Remodeling method of hydrate reservoir associated with underlying gas based on salt concentration and pressure gradient

Abstract

The invention belongs to the technical field of natural gas hydrate exploitation, and particularly relates to a remodeling method of an associated underlying gas hydrate reservoir based on salt concentration and pressure gradient. According to the technical scheme, the method comprises the following steps that 1, a hydrate generation substrate layer and an underlying gas substrate layer are added into a reaction kettle through a triaxial apparatus, and the hydrate generation substrate layer is located on the upper portion of the underlying gas substrate layer; 2) reducing the temperature of the system to 2-4 DEG C through air bath; (3) introducing excessive methane gas into the reaction kettle until the steady-state pressure is 5-25MPa; 4) rotating the reaction kettle to be horizontal, and forming hydrates in the hydrate generation substrate layer; and (5) resetting the reaction kettle to form a composite reservoir structure with an overlying hydrate layer and an underlying free gas layer. According to the invention, the gas-water coexistence state in the underlying gas layer porous medium can be simulated, the generation of the hydrate in the underlying gas layer can be inhibited, and the primary geological contact characteristics of the hydrate layer and the underlying gas layer can be maintained.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of natural gas hydrate exploitation, and particularly relates to a remodeling method for associated underlying gas hydrate reservoirs based on salt concentration and pressure gradient. BACKGROUND

[0002] Natural gas hydrate is widely distributed in permafrost zones and seabed sediments, and its seabed distribution presents a typical layered structure, mainly composed of an upper hydrate layer and a lower free gas layer, as shown in the accompanying drawing. Figure 1 The formation mechanism of the hydrate layer is as follows: when methane gas passes through pore water in the stability zone, if its dissolved concentration exceeds the saturation degree under the current temperature and pressure conditions, methane will crystallize and gradually block the pores, eventually forming a hydrate cap layer with capillary sealing effect; in the lower sediment layer, methane is in a free state due to the temperature and pressure conditions exceeding the hydrate stability zone, and forms a gas-rich reservoir space under the action of the capillary force of the hydrate layer. With the evolution of time, the thickness of the gas layer increases, and the methane concentration continues to enrich, showing significant resource development potential. In order to realize the development and research of such reservoirs, it is necessary to first remodel them in the laboratory. However, the existing experimental remodeling of associated underlying gas hydrate reservoirs has the following technical defects:

[0003] (1) Reservoir construction distortion: Most studies simplify the underlying gas layer into a pure gas cavity without porous medium (such as the hollow reactor separation design by Yang et al., and the direct gas supply through the gas tank by Zhao et al.), ignoring the gas-water two-phase percolation mechanism in the porous medium and the thermal-mass coupling effect between layers, resulting in deviation of heat / mass transfer laws from actual geological conditions.

[0004] (2) Unintended hydrate generation: Existing layered reconstruction techniques (such as the dry-wet glass bead method proposed by Yang et al., and the frozen solid water method used by Li et al.) can inhibit water migration, but may lead to unintended hydrate generation in the lower layer, or artificially eliminate the water-bearing characteristics of the gas layer, thereby affecting the prediction accuracy of the water-gas ratio during the production stage.

[0005] (3) Deviation of thermophysical properties: Models based on anhydrous quartz sand have systematic errors in specific heat capacity and thermal conductivity (the thermal conductivity of quartz sand is about 4 times that of water), resulting in deviation of sensible heat supply evaluation and heat transfer efficiency calculation from actual conditions.

[0006] In recent years, some studies (such as the double reactor system by Shi et al.) have attempted to introduce a water-bearing gas layer, but this system relies on artificial interface connection and cannot truly simulate the original contact relationship between the hydrate layer and the gas layer in the geological environment. SUMMARY

[0007] The application provides a remolding method of a gas-water hydrate reservoir associated with underlying gas based on salt concentration and pressure gradient, which can simulate the gas-water coexistence state in the porous medium of the underlying gas layer, inhibit the generation of hydrate in the underlying gas layer, and maintain the original geological contact characteristics of the hydrate layer and the underlying gas layer.

[0008] The technical scheme of the application is as follows:

[0009] The remolding method of the gas-water hydrate reservoir associated with underlying gas based on salt concentration and pressure gradient comprises the following steps.

