining the hydrate decomposition control mechanism according to claim 1, characterized in that, When the temperature-pressure driving ratio is less than 1, the movement of the hydrate decomposition front is controlled by the flow mechanism; when the temperature-pressure driving ratio is greater than 3, the movement of the hydrate decomposition front is controlled by the heat transfer mechanism; and when the temperature-pressure driving ratio is greater than 1 and less than 3, the movement of the hydrate decomposition front is controlled by both the flow mechanism and the heat transfer mechanism.

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