Method for judging hydrate decomposition control mechanism

Abstract

The invention belongs to the technical field of natural gas hydrate reservoir exploitation, and relates to a method for judging a hydrate decomposition control mechanism, which utilizes a temperature-pressure driving ratio to judge the hydrate decomposition control mechanism, and comprises the following steps: establishing a hydrate reservoir exploitation model, and selecting temperature and pressure at a complete decomposition position in the hydrate reservoir exploitation process; calculating temperature difference driving force and pressure difference driving force of hydrate decomposition according to the temperature and the pressure; calculating a temperature difference driving coefficient and a pressure difference driving coefficient in the hydrate decomposition process; and calculating the change condition of the temperature-pressure driving ratio in the hydrate decomposition process so as to judge the control mechanism of hydrate decomposition. According to the method, the complete decomposition position of the hydrate is selected for research, the hydrate decomposition control mechanism is judged through the temperature-pressure driving ratio, the judgment method is easy and convenient to implement, the control mechanism of hydrate decomposition can be better reflected, dynamic changes of the control mechanism in the hydrate decomposition process at different moments can be considered, and the method is a basis for efficient development of the hydrate reservoir.

Description

Technical Field

[0001] The invention belongs to the technical field of natural gas hydrate reservoir exploitation, and in particular relates to a method for determining a hydrate decomposition control mechanism. Background Art

[0002] Natural gas hydrates, often referred to as "combustible ice," are crystalline compounds formed by gas molecules (primarily methane) and water molecules under high pressure and low temperature. Due to their vast reserves, widespread distribution, shallow burial depth, and favorable physicochemical conditions, they are considered a highly promising strategic alternative energy source worldwide. However, current natural gas hydrate production trials face numerous challenges, including sand production, low gas production per well, and short production stabilization periods. Trial production rates fall far short of meeting the requirements for safe, stable, efficient, and long-term commercial production. A key factor contributing to these issues is the extremely complex natural gas hydrate production process. Its decomposition mechanism involves chemical fields associated with hydrate decomposition, seepage fields associated with post-decomposition gas-water flow, temperature fields associated with the endothermic decomposition process, and mechanical fields associated with hydrate reservoir stability—a typical example of a multi-field coupling process. Therefore, it is crucial to clarify the mechanisms underlying these complex factors in hydrate production. However, our current understanding of the mechanisms controlling hydrate decomposition remains unclear, significantly limiting our understanding and effective control of the hydrate production process.

[0003] During hydrate decomposition, hydrate stability is controlled by both temperature and pressure. Hydrate decomposition and formation, as well as gas compression and expansion, trigger temperature and pressure fluctuations. These changes in temperature and pressure, in turn, affect hydrate decomposition and formation, and thus the stability of hydrate-bearing reservoirs. Therefore, accurately understanding the temperature and pressure variations during hydrate decomposition and clarifying the mechanisms controlling hydrate decomposition are crucial for the efficient recovery of natural gas hydrates.

[0004] Existing research on the mechanisms controlling hydrate decomposition remains unclear and has certain limitations. For example, while conventional experiments and numerical simulations can analyze the changes in pressure and temperature at different locations during hydrate decomposition, the key controlling mechanisms that influence temperature and pressure fluctuations and hydrate decomposition remain unclear, hindering accurate understanding and control of the hydrate decomposition process. Furthermore, the decomposition control mechanisms are constantly changing during hydrate extraction. Currently, there is a lack of an effective method that comprehensively considers temperature and pressure factors to accurately and dynamically determine the controlling mechanisms during hydrate extraction. Summary of the Invention

[0005] To address the above-mentioned issues, the present invention proposes a method for determining the control mechanism of hydrate decomposition. By deeply studying the intrinsic connection between the temperature-pressure driving ratio and the hydrate decomposition control mechanism, a method for determining the control mechanism of hydrate decomposition using the temperature-pressure driving ratio is proposed, thereby realizing dynamic determination of the control mechanism of hydrate decomposition during the mining process.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions: A method for determining a hydrate decomposition control mechanism using a temperature-pressure driving ratio to determine the hydrate decomposition control mechanism includes the following steps: (1) Establish a hydrate reservoir mining model and select the temperature and pressure at the location where the hydrate is completely decomposed during the hydrate reservoir mining process; (2) Calculation of the temperature difference driving force for hydrate decomposition based on temperature and pressure TDF and pressure differential driving force PDF ; (3) Calculation of the temperature difference driving coefficient during hydrate decomposition K 1 and pressure differential drive coefficient K 2; (4) Calculate the change of the temperature-pressure driving ratio during the hydrate decomposition process, and determine the control mechanism of hydrate decomposition based on the value of the temperature-pressure driving ratio.

