CN113656744B - Ocean hydrate zone pit depth estimation and free air layer evaluation method - Google Patents

Abstract

The present invention discloses a method for estimating the depth of pockmarks in an ocean hydrate zone and evaluating a free gas layer, comprising: obtaining gas leakage process parameters, and inputting the parameters into a fluid leakage model in a porous medium, the fluid leakage model in a porous medium outputting leakage fluid velocity; inputting the leakage fluid velocity into a pockmark depth model to obtain the pockmark depth; and inverting the methane concentration in the free gas layer underlying the hydrate through the pockmark depth. The method first predicts the seabed methane leakage through a fluid leakage model in a porous medium, then estimates the leakage according to the leakage, and finally inverts the methane concentration in the free gas layer underlying the hydrate through the pockmark depth. The method is beneficial for predicting seabed methane leakage and disasters on the one hand, and is beneficial for evaluating the amount of seabed hydrocarbon resources on the other.

Description

A method for estimating the depth of pits and evaluating free gas layers in marine hydrate areas

Technical Field

The invention relates to the field of marine geology and natural gas hydrates, and in particular to a method for estimating the depth of seabed pits in a marine natural gas hydrate area and the methane concentration of a free gas layer underlying a hydrate layer.

Background Art

Natural gas hydrate is a solid compound with an appearance similar to ice. It is a cage-like compound composed of low molecular weight gases (mainly hydrocarbon molecules, such as methane, ethane, etc., as well as small molecular gases such as carbon dioxide and hydrogen sulfide) and water molecules under low temperature and high pressure conditions. Natural gas hydrates in nature are mainly formed by methane gas, and because of their appearance similar to ice, they are generally called combustible ice. Methane hydrates are mainly stored in deep-water continental slope environments on the seabed and permafrost areas on land. Natural gas hydrates can release 164 to 180 ^m3 of methane gas and 0.87 ^m3 of water under standard conditions. According to conservative estimates, the content of natural gas hydrates in nature is 21× ^10m3 , which is almost twice the known fossil energy on Earth. It is considered to be an ideal alternative energy source for fossil energy in the 21st century.

When the concentration of dissolved methane in pore water in the hydrate stability zone of the marine environment exceeds the solubility of methane hydrate formation, dissolved methane will crystallize to form hydrates. As the hydrate content increases, a hydrate layer trap is formed, and a free gas layer develops below it. Under certain conditions, the free gas below the hydrate layer leaks upward along the channel into the seabed and forms pits, authigenic carbonate rocks, biomes, and bubble plumes on the seabed, such as the Oregon Hydrate Ridge, the Blake Platform, the North Congo Slope, the Norwegian Sea, and the South China Sea.

The free gas beneath the hydrate layer is sealed by the capillary force of the hydrate layer. As the gas accumulates and the thickness of the gas layer increases, the gas pressure at the interface between the hydrate and the free gas will increase. When the gas overpressure overcomes the capillary seal, it stimulates gas leakage. The overpressure gas pushes the pore water upward to form pits on the seabed. The depth of the pits reflects the destructive intensity of the fluid and the overpressure amplitude of the free gas layer.

Summary of the invention

The present invention aims to provide a method for estimating the depth of pockmarks in marine hydrate zones and evaluating free gas layers, which is beneficial for predicting submarine methane leakage and disasters on the one hand, and for evaluating the amount of submarine hydrocarbon resources on the other hand.

To achieve the above object, the technical solution of the present invention is:

A method for estimating the depth of pockmarks in an ocean hydrate zone and evaluating free gas layers, comprising:

Obtaining gas leakage process parameters, and inputting the parameters into a fluid leakage model in a porous medium, and the fluid leakage model in a porous medium outputs a leakage fluid velocity;

Input the leakage fluid velocity into the pit depth model to obtain the pit depth;

The methane concentration in the free gas layer beneath the hydrate is inverted by the pit depth.

Furthermore, the fluid leakage model in the porous medium is:

In the above equation, ΔP is the total driving force for fluid migration, which is the sum of the pressure drops applied to the gas flow column and the water flow column (ΔP _g +ΔP _w ), equal to the overpressure of the gas reservoir; ρ is the fluid density, d is the thickness of the free gas layer, μ is the fluid viscosity, V is the fluid velocity, k is the permeability of the sediment, k _rg and k _rw are the relative permeabilities of the pore gas and water in the sediment, respectively, and h _g and h _w are the heights of the gas flow column and the water flow column, respectively.

Further, the leakage fluid velocity is:

In formula (2), the approximate relationship It is known that the fluid migration velocity increases with the growth of the air flow column height ( _hg = _hhw ).

