ИНЖЕКЦИЯ ЖИДКОГО ДИОКСИДА УГЛЕРОДА В ПЛАСТ, НАСЫЩЕННЫЙ МЕТАНОМ И ЕГО ГАЗОГИДРАТОМ

Based on the equations of continuous medium mechanics, a mathematical model is constructed for the injection of liquid carbon dioxide into the porous medium saturated with methane and water for the case when the injection is accompanied by the formation of gas hydrates of carbon dioxide and methane. Self-similar solutions to the one-dimensional problem describing the evolution of hydrodynamic and temperature fields are constructed. The dynamics of movement of frontal boundaries of phase transitions with respect to injection pressure and temperature, as well as with respect to permeability and initial pressure of the porous medium is investigated. It is shown that intensity of the formation of methane and carbon dioxide gas hydrates increases with an increase in injection pressure and reservoir permeability, and at rather high values of permeability, pressure drop in the reservoir and injection temperature, the frontal formation boundaries of methane and carbon dioxide gas hydrates may merge. The dependence of the limit values of injection pressure and temperature corresponding to the merger of the phase transition boundaries on the permeability and initial pressure is studied in this work.

Mathematical model of injection of liquid carbon dioxide in a reservoir saturated with methane and its hydrate

Effect of pore water on the depressurization of gas hydrate in clayey silt sediments

Specificities of an injection of "warm" liquid carbon dioxide into a porous medium saturated with methane and its gas hydrate are studied using a mathematical model presented in this work. The release of methane from gas hydrate during injection can proceed in two different regimes. In the first regime, the injection is accompanied by the replacement of methane with carbon dioxide in methane hydrate without the release of free water. In the second one, the injection is accompanied by the decomposition of methane hydrate into methane and water and by the formation of carbon dioxide gas hydrate. For each regime, selfsimilar solutions are constructed and critical conditions separating these regimes are found.

Recently, special attention is paid to the development of deposits of natural gas hydrates due to their wide distribution in nature and the large reserves of hydrocarbons contained in such deposits. The article discusses an innovative gas extraction method based on the injection of carbon dioxide into a gas hydrate formation. This technology makes it possible to accumulate greenhouse gases in solid gas hydrate form at very low economic costs, as well as solve the problem of natural gas extraction. A mathematical model of non-isothermal filtration of carbon dioxide in the gaseous state, methane and water is constructed based on methods and equations of mechanics of multiphase media considering the replacement of methane in the gas hydrate with carbon dioxide to obtain predictive calculations. The model additionally takes into account such important factors as filtration flow of water and gas, real gas properties, Joule–Thomson and adiabatic compression effects; the process of CH4 –CO2 replacement in gas hydrate is assumed to be equilibrium. The equations are presented to calculate the values of the main process parameters (pressure, temperature, mass fraction of gas phase components, phase saturations, etc.) in one-dimensional radial axisymmetric approximation. Discrete analogues of differential equations are written down and an algorithm for numerical solution of the proposed mathematical model is given. Calculation of saturation distributions of methane and carbon dioxide hydrates is carried out considering their phase equilibrium at current values of pressure, temperature and water saturation. Numerical solutions are constructed that describe the distributions of pressure, temperature, mass concentrations of gas phase components and saturations of methane and carbon dioxide hydrates in the reservoir for the problem of injecting carbon dioxide into a reservoir initially saturated with methane and its hydrate.

A mathematical model of injection of liquid sulfur dioxide into a natural layer saturated with ice and methane, accompanied by the formation of SO2 gas hydrate on the front surface is presented. The case when the initial state of the system, as well as the pressure and temperature of the injected liquid sulfur dioxide correspond to the thermobaric conditions of the existence of sulfur dioxide gas hydrate is considered. That, the formation of sulfur dioxide gas hydrate occurs on the front surface, coinciding with the surface of methane displacement by sulfur dioxide. In this case, two characteristic zones are formed in the layer. In the first (near) zone the pores are saturated with sulfur dioxide and its gas hydrate, in the second (far) zone it is saturated with ice and methane. The dependences of the temperature and the coordinates of the formation border of sulfur dioxide gas hydrate on the injection pressure are explored. It is established that the injection pressure increase causes the rise in temperature and movement speed of the surface of the formation of sulfur dioxide gas hydrate. It is shown that at sufficiently high values of injection pressure and initial layer temperature, the temperature rises above the ice melting temperature at the formation border of sulfur dioxide gas hydrate. This corresponds to the formation of an intermediate region saturated with a mixture of water and methane. It is determined the dependence of the limit value of the sulfur dioxide injection pressure, above which an intermediate region saturated with a mixture of water and methane appears on the initial layer temperature.

