RU2403379C1 - Method of gas production from natural accumulations of gas hydrates - Google Patents

Abstract

FIELD: oil and gas industry.

SUBSTANCE: method involves arrangement of dome-shaped gas collector above accumulation of gas hydrates, mechanical destruction of the rock under gas collector, which contains gas hydrates, with its simultaneous loosening and muddying, collection under dome of gas collector of rock destruction products and gas hydrates decomposition products, separation and removal of gas from gas collector.

EFFECT: increasing effectiveness of method owing to reducing the costs for development of underwater deposits of gas hydrates, and method simplification.

9 cl, 7 dwg

Description

FIELD OF TECHNOLOGY

The claimed invention relates to the field of gas production, in particular to methods of gas production from shallow-lying accumulations of gas hydrates in underwater conditions.

BACKGROUND

Gas hydrates are solid compounds of natural gas and water formed under relatively high pressure and low temperature conditions. They are crystalline, macroscopically ice-like substances belonging to a special type of compounds, which are described by the general formula M · nH ₂ O, where M is a hydrate-forming gas molecule and n is the number of water molecules (from 6 to 17), characterizing the composition of the compound and depending on hydrate formation conditions. Most components of natural gas, with the exception of hydrogen, helium, neon, normal butane and heavier hydrocarbons, are capable of forming gas hydrates. The most common natural hydrate forming gas is methane. A unit volume of methane hydrate can contain up to 164 volumes of gas under normal conditions (i.e., at atmospheric pressure and air temperature above zero). Gas hydrates are one of the forms of the existence of natural gas in the bowels under certain thermodynamic and geological conditions.

A feature of gas hydrates, in particular underwater, is their distribution mainly in the form of clusters confined mainly to deep-sea water areas and polar shelves, since they can only be stable under conditions of extreme gas saturation (at appropriate pressure and temperature) of pore water coexisting with them. For the formation of a gas hydrate and finding it in a stable state for a long time, the following conditions must be met: stable temperature and pressure, as well as a sufficient amount of gas for supersaturation of pore water and precipitation in hydrated form. Such conditions are possible only in certain geological environments in which a relatively constant migration of gas into the hydrate formation zone is provided in one form or another.

The term "accumulation of gas hydrates" (GHG) should be understood as a certain volume of rock, the pore space of which is more or less occupied by hydrates. The sizes of such accumulations can be any, and they are limited only by the distribution region of extremely gas-saturated pore water, since under the conditions of under saturation gas hydrates cannot exist. The position of hydrate accumulations is controlled by geological heterogeneities, such as:

temperature field determining the solubility of the gas in water;

a permeability field that determines the conditions for the migration of fluids (gas and water);

the salinity of pore water, also affecting the solubility of the gas;

gas generation conditions (determine the presence of a sufficient amount of gas for supersaturation of pore water).

An analysis of the data related to hydrate occurrences known today and their signs in the oceans and lakes allows us to conclude that underwater gas hydrates can form clusters of two types. The first type includes clusters located at a significant sub-basin depth (hundreds of meters) and controlled by permeability zones under the conditions of dispersed fluid filtration, which is confirmed by the results of deep-sea drilling. Clusters of the second type are located in the immediate vicinity of the bottom, at the bottom, or at a very shallow bottom depth (first meters) in the zones of concentrated (concentrated) discharge of gas-containing fluids and are controlled by fluid conductors (faults, mud volcanoes and diapirs). Within such zones, part of the gas rising from deep horizons through cracks and faults and by diffusion passes into the solid phase. This happens near the water-bottom section due to a decrease in the temperature and solubility of the gas in the pore water near the bottom, which contributes to the supersaturation of pore water by the gas and the precipitation of hydrates. When the gas flow ceases, the stability conditions are violated and the solid hydrate gradually decomposes into gas and water. The main factor controlling the formation and presence of a hydrate in a stable state is the equilibrium gas concentration (or its solubility). It is this factor that determines the minimum amount of gas required to maintain gas hydrate in a stable state. Once a hydrate has formed in unconsolidated marine sediments, the concentration of gas in pore water is fixed by equilibrium concentrations. As a result of this, a change in the solubility with depth controls the loss of gas by diffusion from the places where the hydrate was formed. Thus, to maintain a constant volume of the hydrated aggregate at the place of its formation, a more or less constant gas flow to this place is necessary. The hydrate being in a stable state is determined by the shape of the graph of gas solubility in water within the stability zone of the gas hydrate, which, in turn, is determined by the values of external temperature and pressure.