Further, the step of inputting the leakage fluid velocity into the pit depth model to obtain the pit depth includes:

The leakage fluid velocity satisfies After the quicksand deposits are removed by the ocean current, pits are formed on the seabed. The bottom boundary of the quicksand deposit body determines the depth of the pit. In formula (2), Replacing the fluid velocity and the water column height h _w with the pit depth h _pm , we get the pit depth model:

In formula (3) Simplified to:

The density and viscosity of the fluid are constant under the conditions of temperature and pressure.

Further, the inversion of the methane concentration in the free gas layer beneath the hydrate by the pit depth includes:

Through the methane seepage and pit depth model, it is known that pits are formed in the relationship between gas overpressure, hydrate layer bottom depth and sediment density:

By transforming the equation, the thickness equation of the underlying free gas reservoir sealed by hydrate is obtained. The thickness of the gas layer depends on the depth of the seabed pit and the fluid density in the gas reservoir:

The average fluid density in the gas reservoir is related to the fluid saturation and the overburden pressure. The calculation equation for predicting the fluid density in the gas reservoir is obtained by transforming equation (6):

Under the appropriate conditions, the density of water is considered to be a constant, and the density of the gas is approximately a function of the hydrostatic pressure. Therefore, the gas saturation calculation equation is expressed as:

Formula (8) is used to estimate gas saturation, where the depth of seafloor pits and the thickness of the underlying gas layer under the sealing layer can be obtained through ocean acoustic surveys.

Compared with the prior art, the present invention has the following beneficial effects:

This method first predicts the submarine methane leakage through the fluid leakage model in the porous medium, then estimates the leakage situation based on the leakage situation, and finally inverts the methane concentration in the free gas layer under the hydrate through the pit depth. This method is beneficial for predicting submarine methane leakage and disasters on the one hand, and for evaluating the amount of submarine hydrocarbon resources on the other hand.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a flow chart of a method for estimating the depth of pockmarks in an ocean hydrate zone and evaluating free gas layers provided by an embodiment of the present invention;

FIG. 2 is a schematic diagram showing the process of seafloor fluid leakage and seafloor pit formation.

DETAILED DESCRIPTION

Example:

The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments.

Referring to FIG. 1 , the present embodiment provides a method for estimating the depth of a marine hydrate zone pit and evaluating a free gas layer, which mainly includes the following steps:

101. Obtain gas leakage process parameters, and input the parameters into a fluid leakage model in a porous medium, and the fluid leakage model in a porous medium outputs a leakage fluid velocity;

102. Input the leakage fluid velocity into the pit depth model to obtain the pit depth;

103. The methane concentration in the free gas layer beneath the hydrate is inverted by the depth of the pit.

It can be seen that this method first predicts the submarine methane leakage through the fluid leakage model in the porous medium, then estimates the leakage situation based on the leakage situation, and finally inverts the methane concentration in the free gas layer under the hydrate through the pit depth. This method is beneficial for predicting submarine methane leakage and disasters on the one hand, and for evaluating the amount of submarine hydrocarbon resources on the other hand.

During gas leakage, both the gas column and the water column flow under the overpressure of the free gas enclosed by the hydrate, and the total driving force of fluid migration is equal to the gas overpressure (ρ _w -ρ _g )gd. The gas flow column continues to increase and pushes the overlying pore water in the leakage channel upward at the same speed. Assuming that the hydrate stability zone is a homogeneous body, the fluid leakage rate in the leakage channel is the same. Therefore, the fluid leakage model in the porous medium is:

In the above equation, ΔP is the total driving force for fluid migration, which is the sum of the pressure drops applied to the gas flow column and the water flow column (ΔP _g +ΔP _w ), and is equal to the overpressure of the gas reservoir. ρ is the fluid density, d is the thickness of the free gas layer, μ is the fluid viscosity, μ _g and μ _w are the viscosities of gas and water respectively, V is the fluid velocity, k is the permeability of the sediment, k _rg and k _rw are the relative permeabilities of gas and water in the sediment pores respectively, and h _g and h _w are the heights of the gas flow column and the water flow column respectively. Since it is assumed that the saturation of gas in the gas flow column and the saturation of water in the water flow column are both 1, the relative permeability of gas and water is 1. Fluid leakage migration velocity:

In equation (2), the approximate relationship It can be seen that the fluid migration velocity increases with the growth of the air flow column height ( _hg = _hhw ).

The impedance of the fluid per unit length in the porous medium increases with the increase of the height of the airflow column, that is, the pore fluid pressure on the sediment frame gradually increases. When the fluid impedance exceeds the static rock pressure of the corresponding sediment body, the corresponding sediment layer will be fluidized and become quicksand, that is, the leakage fluid velocity must satisfy When quicksand deposits are removed by the ocean current, pits are formed on the seafloor. The bottom boundary of the quicksand deposit determines the depth of the pits. Replacing the fluid velocity and the water column height h _w with the pit depth h _pm , we can get the pit depth model:

In equation (3) The equation can be simplified to:

Under certain temperature and pressure conditions, the density and viscosity of the fluid are constant.