[0010] 1) A triaxial apparatus is used to add a hydrate generation matrix layer and an underlying gas matrix layer into a reaction kettle, the hydrate generation matrix layer is located on the upper part of the underlying gas matrix layer, the hydrate generation matrix layer is prepared by seawater salinity solution and quartz sand, and the underlying gas matrix layer is prepared by high-salt solution and quartz sand.

[0011] 2) The system temperature is reduced to 2-4 DEG C by an air bath.

[0012] 3) Excessive methane gas is introduced into the reaction kettle until the steady-state pressure is 5-25 MPa.

[0013] 4) The reaction kettle is rotated to be horizontal, and hydrate is formed in the hydrate generation matrix layer.

[0014] 5) The reaction kettle is reset, and a composite reservoir structure of the overlying hydrate layer-underlying free gas layer is formed.

[0015] Further, in the remolding method of the gas-water hydrate reservoir associated with underlying gas based on salt concentration and pressure gradient, the volume ratio of the quartz sand in the hydrate generation matrix layer to the quartz sand in the underlying gas matrix layer is 15-85:85-15.

[0016] Further, in the remolding method of the gas-water hydrate reservoir associated with underlying gas based on salt concentration and pressure gradient, the NaCl concentration in the seawater salinity solution is 3.35-3.5 wt%, and the NaCl concentration in the high-salt solution is 24-26 wt%.

[0017] Further, in the remolding method of the gas-water hydrate reservoir associated with underlying gas based on salt concentration and pressure gradient, the hand push pump of the triaxial apparatus is used to adjust the axial pressure and the confining pressure, so as to simulate the stress condition of the natural hydrate reservoir; the axial pressure reaches 8.0-9.0 MPa; and the confining pressure reaches 7.0-8.0 MPa.

[0018] The working principle of this invention is as follows: In the upper hydrate-forming matrix layer, the NaCl concentration is 3.35–3.5 wt%, and hydrates can form when the temperature is 2–4°C and the methane gas pressure is greater than 4.7 MPa. In the lower underlying gas matrix layer, the NaCl concentration is 24–26 wt%, and hydrates cannot form when the temperature is 2–4°C and the methane gas pressure is less than 26.0 MPa. A methane gas pressure of 5–25 MPa ensures that hydrates are formed only in the upper hydrate-forming matrix layer. During hydrate formation, the triaxial reactor is placed horizontally to avoid interference from salt concentration migration across layers, thus preventing aqueous phase migration. The hydrate layer and the underlying gas layer maintain their original geological contact characteristics, avoiding interface distortion caused by human intervention.

[0019] The beneficial effects of the present invention are as follows: the reshaping method of the present invention can simulate the gas-water coexistence state in porous media in the underlying gas matrix layer, while inhibiting hydrate formation; it avoids the deviation of thermophysical properties between the reshaped sample and the original associated underlying gas hydrate reservoir, and ensures that the hydrate layer and the underlying gas layer maintain the original geological contact characteristics. Attached Figure Description

[0020] Figure 1 A schematic diagram showing the composition of the upper hydrate layer and the lower free gas layer in the natural environment;

[0021] Figure 2 A schematic diagram of the preparation process for associated underlying gas hydrate reservoirs;

[0022] Figure 3 This is a graph showing the temperature and pressure changes during the hydrate formation process.

[0023] Figure 4 This is a graph showing the temperature and pressure changes during the decomposition of hydrates. Detailed Implementation

[0024] like Figure 2 As shown, the method for reshaping associated underlying gas hydrate reservoirs based on salt concentration and pressure gradient includes the following steps:

[0025] 1) Using a triaxial apparatus (manufacturer: Hai'an Huafeng Scientific Instruments Co., Ltd., model: HAHF 200-100); add a hydrate formation matrix layer and a lower gas matrix layer to the reactor. The hydrate formation matrix layer is located above the lower gas matrix layer. The hydrate formation matrix layer is prepared by mixing seawater salinity solution and quartz sand. The lower gas matrix layer is prepared by mixing a high-salt solution and quartz sand. The volume ratio of quartz sand in the hydrate formation matrix layer to the lower gas matrix layer is 21:79. The NaCl concentration in the seawater salinity solution is 3.5 wt%. The NaCl concentration in the high-salt solution is 25 wt%.

[0026] 2) The system temperature was reduced to 3℃ by air bath; the axial pressure and confining pressure were adjusted using a hand pump of a triaxial apparatus to simulate the stress conditions of a natural hydrate reservoir; the axial pressure reached 8.5 MPa; the confining pressure reached 8.0 MPa.

[0027] 3) Introduce excess methane gas into the reactor until the steady-state pressure reaches 6.8 MPa;

[0028] 4) Rotate the reactor to a horizontal position, and hydrates will form in the matrix layer.

[0029] 5) The reactor is reset to form a composite reservoir structure consisting of an overlying hydrate layer and an underlying free gas layer.

[0030] like Figure 3 As shown, temperature sensors T1 and T2 were used to measure the temperature of the hydrate formation matrix layer, while T3 and T4 were used to measure the temperature of the underlying gas matrix layer. Under the initial 21% HBS conditions, the temperature changes in the upper and lower layers during the hydrate formation stage were significantly different. Taking the first gas injection as an example, the temperatures of T1 and T2 rose from 5.4℃ and 5.0℃ to 11.9℃ and 11.3℃, respectively, after the injection; while the temperatures of T3 and T4 rose from 4.6℃ and 4.4℃ to 6.9℃ and 8.5℃, respectively. The comparison shows that the temperature rise in the upper region of the reservoir was more significant after gas injection. This difference stems from the hydrate formation reaction occurring in the upper layer of the reservoir, whose accompanying exothermic process significantly promoted the temperature recovery in the upper layer. Therefore, the more pronounced temperature recovery process in the upper layer after gas injection confirms the successful formation of a complex hydrate reservoir structure with an upper hydrate layer and an underlying gas layer.

[0031] like Figure 4 As shown, when the system pressure drops from 6.8 MPa to 3.5 MPa, the system temperature exhibits a trend of first rapidly decreasing and then slowly recovering. Furthermore, compared to the lower layer temperature, the upper layer temperature not only decreases more significantly but also recovers more slowly. In addition, the starting point of the temperature change in the upper layer lags behind that in the lower layer, indicating that the temperature change in the upper layer is more significantly affected.

[0032] Since the decomposition of hydrates is endothermic, the above temperature change characteristics (especially the significant difference between the upper and lower layers) confirm that hydrates were formed in the upper layer of the reservoir while no hydrates were formed in the lower layer.

Claims

1. Method for the remodelling of associated underlying gas hydrate reservoirs based on the concentration of salts and the pressure gradient, characterised in that, The method comprises the following steps: 1) using a triaxial apparatus, adding a hydrate-forming substrate layer and a underlying gas substrate layer into a reactor, the hydrate-forming substrate layer being on the upper part of the underlying gas substrate layer; the hydrate-forming substrate layer being prepared by seawater salinity solution and quartz sand; the underlying gas substrate layer being prepared by high-salinity solution and quartz sand; 2) reducing the system temperature to 2-4℃ by air bath; 3) introducing excess methane gas into the reactor until the steady-state pressure is 5-25 MPa; 4) rotating the reactor to be horizontal, and forming hydrates in the hydrate-forming substrate layer; 5) resetting the reactor to form a composite reservoir structure of an overlying hydrate layer and an underlying free gas layer.

2. The method for remodelling of salt concentration and pressure gradient dependent underlying gas hydrate reservoirs according to claim 1, characterized in that, The volume ratio of the hydrate-forming substrate layer to the underlying gas substrate layer is 15-85:85-15.

3. The method for remodelling of salt concentration and pressure gradient dependent underlying gas hydrate reservoirs according to claim 1, characterized in that, The NaCl concentration in the seawater salinity solution is 3.35-3.5 wt%; the NaCl concentration in the high-salinity solution is 24-26 wt%.

4. The method for remodelling of salt concentration and pressure gradient dependent underlying gas hydrate reservoirs as claimed in claim 1 wherein, The hand pump of the triaxial apparatus is used to adjust the axial pressure and the confining pressure to simulate the stress condition of a natural hydrate reservoir; the axial pressure reaches 8.0-9.0 MPa; and the confining pressure reaches 7.0-8.0 MPa.

Images