The gas in the SGH is in a solid state associated with water and requires unconventional methods for their development. There are various known methods of gas production from SHG generated in subaquatic environments, however, the choice of technology for the development of SHG depends on the specific geological and physical conditions of their occurrence. Three main methods are known for inducing gas inflow from hydrate-bearing strata: lowering the pressure below the equilibrium pressure of hydrate formation at a given temperature, heating hydrate-bearing rocks above equilibrium temperature, and also their mechanical destruction. In addition, solutions are known in which it is proposed to use reagents that can affect the chemical activity of water and gas, which leads to a shift in the equilibrium state of the formation and decomposition of gas hydrates to lower temperatures (the so-called inhibitors - methanol, ethylene glycol, electrolyte solutions and other). However, the use of such reagents for the purpose of gas production still does not find practical application in connection with their high cost or low efficiency of this method. Other proposed methods of exposure, in particular electromagnetic, acoustic and carbon dioxide injection into the reservoir, have not yet been studied experimentally. Most of the proposed methods for the development of SGH involve a combination of the above methods.

Pressure reduction is achieved by taking free gas from horizons that are hydrate-containing and are outside the stability conditions of gas hydrates. The method described in http://www.gazeta.ru/news/lenta/2009/04/13/n_1351615.shtml is based on the fact that a decrease in pore pressure below the equilibrium pressure of hydrate formation leads to decomposition of a part of gas hydrates and gas evolution to a free state .

One example of methods for the development of SGH using pressure reduction is the method described in international application WO 2007/072172, in which pressure reduction is achieved by pumping gas from lower horizons. This method also involves the injection into the reservoir of water followed by its pumping out using a special pump in order to control the pressure in the hydrate-bearing formation. However, this method is suitable for formations where the hydrate saturation is low and the gas or water has not lost its mobility. Naturally, with an increase in hydration saturation (and hence a decrease in permeability), the effectiveness of this method drops sharply. So, at a pore saturation with hydrates of more than 80%, it is practically impossible to obtain inflow from hydrates by reducing bottomhole pressure. Another disadvantage of the method based on reducing the pressure in the hydrate-bearing formation is associated with secondary technogenic formation of hydrates in the bottomhole zone due to the Joule-Thomson effect.

Known methods for the development of SHG, carried out by heating the deposits. So, in patent US 6192691 describes a method of collecting gas released from the bottom of the bottom of the GHG. When implementing the method, an elastic screen is installed above the gas evolution point, which is attached along the perimeter to the bottom. The gas released from the SGH stretches the screen, giving it a domed shape, is collected under the formed dome and is diverted via a pipeline to the vessel. To disturb the equilibrium state of hydrates and to intensify the gas outlet, it is proposed to pump hot water under the dome. However, this method is characterized by low productivity, and to increase it, it is either necessary to increase the area covered by the screen, or, as proposed, to supply hot water, which significantly complicates the equipment and increases energy costs.

In the application US 20050161217 describes a method for the production of methane by heating the productive formations of the SGG using electrical energy supplied from a source placed on the vessel. One of the supply electrodes is installed around the wellbore in the hydrate reservoir, and the other, for example, on the surface of the hydrate reservoir or on the bottom of the vessel. Due to the heating of the hydrate-bearing formation by passing an electric current through it, methane is released, which is fed through the borehole to the vessel into the gas accumulator.

In the international application WO 2007/136485 a gas production method is described which involves heating the SHG by the radiation energy of a diode laser or a solid-state laser pumped by a diode laser.

Known methods for the development of SGH, combining the thermal effect on the hydrate-containing formation and the injection of inhibitors into it. Thus, US Pat. No. 4,424,866 describes a gas production method comprising injecting a hot, saturated solution of CaCl ₂ or CaBr ₂ into a reservoir, which decomposes solid hydrates, and in the method described in US Pat. No. 6,733,573, acid catalysts for hydrate decomposition are proposed for a similar purpose. . However, the use of expensive inhibitors does not allow obtaining the profitability of such methods sufficient for practical use.

Methods involving thermal effects on the SGH are suitable for formations having a high hydrate content in the pores. The main disadvantage of thermal methods for the development of SHG should be attributed to the large energy costs associated with heating the deposits. So, only the heat of phase transition of methane hydrate is at least 7% of the calorific value of the released gas. Moreover, in real conditions, before the decomposition of the hydrate begins, the reservoir must be warmed up to equilibrium temperature. However, due to the low thermal conductivity of the SGH and large heat losses to the surrounding rock and water, the use of downhole heaters for this purpose is inefficient. A significant part of the energy is spent on overheating of rocks adjacent to the source of thermal energy, while the heating area remains small, and the energy costs are significant. To ensure greater efficiency of the thermal effect on the SHG, a significant contact surface of the heat source with the SHG is necessary, which is associated with significant equipment costs.

Known methods for the career development of oceanic GHGs, in which hydrates are either in loose, completely non-cemented deposits, or they themselves are cemented by mechanical destruction.

So, in the application US 2008/0088171 describes a method for the career development of GHG from the bottom of the sea, carried out by taking hydrated sediments near the water-bottom section using special means resembling underwater excavators, then lifting the hydrate-containing sediment to the surface in containers (tubs) and accumulating hydrated gas under dome, arranged in the bottom of the vessel. Despite the apparent simplicity of the method, the implementation of this invention requires the creation of complex and expensive equipment. In addition, significant losses of gas from decomposed hydrates are possible on the way to the place of its accumulation due to oxidation, diffusion, and carryover.

International application WO 00/47832 describes a method for producing hydrates formed directly at the bottom and in bottom sediments. The method provides for the supply of compressed air and a special solution with a high density through the pipe (chemical composition is not indicated) in the direction of the seabed, as a result of which it is assumed to destroy the SHG, tear off and float pieces of hydrate into the overlying water layers, their subsequent collection and separation of gas and water. It is also proposed to improve the described procedures by additional heating of compressed air and the injected fluid. The disadvantage of this method is the need for complex and expensive equipment and significant energy costs. And given the fact that often the surface of gas discharge centers, to which the SHG is confined, is covered with solid crusts and plates of chemogenic carbonate minerals deposited from carbon dioxide in places of gas discharge, the application of a method involving their destruction with an air jet seems to be ineffective. It is also doubtful that when using this development, a significant depth of effect on hydrate-containing deposits can be achieved.

The technical problem to be solved by the claimed invention is directed is to reduce the cost of developing underwater gas hydrate deposits by using a simple and effective method.

SUMMARY OF THE INVENTION

The inventive method of producing gas from bottom accumulations of gas hydrates includes placing a domed gas collector above the gas hydrate accumulation, destroying the rock containing gas hydrates under the gas collector, while loosening and stirring it up, collecting rock destruction and decomposition products of gas hydrates under the dome of the gas collector, separation and removal from the gas collector of the released gas.

Due to the indicated destruction of the rock containing gas hydrates, its loosening and agitation, the products of rock destruction are concentrated under the dome of the gas collector, in which the intensive decomposition of hydrates occurs with the release of gas, which is then separated and discharged. Intensive mixing of the disintegrated material with water violates the stability conditions of hydrates, which leads to the decomposition of the hydrate into gas and water. As a result of this effect on hydrated rock, several stability parameters are violated at once. As a result of the separation of hydrate aggregates from the substrate and its ascent upward under the gas collector dome, as well as loosening and stirring, the temperature rises and the hydrostatic pressure decreases and at the same time the gas inflow from the underlying sediments ceases. Under these influences on the rock, the solubility of gas for pop-up products increases, causing decomposition of the hydrate.

Usually, to isolate gas from hydrates, they try to upset the equilibrium state of gas hydrates by changing the temperature or pressure inside the reservoir, without taking into account the influence of gas solubility in water on their stability. It has been theoretically and experimentally proved that it is the change in the parameters of gas solubility in water that is one of the factors controlling both the formation and decomposition of the hydrate. This is determined by an important feature of the solubility of a hydrocarbon gas in water in equilibrium with hydrate, which is little dependent on external pressure, and is controlled mainly by the pressure of hydrate formation, equilibrium for a given temperature. Since the equilibrium pressure of gas hydrate formation also becomes large with increasing temperature, the solubility of the gas in water in equilibrium with hydrate also increases, which leads to the decomposition of the latter.

The destruction of the rock containing gas hydrates with its simultaneous loosening and agitation is carried out under the dome of the gas collector, where the released gas is separated and taken to its place of collection, preventing gas loss.

The destruction of the rock, loosening and stirring up can be done mechanically, for example, using a drill, auger drill, auger. The choice of a tool for destruction in this case is determined by the fact that the tool not only destroys, but also loosens and stirs up the loosened rock, contributing to the release of gas.

Also, the destruction of hydrated rocks, loosening and agitation can be done using a high-pressure water jet.

Additionally, water can be supplied under the dome of the gas collector. This leads to a decrease in the gas concentration in the water under the dome and to a change in the stability conditions of the hydrate, and therefore to a more intense gas evolution. A decrease in the concentration of gas in water prevents the formation of secondary hydration films on the bubbles of free gas, which can occur due to the release of it in significant quantities.

The water supplied under the dome of the gas collector can be taken from the upper layers of the water column. In this case, the efficiency of the method is increased due to the higher temperature of the surface water relative to the bottom, which has an additional destabilizing effect on the gas hydrate.

In the particular case, a gas collector in the form of a folding dome can be used as a gas collector. This design simplifies the procedure for lowering the gas collector overboard the floating means, facilitates maintaining a stable position of the floating means during the installation of the gas collector at the bottom, which allows to increase the size of the gas collector, providing the overlap of a larger area of the developed reservoir.

In the particular case, the evacuation of gas from the gas collector is carried out in containers placed on a floating vessel, for example, on a ship.

Additionally, the position of the gas collector over the accumulation of gas hydrates relative to said floating means can be monitored. Monitoring the position of the gas collector can be carried out using devices installed on the bottom.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated by the following drawings.

Figure 1 conditionally shows a diagram of the occurrence of bottom accumulations of gas hydrates.

Figure 2 illustrates the process of gas extraction from the bottom accumulation of gas hydrates in accordance with the claimed method.

Figure 3 and Figure 4 shows the design of a stationary domed gas collector with two types of mechanical tools for rock destruction, loosening and agitating.

Figure 5 shows the folding gas collector before installation.

Figure 6 shows an example of a method with a folding gas collector and a screw tool with a horizontal axis of rotation.

7 shows an example with a folding gas collector and a screw tool with a vertical axis of rotation.

DETAILED DESCRIPTION OF THE INVENTION

To select the object of development - the accumulation of gas hydrates, a preliminary geophysical survey is necessary. Typically, clusters are structures at the bottom with diameters of 50-1000 m, slightly expressed in relief (excess above the bottom is on average 0-100 m). The shape of the accumulation of 3 gas hydrates (Figure 1) on the bottom 1 in the plan is usually approximated by a circle, in some cases a more complex isometric shape may take place. The shape of the bottom accumulations of 3 gas hydrates, in the particular case, corresponds to a truncated cone and is determined by the shape of the diffusion halo of scattering of the upward gas stream 4, part of which accumulates in the rocks in the form of hydrate, the other part leaves the bottom surface 2 in the form of free gas bubbles 5.