[0007] In the step (1), the temperature and pressure changes at the position where the hydrate is completely decomposed are relatively large, and this position can better reflect the control mechanism of the hydrate decomposition.

[0008] In the step (1), the temperature and pressure at the position where the hydrate is completely decomposed at different times during the mining process are selected.

[0009] In step (2), the temperature difference driving force for hydrate decomposition is TDF is the difference between the hydrate storage temperature and the phase equilibrium temperature.

[0010] TDF = T - T e ( P ) (1) Where, T is the hydrate storage temperature, in °C; T e ( P ) is the pressure P The phase equilibrium temperature at , in °C.

[0011] Pressure differential driving force for hydrate decomposition PDF is the difference between the phase equilibrium pressure and the hydrate reservoir pressure.

[0012] PDF = P e ( T )- P (2) Where, P is the hydrate reservoir pressure, in MPa; P e ( T ) is the temperature T The phase equilibrium pressure at , in MPa.

[0013] Temperature difference driving force TDF and pressure differential driving force PDF Schematic diagram as Figure 1 shown.

[0014] In step (3), the temperature difference driving coefficient K 1 is the ratio of the temperature difference driving force of hydrate to the phase equilibrium temperature, which reflects the degree of temperature driving force on hydrate decomposition.

[0015] K 1= TDF / T e ( P ) (3) Where, TDF is the temperature difference driving force for hydrate decomposition, in °C; T e ( P ) is the pressure P The phase equilibrium temperature at , in °C.

[0016] Differential pressure drive coefficient K 2 is the ratio of the pressure difference driving force of hydrate to the phase equilibrium pressure, which reflects the degree of pressure driving force on hydrate decomposition.

[0017] K 2= PDF / P e ( T ) (4) Where, PDF is the pressure differential driving force for hydrate decomposition, in MPa; P e ( T ) is the temperature T The phase equilibrium pressure at , in MPa.

[0018] In the step (4), the hydrate decomposition temperature and pressure driving ratio Kis the ratio of the temperature-difference driving coefficient to the pressure-difference 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 flow mechanisms; when the temperature-pressure driving ratio is greater than 3, the movement of the hydrate decomposition front is controlled by heat transfer mechanisms; and when the temperature-pressure driving ratio is greater than 1 but less than 3, the movement of the hydrate decomposition front is controlled by both flow and heat transfer mechanisms.

[0019] (5) Where, PDF is the differential pressure driving force of hydrate, in MPa; P e ( T ) is the temperature T The phase equilibrium pressure at , in MPa; TDF is the temperature difference driving force of hydrate, in °C; T e ( P ) is the pressure P The phase equilibrium temperature at , in °C.

[0020] In the past, the control mechanism of hydrate decomposition was analyzed by calculating and comparing characteristic times. This has two main problems: first, the calculation of characteristic times requires many parameters, and the values ​​of some parameters are difficult to determine, resulting in poor reliability of the calculated characteristic times; second, the decomposition control mechanism during hydrate decomposition is not static, and the characteristic time method does not consider the dynamic changes of the decomposition mechanism. The present invention selects the position where hydrates are completely decomposed for research and uses the temperature-pressure driving ratio to determine the hydrate decomposition control mechanism. This judgment method is simple and easy to use, better reflects the control mechanism of hydrate decomposition, and can consider the dynamic changes of the control mechanism during the hydrate decomposition process at different times. The results are more reliable, laying the foundation for the efficient development of hydrate reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present application. The exemplary embodiments of the present invention and their description are used to explain the present invention and do not constitute an improper limitation to the present invention.

[0022] Figure 1 Schematic diagram of the temperature difference driving force and pressure difference driving force for hydrate decomposition; Figure 2 This is a schematic diagram of the hydrate production model of Example 1; Figure 3 This is a graph showing the temperature and pressure changes over time at the position where the hydrate is completely decomposed in Example 1; Figure 4 Graph showing the temperature difference driving force and pressure difference driving force over time in Example 1; Figure 5Graph showing the change of the temperature-pressure driving ratio over time in Example 1; Figure 6 This is a graph showing how the temperature-pressure driving ratio of Example 2 changes with time. DETAILED DESCRIPTION

[0023] The present invention will be further described below.

[0024] Example 1 The method for determining the control mechanism of hydrate decomposition by using the temperature-pressure driving ratio comprises the following steps: (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 hydrate reservoir mining process.

[0025] Referring to the geological data of hydrate reservoir at SH7 station in Shenhu area of ​​South China Sea, a geological model of hydrate depressurization mining was established. The model size is 400 m×400 m×332 m. The model schematic is shown in the figure below. Figure 2 As shown in the figure, vertically, it is divided into a 155m upper cap layer, a 22m hydrate layer, and a 155m underlying layer from top to bottom. The hydrate saturation of the hydrate layer is 0.438. The depth of the seabed mudline is 1108 meters, the reservoir porosity is 0.41, and the initial permeability is 75×10 -3 μm 2 The initial pressure at the bottom of the hydrate layer was 13.83 MPa, and the initial temperature was 14.15°C. The upper and lower boundaries of the model were set to constant pressure to simulate the pressure environment of the actual reservoir. A vertical production well was set in the center of the model, and the entire hydrate layer was perforated for production. The bottomhole pressure was set to 3 MPa.

[0026] The grid division used in the numerical simulation of depressurized production of hydrate reservoirs is as follows: 79 grids are divided in the X and Y directions on the plane, with grid sizes of 14×10m (i.e., 14 10m grids, the same below), 3×5m, 45×2m, 3×5m, and 14×10m, respectively. This ensures that the grid near the center well of the model is dense and the grid away from the well is sparse, thereby reducing the computational workload while meeting the calculation accuracy. The Z direction is divided into 53 grids, which, from top to bottom, are 10×10m, 11×5m, 11×2m, 11×5m, and 10×10m. The 22-meter-thick hydrate layer is the focus of this study, and the grid division is relatively dense. The simulation lasted 15 years. Taking the middle layer of the hydrate layer (the middle layer is the 6th of the 11 hydrate layers) and the bottom layer (the 11th layer) as examples, the temperature and pressure values ​​at the location where the hydrate is completely decomposed at different times were extracted, as shown in the following figure: Figure 3As shown in Figure 2, the temperature and pressure at the hydrate decomposition front gradually increase as mining progresses. The bottom layer has higher temperature and pressure than the middle layer. This is because the bottom layer is closer to the underlying layer, where the pressure drop effect is less severe and the temperature is more easily replenished.

[0027] (2) Based on the temperature and pressure at the position where the hydrate is completely decomposed at a certain moment, the temperature difference driving force TDF and the pressure difference driving force PDF of the hydrate decomposition are calculated.

[0028] Taking the bottom layer of hydrate as an example, with a one-year mining period, numerical simulation results show that the temperature at the location where the hydrate is completely decomposed is 11.33°C (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°C). Therefore, the calculated results for the temperature differential driving force (TDF) and the pressure differential driving force (PDF) are as follows: , ; These two values ​​reflect the magnitude of different driving forces in the hydrate decomposition process.

[0029] Taking the bottom layer of hydrate as an example, the temperature driving force and pressure driving force at the position where hydrate is completely decomposed at different times during the mining process are statistically calculated, and the temperature driving force and pressure driving force of the bottom layer of hydrate reservoir vary with time as shown in the following figure: Figure 4 It can be seen that after hydrate depressurization, the temperature difference driving force and the pressure difference driving force both decrease rapidly and then gradually stabilize.

[0030] (3) Calculation of the temperature difference driving coefficient during hydrate decomposition K 1 and pressure differential drive coefficient K 2。

[0031] The calculation is still based on the data when the lowest layer of hydrate is mined for one year: , ; The temperature difference driving coefficient reflects the extent to which hydrate decomposition is driven by temperature, and the pressure difference driving coefficient reflects the extent to which hydrate decomposition is driven by pressure difference.

[0032] (4) Calculate the change of the temperature-pressure driving ratio during the hydrate decomposition process, and determine the control mechanism of hydrate decomposition based on the value of the temperature-pressure driving ratio.

[0033] The calculation is still based on the data when the lowest layer of hydrate is mined for one year: ; The temperature-pressure driving ratio reflects the ratio of the degree to which hydrate decomposition is controlled by heat transfer to that by flow. When the temperature-pressure driving ratio is less than 1, the decomposition of hydrates is mainly controlled by the flow mechanism; when the temperature-pressure driving ratio is greater than 3, the decomposition of hydrates is mainly controlled by the heat transfer mechanism; and when the temperature-pressure driving ratio is greater than 1 but less than 3, the decomposition of hydrates is controlled by both the flow mechanism and the heat transfer mechanism.

[0034] Draw a graph showing the temperature and pressure driving ratio of the bottom and middle layers of the hydrate reservoir over time. Figure 5 It can be seen that at the initial stage, hydrate decomposition is controlled by both heat transfer and flow mechanisms. As production progresses, the temperature-pressure driving ratio gradually decreases, and the degree to which hydrate decomposition is controlled by flow mechanisms gradually increases. The temperature-pressure driving ratio in the middle layer of the hydrate reservoir is greater than that in the bottom layer of the hydrate reservoir. This is because the middle layer is less susceptible to heat replenishment, resulting in a lower temperature in the middle layer than in the bottom layer, and the degree to which hydrate decomposition is controlled by heat transfer mechanisms is relatively large.

[0035] Example 2 In order to confirm the accuracy of the method of using the temperature-pressure driving ratio to judge the control mechanism, the heat transfer control model is taken as an example and the previous analysis method using dimensionless characteristic time is used for verification.

[0036] First, the dimensionless characteristic time analysis method of the predecessors is briefly described: hydrate decomposition is controlled by three mechanisms: flow capacity, decomposition capacity, and heat transfer capacity. The characteristic time of different control mechanisms is defined as follows: ① Characteristic time of decomposition control τ 1: The time required for the complete decomposition of hydrates per unit volume of hydrate reservoir; ② The characteristic time of flow control is τ 2: The time it takes for the gas phase to flow through the entire hydrate reservoir; ③ The characteristic time of heat transfer control is τ 3: The time required for gas-phase heat diffusion transfer per unit length within a hydrate reservoir. During depressurization of a hydrate reservoir, the longer the characteristic time of a controlling mechanism, the more strongly that mechanism controls the movement of the hydrate decomposition front.

[0037] 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.

[0038] Table 1 Main parameters and characteristic time table of the model in Example 2 ; It can be seen that the characteristic time satisfies and Under these two conditions, according to the dimensionless characteristic time analysis method, it can be seen that hydrate decomposition is controlled by the heat transfer mechanism.

[0039] The temperature-pressure driving ratio proposed in this paper is then 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.

[0040] The process is the same as that of step 1 to step 4 in Example 1. Taking the bottom layer of the hydrate layer as an example, the temperature and pressure driving ratio changes at the position where the hydrate is completely decomposed at different production times of the heat transfer control mechanism model are calculated as follows: Figure 6 The temperature-pressure driving ratio (TPR) could not be calculated for the first three years of mining because the hydrates in the bottom layer were not completely decomposed. After the hydrates were completely decomposed, the TPR at the location of complete hydrate decomposition was greater than 3, indicating that hydrate decomposition was primarily controlled by heat transfer. This analysis result is consistent with that of the dimensionless characteristic time method, thus verifying the reliability of this method.

[0041] Although the above describes the specific implementation methods of the present invention, it does not 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 on the basis of the technical solution of the present invention without creative work 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: The control mechanism of hydrate decomposition is determined by using the temperature-pressure driving ratio, which includes the following steps: (1) Establish a hydrate reservoir mining model and select the temperature and pressure at the location where the hydrate is completely decomposed during the hydrate reservoir mining process; (2) Calculation of the temperature difference driving force for hydrate decomposition based on temperature and pressure TDF and pressure differential driving force PDF ; (3) Calculation of the temperature difference driving coefficient during hydrate decomposition K 1 and pressure differential drive coefficient K 2; (4) Calculate the change of the temperature-pressure driving ratio during the hydrate decomposition process and determine the control mechanism of hydrate decomposition based on the value of the temperature-pressure driving ratio; Temperature difference driving force for hydrate decomposition TDF is the difference between the hydrate reservoir temperature and the phase equilibrium temperature; the pressure difference driving force for hydrate decomposition PDF is the difference between the phase equilibrium pressure and the hydrate reservoir pressure; the temperature difference driving coefficient K 1 is the ratio of the temperature difference driving force of hydrate decomposition to the phase equilibrium temperature; the pressure difference driving coefficient K 2 is the ratio of the pressure difference driving force of hydrate to the phase equilibrium pressure; the temperature and pressure driving ratio of hydrate decomposition is K is the ratio of the temperature difference driving coefficient to the pressure difference driving coefficient of hydrate decomposition, and the calculation formula is: ； Where, PDF is the pressure differential driving force for hydrate decomposition, in MPa; P e ( T ) is the temperature T The phase equilibrium pressure at , in MPa; TDF is the temperature difference driving force for hydrate decomposition, in °C; T e ( P ) is the pressure P The phase equilibrium temperature at , in °C.

2. The method for determining the hydrate decomposition control mechanism according to claim 1, wherein: 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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