Natural gas hydrate is a crystalline compound composed of water and natural gas. In the process of depressurized production of gas wells, hydrates are usually easy to form in the wellbore. Once these hydrates are formed, they will lead to liquid accumulation at the bottom of the well, which will bring great harm to the normal production of gas wells. Therefore, it is very important to understand the conditions of hydrate formation in wellbore and to predict and prevent hydrate formation. The formation of natural gas hydrate is related to natural gas composition, certain low temperature and high pressure conditions, high speed flow of gas flow, pressure fluctuation and other factors. Through the analysis of physical and chemical properties of natural gas hydrate, various factors affecting the formation and decomposition of natural gas hydrate are summarized in this paper. Taking one breath well as an example, Pipesim software establishes well model, predicts the formation of gas hydrate in wellbore under different tubing sizes, different bottom hole flowing pressure and different gas production. Finally, according to the hydrate control mechanism, the concrete measures to control the formation and decomposition of gas hydrate in wellbore are put forward.

Submarine cold seeps have recently attracted significant attention and are among the most effective indicators of gas hydrate in the oceans. In this study, remotely operated vehicle (ROV) observations, seismic profiles, core sediments, bottom seawater, and fluid vented from cold seeps in the deep-water Qiongdongnan Basin were used to investigate the origin and evolution of cold seeps and their relationships with gas hydrate. At stations A, B, and C, inactive cold seeps with dead clams, cold seep leakage with live clams, and active cold seeps with a rich mussel presence, respectively, were observed. The salinity and Na+ and Cl- concentrations of the cold seeps were different from those of typical seawater owing to gas hydrate formation and decomposition and fluid originating from various depths. The main ion concentrations of the bottom seawater at stations B and C were higher than those at station A, indicating the substantial effects of low-salinity cold seep fluids from gas hydrate decomposition. The Na+-Cl-, K+-Cl-, Mg2+-Cl-, and Ca2+-Cl- diagrams and rare earth element distribution curves of the water samples were strongly affected by seawater. The concentrations of trace elements and their ratios to Cl- in the bottom seawater were high at the stations with cold seeps, suggesting the mixing of other fluids rich in those elements. Biochemical reactions may also have caused the chemical anomalies. Samples of HM-ROV-1 indicated a greater effect of upward cold seep fluids with higher B/Cl-, Sr/Cl-, and Ba/Cl- values. Moreover, the Re/Cl- value varied between fluid vents, possibly due to differences in Re precipitation strength. Differences in cold seep intensity are also believed to occur between areas. The cold seep fluxes changed from large to small before finally disappearing, showing a close connection with gas hydrate formation and decomposition, and influenced the local topography and ecosystems.

Effect of gas hydrate formation and decomposition on flow properties of fine-grained quartz sand sediments using X-ray CT based pore network model simulation

The scheme and the corresponding theoretical model of process of methane replacement by liquid carbon dioxide at his injection in a gas hydrate layer are offered. The flat one-dimensional automodel solution corresponding to injection of carbon dioxide in semi-infinite layer through flat border is constructed. It is fixed that such solution can contain three characteristic zones in general case: the far zone, where there is a methane filtration in the porous medium which is partially saturated by methane hydrate in the absence of phase transitions; the melted zone of products of decomposition of methane hydrate (water and methane) can be formed in the intermediate zone (this zone is formed because of heating owing to injection of warm carbon dioxide); the near zone where there is a filtration of liquid carbon dioxide in the layer which is partially saturated by hydrate of carbon dioxide. The analysis of influence of parameters of layer, his initial state (temperature, pressure and hydrate saturation) and the parameters of the injected dioxide of carbon on the various modes of filtration is conducted.

A developed mathematical model offers a study of methane-carbon dioxide replacement in gas hydrate through injection of liquid carbon dioxide into a porous stratum with final sizes. Depending on the parameters of injected carbon dioxide flow and depending on the stratum boundary conditions, the process of formation of the gas hydrate comprising carbon dioxide can occur through methane replacement in the gas hydrate or through the formation of zones with free methane and water. The critical diagrams are presented that map the conditions for both regimes.

Pore waters have been extracted from sediments in the permafrost interval (110-176 m) and the gas hydrate interval (886-952 m) of the JAPEX/JNOC/GSC Mallik 2L-38 drill core and analyzed for d18O, d2H, and geochemistry. Pore waters from the permafrost interval have d18O values of -19.5 ± 0.5 o/oo (upper permafrost) and -23.1 ± 1.0 o/oo (lower permafrost) indicating the likely origin to be local, contemporary meteoric waters infiltrating these sediments during a period of subaerial exposure. Pore waters in the subpermafrost gas hydrate zone are isotopically depleted from seawater values, with d18O ranging between -14 o/oo and -8 o/oo. A weak correlation between d18O and Cl- exists in the gas-hydrate-bearing sands, consistent with the combined effect of isotopic depletion during gas hydrate formation, and enrichment associated with gas hydrate decomposition. The upper silt and deeper clayey silt sections also retain a minor correlation between isotopes and Cl-, and show strong variability in both d18O and Cl- with depth, suggesting a history of gas hydrate formation, decomposition, and fluid migration. The Cl--d18O relationships demonstrate that the original pore waters are a mixture of seawater with greater than 50% meteoric water.

Naturally occurring gas hydrates have recently become one of the most interesting and rapidly expanding research topics in the geologic oceanographic and engineering sciences. These frozen mixtures of hydrocarbon gas (mostly methane) and water occur over vast areas of the ocean floor in special temperature and pressure regimes. They are important because they:represent an enormous potential energy source,have been implicated as drivers of global climate change by releasing large volumes of methane (a greenhouse gas) into the atmosphere, andpresent potential engineering geology problems for man's operations in deep water and may be placed in the category of a geohazard. However, the identification of gas hydrates in the shallow subsurface from remotely sensed acoustic data is not straight-forward in complex geological provinces like the continental slope of the northern Gulf of Mexico and essentially no definitive data on seabed methane flux from dissociation of gas hydrates occurring at or near the seabed exists in the scientific literature. Repeated direct observations (manned submersible) of sites in the deepwater northern Gulf of Mexico where gas hydrates have been identified as exposures on the seafloor indicate that hydrates can form and decompose within the return observation period of 1-year. A 1-year in situ data collection experiment reported on in this document was designed to produce data for developing a better understanding of surficial gas hydrate formation and decomposition cycles and their possible links to oceanographic forcing. Current meter data and records of gas emission from a site of exposed gas hydrate suggest a coupling of temperature excursions of < 2°C and pulses of gas emission (gas hydrate decomposition). Increases in water temperature are strongly coherent with north-south pulses in the current field. Higher frequency cycles of gas emission also occur at the tidal time scale. However, these small variations are superimposed on longer-term events of 1–2 weeks. These in situ collected data indicate that intrusions of Loop Current water are not needed to force dissociation of surface and near-surface gas hydrates. Surface sediments associated with areas characterized by exposures of gas hydrate have extremely variable geotechnical properties. Much of this variability relates to shell content and products of early diagenesis. Introduction At present, knowledge of gas hydrates in natural settings outside of permafrost areas is just starting to be acquired. The most sitespecific data have been collected from a gas hydrate field on the Blake Ridge off the coast of North Carolina, ODP Leg 164 (Holbrook et al., 1996; Dickens et al., 1997). By using a special pressurized sampling device, gas hydrates and host sediments were kept at their in situ pressures and temperatures until they could be studied in the laboratory onboard ship. It was found that a vertical profile of samples through the hydrate zone indicated that 0-9% of the pore volume in the hydrate zone is occupied with gas hydrate and that gas comprises up to 12% of the pore volume in the underlying free-gas zone. Gas hydrate was found to occur as finely disseminated pieces in pore spaces to large nodular masses (up to 30 cm diameter). The methane in these hydrate deposits is of biogenic origin and estimates of gas in the Blake Ridge hydrate field suggest that this area alone contains enough methane to supply the United States needs for over 100 years (Dickens et al., 1997).

Mathematical model for carbon dioxide injection into porous medium saturated with methane and gas hydrate

Experimental study of the formation and decomposition of mixed gas hydrates in water-hydrocarbon system