The method is as follows. Preliminary work (soil sampling) estimates the minimum bottom depth of hydrates and cluster boundaries. The domed gas collector 6 (FIG. 2) is lowered overboard the vessel 10 using a standard ship hoisting device, such as a winch 11. The gas collector 6, including a gas separator (not shown in FIG. 2), is connected to a gas storage tank (in FIG. 2 not shown) using a hose 9 for pumping gas from the gas collector. The rock cutting tool 7, which in this example is a screw mill with a vertical axis of rotation, destroys the hydrated rock within the accumulation of gas hydrates, limited by the dashed line in FIG. 2, while loosening it and stirring it up. As a tool driven by engine 8, a screw mill with a horizontal axis of rotation (FIG. 4, FIG. 6) with carbide or diamond inserts on the forming screws can be used. As a tool, you can also use a drill, auger drill or auger, which provide not only the destruction of the rock, but also loosening and stirring. An engine 8 or several engines driving a tool may receive power from a ship.

When implementing the inventive method, various methods of mechanical destruction of the rock can be used: planing, milling, drilling and chipping, it is important that at the same time loosening and stirring is also carried out. Destruction of the breed can be done with some tools, and loosening and stirring up with others.

Simultaneously with the above actions, the violation of the stability conditions of the gas hydrate can be additionally carried out by pumping water from surface horizons (for example, sea water near the vessel) under the dome of the gas collector.

The hydrate begins to decompose into gas and water is already in the sediment, then it breaks away from the substrate and floats, continuing to decompose in water. In this case, part of the gas is dissolved in water, the rest is released into the gas phase. Water-soluble gas and gas in the free phase (in the form of bubbles) accumulates under a domed gas collector 6, is separated and pumped through a hose 9 to a container located on the vessel 10. Gas can be separated from the liquid using a standard separator installed in the upper part of the gas collector 6 ( figure 2 is not shown). The collection of evolved gas can also be organized in tanks located at the bottom.

The effectiveness of this method has been proven in the process of experimental work in places of known near-surface accumulations of gas hydrates in the Gulf of Mexico, the coast of about. Vancouver The essence of the experiment was the impact on the hydrated plates and hills formed on the surface by remotely controlled devices (loosening the soil). Due to the physicochemical properties of hydrate aggregates, similar to the properties of ordinary ice, pieces and hydrate aggregates break away from the substrate with an insignificant effect, float and begin to decompose, emitting gas bubbles.

The process can be controlled through direct observations using a video camera and sensors: a sensor measuring gas concentration of methane in water, a pressure sensor, and a temperature sensor installed in the upper part of the gas collector dome 6.

In this case, secondary hydrate formation, which can occur during the supersaturation of sea water located under the dome of the gas collector 6 with gas from decomposed hydrates, is prevented by lowering the concentration of water-dissolved gas during its separation and pumping into the tank and, in addition, when water is injected from the sea surface under the gas collector .

The vessel 10 (see FIG. 2) is held in position above the gas collector 6 using an underwater navigation system that includes bottom repeaters 13, a receiver 12 on the vessel 10 and an emitter 14 (FIG. 3, FIG. 4) mounted on the gas collector 6 .

This method can be successfully implemented at depths of 390-2000 m, water temperature 1-15 ° C.

Claims (9)

1. A method of producing gas from bottom accumulations of gas hydrates, including placing a domed gas collector above the gas hydrate accumulation, mechanically breaking rock under gas collector containing gas hydrates, while loosening and stirring it up, collecting rock destruction and decomposition products of gas hydrates under the dome of the gas collector, separation and removal of gas from the gas collector. 2. The method according to claim 1, characterized in that the mechanical destruction of the rock, loosening and agitation is carried out using a drill. 3. The method according to claim 2, characterized in that the mechanical destruction of the rock is carried out using a screw drill, auger. 4. The method according to claim 1, characterized in that additionally under the dome of the gas collector serves water. 5. The method according to claim 4, characterized in that the water supplied under the dome is taken from the upper layers of the water column. 6. The method according to claim 1, characterized in that they use a folding gas collector. 7. The method according to claim 1, characterized in that the exhaust gas from the gas collector is carried out in containers placed on a floating vessel, for example, on a ship. 8. The method according to claim 1, characterized in that it further control the position of the gas collector over the accumulation of gas hydrates relative to the aforementioned floating means. 9. The method according to claim 8, characterized in that the position of the gas collector is carried out using devices installed on the bottom.

Images