Experimental Study of Brine Injection Depressurization of Gas Hydrates Dissociation of Gas Hydrates

Abstract

Summary. The two most promising techniques for producing natural gas from hydrate reservoirs are depressurization and brine injection. This paper examines the dissociation characteristics of methane hydrates during these processes. A correlation for the rate of hydrate dissociation during brine injection as a function of salinity, brine temperature, brine injection rate, pressure, and hydrate/brine surface area is presented. Depressurization experiments show that hydrate dissociation results in a decrease in the rate of pressure decline and contributes significantly (15% to 70%) to the cumulative gas production. Introduction Extensive deposits of naturally occurring gas hydrates have been found in many regions of the world. Because the formation of gas hydrates requires such favorable thermodynamic conditions as low temperatures and high pressures along with physical contact of natural gas with water, in-situ natural gas hydrates occur in three types of environments:Arctic onshore sediments overlain by a continuous thick permafrost layer (e.g., Prudhoe Bay and Kuparuk River area of Alaska's North Slope, Canada's Mackenzie delta and Northwest Territories, and Messoyakha and other arctic regions of Russian Siberia);sediments underneath arctic offshore waters (e.g., Beaufort Sea shelf and slope, Bering Sea slope, Navarin and Aleutian basins, North Pacific Ocean, and the Gulf of Alaska; anddeep marine offshore sediments in tropical regions (e.g., off-shore Guatemala, Gulf of Mexico, Blake Bahamas, Baltimore Canyon, North Sea, and offshore California). Because of the potentially large size of this resource, gas hydrates are considered as a future, unconventional resource of natural gas. Recovery Methods for Hydrate Reservoirs Gas hydrates are relatively immobile and impermeable; hence, it is essential to decompose them into gas and water before gas can be produced from hydrate reservoirs. The transformation of solid gas hydrates to gas and water requires latent heat of hydrate dissociation. Several techniques proposed for decomposing natural gas hydrates are based on three common principles:reduction in reservoir pressure below the equilibrium hydrate-dissociation pressure, as in the depressurization method;heating the reservoir above the hydrate-dissociation temperature by external means, as in such thermal recovery methods as hot-water, steam, or brine injection; andinjection of chemicals, such as methanol and glycol, that act as hydrate inhibitors and reduce the hydrate-dissociation temperature. Each method has merits and disadvantages. Holder et al. reported that the heat required to dissociate hydrates is only 10% of the heating value of the produced natural gas. Thus, the thermal recovery methods are energy efficient from the thermodynamic viewpoint. In steam injection, however, the heat losses in the well-bore and reservoir can be severe, especially for thinner hydrate zones. Hot-water injection will yield lower heat losses than steam injection, but the injectivity of water in the hydrate reservoirs will govern the applicability of this method. Fracturing can be used to improve water injectivity, but that may result in lower heat-transfer efficiencies because of channeling effects. Use of glycol and methanol for dissociating hydrates is governed by economics because large quantities of these expensive chemicals are needed to ensure sufficient gas production. With results of the mathematical model for the hot-brine simulation technique, Kamath and Godbole showed that injection of hot brine to dissociate hydrates is thermally more efficient than steam or hot water because brine also acts as a hydrate inhibitor. Brine causes reduction in dissociation temperature which reduces reservoir heating, causes latent heat of dissociation and heat losses, and increases the gas production rate. In the depressurization method, hydrates dissociate by means of heat conduction from the surrounding formations. The rate of dissociation, however, is controlled by the thermal conductivity of the surrounding formations. In cases where hydrates overlie a free-gas zone, this technique is advantageous because hydrate dissociation can contribute significantly to gas production from a free-gas zone. This method has been used commercially to produce natural gas from hydrates in the Messoyakha field in the USSR. Hydrate-Dissociation Phenomenon The hydrate-dissociation phenomenon is more complex than melting or sublimation processes because it involves the presence of three phases: gas, water, and solid hydrates. Experimental data on dissociation characteristics of propane and methane hydrates during hot-water injection showed that the rate of hydrate dissociation is a function of the temperature-driving force at the hydrate/water interface and interfacial area. The heat transfer to dissociating hydrates is considered to be analogous to the nucleate boiling phenomenon. Selim and Sloan considered the hydrate-dissociation process under thermal stimulation to be a moving-boundary ablation process and presented a mathematical model that assumes that the water formed by dissociation is blown away from the hydrate surface by the produced gas. Ullerich et al. conducted an experimental study to measure methane hydrate-decomposition -rates under constant heat-flux conditions. Their experimental results agreed with Selim and Sloan's model within 10%. Selim and Sloans extended their earlier model to consider hydrate dissociation in porous media. Roadifer et al. presented a 2D finite-difference thermal model to simulate dissociation of hydrates in porous media in cylindrical geometry by considering the process to be a classic movingboundary Stefan problem. Kim et al. experimentally studied the intrinsic kinetics of the decomposition of methane hydrates with a semicontinuous stirred tank reactor. They described the process as destruction of the host (water) lattice at the surface of a hydrate particle and desorption of the guest (methane) molecules from the surface. Jamaluddin et al. 11 recently developed a mathematical model for the decomposition of a hydrate block under thermal stimulation conditions by coupling the intrinsic kinetics with the heat-transfer rates. Other hydrate-reservoir-simulation studies under thermal stimulation conditions include frontal sweep and fracture flow models for hot-water injection, 12 a single-well cyclic-steam simulation model, and a hot-brine stimulation model. Mc Guire also presented a model for hydrate dissociation under a depressurization scheme. Holder et al. presented simulation results for depressurization of a hydrate reservoir overlying a free-gas zone. SPEFE, December 1991 P. 477^