Through the above-mentioned methane leakage and pit depth model, it can be seen that the formation of pits is related to gas overpressure, hydrate layer bottom depth and sediment density.

By transforming the equation, we can get the thickness equation of the underlying free gas reservoir sealed by hydrate. The thickness of the gas layer depends on the depth of the seabed pit and the fluid density in the gas reservoir.

The average fluid density in the gas reservoir is related to the fluid saturation and the overburden pressure. By transforming equation (6), we can get the calculation equation for predicting the fluid density in the gas reservoir.

Under certain circumstances, the density of water is considered to be a constant, and the density of the gas can be approximated as a function of the hydrostatic pressure. Therefore, the gas saturation calculation equation can be expressed as:

Equation (8) can be used to estimate gas saturation, where the depth of seafloor pockmarks and the thickness of the underlying gas layer can be obtained through ocean acoustic surveys, and the relevant data are relatively common.

FIG2 shows a schematic diagram of the principle of free gas leakage below hydrate and formation of seafloor pits, where Z is the depth below the seafloor and h is the thickness of the hydrate stability zone (or the depth of the hydrate sealing layer). The process is as follows.

At t _0, the free gas is trapped below the hydrate layer.

At t ₁ , the gas pierces the sealing layer and begins to leak.

At t ₂ , the height of the air column increases, pushing the water to flow outward, the height of the water column shortens accordingly, and the fluid migration speed continues to increase.

At time t ₃ , the pore pressure in the water-bearing flow deposits exceeds the static rock pressure, and pits appear on the seabed, forming a single air flow channel.

At t ₄ , the natural gas in the free gas reservoir is gradually drained, the pore overpressure disappears, and the gas flow column in the fluid channel gradually degenerates.

At t ₅ , the air flow column completely disappears, leaving a gas chimney on the seabed. The hydrate sealing effect is restored, and new gas accumulation begins.

The depth of the seafloor pit can be calculated using equation (4). The unknown variables are the thickness of the free gas layer and the depth of the methane sealing layer.

Equation (8) can be used to calculate the methane saturation in the free gas reservoir underlying the hydrate. The unknown variables are the thickness of the free gas layer, the depth of the pit, and the depth of the sealing layer.

The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable ordinary technicians in the field to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made based on the essence of the content of the present invention should be included in the protection scope of the present invention.

Claims (1)

1. A method for estimating pit depth and evaluating free gas layer in a marine hydrate region, comprising:

Acquiring a gas leakage process parameter, inputting the parameter into a pore medium fluid leakage model, and outputting leakage fluid speed by the pore medium fluid leakage model;

inputting the seepage fluid speed into a pit depth model to obtain pit depth;

inverting the methane concentration in the hydrate subsurface free gas layer by pit depth;

The pore medium fluid leakage model is as follows:

Δp in the above equation is the total driving force of fluid migration, which is the sum Δp _g+ΔP_w of the pressure drops applied to the airflow column and the water flow column, and is equal to the overpressure of the gas reservoir; d is the thickness of the free gas layer, V is the seepage fluid velocity, k is the permeability of the sediment, k _rg and k _rw are the relative permeabilities of the sediment pore gas and water respectively, and h _g and h _w are the heights of the gas flow column and the water flow column respectively;

The leakage fluid velocity is:

In the formula (2) in approximate relation Knowing that the leakage fluid velocity increases with increasing air column height h _g＝h-h_w;

the inputting the leakage fluid velocity into the pit depth model to obtain the pit depth comprises:

The leakage fluid speed is satisfied Forming a pit on the sea floor after the quicksand sediment is removed by ocean currents, wherein the pit depth is determined by the bottom boundary of the quicksand sediment; in formula (2)Replacing the seepage fluid speed, and replacing the water flow column height h _w with the pit depth h _pm to obtain a pit depth model:

in (3) The simplification is as follows:

the density and viscosity of the fluid are constant under the condition of meeting the temperature and pressure;

The inverting methane concentration in the hydrate subsurface free gas layer by pit depth comprises:

by a pit depth model, pit formation is known to be related to gas overpressure, hydrate bed bottom boundary depth and sediment density:

And obtaining a thickness equation of the hydrate-enclosed down-cover free gas reservoir through equation deformation, wherein the thickness of the gas layer depends on the depth of the submarine pit and the density of fluid in the gas reservoir:

the average fluid density in the gas reservoir is related to fluid saturation and overburden pressure; obtaining a calculation equation for predicting the fluid density in the gas reservoir by the deformation of the formula (6):

In a satisfactory environment, the density of water is considered to be constant and the density of gas is approximated as a function of the hydrostatic pressure, so the gas saturation calculation equation is expressed as: