Evaluation of Hot-Brine Stimulation Technique for Gas Production From Natural Gas Hydrates

Abstract

Summary. Thermally efficient production of natural gas can be accomplished by the use of hot brine to dissociate solid gas hydrate deposits in the earth. The advantages of brine stimulation over steam or hot-water injection are lower energy requirements for reservoir heating and hydrate dissociation, reduced heat losses, higher gas production, and improved thermal efficiency. In addition, the problems of blockage of rock pores and wellbore because of reformation of hydrates during gas production can be avoided. A mathematical model for a hot-brine stimulation technique was developed to compute gas recovery and the energy-efficiency ratio (i.e., the ratio of energy content of produced gas to heat injected) for a reservoir containing gas hydrates. The effects of variations in reservoir porosity, hydrate-zone thickness, depth, salinity of brine, brine temperature, and brine injection rate on the energy-efficiency ratio and gas production were determined. A comparison brine and steam injection cases for the same heat injection rate shows higher gas production and energy-efficiency ratio for the brine case. Introduction Huge quantities of natural gas in the form of gas-hydrate deposits exist in many regions of the world. These deposits occur in the suboceanic sediments, as well as in the arctic regions. Gas hydrates contain about 170 to 180 std ft natural gas/ft [170 to 180 std m /M ] hydrate and represent a potential unconventional source of natural gas. Gas hydrates are relatively immobile and impermeable; hence they need to be dissociated into gas and wateto produce natural gas from hydrate reservoirs. To decompose hydrates, heat must be added to a hydrate reservoir. A portion of this heat is required to raise the temperature of the reservoir to the dissociation temperature. and another portion is required for converting hydrates into gas and water. The wellbore heat losses and heat losses to overburden/underburden formations should he considered in the calculation of the actual heat requirement. The heat required to dissociate hydrates is only a small fraction (about one-tenth) of the heat obtained by burning the gas produced from hydrates. In other words, the production of natural produced from hydrates. In other words, the production of natural gas from hydrates is a thermodynamically energy-efficient process. Several ways of dissociating natural gas hydrates have been suggested. These include thermal recovery techniques, such as steam injection, hot-water injection, and fireflooding; depressurization; and injection of chemicals, such as methanol or glycol, which cause hydrate destabilization. In thermal recovery techniques, the heat required to dissociate hydrates is supplied from an external source, whereas in other techniques, this energy comes from the surrounding formations. Each of these methods has its own merits and demerits. For example, in steam injection and fireflooding, heat losses can be severe for thinner hydrate zones, but for thicker zones (greater than 150 ft [15 m]) these techniques can be thermally efficient. Fire-flooding can cause dilution of gas to be produced and can result in a reduction in its energy value. Hot-water injection will yield lower heat losses than steam injection or fireflooding, but injectivity of water in hydrate reservoirs will govern the applicability of this method. The injection-water temperature should be low enough to avoid excessive heat losses, yet high enough to avoid unrealistically high injection rates. Hydraulic fracturing can be used to improve water injectivity but can result in lower heat-transfer efficiencies because of channeling effects. Use of methanol or glycol will be governed by economics, because large quantities of these expensive chemicals will be needed to ensure sufficient gas production. production. In the depressurization scheme, pressure reduction causes destabilization of hydrates. As hydrates dissociate, they absorb heat from the surrounding formations. The hydrates continue to dissociate until they generate enough gas to raise reservoir pressure to the equilibrium pressure of hydrates at the new temperature, which is lower than the original value. A temperature gradient is thus generated between hydrates (sink) and surrounding media (source), and heat flows to hydrates. The rate of dissociation of hydrates, however, is controlled by the rate of heat flux from the surrounding media or by the thermal conductivity of the surrounding rock matrix. Holder and Angert have shown that this technique yields excellent results, if hydrates are in conjunction with the free-gas zone. Combining these recovery schemes could also be beneficial; e.g., it may be possible to produce gas from hydrates by thermal stimulation followed by depressurization. Many questions need to be answered if gas is to be produced from hydrates. First. the form in which hydrates exist in a reservoir should be known. Hydrates may exist in different forms (all hydrates, excess water, excess ice, in conjunction with free gas, or in conjunction with oil) and in different types (massive, nodular, laminated, or dispersed). Each case will have a different effect on the method of production and on economics. Second, the saturation of hydrates in reservoir rock pores is unknown. Recently, well log interpretations have been used to determine porosity and hydrate saturation. Third, there could be several problems associated with gas production, such as pore blockage by hydrates or ice and blockage of production, such as pore blockage by hydrates or ice and blockage of the wellbore resulting from re-formation of hydrates during flow of gas through the production well. Despite these concerns, hydrates in the earth exhibit several char-acteristics, especially when compared with other unconvention a natural gas resources, that increase their importance as a potential energy resource and make their future production likely. These include higher concentration of gas in hydrated form, enormously large deposits of hydrates, and their widespread existence in the world. In addition, hydrates are likely to exist in conjunction with conventional gas and oil. The production of gas from such hydrate occurrences may be possible as a bonus during conventional production of gas or oil. It is in this situation that the first venture production of gas or oil. It is in this situation that the first venture of gas production from hydrates is expected. Brine Stimulation The use of hot brine for dissociation of hydrates seems to be an attractive recovery scheme for the following reasons. JPT P. 1379